Monthly Sky Notes November 2006 (2 of 3)

The Summer Triangle
Comet C/2006 M (SWAN)
Leonids
Double Stars
Deep Sky Objects

Double Stars

Albireo - Beta Cygni 3.0mv 3.3 5.5 K3II+B0V 35".0 380ly warm orange red & bluish-white colour contrast topaz & sapphire orbital period >75000 years. Albireo ia actually triple, Albireo A is a close binary spectral class K3 3.3 stable helium fusing giant and a hot but dimmer B9 5.5 hydrogen fusing blue dwarf. Not readily resolveable in amateur 'scopes, more a spectroscopic binary. The K3 giant has a temperature around 4400K and luminosity 950 Suns, radius 50 Suns, 5 solar masses. On average separated from its blue dwarf B9 companion by 40au, orbital period ~100 years in a highly eccentric orbit. The visually seen blue star, Albireo B is similar to Albireo A's companion, and is a B8 dwarf surface temperature 12,100K, equatorial velocity >250 km/sec, rotation period ~14hrs. Albireo B is a "B-emission star" that is loosing mass and is surrounded by a disk of gas of its own making. From Albireo B, Albireo A would appear as brilliant orbiting orange and blue points half a degree apart, the K3 giant shining with the light of 35 Full Moons, the close B9 companion at about half that.

Epsilon 1 - 2 Lyrae (Double - double) 3.88mv 5.1 6.5 Epsilon 1 north of Epsilon 2, separation 208"arc (3'.5). Even a humble 60mm refractor will split each into two more nearly identical class A stars, those of Eps-1 by 2.8 arcseconds, those of Eps-2 by 2.2 arcseconds, all at 160ly. From east to west, the four are labelled A through D as Eps-1 A, Eps-1 B. Eps-2 C, Eps-2 D. The stars of each of the pairs orbit each other with periods estimated at around 1000 years, placing them some 140au apart. Eps-1 and Eps-2 are vastly too far apart for any orbital motion to be noticed, as they are separated by at least 10,000au and must take at least half a million years to make a complete circuit. While they look similar, there really are differences among them all. From A thru' D, we find apparent magnitudes 5.1, 6.0, 5.1, 5.4; spectral lasses A3, A7, A5, A5; surface temperatures 8000K, 7700K, 8200K, 8200K; luminosities 18S, 8S, 17S, 14S; masses 1.9S, 1.5S, 1.9S, 1.8S. The whole system is probably 800 or so million years old. We still do not know just how such quadruples are formed, with pairs so reasonably close and the pairs of pairs so far apart. (Mizar - Zeta Ursa Majoris and Castor - Alpha Geminorum) are similar but the individual pairs are spectroscopic binaries). From each of the pairs, an observer would see the other shining with the light of a quarter Moon under a degree away from each other. The only way such a quadruple star system can survive for any period of time is to be in a double-double hierarchy; if the four are mixed up close together, the combined gravitational influences will kick out first one, then another, leaving just a double behind. Eventually, gravitational influences of passing stars and tides raised by the Milky Way galaxy will most likely separate Eps-1 and Eps-2 into doubles that go their separate ways.

Gamma Delphini 3.87mv 5.14 4.27 F7+K1 Gamma-1 F7 dwarf Gamma-2 K1 subgiant Separation 9".2arc reddish-yellow and greyish lilac colour contrast pair lying at 104ly. Gamma-1 6600K Gamma-2 4700K; luminosity 7.5S & 26S; radii 2.5S & 7.5S; masses 1.5S & 1.7S. An interesting pair, the cooler Gamma-2 is the brighter not only because its mass of 1.7S is somewhat higher than Gamma-1's, but it has also recently given up hydrogen fusion in its core and is now in the process of expanding and slowly brightening with a dead helium core as it works towards becoming a helium fusing giant. Gamma-1, with a mass 1.5S, is a little behind Gamma-2 in its life cycle. Though still fusing hydrogen in its core, it does not have long to go before it follows its mate in becoming a giant, the pair about 2 billion years old. Over the past 200 years we have watched them move enough around each other to enable an approximate orbit to be calculated. They revolve with a period of 3200 years at an average distance of 330au. A high eccentricity makes them as much as 600au apart, and brings them as close as 40au. On average, from each one, the other would appear as bright as 100 Full Moons.

Alpha Capricornis - Algedi (Arabic - The Kid) Alpha-1 G3 690ly Alpha-2 G8 109ly Separation 5'arc optical pair. The doubling is a remarkable illusion as the two stars have very different distances, making them a line of sight double. They are not otherwise associated at all. The fainter, Alpha-1 is 6 times further away than Alpha-2. Such coincidences among brighter naked eye stars are unusual. Much odder is that the stars themselves each fall into similar and relatively rare categories, both evolved, dying yellow stars class G and 5000K rather similar in temperature to the Sun. Alpha-2, the closer, is a giant, luminosity 43S, while Alpha-1 is a supergiant, luminosity 930S (21 times brighter than Alpha-2), only seeming the fainter because of its greater distance. Alpha-1, the supergiant, is the larger of the two, radius 40S (5 times larger than Alpa-2), making it rather small as supergiants go. It is, as a supergiant (though a lesser one), also the more massive, 5S, double that of Alpha-2. Both stars have quit hydrogen fusion in their cores, and are preparing to fuse their internal helium into carbon, if they have not done so already. Alpha-2, Algedi-the-giant, is deficient in metals, its iron abundance somewhere between a tenth and a half that of the Sun, indicating it comes from an older set of stars, whereas Alpha-1, Algedi-the-supergiant, has a normal, solar, chemical composition.

Gamma Andromedae - Almaak / Almach / Almak / Alamak / Alamaak (Arabic - wild cat) 2.16mv 2.26 5.0 K3+B8V 355ly Separation 9".7arc golden yellow and blue. Through the telescope the star is extraordinarily lovely, even a small instrument showing a superb pair separated by 10 arcseconds, described by Admiral William Henry Smyth (the father of amateur astronomy) as "orange and emerald green." The second magnitude brighter component Gamma-1 is a class K3 bright giant with a surface temperature around 4500K and a star now in the act of dying. From its distance of 355ly, we find a luminosity about 2000S and radius 80S, big enough to take the star to the orbit of Venus. More remarkable, the fainter blue-green component, Gamma-2 is also double, though the duplicity is far more difficult to see. Fifth (5.1mv) and sixth (6.3mv) magnitude white hydrogen-fusing dwarfs (with respective surface temperatures of about 12,000K & 10,000K) orbit each other with a period of 63.7yrs separated on average by 0".3arc (0".12 to 0".95 currently 0".4 Epoch2000.0) which translates to 33au. A fairly high eccentricity takes them as close together as 13au and as far apart as 52au. Yet again the system splits, as the brighter of the two is also double, though detectable only with the spectrograph (spectroscopic binary), the components very close and orbiting every 65hrs, Gamma-2 thus triple. The orbit gives a combined mass for the three stars of 8.7S, which is consistent with advanced B8 & A0 dwarfs joined with a lower mass class A7 (or so) dwarf star about which nothing is known. The naked-eye star we know as Almach is thus quadruple, making it a feast for both the mind and for the eye. One or two other more distant stars might belong to the system as well.

Iota Cassiopeiae 4.52mv Iota-A 4.64 (4.45 - 4.53)A3Vpec Iota-B 6.9 F5V Iota-C 8.4 G7V 140ly Separations A-B 2".7arc (100au) A-C 7".4arc (300au). While A & B are both white and fairly colourless, C is on the orange side. The closeness of the stars leads to a false visual colour contrast that make the system quite pretty and well worth a look. Iota Cas A makes the system yet more interesting, as it displays a wobble in its motion that long suggested that it was itself a close unresolved double, making the whole affair (at this point in the discussion) a quadruple star. Apparent motion of B around A showed an orbit with an uncertain period of 620 to 870yrs, while Iota-C is too far out to show movement. Subject to severe foreshortening, C is certainly farther away from the others than it appears. Iota Cas really reveals one of the great technological advances of the past century, the ability of active "adaptive optics" to "de-twinkle" starlight to allow the observation of greatly improved detail. (As the star jumps one way as a result of refraction in the Earth's atmosphere - - the basis of twinkling - - the telescope optics move in the other way to counter the apparent stellar motion.) Observation of Iota Cas directly shows the very close-in companion to Iota-A (a class G2 star called Iota Cas Aa 0".5arc from A), while multiple observations of the system give the orbit of Aa about A (and thus the system's masses) as well as the relative motion of B relative to the A-Aa binary. Combining all data, Iota Aa goes around A at a distance of 18au with a period of 47yrs, while B seems to be moving in a straight line and may not be part of the system at all, but "just passing by." To add to the complexity, adaptive optics observation of C in Sep 2004 revealed that C also has a close K5 companion called "c" 0".4arc from C, making five stars for the whole system! Even if A, B, & C are actually unbound from each other, they are clearly all close together, as the parallax for A gives luminosities for the other components that are in line with their spectral classes. For A, Aa, B & C (including the companion "c"), we have estimated surface temperatures of 8500K, 5700K, 6300K, & 4900K, luminosities of 21S, 0.7S, 2.7S & 0.7S, and masses for all five, 2.0S, 0.7S, 1.3S, 0.8S & 0.65S (the first two from the orbit, the latter three from the spectral class). Looking past the multiplicity, Iota A (which is also classed as A3 once the effect of Aa is accounted for) is notable in its own right as a "peculiar," slightly variable, magnetic star with strong enhancements of strontium and calcium (making it an "Alpha-2 Canum Venaticorum" star), the effect of separation of chemical elements in a fairly quiet outer atmosphere. Strong magnetic spots that carry the chemical oddness rotate in and out of view with a precisely known period of 1.74085 days.

Zeta Ursa Majoris (Mizar & Alcor) (Arabic - the groin) 2.06mv 78ly separation 12'arc. The two, Mizar and Alcor, termed the "horse & rider" by the Arabians, are a good test for minimal vision. Mizar takes its place in the celestial hall of fame as the first known "double star," one that consists of a pair of stars that orbit each other. Found to be double by Riccioli in 1650, Mizar is a prime target for someone with a new telescope, as the components are an easy 14"arc apart (at least 500au), the two taking at least 5000yrs to orbit each other. More remarkably, each of these two components is again double. The brighter of the two (2.27mv) contains a very close pair a mere 7-8milliarcsec apart (an angle subtended by a penny at 300 miles) that has an orbital period of 20.5 days; the fainter (3.95mv) contains a pair with a period of about half a year. Mizar is thus actually a quartet of stars, a double-double. It is moving through space together with its more distant companion, Alcor. Mizar and Alcor together therefore probably make a quintuple system, Alcor taking at least 750,000 years to make a single round trip around its quadruple companion. All of the stars are similar, all "main sequence" hydrogen-fusing stars like the Sun, but of white class A (the brighter both A2,the fainter probably both A5 or A7) with surface temperatures ranging between around 7500K and 9000K and luminosities from 10S to 30S. The orbit of the brighter double that makes Mizar has been observed with a sophisticated "Interferometer". Analysis of the stars' light shows the components to have masses 2.5S; the masses of the fainter pair are estimated at around 1.6S. The stars have odd chemical abundances as a result of slow rotation, which allows for quiet atmospheres and chemical separation. The brighter of the pair seen through the telescope is rich in silicon and strontium, whereas the fainter is a "metallic line star" that is deficient in aluminium and calcium but high in silicon and in rare earths like cerium and samarium.

Delta Cephei 4.1mv 3.5 - 4.3 F5V - G7V 300ly Amongst the most famed of all stars does not have a proper name. It is, however, the only star that has given its constellation name over to represent a whole class of stars, the "Cepheids." While most stars look down steadily upon us, Delta Cephei is one of the few easily-visible variables, its magnitude changing from 3.5 to 4.3 and back over an amazingly regular period of 5d:8h:47m:32s, the star acting like a natural clock. We can't even pin down its spectral class. Listed as a yellow-white class F5 supergiant, the star actually changes from F5 to cooler G7 in synchrony with its light variations, the temperature changing from 6800K at the warmest to 5500K at the coolest. The term "supergiant" is apt, as at a distance of around 300ly the star pours out an average of 2000S luminosities into space from a surface swollen to some 80S radii. At the pinnacle of a vast class of stars, Delta Cephei has naked-eye cousins that include Mekbuda (Zeta Geminorum) and Eta Aquilae, both of which are just somewhat brighter. All Cepheids are dying high mass stars (Delta Cep around 5S mass) with helium cores that have lost a sense of equilibrium, and regularly expand and contract, pulsing like celestial hearts. Their deep astronomical importance lies in the observation that their luminosities are directly related to their periods of pulsation (which run from about a day to over 50 days). Since the period gives the luminosity, we need only measure the apparent luminosity (the visual magnitude) to find the distance. Cepheids are so luminous that they are easily seen in nearby galaxies, their presence then giving the galaxies' distances. Edwin Hubble's discovery of a Cepheid in M33, the Andromeda Galaxy allowed the first firm distance to be determined. The observation of Cepheids in more distant systems was a "key project" of the Hubble Space Telescope, one that has allowed the establishment of an accurate cosmological distance scale. Delta Cephei does not pulse in lonely splendour, but has a 6.3mv B7 500S luminosity companion at 41"arc separation. Less than 100 million years ago, Delta Cephei began its journey as a hotter and more massive star than its four-solar mass neighbour, which is still a normal hydrogen-burner, but which will soon follow Delta Cep's path when its internal hydrogen gives out. Separated by at least 12,000au, the two take at least half a million years to orbit each other. Even at that distance, the companion would shine in Delta Cep's sky about as bright as our Venus. Watching Delta Cephei vary over a full magnitude from the companion would be a fascinating sight, not that there would be anybody to watch, the stars being much too youthful to have any life on any planets that are probably not there in the first place.

Delta Orionis - Mintaka Orion is defined by his great belt, three bright second magnitude stars in a row that the ancient Arabs called "the string of pearls," which is the meaning of the name of the middle star, Alnilam. The two flanking stars, eastern Alnitak and western Mintaka, both come from Arabic phrases that mean "the belt of the Central One," the Central One the Arabic personification of our Orion, a woman lost to history. Though (at magnitude 2.21) Mintaka is the seventh brightest star in Orion, and the faintest of the three belt stars, it still received the Delta designation from Bayer, who lettered the belt stars in order from west to east before dropping down to Orion's lower half to continue the process. Of the sky's brightest stars, first through third magnitude, Mintaka is closest to the celestial equator, only a quarter of a degree to the south, the star rising and setting almost exactly east and west. The star is wonderfully complex. A small telescope shows a seventh magnitude companion separated by almost a minute of arc. At Mintaka's distance of 915 light years (very nearly the same as Alnitak at the eastern end of the belt), the faint companion orbits at least a quarter of a light year from the bright one. In between is a vastly dimmer 14th magnitude component. The bright star we call Mintaka (whose solo magnitude is 2.23) is also double, and consists of a hot (30,000K) class B, slightly evolved, giant star and a somewhat hotter class O star, each radiating near 90,000 times the solar luminosity (after correction for a bit of interstellar dust absorption), each having masses somewhat over 20 times the solar mass. This pair is too close to be separated directly. The duplicity is known through the star's spectrum (its rainbow of light), which detects two stars orbiting each other every 5.73 days, and also because the stars slightly eclipse each other, causing a dip of about 0.2 magnitudes. Mintaka is most famed. however, as a background against which the thin gas of interstellar space was first detected, when the German astronomer Johannes Hartmann in 1904 discovered absorptions in the star's spectrum that could not be produced by the orbiting pair. From this discovery, and others that followed, we now know that all of the Galaxy's interstellar space contains an enormously complex medium of gas and dust that is the birthplace of new stars. Mintaka will also, to some distant generation of astronomers, be famed in death, as each of its components is so massive that their only fate is to explode violently as supernovae.

Sigma Orionis Double stars are among the amateur's favorite targets, Albireo, Mizar always on the list. Multiples are better yet, and there are few more attractive than Sigma Orionis (which has no proper name), where you see a quartet of stars, the brightest of which is also a close double. Indeed, Sigma Ori, whose five stars together shine in Orion at bright fourth magnitude (3.66mv) just south of Alnitak in Orion's belt, is really at the pinnacle of a small star cluster that lies a somewhat-uncertain 1150 light years away. In turn, the stars and the cluster are a part of the Orion OB1 association, which includes many of the other stars in the constellation. Sigma's main component, "AB," dominates, the two a mere 0.25 seconds of arc apart shining at magnitudes 4.2 and 5.1. Both very young hydrogen-fusing dwarfs only a few million years old, the brighter is a magnificent blue class O (09.5) star, while the lesser is class B (B0.5). The pair orbit every 170 years at a distance of about 90au. After correction for ultraviolet light from very hot (32,000K and 29,600K) surfaces, they respectively radiate at a rate of 35,000 and 30,000 Suns. Temperature and luminosity give masses of 18S and 13.5S, the sum of nearly 32 solar masses making the close AB pair among the most massive of visual binaries. Together they illuminate their surroundings, causing interstellar gas to glow. The next brightest stars in the system are Sigma Ori "D" and "E," bright seventh magnitude class B (B2) dwarf stars that at magnitudes 6.62 and 6.65 are nearly identical in brightness and have masses around 7S. The similarity stops there. "E" is the prototype of the weird "helium-rich" stars that have strangely elevate abundances of helium. Even odder, the helium in "E" seems to be concentrated toward particular patches that involve a combination of the rotational and magnetic field axes. They may be related to cooler magnetic stars such as Cor Caroli, but no one really understands them. The last of Sigma's stars, "C," appears to be a normal ninth magnitude class A dwarf. In projection on the sky, "C" is the closest to the AB pair, and is at least 3900au away, while "D" and "E" lie at least 4600au and 15,000au distant. While the orbit of the AB pair is stable, the orbits of the other three are not, and long before they die they will probably be gravitationally sped up and forced out. "A" will explode first and may even kick "B" (which will explode next) out of the system. The other stars, wherever they wind up, will die as white dwarfs. The cluster seems also to contain numerous low-mass stars, brown dwarfs, that have masses only a few times that of Jupiter.

Theta-1 Below Orionis Orion's brilliant three-star Belt (from right to left Mintaka, Alnilam, and Alnitak, or Delta, Epsilon, and Zeta Orionis) is the Hunter's three-star "Sword," in the middle of which is one of the great sights of the telescopic sky, the Orion Nebula. Also known as Messier 42, the Nebula is an immensely complex cloud of dusty gas 1400-1500 light years away and 20 light years across (depending on just where you draw the ill-defined boundaries) that is made to fluoresce by the hot stars of Theta-1 Orionis, which are situated directly in front of it. (The Orion Nebula is a "blister" on the face of the great Orion Molecular Cloud, which lies behind the Hunter and hosts several sites of active star formation. Though invisible to the eye, it glows brightly in the radio spectrum). Even a small telescope shows Theta-1 to be a quartet, which carries a group name, the "Trapezium," from west to east labelled Theta-1 A, B, C, and D. Clumped within a span of 22 seconds of arc (10,000au), all are hot class O and B stars that together make an apparent "single star" of magnitude 5 (4.7mv). Taken separately, from A through D, they are of magnitude 6.7, 8.0, 5.1, and 6.7 and blue spectral classes B1, B0, O6, and B0.5. All contribute to the energetic ultraviolet light that energizes (ionizes, stripping electrons from atoms) the Orion Nebula. By far the leader of the pack is Theta-1 C, a great 40-solar-mass star with a temperature of 40,000K (making it the hottest "naked eye" star, though the 4 are inseparable without optical aid), a huge luminosity 210,000 times that of the Sun (85 percent of the Trapezium's total), and a 1000 kilometer/second wind with 100,000 times the flow rate of the solar wind. The power of the star is such that it is evaporating dusty disks around nearby new stars that in other settings might form planets. The other members of the Trapezium pale only in comparison with "C," all containing over 10 solar masses. A main interest lies in their multiplicity. Theta-1 A is an eclipsing double also known as V 1016 Ori. Every 65 days, the star dips by a magnitude as a star still in the process of formation just 1au away passes in front of the bright component, the whole thing watched by another companion 100au off. Theta-1 D seems to have a companion as well. The champion in this contest, however, is Theta-1 B, which has a companion 60au away called "B1." "B" itself is another eclipser (known also as BM Ori) that drops by nearly a magnitude every 6.5 days, the companion probably much like the Sun. Since "B1" is also double, Theta-1 B is quadruple. Adding them all up (and including fainter Theta-1 E, which lies close by), the Trapezium is a complex multiple of 11 stars! And that is not really the end, as the Trapezium is really the core of an incredibly dense star cluster that seems to fill the background of the Orion Nebula, all of it born less than a million years ago. Most multiple stars are hierarchical, a distant star going around a close double (like Theta-1 B), or two close doubles going around each other (like Mizar or Epsilon Lyrae), which gives great stability. The Trapezium, on the other hand, is gravitationally unstable, the stars all too close together. As a result, one after the other will be ejected from the group. After only a few million years, the leader of them all, Theta-1 C, will inevitably explode as a great supernova, the others probably doing so as well, all lighting the dusty gases of interstellar space, all providing shock waves that will promote new star formation within the local molecular clouds.

Gamma Leonis - Algieba (Arabic "the forehead") 2.01mv 2.6 3.8 K0III+G7III Separation 5arcsecs 126ly. Algieba is a magnificent double star with orange-red and yellow components, others see it yellow and greenish. Named after its place in the foreparts of Leo the Lion, originally the name was applied to several stars of the "Sickle." The star marks the radiant of the Leonid meteor storm (the debris of Comet Temple-Tuttle), which returns with varying intensity every 33 years, the last in 1998. In 1833 the storm produced a fall of 100,000 per hour. To the eye, Algieba shines at second magnitude (2.01 for the combined stars), but even a modest telescope under good atmospheric conditions will allow you to split the pair, one appearing a bright third magnitude (2.61mv), the other at bright fourth (a rather uncertain 3.5mv), separated by just under 5 arcsecs. The brighter, the more orange of the two, is a class K1 giant with a surface temperature 4400K, the fainter a somewhat warmer 4900K class G7 giant, making it the yellower one. Such giant pairings (like Capella - Alpha Aurigae) are rather unusual. The orbital period is so long, over roughly 620 years, that only a fraction of the full path has been seen since discovery. At the star's distance of 126ly, the components are at least 97au apart, over twice the distance of Pluto from the Sun. A very high eccentricity takes the two between 15au and 180au apart. Both stars are quite luminous, the brighter 180S, the other 50S (when invisible infrared is taken into account - Bolometric Magnitude), leading to respective radii 23S & 10S. Both are true giants, meaning that they have stopped fusing hydrogen to helium in their cores and have expanded to their great proportions. Evolutionary calculations suggest that each are about double the solar mass, while the still-uncertain orbit gives a total mass of 2.35S. Born from the same interstellar cloud perhaps 2 billion years ago, they are each deficient in metals, their iron abundance about a third that of the Sun. It is hard to tell how far along they might be in their evolution. They are both fusing helium in their cores or they might be giants in development, with quiet helium cores that are waiting to fire up, or each may be in a different stage, the chemical composition at the surface, which is influenced by age, suggesting the former. The pair move with rather high speed, 71 km/sec, relative to the Sun, over four times normal suggesting that they come from a different part of the Galaxy. Some 6'arc away are two tenth magnitude "companions," one of which may actually belong to the system. If so, it at least 14,000au away and would take roughly a million years to orbit Algieba proper. From there, the binary would shine with the brightness of a couple of Full Moons half a degree apart.