Epsilon Aurigae

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**Epsilon Aurigae A**
Observation data Epoch J2000
Constellation (pronunciation)	Auriga
Right ascension	05^h 01^m 58.1^s
Declination	+43° 49′ 24″
Apparent magnitude (V)	3.04
Characteristics
Spectral type	A8 Iab
U-B color index	0.30
B-V color index	0.54
Variable type	Eclipsing binary
Astrometry
Radial velocity (R_v)	−2.5 km/s
Proper motion (μ)	RA: 0.18 mas/yr Dec.: −2.31 mas/yr
Parallax (π)	1.60 ± 1.16 mas
Distance	approx. 2000 ly (approx. 600 pc)
Absolute magnitude (M_V)	−5.95
Details
Mass	15–19 M_☉
Radius	100 R_☉
Luminosity	47,000 L_☉
Temperature	7,800 K
Metallicity	?
Rotation	?
Age	? years
Other designations
Haldus, Almaaz, Al Anz, 7 Aurigae, HR 1605, BD +43°1166, HD 31964, SAO 39955, FK5 183, GC 6123, ADS 3605, CCDM 05020+4350, HIP 23416.
Database references
SIMBAD	data

Epsilon Aurigae (ε Aur / ε Aurigae) is a star in the constellation Auriga. It also has the traditional names Haldus, Almaaz, or Al Anz. Epsilon Aurigae is an eclipsing binary, whose brightness varies in apparent visual magnitude between +3.0 and +3.8 with a period of about 9,890 days (~27.1 years). It is approximately 2,000 light years distant.

The visible component is a supergiant belonging to the spectral class A8 and is among some of the most luminous stars within 1000 parsecs of the Sun. Its diameter corresponds to about 100 solar diameters. The other (eclipsing) component is not visible.

The most-favored speculation is that it consists of one or two small stars with a ring of opaque dust set nearly edge-on, producing the observed eclipses of 2 years' duration.
The alternate hypothesis is that it is immense, but of low density, and thus semi-transparent. If this is so, then it will be the largest known object in the universe.

One of the phenomena exhibited during the eclipse is that the level of obscuration of the A star by the companion diminishes briefly, as though there were a hole in the companion.

This system was an early candidate for containing a black hole but the two models given above are now preferred. See [1] and [2]

The next eclipse occurs from 2009-2011, and it is hoped that the extensive observations that will likely be carried out may yield further insights into the nature of this system. Finding the star is not difficult; it is the apex of the isosceles triangle formed as the 'nose' of the constellation Auriga, and is visible throughout the winter at northern latitudes, not far from brilliant, yellow Capella.