Defining the three categories by their similarity to the Sun reflects the evolution of astronomical observational techniques. Originally, solar-type was the closest that similarity to the Sun could be defined. Later, more precise measurement techniques and improved observatories allowed for greater precision of key details like temperature, enabling the creation of a solar analog category for stars that were particularly similar to the Sun. Later still, continued improvements in precision allowed for the creation of a solar-twin category for near-perfect matches. Similarity to the Sun allows for checking derived quantities—such as temperature, which is derived from the color index—against the Sun, the only star whose temperature is confidently known. For stars that are not similar to the Sun, this cross-checking cannot be done. Solar-type stars show highly correlated behavior between their rotation rates and their chromospheric activity (e.g. Calcium H & K line emission) and coronal activity (e.g. X-ray emission) Because solar-type stars spin down during their main-sequence lifetimes due to
magnetic braking, these correlations allow rough ages to be derived. Mamajek &
Hillenbrand (2008) However, there are some stars that come very close to being identical to the Sun, and thus are considered solar twins by members of the astronomical community. An exact solar twin would be a G2V star with a 5,778K surface temperature, be 4.6 billion years old, with the correct metallicity, and a 0.1%
solar luminosity variation. The stars below are more similar to the Sun and having the following qualities: •
Axial rotation approximately once every 27 days or • Radius of • Chemical composition by mass:
hydrogen (73.4%);
helium (25%);
carbon (0.2%);
nitrogen (0.09%);
oxygen (0.80%);
neon (0.16%);
magnesium (0.06%);
silicon (0.09&);
sulfur (0.05%);
iron (0.003%). The following are the known stars that come closest to satisfying the criteria for a solar twin. The Sun is listed for comparison. Highlighted boxes are out of range for a solar twin. The star may have been noted as solar twin in the past, but are more of a solar analog. Some other stars are sometimes mentioned as solar-twin candidates such as:
Beta Canum Venaticorum; however it has too low metallicity (−0.21) to be a solar twin.
16 Cygni B is sometimes noted as twin, but is part of a triple star system and is too old to be a solar twin at 6.8 Ga. == By potential habitability ==