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Dupree, Ronald Gilliland,
NASA and ESA
(Larger ultraviolet image -- more).
As highly evolved, yellow-orange giant stars, Capella Aa
and Ab are still much smaller than the red supergiant
Betelgeuse, at left. (A Digitized Sky Survey image of these
binary giants may be available at the Nearby Stars Database.)
The Capella, or Alpha Aurigae, system is located about 42.2 light-years from Sol. It is the brightest star (5:16:41.4+45:59:52.8, ICRS 2000.0) of Constellation Auriga, Latin for the Charioteer. The name "Capella" derives from the Latin for "Little Nanny" (or She-Goat) and refers to the goat carried on the left shoulder of the Charioteer. A bright star of the northern hemisphere in Winter, Capella is the closest first-magnitude star to the North Pole (and dimmer Polaris). Viewed as the sixth brightest star in the Earth's night sky, Capella is actually composed of four components, of which giant stars Aa and Ab constitute the second brightest spectroscopic binary -- first detected by William Wallace Campbell in six spectrum plates taken between 1896 and 1897. The system is a member of the Hyades group. (See Akira Fujii's color photo of Capella -- at the top left of photo.)
See an animation of the planetary and potentially habitable zone orbits of the Capella Aab system or of the Capella CD system, with a table of basic orbital and physical characteristics. Also, take a "trip to Capella" using hypothetical images from an x-ray interferometer (created by the Center for Astrophysics and Space Astronomy at the University of Colorado).
Capella Aa is a yellow-orange giant star of spectral and luminosity type G8-K0 IIIe. The star has about 2.7 times the mass of Sol, 12.2 times its diameter, and around 78.5 times its luminosity (Hummel et al, 1994; and Mewe et al, 2001; in pdf). According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 1708, Magnesium-II emissions were detected by balloon-borne spectrometer and soft X-ray emission was also detected. It may be around 525 million years old (Hummel et al, 1994). Star Aa is a RS CVn type variable, a New Suspected Variable star designated as NSV 1897. Useful catalogue numbers and designations for the star include: Alf or Alp Aur A, 13 Aur A, HR 1708*, Gl 194 A, Hip 24608, HD 34029, BD+45 1077, SAO 40186, FK5 193, LTT 11619, and ADS 3841 A.
Larger optical interferometric,
contour, and other images
(see Balwin et al, 1996 -- also
Giant stars Aa and Ab are
separated by less than the
Earth-Sun orbital distance
(around 0.73 AU).
A double-lined spectroscopic binary, Star Aa appears to be slightly more massive and so more highly evolved and cooler than companion Star Ab. Both appear to be on their first ascent of the red-giant branch, having probably both evolved from A-type dwarf stars with only a small difference in mass. Star Aa has almost certainly begun the fusion of its internal helium into carbon (as a helium-burning "clump" giant), because its low surface lithium abundance is indicative of dilution by a fully convective, stellar envelope (Pilachowski and Sowell, 1992). However, the deeper, hot corona of Star Aa is much more variable than that of Star Ab. The extreme ultraviolet spectra of Stars Aa and Ab display the presence of iron emission lines from Fe XV to Fe XXIV (more). The two stars have a combined luminosity of over 150 times that of Sol.
NASA, Chandra X-Ray Observatory -- ring and larger images (more)
Magnetic field interactions between giant stars Aa and Ab are thought to energize the corona
of the larger star (Aa), generating more x-rays than would be expected for a single star (more)
and creating an x-ray luminosity that is 10,000 times that of the Solar corona.
According to Hummel et al (1994) in the new Sixth Catalog of Visual Orbits of Binary Stars, Stars Aa and Ab are separated by less than two-thirds (0.730) of an AU (68 million miles) and move with a circular orbit (e= 0.000) with a period of only 104.0 days and an orbital inclination of about 137.2° from the perspective of an observer on Earth. (See an animation of the planetary and potentially habitable zone orbits of the Capella Aab system, with a table of basic orbital and physical characteristics.) The two bright stars have another pair of dim stellar companions C and D at a current separation of about 11,000 AUs, or 0.17 light-years.
Capella Ab is a yellow-orange giant star of spectral and luminosity type G1 III, with rotationally broadened spectral lines. Although it has been evolving out of the main sequence into the red-giant phase, the star has a contracting helium core that may not have as yet ignited helium fusion. Star Ab has about 2.6 times the mass of Sol, 9.2 times its diameter, and around 77.6 times its luminosity (Hummel et al, 1994; and Mewe et al, 2001; in pdf). It rotates much faster than Star Aa, resulting in enhanced chromospheric activity, but its high surface lithium abundance suggests that the star has not formed a deep convective envelope. Useful catalogue numbers and designations for the star include: Alf or Alp Aur B, Gl 194 B, and ADS 3841 P. (According to Robert Burnham, Jr. (1931-93), the designation "B" was already used for a nearby faint field star that was found later to be unrelated to the system gravitationally.)
Any Earth-type planets that may orbited Capella Aa or Ab during their youth probably have been burnt to a cinder. Currently, the water zones for either Capella Aa or Ab are farther out than the average orbital distance between these two stars. (As single stars, the orbit of an Earth-like planet with liquid water around Star Aa currently would be centered around 8.7 AU -- just inside the orbital distances of Saturn in the Solar System, while Star Ab's water zone orbit would be entered around 7.8 AU, between the orbital distances of Jupiter and Saturn.) During the relatively brief, combined giant phases of the two stars at present, however, a planet could orbit the Aab pair far enough out for the two stars to act as a single gravitational source and near enough for it to receive enough energy to sustain life, possibly around 12.5 AUs out from the binary. Astronomers would find it very difficult to detect an Earth-type planet in the water zone of this binary using present methods.
Eventually, Stars Aa and Ab will lose much of their current mass, from intensified stellar winds that eventually puff out their outer gas envelopes of hydrogen and helium (and lesser amounts of higher elements such as carbon and oxygen) into interstellar space as planetary nebulae. The result will be planet-sized, white dwarf cores that gradually cool and fade in brightness from the shutdown of thermonuclear fusion. (Nearby white dwarfs include solitary Van Maanen's Star and the dim companions of Sirius, Procyon, and 40 (Omicron2) Eridani.)
Discovered to be a faint companion of Stars Aab by Ragnar Furuhjelm, Capella C is a red dwarf star of spectral and luminosity type M1 V. It has about 30 to 40 percent of Sol's mass, 56 percent of its diameter, and 1.2 to 1.4 percent of its luminosity. Determined to be a binary by Carl L. Stearns in 1936, the combined visual luminosity of Stars C and D is about one percent of Sol's. According to Robert Burnham, Jr. (1931-93), this binary system was often designated as "Capella H" because the letters B through G were already used for faint field stars that are not gravitationally bound.
Stars C and D are at located at about: 05:17:23.7+45:50:23 (ICRS 2000). According to Wulff Dieter Heintz (1975) in the new Sixth Catalog of Visual Orbits of Binary Stars, Stars C and D have an average separation of about 48.1 AUs (a= 3.72") and move in a circular orbit (e= 0) with a period of about 388 years and an orbital inclination of 65.0° from the perspective of Earth. (See an animation of the planetary and potentially habitable zone orbits of the Capella CD system, with a table of basic orbital and physical characteristics.)
The orbit of an Earth-like planet (with liquid water) around Star C would be centered around 0.11 AU -- well inside the orbit of Mercury in the Solar System -- with an orbital period of 24.4 days. Useful catalogue numbers and designations for the star include: Gl 195 A, G 96-29, LTT 11622, AC+45 217-363 A, and ADS 3841 H.
NASA -- larger image
Capella CD are dim, red
dwarf stars, like Gliese
623 B (M5.8Ve) at lower
right of Gl 623 A (M2.5V).
Capella D is a red dwarf star of spectral and luminosity type M4-5 V. It has about a tenth of Sol's mass, 25 to 30 percent of its diameter, and 0.05 percent of its luminosity. The orbit of an Earth-like planet (with liquid water) around Star C would be centered around 0.022 AU -- well inside the orbit of Mercury in the Solar System -- with an orbital period of 7.8 days. Useful catalogue numbers and designations for the star include: Capella L, Gl 195 B, AC+45 217-363 B, and ADS 3841 L.
The following table includes all star systems known to be located within 10 light-years (ly), plus more bright stars within 10 to 20 ly, of Capella.
|Star System||Spectra &|
|L 1672-14||M2 V||3.9|
|Lambda Aurigae||G0-1.5 V-IV||4.5|
|G 81-39||K V||5.4|
|BD+53 935||M1 V||6.3|
|BD+53 934 AB?||K1 Ve||6.5|
|BD+52 911||M0 V||6.6|
|G 96-45||K-M V||7.6|
|Ross 28||M3.5 V||9.5|
|* plus bright stars *||. . .|
|Iota Persei||G0 V||16|
|Psi5 Aurigae||G0 V||18|
|Theta Persei 2?||F7 V||18|
|BD+37 783||G5 V||19|
Up-to-date technical summaries on this star can be found at: the Astronomiches Rechen-Institut at Heidelberg's ARICNS for Star A, Star B, Star C, and Star D; and the Nearby Stars Database. Additional information may be available at Roger Wilcox's Internet Stellar Database.
The Babylonians, Greeks, Arabs, and Chinese have all associated Constellation Auriga with a charioteer. According to one Greek myth, the head goddess Hera had a child who was born lame. Disgusted, she threw him out of Heaven to Earth, where he became the famous lame smith, Hephaestus, who fashioned beautiful ornaments and armor for the gods and who came to Orion's rescue after Orion had been blinded. Hephasetus invented the chariot to move around better despite his lameness. For more information about the stars and objects in this constellation and an illustration, go to Christine Kronberg's Auriga. For an illustration, see David Haworth's Auriga.
For more information about stars including spectral and luminosity class codes, go to ChView's webpage on The Stars of the Milky Way.
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