Our close neighbor Alpha Centauri A, which along with Alpha Centauri B comprises the Alpha Centauri binary system, is another type G2V star. About 10% heavier than the sun, and nearly 25% larger in diameter, it is 4.365 light-years away. It is slightly older than the Sun, at 4.85 billion years, and its extra mass means it will not survive quite as long. The two components orbit each other on an eccentric orbit where their separation varies between 1.67 and 5.3 billion km (c. 1 to 3.3 billion miles) over a period of about 79.9 years. A third star, Proxima Centauri, which is the closest star to us apart from the Sun, may be gravitationally bound to the Alpha Centauri system, but is about one fifth of a light year away from it. In about 28,000 years, Alpha Centauri will be about 3.26 light years away from us, after which it will recede until in about 100,000 years it will be so far away it will cease to be visible to the naked eye.
51 Pegasi is very close to being a copy of the sun; just about 25% larger, 6% heavier and 30% brighter. It is also about 60% richer in elements heavier than hydrogen. It is borderline between being a sub-giant and on the main sequence so is listed as either a type G2.5IVa or a type G4-5Va. About 51 light-years away, it is estimated to be between 7.5 and 8.5 billion years old. It is further advanced in its evolution than the sun, and beginning to run out of hydrogen. It has a planet 51 Peg b, unofficially called Bellerophon, with about half the mass of Jupiter, that orbits incredibly close to its parent; once every 4.2 days at a distance of only about 4.6 million miles. It has a surface temperature in excess of 1,000ºK. The image to the right is an artist's impression of what the surface could look like if the planet were a large rocky planet rather than a gas giant.
Type G Stars
The most famous G type star is our own Sun (type G2V), a full description of which is available in the Solar Ssystem section. These type of star typically weigh about the same as the sun; perhaps 0.75 to 1.25 times the mass of the Sun, but, yet again, not all are typical as you will see.
Alpha Centauri A
Tau Ceti is a type G8V star that is very similar to the Sun at a little less than 80% of the diameter and mass, and about half the luminosity. It is about 12 light-years away, and is much less rich in heavier elements than the Sun, so is unlikely to have formed rocky, earth type planets. However, it also has an extensive debris disk, analogous to our Kuiper Belt, which raises the possibility of planetary formation, though they would be well outside of the habitable zone. The extensive debris disk, with perhaps 10 times the amount of material that is in the Kuiper Belt, would result in many more asteroid impacts than we see on Earth. There may be a Jupiter sized gas giant that would protect any inner planets in the same way Jupiter does in our solar system.
Estimates of the age of Tau Ceti vary, but center on a range of between six and ten billion years old. It will follow a similar evolutionary pattern to the sun, but will survive longer, perhaps 12 billion years on the main sequence, due to its being lighter. It is a very stable star, even more so than the Sun, in fact.
Rather than a single star, the Capella, or Alpha Aurigae, star system comprises two binary pairs, and is one of the brightest stellar objects seen in the northern hemisphere. The brighter pair comprises two type-G giant stars both around 42 light-years away, and the other pair comprises two small red dwarfs. The primary star of the brighter pair, a type G8III that is cooling and evolving into a type K0III on its way to becoming a red giant, is about 2.7 times the mass of the Sun, and 12 times larger in diameter, with a surface temperature of approximately 4,900 K. The other star of the pair, a type G1III, is a little less than 2.6 times the mass of the Sun, and 9 times larger in diameter, with a surface temperature of approximately 5,700 K. Both are intrinsically nearly 80 times brighter than the Sun. They are very close together, orbiting about 100 million km (62.5 million miles) from each other once every 104 days. Although only about 500 million years old, both probably started as type A stars and have have started to expand as they start changing into red giants. The larger of the type G stars has probably started to burn helium, and is not likely to last more than another 50 million years or so. Being only about .73 astronomical units apart, when the larger star stops burning helium and starts to expand dramatically, it will almost certainly disrupt the other star of the pair.
Astronomy & Cosmology
Stars - Stellar Classes