User:Pfhreak/Solar Twin

Solar-Type, Solar Analog, and Solar Twin stars are those stars that are particularly similar to the Sun, with solar twin being more similar than solar analog, and solar-type being less similar than solar analog. Observations of these stars are important for understanding better the properties of our own Sun in relation to other stars and the habitability of planets.

By Similarity to the Sun
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 — like temperature, which is derived from the color index — against the Sun, the only star whose temperature is confidently known. For stars which aren't similar to the Sun, this cross-checking can't be done.

Solar-Type
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 F8 V through K2 V, 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 Analog
These stars are photometrically similar to the Sun, having the following qualities:


 * Temperature within 500 K Solar (roughly 5200 to 6300 K)
 * Metallicity of 50—200% Solar, meaning the star's proplyd would have had similar amounts of dust for planets to form from
 * No close companion (orbital period of ten days or less), as such a companion stimulates stellar activity

Solar analogues which don't meet the stricter solar twin criteria include:


 * Alpha Centauri A
 * Alpha Centauri B
 * Tau Ceti
 * 40 Eridani A
 * 70 Ophiuchi A
 * Eta Cassiopeiae A
 * Delta Pavonis
 * Xi Ursae Majoris A
 * Xi Ursae Majoris B
 * 61 Virginis
 * Zeta Tucanae
 * Chi1 Orionis
 * Beta Comae Berenices
 * Kappa1 Ceti
 * HR 4523
 * 61 Ursae Majoris
 * HR 4458 A
 * HR 511
 * 51 Pegasi

Solar Twin
These stars are more similar to the Sun still, having the following qualities:


 * Temperature within 50 K Solar (roughly 5720 to 5830 K)
 * Metallicity of 89—112% (± 0.05 dex) Solar, meaning the star's proplyd would have had almost exactly the same amount of dust for planets to form from
 * No stellar companion, because the Sun itself is solitary
 * An age within 1 billion years Solar (roughly 3.5 to 5.6 Ga)

Solar twins include:


 * Beta Canum Venaticorum
 * 18 Scorpii
 * 37 Geminorum
 * HD 98618

By Potential Habitability
Another way of defining solar twin is as a "habstar" — a star with qualities believed to be particularly hospitable to an Earth-like planet. Qualities considered include variability, mass, age, metallicity, and close companions.