Solar-type stars, 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.^[1] Observations of these stars are important for understanding better the properties of the Sun in relation to other stars and the habitability of planets.^[2]

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.^{[citation needed]}

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.^[1]

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 F8V through K2V, which would correspond to B−V color of 0.50 to 1.00.^[1] This definition fits approximately 10% of stars, so a list of solar-type stars would be quite extensive.^[3]

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)^[4] 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)^[5] 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).^{[citation needed]}

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):

Solar analog

These stars are photometrically similar to the Sun, having the following qualities:^[1]

• Temperature within 500 K from that of the Sun (5278 to 6278 K)
• Metallicity of 50–200% (± 0.3 dex) of that of the Sun, meaning the star's protoplanetary disk would have had similar amounts of dust from which planets could form
• No close companion (orbital period of ten days or less), because such a companion stimulates stellar activity

Solar analogs not meeting the stricter solar twin criteria include, within 50 light years and in order of increasing distance (The Sun is listed for comparison.):

Solar twin

To date no solar twin that exactly matches the Sun has been found.^[48] However, there are some stars that come very close to being identical to the Sun, and are such 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.^[49] Stars with an age of 4.6 billion years are at the most stable state. Proper metallicity, radius, chemical composition, rotation, magnetic activity, and size are also very important to low luminosity variation.^{[50][51][52][53]}

The stars below are more similar to the Sun and having the following qualities:^[1]

• Temperature within 50 K from that of the Sun (5728 to 5828 K)^[a] (within 10 K of sun (5768–5788 K)).
• Metallicity of 89–112% (± 0.05 dex) of that of the Sun, meaning the star's proplyd would have had almost exactly the same amount of dust for planetary formation
• No stellar companion, because the Sun itself is a solitary star
• An age within 1 billion years from that of the Sun (3.6 to 5.6 Ga)

Other Sun parameters:^[54]

• Sun rotates on its axis once in about 27 days or 1.997 kilometres (1.241 mi)/s
• Sun radius is 700,000 kilometres (430,000 mi)
• Sun 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%).^[55]

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.