[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, November 2, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. The Moon fades this week from near full to its third quarter, reaching the phase about the time of moonset the night of Wednesday the 7th (or the morning of Thursday, the 8th). As it goes, it will be seen to the west of Saturn the night of Friday, the 2nd, to the east of it and closer the night of Saturday, the 3rd. The night of Monday, the 5th, be sure to watch the Moon play closely with Jupiter, our satellite less than two degrees north of the giant planet around midnight. The two brightest planets in the sky, Venus and Jupiter, are highlighted this week. (Yes, Mars can get brighter than Jupiter, but for only a very short amount of time near its favorable oppositions.) The morning of Friday, the 2nd, Venus stands four degrees north of Spica, which the Sun has just cleared, meaning that Venus is now rising rather late, about 5 AM Standard Time, just after the birth of morning twilight. On the same day, Jupiter becomes momentarily stationary, that is, it ceases its normal forward motion easterly through the stars, and reverses into retrograde, as the Earth prepares to come between it and the Sun. Two days later, on the morning of Sunday, the 4th, Mars passes 2 degrees south of Neptune, an event only visible if you have a telescope, Neptune rather far below naked-eye brightness. Since Venus and Mercury maintain their close connection this week, the two less than a degree apart for most of this period, both actually are seen north of Spica. Go look, and use Venus to find the smallest inner planet (Mercury), which will be the brightest body close to bright Venus, both notably bright in morning twilight. With the Moon now gone from the evening sky, we can look again at the stars. Even in early evening, the Great Square of Pegasus can be seen moving high in the southeastern sky. The Square's northeastern star is part of both Pegasus and Andromeda, which climbs in streams of stars to the northeast. In the middle of Andromeda, if you have a dark sky, you might spot the fuzzy patch of the Andromeda Nebula, which in the early twentieth century was discovered to be a large nearby galaxy comparable to our own (our 200-billion-star assembly that makes the Milky Way). Nearby here takes on a relative meaning, as this great spiral galaxy, also called M 31, is two million light years away, the farthest thing visible to the naked eye. Comparable in distance is another, M 33, the great spiral galaxy in Triangulum, which under ideal circumstances (which includes being young!) is also visible to the naked eye, though just barely. The southern hemisphere contains two more naked-eye galaxies, the Large and Small Magellanic Clouds in the constellations Dorado (the Swordfish) and Tucana (the Toucan), these two requiring the observer's latitude to be well south of 20 degrees north latitude. STAR OF THE WEEK. MATAR (Eta Pegasi). Pegasus (the Flying Horse) is so well known for its Great Square that we sometimes give the other stars little thought. Coming off the northwestern star of the Square, Scheat, is a pair of stars that with Scheat make a rather prominent triangle, the northern one Matar, which Bayer called Eta, and which (at mid third-magnitude, 2.95) actually ranks fifth in brightness rank (ignoring Alpheratz, Delta Pegasi, which is actually Alpha Andromedae). Matar, from Arabic, has to do not with a horse, but with rain, though just what is unclear, one source suggesting lucky rain. At a distance of 215 light years, Matar is double and may well be quadruple, consisting of a very unequal pair of pairs, an unbalanced double-double. The bright naked-eye star is actually a close pair separated on the average by only three astronomical units (a bit over half the size of Jupiter's orbit). The brighter, 262 times the luminosity of the Sun, is an evolving class G (G2) 5100-Kelvin giant with a quiet, contracting helium core, the fainter a hotter (7800 Kelvin) class A (A5) hydrogen-fusing solar type dwarf. The measured orbit (its period 2.24 years) reveals the stars to contain respective masses 3.2 and 2.0 times the mass of the Sun. Ninety seconds of arc away is a much fainter (ninth magnitude) class G (G5, a bit cooler than the Sun) star that separates into another pair only 0.2 seconds of arc (at least 13 astronomical units) apart that take at least 34 years to orbit. That the two doubles are actually related is not fully known, some say yes, others no, that they are a line-of-sight coincidence. The luminosity of the dim pair, however, is close to being right for G stars if assumed to be at Matar's measured distance, so they are
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Sunday, August 24 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. The Moon passes through its first quarter early in the week, on Saturday, the 25th, thereafter waxing toward its full phase, brightening as it goes through Scorpius and Sagittarius, bottoming out at its most southerly position of the month the night of Tuesday, the 28th. The night of Saturday, the 25th, the Moon will pass 12 degrees north of Antares in Scorpius, while the following night it will be approaching its passage to the north of Mars. The night of Thursday the 30th, it is Neptune's turn to be visited. Mars is now transiting the meridian to the south in bright twilight; by darkness it has moved into the southwest, where, as seen in the early evening, it will remain throughout the year. For observers in the mid-northern hemisphere, setting time has moved to just after midnight daylight time. But as Mars prepares to set, Saturn rises in the northeast, followed around 2 AM by Jupiter, which has now pulled rather far to the west of Venus, the brilliant morning star rising before the onset of twilight until the end of October (by which time Saturn and Jupiter will be rising in early- to-mid evening). Not that anyone will notice, but Pluto ceases its westerly retrograde movement against the stars of southern Ophiuchus this week, on Saturday the 25th. Three days later, Ceres, the largest asteroid (570 miles -- 910 kilometers -- wide and also invisible without a telescope), does the same thing. The orbits of asteroids are commonly more highly tilted than are those of the planets (Pluto excepted). Ceres, now beneath the Little Milk Dipper in Sagittarius, is about as far south as it gets, some 8 degrees below the ecliptic (the apparent path of the Sun). As August heads towards September, the sky's fifth brightest star, Vega in Lyra, passes nearly overhead in early evening for those in mid-northern latitudes. A bit farther north (and a bit west) is the much fainter head of Draco, the Dragon. A line drawn south from Vega passes through the line of stars that makes the tail of Serpens the Serpent (the only constellation that comes in two parts, the head and tail divided by Ophiuchus), then much farther down back to Sagittarius, which sits atop Corona Australis, the Southern Crown. Once the Moon gets out of the way, you can admire the great star clouds of the Milky Way that seem to blanket Sagittarius, the celestial archer, one of two mythological centaurs in the sky, the other Centaurus, a much larger constellation now escaping to the west, its southern portions far below the horizon for most people in the northern hemisphere. STAR OF THE WEEK. MENKENT (Theta Centauri). Centaurus is dominated by its two brightest stars, Rigil Kentaurus (Alpha Centauri, third brightest star in the sky and the nearest star to the Earth) and first magnitude Hadar (Beta Centauri). Though these two shine brilliantly to the lucky residents of the southern hemisphere, neither is visible from mid-northern latitudes, so if those that live there wish to know Centaurus, they must begin with the third brightest star in the constellation, Menkent, to which is assigned the rather lowly letter Greek letter Theta (Gamma Centauri rather oddly coming in second). The name comes from an Arabic word for shoulder (of the Centaur), to which is attached the Latin abbreviation for Kentaurus for Centaur, tying Menkent back to the constellation's luminary, Rigil Kentaurus. Menkent, at mid-second magnitude (2.06) is but four percent fainter to the eye than Polaris. Much closer than Polaris, however, only 61 light years away, it is intrinsically much less luminous. At the warm end of class K (K0), the star is a near-clone of the northern hemisphere's Pollux, just fainter to the eye than Pollux because it is 80 percent more distant. Menkent, with no known or even suspected companions, is about as pure a sample of its class you can come by. From its 4780 Kelvin surface, this yellow-orange star radiates at a luminosity 60 times that of the Sun, the star's radius 11 times solar. Well along in its evolution, Menkent is now fusing helium into carbon and oxygen in its deep core. The star's only offbeat property is its rather high proper motion, its speed across the line of sight. Approaching us at only one kilometer per second, it is speeding past us at 65 kilometers per second, about twice normal, suggesting that the star really belongs to the outer part of the Galaxy's disk and is only visiting the solar neighborhood. Jim Kaler Professor of Astronomy Phone: (217) 333-9382 University of
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the 5-day period starting Sunday, August 19 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. During this shortened period, the Moon waxes through its crescent phase, first quarter reached next Saturday, August 25th. The night of Wednesday, the 22nd, the Moon will make a nice pass to the north of the star Spica in Virgo, which is now seen well to the southwest as twilight draws to a close. In spite of the growing Moon, the evening still belongs to Mars, which shines brightly just to the west of south as the sky darkens. Follow its progress as it ever-so-slowly (but with increasing speed) pulls to the east of reddish Antares in Scorpius, the star not very far to the right of the red planet. Three planets ride the morning sky, the leader Venus. Moving to the east against the stars of Gemini, the brilliant planet passes seven degrees to the south of Pollux, Gemini's brightest star, on Wednesday, the 22nd. To the west of Venus, find bright Jupiter, and rather well to the west of Jupiter is Saturn, the two giant planets of course far beyond Venus, Jupiter almost 5 times farther away, Saturn nearly eight. The night of Tuesday, the 21st, the Winter Solstice in Sagittarius will be directly south at 9 PM Daylight Time. The Solstice, the most southerly point of the ecliptic, marks the position of the Sun on the first day of northern winter. Just rising exactly in the east at that time will be the Vernal Equinox, the point at which the solar path crosses the equator. Immediately to the left of the equinox, which lies 23.4 degrees south of the celestial equator, is a small fuzzy spot visible to the naked eye, a large cloud of interstellar gas and dust, the Lagoon Nebula, or Messier 8. Binoculars will make the cloud immediately jump from the background. Almost directly north, and nearly overhead at mid-northern latitudes, lies one of the most ancient constellations of the sky, Hercules, the celestial memorial to the great hero, the figure originally known as the Kneeler. Toward the northwestern corner lies another fuzzy spot, the greatest of the northern globular clusters, Messier 13, the Great Cluster in Hercules. The telescopic view of the cluster, which contains somewhere around a million stars, is stunning. To the west of Hercules is the semi-circle that makes Corona Borealis, the Northern Crown; its southern hemisphere counterpart, Corona Australis, the Southern Crown, lies south of Sagittarius. To the east of Hercules find Lyra, the Harp, brilliantly marked by Vega, the fifth brightest star in the sky. STAR OF THE WEEK. ZETA HER (Zeta Herculis). With the exception of the brightest stars, star names were handed out more by position than brightness, as attested to by Zeta Herculis. At bright third magnitude (2.81), just barely the second brightest star in the constellation Hercules (right behind Kornephoros, Beta Herculis), Zeta Herculis was ignored by the ancients. Even Bayer rather ignored it by giving it the sixth letter in the Greek alphabet, the Alpha designation going to faint-third-magnitude Rasalgethi clearly because of its position in the Hero's head. In spite of the star's lack of public prominence, it has a lot to recommend it. Zeta Her is actually double, a modestly bright third magnitude star orbited by a sixth (5.53) companion only a second of arc or so away. The brighter star, Zeta Her A, is a class G (G2) subgiant with the same temperature (5780 Kelvin) as the Sun (which is also a G2 star). With a mass some 50 percent greater than the Sun, however, and beginning its evolution toward gianthood (its core hydrogen fusion likely shut down), Zeta Her A is 6 times more luminous than the Sun with a radius 2.5 times as large. Nevertheless, the star gives a good idea of what the Sun would look like from a great distance, in Zeta Her's case 35 light years. The companion, a cooler class G (G7) hydrogen-fusing dwarf with a luminosity only 65 percent that of the Sun and a mass about 85 percent solar, orbits with a period of 34.5 years at a mean distance of 15 Astronomical Units (over 50 percent farther than Saturn is from the Sun). A rather high eccentricity takes the two as far apart as 21 AU and as close as 8 AU. Under such conditions, planets would very likely be impossible. Astronomers have identified a number of extended moving groups of stars that seem to have some common origin (the most famed the stars that are related to the Ursa Major cluster). The Zeta Herculis moving group, of which the star is the leader, contains stars as far removed as Perseus, Lupus, and Octans, the dim constellation that surrounds the South Celestial Pole.
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, July 6, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. The Moon falls between quarters this week, spending the entire seven day period in its waning gibbous phase, full moon having taken place last Thursday, July 5th, the third quarter not being passed until next Friday, July 13. As it moves through the constellations of the Zodiac, it passes south of Neptune on Saturday the 7th, south of Uranus on Sunday the 8th (both planets in Capricornus), and its apogee (where it is farthest from the Earth) on Monday the 9th. The planets put on quite a show for us this week. In the evening, Mars, four degrees south of the ecliptic and about as southerly as it can get, glows a brilliant red just to the east of the bright reddish star Antares in Scorpius. The planet, brighter than the brightest star and comparable to Jupiter, lies above the great curve of stars that make the Scorpion's tail. Since Mars moves in orbit only a bit slower than Earth, we will have it with us in the evening for the rest of the year. The big event, however, takes place the morning of Friday, the 13th (a lucky day!) when Venus, Saturn, and the bright star Aldebaran of Taurus all gather together into a tight equilateral triangle below the Pleiades star cluster (Saturn down and to the left of Venus, Aldebaran below, to the south of Saturn). Venus is now at its earliest rising of the year, coming up just before 3 AM daylight time, about an hour ahead of twilight. As the sky grows light, you might then glimpse Jupiter and Mercury just above the east- northeast horizon, Mercury (the fainter) about two degrees down and to the right of Jupiter. The pair comes into conjunction the day before, on Thursday the 12th. Three days before, on Monday the 9th, Mercury passes its greatest western elongation, the little planet 21 degrees to the west of the Sun. Even the asteroids get into the act, as Ceres, the largest of them (though only 900 kilometers across and invisible to the naked eye) passes opposition with the Sun on Saturday, the 7th. Only number 4 in discovery order, 500-kilometer wide Vesta (the third largest of them), can be seen without optical aid, and then just barely. Nearly 100,000 of the critters have been discovered. Vast numbers of tiny ones small enough to hold in your hand hit the Earth every day as meteorites. Two crowns ride the nightly sky, one in the north, the other in the south. As the sky darkens, look 30 or so degrees north of the equator (nearly overhead for mid-northern latitudes) to find the gentle curve that makes Corona Borealis, the Northern Crown. If you are far enough south, around 1 AM you can watch the passage of Corona Australis, another curve of stars that makes the Southern Crown, which lies directly below the Little Milk Dipper of Sagittarius. STAR OF THE WEEK. BETA COM (Beta Comae Berenices). The naked-eye sky is dominated by luminous stars, stars that are far brighter than the Sun. Only a few stars like the Sun and fainter sneak through. They simply do not have enough radiative power to be visible unless they are quite close to us. (Selection effects like this one, in which Nature shows us what she wishes, pervade science: the population of microbes far exceeds that of elephants, yet only the elephants are visible without some kind of aid.) It is then quite surprising to find the luminary of a constellation to be a near-solar clone. Fourth magnitude (4.26) Beta Com, with no proper name at all, just barely beats out Diadem (Alpha Comae Berenices) for the honor (such as it is) of being the brightest star within the faint but glorious constellation Coma Berenices (Berenices Hair). Though within the formal constellation boundaries, Beta Com is not a part of the star cluster that makes the constellation's heart, its distance of only 30 light years placing it 1/9 as far as the cluster and half as far as Diadem. At 6000 Kelvin, this class G (G0, alternatively classed as F9.5) star is only slightly warmer than our class G2 (5780 Kelvin) Sun. A hydrogen-fusing dwarf like the Sun, Beta Com is only 37 percent more luminous than is the Sun and but 10 percent larger, the result of 10 percent greater mass. There is some suggestion that the star might have a close companion (detectable only via spectrograph), though such a neighbor is unconfirmed and probably unlikely. How sunlike is Beta Com? It is a bit metal-rich, containing perhaps 7 percent more iron (relative to dominant hydrogen) than the Sun. No planets have yet been spectroscopically detected as they have for several similar stars. A search for a residual dusty disk (one left over from planet formation) around the star has also
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the short week starting Sunday, June 10, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. The Moon, beginning the week in its waning gibbous phase, passes third quarter the evening of Wednesday, the 13th, and thereafter wanes through crescent, every day rising later after midnight. It passes south of Neptune, in Capricornus, on Sunday the 10th, and south of Uranus, in the far eastern end of the same constellation, the following day. The two planets outward from the Earth provide a wonderful contrast. Jupiter, the Solar System's giant, and ordinarily the second brightest of planets, passes conjunction with the Sun on Thursday the 14th, and will be quite invisible. Mars, on the other hand, is now at its best, passing opposition to the Sun the day before, Wednesday, the 13th. On opposition night, Mars will rise at sunset, set at sunrise, cross the celestial meridian to the south at midnight (1 AM daylight time), and have its greatest angular retrograde (westerly) speed. It will also about as close to the Earth as possible during this orbital round, 68.6 million kilometers, or 42.6 million miles. Unfortunately, the red planet, which now shines brighter than any star in the sky (and is now exceeded only by the morning's Venus, the Moon, and the Sun), will also be at about its most southerly position (just to the west of Sagittarius), and for northern observers about as low as it can get. Turbulence in the Earth's atmosphere then makes detail more difficult to view by telescope. After opposition, Mars will begin to rise before sunset, will move more and more into the early evening sky, and will be with nicely us for the remainder of the year. Mars passes opposition to the Sun every 780 days. The orbit is so eccentric that opposition distance vary considerably. The next one will be better. The minimum distance is 56 million kilometers (35 million miles), the maximum almost twice as great. Only Venus comes closer. Mars actually lies in far southern Ophiuchus between Sagittarius and Scorpius, providing a good chance to compare the color of the planet with its namesake Antares (Ares the Greek version of the war god), which is nicely visible to the far right of Mars. As spring slowly blends into summer, these beautiful summertime constellations, which contain the Milky Way, will begin to overtake the more drab spring skies, which now still dominate in the early evening, Virgo and Spica nearly due south as night falls. Watch as they slip away a degree per day to the west as the sky reflects the degree-per-day motion of the Earth around the Sun. If your skies are dark, you might make out the dim box of stars that forms Libra, the scales, the zodiacal constellation that lies between Virgo and Scorpius. STAR OF THE WEEK. ZANIAH (Eta Virginis). An old term -- before we understood that stars do move -- referred to the fixed stars, the phrase really meant to distinguish real stars from the wandering stars, the planets. The names applied to stars sometimes appear about as fixed as the stars themselves, that is they, or at least some of them, move around too. Zaniah, the Eta star of Virgo, the Virgin, refers an angel, the angel of a-awwa (the meaning unknown, and with apologies to Arabic readers for leaving out the necessary accents), and was originally applied to Porrima (Gamma Virginis), and later fell to dimmer (mind-fourth magnitude, 3.89) Eta. In a rather special place, Zaniah, to the west of Porrima, is squeezed between the celestial equator and the ecliptic. Of the brighter stars that make the constellation figures, Zaniah is one of the closest to the equator, only 2/3 of a degree to the south of it, and only 10 degrees to the east of the autumnal equinox, the point where the Sun crosses the equator on its way south in September. Zaniah, 250 light years away, is classed as an A (A3) subgiant, the latter meaning that the star seems to be about to give up its central hydrogen fusion, if it has not already. From its surface temperature of 8800 Kelvin (and of course its distance) we can calculate a luminosity 130 times that of the Sun. The star's status and properties, however, are seriously compromised by its seeming triple nature. None of the components can be resolved by eye at the telescope. Ultrashort imaging (to avoid smearing of the image by twinkling) in addition to occultations by the Moon reveal a pair of stars (one fourth magnitude, the other fifth) separated by but 0.12 seconds of arc, or around 10 Astronomical Units. One of them, probably the brighter, is revealed by the spectrograph to be a much closer double with a period of 72 days and an average separation of only half an astronomical unit.
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, June 1, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. The next Skylights will appear Sunday, June 10. The Moon passes through its full phase this week, when it is opposite the Sun, on Tuesday, June 5. It will on that day rise near sunset and set near sunrise. Thereafter, it begins to thin through its waning gibbous phase. The night of June 5 the Moon will appear up and to the right of the planet Mars, while the following night it will have moved to appear up and to the left of the red planet. As we approach the beginning of summer in the northern hemisphere, when the Sun will be as far north as it can get (and as high as it can get for northerners), this full Moon will be the year's second-most southerly, the Rose Moon rising in the southeast, setting in the southwest. The brightest and dimmest planets (as seen from Earth) make the rest of the planetary news. Venus, very slightly dimming, reaches its greatest elongation west, when it is 46 degrees to the west of the Sun. This lovely planet, third only to the Sun and Moon in apparent brightness, now rises in the east just ahead of morning twilight. Even though the angle between it and the Sun now decreases, however, Venus will continue to rise earlier, and until mid-August into ever darker skies. Earliest Venus-rise will occur around mid-July. At the same time, dim Pluto, not visible without a good-sized telescope, is in opposition to the Sun the night of Monday, June 4. When the Moon reaches its full phase, it will lie roughly 10 degrees below the frigid outer planet, which some take not to be a planet at all. In truth, Pluto appears to be some kind of hybrid object that bridges the gap between the outer planets and the building blocks (the comets) that created them. Apparently there was just not enough raw material in these distant reaches of the Solar System to make a respectable planet like Neptune or Uranus. The early evening presents us with the tail of the longest constellation in the sky, Hydra , the Water Serpent, which wraps itself a third of the way around the celestial sphere. Find Corvus, the Crow, a small irregular box of stars that for northerners appears rather low in the south around 9 PM. The top two stars point leftward to Spica in Virgo, while the bottom two point to otherwise un-named Gamma Hydrae, the next-to-the-last bright star (such as it is) that lies in the celestial snake. Snakes of some sort are quite popular, others being summer's Serpens (the Serpent), which comes in two parts, the southern hemisphere's Hydrus (another water snake), and, if you wish to stretch the definition a bit, the northern hemisphere's Draco, the Dragon, whose tail winds between the Dippers. STAR OF THE WEEK. GIAUSAR (Lambda Draconis). The front bowl stars of the Big Dipper are famed for pointing at Polaris in Ursa Major's Little Dipper. What, however, of the stars along the way? The path to Polaris is so familiar that we rarely stop to see the other sights that lie along it. About a third of the way from Dubhe (the Big Dipper's front bowl star) to Polaris (and a just a bit to the east) lies Giausar, the tail star of Draco the Dragon, to which Bayer assigned the Greek letter Lambda. The Arabic name of this mid-fourth magnitude (3.84) star is confusing at best. At times thought to refer to a central one much as does the Arabic name of Orion, the word actually refers to the nodes of the lunar orbit, the points at which the Moon crosses the ecliptic plane twice a lunar month -- which makes little sense, since Draco contains the ecliptic POLE, and is therefore quite distant from the ecliptic itself. Rather clearly, the name was applied in error. The star is about as neglected by research astronomers as it is by even dedicated skywatchers, rather too bad as it has -- as a class M (M0) red giant -- one of the rarer of naked-eye types. Over the past 20 years it has been mentioned less than 40 times. It is neglected by other stars too, as it has no known companions. Giausar is one of the sky's cooler and larger stars. With a temperature of 3525 Kelvin, it shines to us (if the estimate of invisible infrared radiation is correct) from a distance of 335 light years with a luminosity 1870 times that of the Sun, which leads to a radius of 0.55 astronomical units, half the size of Earth's orbit. Large enough to have had its angular diameter measured (at 0.0073 seconds of arc), direct measure of radius makes it somewhat smaller, a mere 0.37 astronomical units, about the size of Mercury's orbit. Even the star's general behavior is obscure. Classified as a semi-regular variable, there is some indication that it changes
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, May 25, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. Skylights appears a day early this week. The Moon passes through its first quarter this week on Tuesday, May 29. The night of Friday, the 25th, the slim crescent will (for those in the Americas) stand smack in the middle of Gemini with Castor and Pollux right above it as it sets. Two days before the quarter, the Moon passes its perigee point, where it is closest to the Earth. The dark markings on the lunar surface, the maria, or lunar seas, are huge lava-filled impact basins -- just very large craters. Three of these circular features are readily visible at first quarter. From upper left to lower right, see if you can make out Mare Serenitatis (the Sea of Serenity), Mare Tranquillitatis (the Sea of Tranquillity), and Mare Foecunditatis (the Sea of Fertility). Toward the right edge of the Moon perhaps you can also make out smaller Mare Crisium (the Sea of Crises). Apollo 11, carrying the first lunar voyagers, set down on Mare Tranquillitatis on July 20, 1969. Saturn is truly gone from sight as it passes conjunction with the Sun this week, on Friday, the 25th. You can forget Jupiter too, at least for now. Both planets, however, will make nice display pieces in the morning sky by mid-summer. By odd coincidence (such abound in astronomy, as there are so many events going on), the planet Uranus enters retrograde motion in eastern Capricornus at almost exactly the moment that the Moon passes first quarter. At bright sixth magnitude, Uranus is just visible to the naked eye in a dark site with no moonlight present. Neptune (a telescopic planet), to the west of Uranus, began its retrograde movement earlier in the month. Mars is moving backwards too though, since it has just reversed its direction, at a not-very-fast pace. That will pick up as the red planet approaches its very bright opposition to the Sun on June 13. By then it will have moved from its current residence in Sagittarius into Ophiuchus, the only constellation not of the classical zodiac through which runs the ecliptic (the apparent path of the Sun). If you are far enough south, this is the moment for the Southern Cross, which crosses the meridian in early evening around the time that northern observers see Corvus the Crow to the exact south. In between is the tail of Hydra, the Water Serpent, and the bright stars of western Centaurus. The top two stars of Corvus point leftward to Spica, in Virgo. If decently south of 40 degrees north latitude, look about 35 degrees due south of Spica to see the fuzzy ball made by the greatest star cluster in the Galaxy, the magnificent globular cluster Omega Centauri. Corrections: In last week's Star of the Week, Alkes, note that the star is converting helium into carbon and oxygen (not helium into hydrogen as stated!) and that the proper motion is given 1000 times too high. Thanks to those who caught the errors. STAR OF THE WEEK. MIMOSA (Beta Crucis). Few are the first magnitude stars, as their apparent brightness requires either great luminosity, closeness, or both. Mimosa satisfies the first requirement. Tied for apparent brightness at number 19 in the sky with Deneb (apparent magnitude 1.25), among the stars of first magnitude it ranks number 10 in luminosity. The second brightest star of Crux, the Southern Cross, Mimosa is too far south to have a traditional proper name, one assigned by the ancients. The origin of its name, which comes from Latin and means actor (the word also used in botany), is unknown. Mimosa is a magnificent blue-white, very hot class B (B0.5) giant star with a temperature that soars to 27,600 Kelvin. Such heat causes the star to radiate most of its light in the invisible ultraviolet. To the eye, Mimosa would appear 3000 times brighter than the Sun; if all the radiation is taken into account, the luminosity climbs over 11 times higher to 34,000 solar. From these figures we calculate a radius of 8.1 times that of the Sun, in good agreement with the value of 8.4 solar found from the small angular radius. Mimosa is clearly a close double, but one whose components are too near to each other to resolve separately and that take almost exactly 5 years to orbit. The nature of the second star is unknown, but the agreement between the two radii above suggests that it does not add much to the total luminosity. If it does not, Mimosa has a mass of 14 times that of the Sun, and the pair is separated by around 8 astronomical units. Mimosa is also a multiply-periodic Beta Cephei star (named after Alfirk) that varies between magnitudes 1.23 and 1.31 with periods of 5.68, 3.87, and 2.91 hours. Though the
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, May 18, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. Our Moon passes through its new phase this week on Tuesday, the 22nd. Watch as the waning morning crescent, seen in eastern dawn, thins early in the week and then appears on the other side of the Sun to the west the evening of Thursday, the 24th. The morning of Saturday the 19th, the Moon will appear a few degrees to the south of brilliant Venus. Saturn is now gone, lost to evening twilight, and Jupiter, just a little behind, is quite difficult to find, as it now sets just a bit over an hour after the Sun. The evening sky does present us with a fine apparition of Mercury, however. Only a day before the Moon passes new (on Monday the 21st), the little planet, the one closest to the Sun, passes its greatest eastern elongation, when it is 22 degrees to the east of the Sun and maximally visible. For those in North America, the Moon will provide a fine guide, as the lunar crescent will be just a few degrees to the left of the planet the night of Thursday, the 24th. Look in bright twilight and follow the Moon as it sets and the sky darkens to see Mercury emerge from the fading glow. The planet remains mysterious. Of the nine planets, only Pluto is smaller. Not quite 40 percent the size of Earth, Mercury has the largest iron core relative to its size of any of them. Dangerously close to the Sun, it has been visited by but one spacecraft (Mariner 10 passing it 3 times in 1974 and 1975), and only about half has been imaged. So close in angle to the Sun that it is visible only in the daytime or in twilight, surface features are nearly impossible to see from Earth. Once Mercury sets, the evening sky awaits the rising of Mars, which climbs above the southwestern horizon around 10:30 PM Daylight Time. Now moving retrograde in far western Sagittarius, the red planet, fourth from the Sun, and the last of the terrestrial planets (those constructed like Earth), will be rising in evening twilight by the end of the month. In the early evening, for those in mid-northern latitudes, look out perpendicular to the plane of our Galaxy. The Milky Way is about as absent as it can get, and lies around the horizon where it is invisible. Our view is unobstructed by the dust in the Galactic plane, allowing us to see outward as far as our instruments will carry us, to distant galaxies billions of light years away. From the southern hemisphere, however, the Milky Circle is high and spectacular as it passes through Centaurus and Crux, the Southern Cross. STAR OF THE WEEK. ALKES (Alpha Crateris). Among the dimmest of all classical constellations is Crater, the Cup, which with Corvus, the Crow, rides the back of Hydra, the Water Serpent. The only proper name in the constellation, Alkes comes from Arabic and means the wine cup, the star standing in for the whole figure (the name also clearly related to the English alcohol.) Though mid-fourth magnitude (4.07), Alkes received the Alpha designation from Bayer. It takes second place to the un-named Delta star (that is, to Delta Crateris), and is in a virtual dead heat with Gamma Crateris. Alkes is yet one more orange class K (K0) giant star, though one with an interesting difference. At a distance of 175 light years, Alkes shines 80 solar luminosities into space from a 4725 Kelvin surface, giving the star a calculated radius 13 times that of the Sun. Alkes, with a mass estimated at around 2.5 times solar, is clearly in the clump, a set of stars that all have about the same characteristics of luminosity and temperature and that are all fusing helium to hydrogen in their cores, Arcturus and Aldebaran bright examples. Unlike most helium-burning clump stars, unlike most stars around us, Alkes is also a modest high velocity star. Most of our neighbors are going around the Galaxy at a speed somewhat in excess of 200 kilometers per second. However, all the orbits are a bit different, so they drift relative to each other at speeds of 20 to 40 or so kilometers per second. From its rate of angular motion across the sky (480 seconds of arc per year relative to the distant background) and its speed away from us of 47 km/s, Alkes is moving relative to the Sun at 130 km/s, showing it to be a visitor from a different part of the Galaxy. The star has on occasion been placed into the group of super-metal-rich stars. Though the metal content is probably more solar, it is clear that the star has come to us from the inner metal-rich part of the Galaxy, the so-called bulge. Consistently, Alkes has also been dropped into an odd category of 4150 stars, which seem to have a high abundance of cyanogen, the CN molecule.
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, May 4, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. The Moon approaches full the early part of the week, passing that brilliant phase on Monday May 7th among the dim stars of Libra, thereafter waning through gibbous, rising after sunset. Sending so much sunlight back to Earth, the full (or near-full) Moon is so bright that the surrounding stars are all but invisible. In eastern dawn, Venus also passes through its greatest possible brightness, on Friday the 4th, the planet shining brightly amidst the equally dim stars of Pisces and very close to the vernal equinox, the point on the celestial equator passed by the Sun on the first day of spring. A view through the telescope shows Venus as a large crescent. Though we see only a small section of Venus's daylight side, its proximity to us still throws a maximum amount of sunlight in our direction. The observations of Venus's phases was one of the key proofs to Galileo that Copernicus was right, that the Earth really does go about the Sun. Western twilight holds the disappearing treasure of bright Jupiter, Saturn, an hour to the west and in a bright sky, difficult to view. If you can find it, perhaps you can also find Mercury down and to the right early in the week. Jupiter will soon follow into near- invisibility. Brightening Mars, however, makes up the difference, as it now rises in Sagittarius around 11:30 PM Daylight Time. Mimicking Venus as a celestial marker, Mars now lies only a couple degrees above the center of the Galaxy, the center of the Milky Way. The range of celestial brightness is astonishing. As bright as Venus may appear to us, the full Moon is 1000 times brighter, and the Sun a million times brighter yet (so bright as to be able to damage the eyes). Venus, however, is at maximum 15 times brighter than the brightest star, Sirius, and a remarkable 25,000 times brighter than the faintest star we can see without optical aid. With telescopes the range is even more astonishing: the Hubble Space Telescope can go another 2.5 billion times fainter! Two other items of note include Neptune, which begins retrograde motion in Capricornus on Thursday the 10th. The most distant large planet, Neptune is now 90,000 times fainter than Venus, and requires a telescope to see. The bright Moon will unfortunately wash out one of the year's better meteor showers, the Eta Aquarids, which will peak the morning of Sunday, May 5. The debris of Halley's Comet, the shower typically produces some 30 meteors per hour in a dark sky. In spite of the Moon's brightness, look to the south in late twilight to find the great figure of northern spring, Leo the Lion, the bright star Regulus at the end of the Sickle that makes the great beast's head. STAR OF THE WEEK. PRAECIPUA (46 Leonis Minoris). While Leo is easy to admire, its modern-constellation counterpart, faint Leo Minor, which rides the back of the Zodiac's King of the Beasts, is not. Leo Minor, the Lesser Lion, is so dim that few bother with him, and is so unimportant a constellation that no one would think of making Leo into Leo Major, thus raising Leo Minor's rank. Because the modern constellations were invented long after proper names were assigned, most well after Bayer assigned Greek letters (and to some degree were carved from other constellations), the names of their stars can be a bit of a mess. Only one star in Leo Minor carries a Greek letter name, and that is Beta; there is no Alpha, and Beta Leonis Minoris is not even brightest, coming in second. The little constellation does have its charm however, mostly in a flat quadrilateral with the constellation's luminary, bright fourth magnitude (3.83) 46 Leonis at the eastern end (the 46 a Flamsteed number, the only Flamsteed number whose star ranks number 1 in a constellation). Leo Minor is also one of the few modern constellation whose brightest star carries a proper name, 46 Leo Minoris also called Praecipua, or Chief, a modern term from Latin telling that 46 is the brightest star. Praecipua is otherwise ordinary, an orange class K (K0) giant-subgiant with a temperature of 4690 Kelvin. At a distance of 98 light years, it is not quite up to average giant brightness, radiating 32 solar luminosities into space, from which we derive a modest diameter (for a giant) 8.5 times that of the Sun. A star of around 1.5 solar masses, once a hydrogen-fusing cool class A star, it is now evolved, and is quietly fusing helium to carbon in its core. The star is known to be somewhat metal poor compared with the Sun, its iron content down by about a third. Of most interest perhaps is how well we know it. Recent accurate measures of
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, April 27, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. The Moon, in its perpetual journey around the Earth, passes through its first quarter this week, when it is 90 degrees to the east of the Sun, on Monday, April 30, thereafter waxing toward full. Two days after the quarter, it passes perigee, its closest point to the Earth (typically 363,000 kilometers, or 226,000 miles, 5.5 percent closer than average). The night of Friday, the 27th, the crescent will occult, or cover, two modestly bright stars of Gemini. Eta Geminorum will be occulted for east and central North America around 9:30 Central Time (add an hour for the east coast), while Mu Geminorum (Tejat) will be occulted for westerners around 10:45 Pacific Time. The exact time depends on location. As May begins the giant planets slip away to the west. On Sunday April 29th, Saturn sets just as twilight ends around 9:30 PM daylight time, though Jupiter lingers another hour. Mars, however, is quickly rising ever-earlier, and is now up in the southwest in Sagittarius by midnight. In the morning, Venus rises in the east just minutes after twilight commences. On Friday May 4th, this closest of all planets reaches greatest brilliancy for its current morning appearance. Since Venus has just passed between us and the Sun, we are still mostly seeing its nighttime side, and the planet appears in the telescope as a small crescent. Twilight is caused by sunlight that illuminates the Earth's atmosphere after sunset or before sunrise. There are three definitions. As the Sun sets, civil twilight ends when the Sun reaches 6 degrees below the horizon and it becomes too dark for ordinary outdoor activities. At 12 degrees we reach the end of nautical twilight, when the seagoing navigator can no longer see the ocean horizon (against which to measure the altitudes of stars to determine latitude and longitude). At 18 degrees, the end of astronomical twilight, the sky becomes as dark as it can get. That is the time for the astronomer to get to work. The sequence is repeated backwards in the morning. Professional observatories run on -18 degrees. As twilight ends, the Big Dipper of Ursa Major is now high overhead for those in the mid-northern hemisphere, the Little Dipper of Ursa Minor pointing up to meet it. If far enough north, above 35 degrees north latitude, you can see Cassiopeia swing beneath the pole opposite the Dipper. To the west of the Dippers and around toward Cassiopeia is an immense area of sky so devoid of bright stars that the ancients invented no constellations within it. Not until after 1600 did astronomers need to organize the sky better and fill in the blanks, giving us (among many others) the obscure and large northern figures of Camelopardalis (the Giraffe, which lies between Auriga and the Pole) and Lynx (the obvious Lynx), which sprawls in a long line from east of Auriga to nearly under the Big Dipper. STAR OF THE WEEK. BETA CAM. Camelopardalis, the Giraffe, a huge modern northern constellation invented only about 400 years ago, is so obscure that no star within it carries a proper name, and only three carry Greek letter names. Only four of its stars are even as bright as fourth magnitude, the brightest, Beta Camelopardalis, in the middle of the range (4.03). Nobody points out the dim figure at astronomy open houses. Yet the ghostly Giraffe is not without its highlights, one of which is its luminary, Beta. This great star looks faint only because of its large distance of 1000 light years. In truth it is a class G (G0) yellow-white supergiant (though of a somewhat fainter variety) that shines with a luminosity 3300 times that of the Sun from a surface just a bit cooler than solar (about 5500 Kelvin) that would stretch three-tenths of the way from the Sun to the Earth. The star is far enough away, and close enough to the Milky Way, that it is dimmed some 15 percent by interstellar dust. With a mass around 7 times that of the Sun, Beta Cam is only around 40 million years old. Lying just over a minute of arc away is a companion that is itself double, about which nothing is known except that the brighter is class A, the dimmer probably F. Separated by at least 25,000 Astronomical Units, the small double takes at least a million years to orbit the supergiant (which from the little double would shine with the brightness of four full Moons). Beta Cam is also a double mystery. It is most likely making the transition from being a hydrogen fusing dwarf (of hot class B) to a larger helium-fusing red giant. Whatever its status, it falls into a zone of temperature and luminosity in which stars become unstable and
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, April 20, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. The Moon passes through its new phase this week on Monday the 23rd. As the waning crescent thins, it passes ten degrees south of Venus around mid-day on Friday the 20th, and on the morning of Saturday the 21st will be well below the bright planet, which now lights up the eastern dawn sky. After passing the Sun, the waxing lunar crescent will be barely visible in western twilight the evening of Tuesday the 24th. The following night, Wednesday the 25th, the crescent, glowing softly with earthlight, will make a fine four- part configuration with Saturn, Jupiter, and Taurus's bright star Aldebaran, the Moon positioned about halfway between the two planets. By the evening of Thursday, the 26th, the Moon will appear well up and to the left of Jupiter, having passed south of it during the day (for those in the Americas). The two bright evening planets will not be with us too much longer, as Saturn now sets around 9 PM (daylight time), Jupiter an hour later. Mars, however, takes over, the red planet now rising around midnight between the classical figures of Scorpius and Sagittarius. Mercury is completely out of sight, as it passes in back of the Sun in superior conjunction on Monday, the 23rd. The minor bodies of the Solar System weigh in this week in the form of the Lyrid meteor shower, which peaks the morning of Sunday the 22nd. The Lyrids, which appear to emanate from the constellation Lyra, are the rocky debris of Comet Thatcher (1861 I), whose orbit we are passing. It is usually a modest shower, producing perhaps 10 meteors per hour, but is capable of up to 100. The summer stars, represented here by Lyra, are yet to come however. Now it is time to admire those of spring. Just after the end of twilight look high to the south to find the great figure of Leo, the Lion, this zodiacal constellation looking for all the world like a great beast stalking the sky, his head outlined by the "Sickle," which ends in Regulus. His hindquarters are marked by a prominent triangle whose eastern star is Denebola. Then follow the curve of the handle of the Big Dipper (nearly overhead for most in North America) to find Arcturus in Bootes and then Spica in the Zodiac's Virgo, this lonely star seen to the left of the box that makes Corvus the Crow. Almost exactly between Regulus and Spica lies the autumnal equinox, the point where the Sun crosses the celestial equator to mark the beginning of fall (which this year will happen on September 22). Since the Sun has recently passed the Vernal Equinox in Pisces, its opposite is nicely "visible" (in our imaginations) at night. Down and to the right of Leo is Alphard in Hydra, the water serpent, and just up and to the left of Alphard is the modern constellation Sextans, which honors the venerable sextant, the prime tool of the celestial navigator. STAR OF THE WEEK. ADHAFERA (Zeta Leonis). The Sickle of Leo is known for the bright star Regulus and the famous double Algieba. Just above Algieba, you can admire a fainter star, third magnitude (but just barely so, 3.44) Adhafera. The name, which comes directly from Arabic and means "the lock of hair," makes sense, as the star lies in Leo's great mane. Unfortunately, however, "Adhafera" actually refers to nearby Coma Berenices (Berenices Hair), and was given to Leo's star in error, a common failing of those who applied star names. Adhafera, a yellow-white class F (F0) giant star 260 light years away, actually shines 207 times more brightly than does the Sun. With a temperature of 7030 Kelvin, almost of its radiation pours out in the visible, where we can see it (that is, there is almost no correction for invisible ultraviolet or infrared, quite like our Sun), from which we find a radius 10 times solar. Spinning with an equatorial velocity of 84 kilometers per second (48 times solar), this three solar mass star takes less than six days to make a full rotation. Adhafera's real interest lies in its very clean state of evolution. Class F giants are rare, as they are making a remarkably fast transition from the main sequence (where they once fused hydrogen to helium in their cores) to the giant state (where they will eventually fuse helium to carbon). Only a million years ago, Adhafera was a white class A (almost class B) dwarf. Now with a shrinking, dead helium core, it will become an orange class K giant star in only another million years, and will then make its "run to the top," taking a leisurely 100 million years to expand from 12 solar radii to a to a red class M giant with a radius near that of the Earth's orbit! At that point, it will fire up its
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, January 19, 2001. Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. This week holds the new Moon, passed the morning of Wednesday, the 24th. Six hours later, it passes apogee, when it is farthest from the Earth. Before that date you can see the earthlit waning crescent in the morning hours to the east before sunrise, and afterward the reversed waxing crescent in the west after sunset. The Moon will be technically visible in western twilight the night of Thursday, the 25th, nearly in conjunction with (and below) the planet Mercury, but both will be quite difficult to see. By the night of Friday, the 26th, however, the slim lunar crescent (now well up and to the left of Mercury) will be quite visible. Mercury's visibility will improve until early next week, when it reaches greatest eastern elongation from the Sun. Earlier in the week, on Monday the 22nd, Mercury passes close conjunction with Uranus, a quite-invisible event. At the end of the week (on Thursday the 25th), Uranus's near-twin, more-distant Neptune, finally passes conjunction with the Sun to become a morning object. Uranus will follow in early February. Though Mercury will be, as usual, hard to find, Venus will not, as it continues to blaze away in the southwestern evening sky. In a highly unusual coincidence, both Jupiter and Saturn cease their westerly retrograde motions not only the same day (Thursday the 25th), but within about an hour of each other. It would be difficult to figure how often this happens without running actual orbital calculations, but it must be once in hundreds, if not thousands, of years. Odder still, Jupiter ceases retrograde first even though it is the more-easterly planet (ordinarily, the more- westerly ones finish first), a result of Jupiter's being closer to us than Saturn (4.4 as opposed to 8.6 astronomical units, the AU the average distance between the Earth and the Sun). Both planets will thereafter begin to move in their normal easterly direction against the background stars (for now, of Taurus), Jupiter pulling away from the ringed planet as it heads for the next constellation of the zodiac, Cancer. Taurus (Jupiter and Saturn's temporary home) is at its best in the early evening around 8 PM. Look for the vee-shaped Bull's head made by the Hyades cluster, the Seven Sisters (Pleiades) up and to the right. Aldebaran, the Bull's orange eye, is only situated in front of the Hyades and is not actually a part of it. Taurus is one of the few constellations that link to another, the northern horn (which extends northeast of Aldebaran) tied to bright Auriga, the Charioteer, which with bright Capella, stands above Orion. The other linking constellations are northern autumn's Andromeda with Pegasus and summer's Ophiuchus with Serpens. STAR OF THE WEEK. KEID (Omicron-2 Eridani, or 40 Eridani). An unassuming star, Keid (Omicron-2) of Eridanus (and more commonly known by its Flamsteed number, 40 Eridani) seems to play second fiddle to its somewhat brighter neighbor Beid, Keid the Arabic "egg shells" of Beid, "the eggs." The two stars are not a true binary, Beid seven times farther than Keid. Keid, however, is by far the more interesting star. A modest faint-fourth magnitude (4.43) to the eye, Keid is a triple star, famed not for Keid itself but for the much fainter companions. The 67th closest star and the 50th closest star system, Keid lies a mere 16.5 light years away. Much farther and the cool class K (K1) ordinary dwarf (Keid-A, one of the very few visible to the naked eye) would not be visible. Other than proximity, Keid A has little to offer but a cool temperature of 5100 Kelvin, a low luminosity of 0.4 times that of the Sun, and a mass around three-fourths solar. Such stars abound in space, but they are so faint that few can be seen without a telescope. A little over a minute of arc (83 seconds) away, however, and easily seen with only a small instrument, lies the prize of the system, Keid-B (rather, 40 Eridani B), a tenth magnitude white dwarf, by far the most visible white dwarf of all (though Sirius-B is more famed), its luminosity a mere 0.008 that of the Sun. White dwarfs are the final products of ordinary solar-like evolution, and are the spent cinders of the original stars' cores. Ordinary dwarfs (like the Sun and Keid-A) fuse hydrogen to helium in their cores. When the hydrogen is gone, the stars become giants and fuse the helium to carbon and oxygen. The outer envelope is ejected, and all that remains is the low-mass ultradense carbon-oxygen white dwarf. Typical white dwarfs are only about the size of Earth and have extraordinary average densities of a ton per cubic centimeter.
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, January 5, 2001 Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. Welcome not to the turn of the odometer (to 2000), but to the turn of the Millennium itself, not to mention the 21st Century and the New Year, 2001. We begin with the Moon heading toward its full phase, reached on Tuesday the 9th. Having just had a solar eclipse last Christmas Day, we get a lunar eclipse this full Moon (if conditions are right for a solar eclipse, a lunar eclipse will commonly precede or follow). This one will be total and a beauty, but for Europe, Asia, and Africa, not for the Americas. This full Moon will be the second highest of the year, shining from amidst the stars of Gemini. The highest will occur next December, when the Moon will also be eclipsed, bookending the year. (That eclipse will not be visible in the Americas either, no loss, since it will be a penumbral partial-shadow eclipse, which is essentially a non- event). Lunar perigee takes place only a day after full (Wednesday, the 10th), the combination of full, near-perigee, and near-perihelion (for Earth) producing exceptionally high ocean tides. While approaching full, the Moon will make beautiful configurations with the two giant planets, Saturn and Jupiter, which are both still in retrograde against the stars of Taurus. The night of Friday, the 5th, the waxing gibbous Moon will pass just beneath Saturn, while the following night it will be down and to the left of Jupiter, at the same time just above Taurus's brightest star, first magnitude Aldebaran, all this action taking place above the sky's brightest constellation, great Orion. One can hardly ignore Venus, however, shining brilliantly in the southwest at sundown. If you stay up after midnight, also note Mars, which now rises around 2 AM and continues to brighten among the stars of Libra. Directly north of Jupiter and Saturn, find the hero of the Andromeda myth, Perseus, which passes roughly overhead in mid- northern latitudes and contains a bright portion of the Milky Way. The constellation is especially known for naked-eye clusters. The most famed, the Double Cluster, lies to the northwest of the bright string of stars that make the most prominent part of the constellation. Just barely visible without optical power, it is a marvelous sight in a small telescope. Hardly recognizable as a cluster is central Perseus itself, making Perseus one of the constellations that are not made just of random stars, but of those that are at least in part physically associated. Much the same is true for Orion. Though not a cluster bound by gravity, many of the stars are in loose association, their births connected in both time and space. Look in particular for the famed "belt," the Arabs "string of pearls," which nearly straddles the celestial equator and lies above the most famed of all interstellar gas clouds, the Orion Nebula. STAR OF THE WEEK. GAMMA PER (Gamma Persei). The naming of stars at times seems to have little to do with their brightnesses. All the first magnitude stars, and most of the second magnitude visible from classical lands, have names (an outstanding exception being Gamma Cassiopeiae), but from there on the naming is erratic. Gamma Persei (so-called though actually fifth brightest in Perseus) carries no known western name even though it lies at mid-third magnitude (2.93). One reference calls it "Algenib," an alternative name for Mirfak (Alpha Persei), but that is surely a mistake. And a pity too, as the star fascinates. Lying about 225 light years away, it is a close (only barely separable) double that consists of a class G (G8) giant coupled with an ordinary main sequence class A (A2) dwarf, which together shine 300 times more brightly than the Sun, the yellow giant notably the brighter of the two. Rather overwhelmed in Perseus by Algol, the brightest eclipsing double star in the sky, Gamma Per takes its fame from being the second brightest eclipser, a fact only recently discovered. The star was long known from spectroscopic (Doppler) observations to be a double that takes 14.6 years to orbit. Moreover, the plane of the orbit was found to lie tantalizingly in the line of sight, presenting the small possibility that the two might actually eclipse each other. Diligent observations discovered the eclipse (which produces a 30 percent dip in the light output of the system and is just visible to the naked eye) only in 1990. The event, in which the giant gets in the way of the dwarf, takes somewhat under two weeks. Analysis of the observations shows the average separation of the stars to be 10 Astronomical Units. The orbit, however, is highly elliptical (almost as much as that
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, December 15, 2000 Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. The Moon descends during the very early part of the week through the tail end of its waning gibbous phase. Now rising well after sunset, it then passes through third quarter the night of Sunday the 17th, the phase actually reached a few hours before near- midnight moonrise in North America. As the lunar crescent wanes, it will pass above Mars the night of Tuesday the 19th, and by moonrise will be seen somewhat to the northeast of the red planet. Mars, now in Virgo, just to the northeast of the star Spica, is slowly brightening as the Earth (moving slightly faster around the Sun) creeps up on it, and is now just making the transition from second magnitude to first. Best visibility, Mars opposite the Sun, is still six months off, however. For planets we have instead the glorious evening view of brilliant Venus in the southwest in Capricornus, and Jupiter and Saturn, hosted by Taurus, climbing the eastern sky. The big event this week involves our own planet Earth, which sees the Sun pass the Winter Solstice in Sagittarius at 7:37 AM Central Standard Time (8:37 EST, 5:37 PST). At roughly the time of sunrise, the Sun will have reached its lowest point for the year, 23 degrees 26 minutes south of the celestial equator, and astronomical winter will begin in the northern hemisphere. The northern end of the Earth's axis will be tilted as far as possible away from the Sun, sunlight must spread itself out over a larger area, and the northern hemisphere will receive its minimum solar heat. Since it takes some time for the Sun to climb back up north, the weather, as we all know, will continue to chill. The southern hemisphere, however, now glories in sunlight, with the Sun passing overhead at the Tropic of Capricorn, the parallel of latitude that lies 23 degrees 26 minutes south of the equator. As part of the whole event, the Sun will not rise north of the Arctic Circle, and will not set south of the Antarctic Circle (these located 23 degrees 26 minutes from the north and south poles of the Earth). Winter: with the constellations of autumn starting to make their transition to the west, the Great Square of Pegasus high to the south at 7 PM, Orion and his mighty crew begin to make an impact in the evening sky, the celestial hunter hitting the meridian to the south around midnight. Here we can glory in the "winter-six" of Orion, his two hunting dogs Canis Major and Canis Minor, Gemini (its Summer Solstice now high in the sky), Auriga, and Taurus, the latter carrying the Hyades, the Pleiades, Jupiter, and Saturn to the west. STAR OF THE WEEK. ALGENIB (Gamma Pegasi). Though the brightest star of Pegasus is Enif, the Epsilon star, the stars of the Great Square are of such obvious note that Bayer gave them Alpha through Delta. Delta (Alpheratz), the brightest, actually belongs to Andromeda as Alpha Andromedae, leaving Alpha, Beta, and Gamma Pegasi (Markab, Scheat, and Algenib) ranking 3-2-4 in the constellation and 2-1-3 in the Square, Algenib the faintest of them. The star's name, from Arabic, means "the side," and originally belonged to Mirfak in Perseus (whose alternative name is still Algenib). Second magnitude (2.83) as viewed from Earth, Algenib is a brilliant hot blue class B (B2) star with a high temperature of 21,500 Kelvin. From its distance of 335 light years, and allowing for a lot of ultraviolet radiation, we find a luminosity 4000 times that of the Sun (one suggestion as high as 12,000, though that seems extreme) and a radius of 4.5 solar. The lower figure calls for a mass 7 times solar, the higher up to 10. The star is now beginning slowly to evolve. Though classed as a "subgiant," it is probably still fusing hydrogen into helium in its core. It will evolve into a massive carbon white dwarf rather like Sirius B, the upper mass limit suggesting a more advanced status as a neon-oxygen white dwarf. Algenib is measured by the Doppler effect to be an especially slow rotator, only 8 kilometers per second (4 times solar), unusual for hot class B stars, which are ordinarily high-speed spinners. Most likely, we are looking at the star almost along its axis (its pole pointed at us), so that we do not sense the real rotation. (If a star is exactly pole-on, the Doppler effect, which is sensitive only to line-of-sight motions, would give zero rotation; that is, the star would not appear to rotate at all.) Algenib is also among the collection of Beta Cephei stars (named after Alfirk, Beta Cephei, and which include such luminaries as Mirzam, Hadar, and several others). All hot class B stars, most beginning to evolve in some way,
[ISTA-talk]Skylights
Skylights, University of Illinois Department of Astronomy. Astronomy News for the week starting Friday, December 1, 2000 Phone (217) 333-8789. Prepared by Jim Kaler. Find Skylights on the Web at http://www.astro.uiuc.edu/~kaler/skylights.html, and Stars (Stars of the Week) with constellation photographs at http://www.astro.uiuc.edu/~kaler/sow/sow.html. The Moon, beginning the week as a thick evening crescent, passes its first quarter the night of Sunday, December 3, around the time of moonset in the Americas, and then begins its gibbous growth toward full. The vocabulary of astronomy is often perverse, as at first "quarter," we see "half" the sunlit face of the Moon, the other half in darkness. As a result, "half" and "quarter" seem to mean the same thing. "Half," however, refers to the visibility of the lunar disk, whereas "quarter" refers to the quartering of the orbit, of the phase cycle of 29.5 days. From the whole cycle comes our "month," from the quartering, at least in part, the "week." The week also begins with the Moon passing a couple degrees south of the planet Uranus, now in Capricornus, and the first of the "discovered" planets, found by William Herschel in 1781. Twice as far from us as Saturn (which now is well up in the east at the end of twilight, just up and to the right of bright Jupiter), and only half Saturn's size, Uranus is only barely visible to the naked eye. Currently a bit to the west of Uranus, and half again as far away, 30 times Earth's distance from the Sun, lies Neptune, which requires a small amateur telescope to see. Discovered around 1846 as a result of its gravitational pull on Uranus, Neptune takes 165 years to orbit and will not complete a full turn since discovery for another decade. Finally, at the end of the planetary system is dim Pluto. Only about the size of the western US, Pluto averages 40 times Earth's distance from the Sun (though now it is just beyond Neptune). With a highly tilted orbit that now places the planet in the constellation Ophiuchus, Pluto passes conjunction with the Sun on Monday, the 4th. The Sun, in its inexorable apparent journey around the ecliptic will pick off Neptune and Uranus early next year. Pluto may be the end of the planetary system, but hardly the Solar System. Around and beyond Pluto lie hundreds of discovered small bodies of the "Kuiper Belt" of comets. The planets seem to have assembled from small bodies, which in turn were created from the accumulation of the dust that surrounded the early Sun. Pluto a transition object, seems to have tried to match its bigger brothers but could not quite make it as a result of the lack of raw material. Around 7:15 PM, the "equinoctial colure," the north-south-circle that connects the equinoxes and the poles, rides the celestial meridian. Looking to the south from North America, the colure is bracketed by two lonely stars. To the west of the meridian find first magnitude Fomalhaut in Picsis Austrinus, the Southern Fish, while to the east lies second magnitude Deneb Kaitos, which marks the tail of Cetus the Whale, within a "wet" section of sky that includes Delphinus, Capricornus, Aquarius, and Pisces. STAR OF THE WEEK. KULLAT NUNU (Eta Piscium). If some star names are more obscure than others, this one is surely near the bottom of the list. Unlike most star names, which are from Greek, Latin, or especially Arabic, it seems to have derived from Babylonian and really more refers to the cord that connect the Fishes of Pisces rather than to the star itself. Though Alrescha, the Alpha star, is perhaps best known in this dim constellation (as a result of its placement at the central "kink" in the cord), it is rather well topped by Bayer's Eta star, which at bright fourth magnitude (3.69) is Pisces' brightest, and somehow deserves a name (so "Kullat Nunu," while hardly official, will have to do). Best to think of it more as just plain old Eta Piscium. The star itself is a bit unusual, a bright class G (G7) giant, at 4930 Kelvin a bit cooler than Capella-A. It is one of the few of its class to have had its angular diameter measured. From that and the distance of 294 light years, Eta is 26 times larger than the Sun and would extend 30 percent of the way from the Sun to Mercury. The star's calculated luminosity of 316 Suns and its temperature conspire to give almost the same answer, showing that the measured properties are consistent and correct. In turn, these data indicate a mass between 3.5 and 4 times that of the Sun and that the star is most likely dying and in a state of internal helium fusion. Only about 250 million years ago it was a hot, blue Class B star, and in far less time than that, it will turn into a massive white dwarf rather like Sirius-B. Spinning with an equatorial velocity of at least 8 kilometers per second, the giant may take almost half a year to make a full turn. All this activity has been witnessed by a mysterious, relatively dim
