Solar analog

thumb|This illustration compares the somewhat larger and hotter Sun (left) to the relatively inactive star Tau Ceti.

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.

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)

|style="text-align:left;"|

| 10.4

| K2V

| 5084

|style="background-color:Moccasin;"| -0.13

| 0.4-0.8

|style="text-align:left;"|

| 11.9

| G8V

| 5344

|style="background-color:Moccasin;"| –0.52

|5.8

| 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.

right|thumb|350px|Morgan-Keenan spectral classification of stars. Most common star type in the universe are M-dwarfs, 76%. The Sun is a 4.6 billion year-old G-class (G2V) star and is more massive than 95% of all stars. Only 7.6% are G-class stars.

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%).

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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.

{| class="wikitable sortable mw-collapsible" style="text-align: center; white-space: nowrap;"

!rowspan="2"| Identifier

!colspan="2"| J2000 coordinates

|style="text-align:left;"|

| 208

| G5V

| 5,795

| +0.02

| style="background-color:Moccasin;"|7.1

|style="text-align:left;"|

| 2934

| G5V

| 5,780

| +0.023

|style="background-color:Moccasin;"| ~ 4.2 (± 1.6)

At least 0.5–1 billion years old
On the main sequence
Non-variable
Capable of harboring terrestrial planets
Support a dynamically stable habitable zone
0–1 non-wide stellar companion stars.

The requirement that the star remain on the main sequence for at least 0.5–1 Ga sets an upper limit of approximately 2.2–3.4 solar masses, corresponding to a hottest spectral type of A0-B7V. Such stars can be 100x as bright as the Sun. Tardigrade-like life (due to the UV flux) could potentially survive on planets orbiting stars as hot as B1V, with a mass of 10 M☉, and a temperature of 25,000 K, a main-sequence lifetime of about 20 million years.

Non-variability is ideally defined as variability of less than 1%, but 3% is the practical limit due to limits in available data. Variation in irradiance in a star's habitable zone due to a companion star with an eccentric orbit is also a concern.

Another such example would be HIP 11915, which has a planetary system containing a Jupiter-like planet orbiting at a similar distance that the planet Jupiter does in the Solar System. To strengthen the similarities, the star is class G5V, has a temperature of 5750 K, has a Sun-like mass and radius, and is only 500 million years younger than the Sun. As such, the habitable zone would extend in the same area as the zone in the Solar System, around 1 AU. This would allow an Earth-like planet to exist around 1 AU.

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See also

Footnotes

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Further reading