Solar twin

Solar-type star, solar analogs (also analogues), and solar twins are stars that are particularly similar to the Sun. The stellar classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-type. Observations of these stars are important for understanding better the properties of the Sun in relation to other stars and the habitability of planets.

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.

These stars are broadly similar to the Sun. They are main-sequence stars with a B−V color between 0.48 and 0.80, the Sun having a B−V color of 0.65. Alternatively, a definition based on spectral type can be used, such as F8V through K2V, which would correspond to B−V color of 0.50 to 1.00. This definition fits approximately 10% of stars, so a list of solar-type stars would be quite extensive.

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) have estimated the ages for the 108 solar-type (F8V–K2V) main-sequence stars within 52 light-years (16 parsecs) of the Sun based on their chromospheric activity (as measured via Ca, H, and K emission lines).

The following table shows a sample of solar-type stars within 50 light years that nearly satisfy the criteria for solar analogs (B−V color between 0.48 and 0.80), based on current measurements: (The Sun is listed for comparison.)

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