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Comparative sizes of nearby exoplanetary host stars - click for a diagram showing their location

Five nearby stars that are known or suspected to host exoplanetary systems, shown at their relative sizes. Colors are exaggerated to emphasize differences in spectral classes. Basic details on each system are available here. More extensive discussions are linked to the individual star names in the table below. See also What color are the stars? Image by DEEP FLY 2007.

The size of a star is determined by its mass and its evolutionary phase. In this diagram, all stars except Gamma Cephei are in the main sequence stage of their development, in which they shine by converting hydrogen to helium through nuclear fusion in their cores. Gamma Cephei is a subgiant, meaning that it has exhausted the hydrogen in its core and is now burning hydrogen in its outer shells, resulting in a substantial increase in diameter and a reddening in color. When it was on the main sequence, Gamma Cephei was evidently an F-type star like Upsilon Andromedae.

All stars rotate, just like planets and moons. Younger stars like Vega (about 200 million years old) rotate faster than older stars like the Sun or Gamma Cephei (4.6 and 6.6 billion years old, respectively). Vega's rotational period of about 12.5 hours is so fast that it has resulted in an equatorial bulge, such that the star is 23% wider at the equator than at the poles (Aufdenberg et al. 2007).

For stars more massive than spectral type M, the evolutionary phase is indicated by appending the Roman numerals I-V to the star's spectral class. V refers to the main sequence; IV refers to the subgiant phase; III refers to the giant phase; and I and II refer to the rare class of high-mass supergiants. (M dwarf stars do not follow the same evolutionary path as more massive stars.)



GJ 581 Epsilon
Eridani 		Sun Upsilon
Andromedae 		Vega Gamma
Cephei
Spectral Type M3 K2 V G2 V F8 V A0 V K1 IV
Mass 0.31 0.86 1.00 1.32 2.14 1.40
Diameter 0.29 0.77 1.00 1.64 2.82 4.80
Luminosity 0.01 0.34 1.00 3.4 37 11

Sources: Bonfils et al. (2005) The HARPS search for southern extra-solar planets VI. A Neptune-mass planet around the nearby M dwarf Gliese 581 (all data on GJ 581). Takeda et al. (2007) Structure and evolution of nearby stars with planets II: Physical properties of ~1000 cool stars from the SPOCS catalog (Epsilon Eridani and Upsilon Andromedae). Aufdenberg et al. (2006) First results from the CHARA Array VII: Long-baseline interferometric measurements of Vega consistent with a pole-on, rapidly rotating star (Vega); Yoon et al. (2010) A new view of Vega's composition, mass, and age (Vega). Neuhauser et al. (2007) Direct detection of exoplanet host star companion g Cep B and revised masses for both stars and sub-stellar object (Gamma Cephei). Kaler J. (2007) Stars (luminosities for e Eridani, u Andromedae, and g Cephei).










All text is copyright Raymond Harris 2006-2010. Credits for each image are listed in the accompanying caption.