Capella 4

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).

NASA Observatorium




See a discussion of
the "main sequence"
as part of stellar
evolution and death.

Alpha Aurigae Aa (or A)

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.

© MRAO Cambridge, COAST


Larger optical interferometric,
contour, and other images
(see Balwin et al, 1996 -- also
at COAST).



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.

Alpha Aurigae Ab (or B or P)

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.)

NASA Observatorium


See a discussion of
"core helium burning"
as part of stellar
evolution and death.

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.

H. Bond (STSci), R. Ciardullo (PSU), WFPC2, HST, NASA -- larger image
(White dwarfs are remnant stellar cores that have cast off their outer gas
layers, like planetary nebula NGC 2440.)

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.)

Alpha Aurigae C (or H)

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).

Alpha Aurigae D (or L)

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.