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]
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]
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):
Identifier | J2000 coordinates [6] | Distance [6] (ly) | Stellar class [6] | Temperature (K) | Metallicity (dex) | Age (Gyr) | Notes | |
---|---|---|---|---|---|---|---|---|
Right ascension | Declination | |||||||
Sun | — | — | 0.0000158 | G2V | 5778 | +0.00 | 4.6 | [7] |
Rigil Kentaurus [8] | 15h 49m 36.49400s | −60° 50′ 02.3737″ | 4.37 | G2V | 5790 | +0.20 | 4.4 | [9] [10] [11] [12] |
Toliman | 4.37 | K0V | 5260 | 4.4 | ||||
Epsilon Eridani [13] | -09h 27m 29.7s | 03° 32′ 55.8″ | 10.4 | K2V | 5084 | -0.13 | 0.4-0.8 | |
Tau Ceti [14] | 01h 44m 04.1s | −15° 56′ 15″ | 11.9 | G8V | 5344 | –0.52 | 5.8 | [15] |
82 Eridani [16] | 03h 19m 55.7s | −43° 04′ 11.2″ | 19.8 | G8V | 5338 | –0.54 | 6.1 | [17] |
Delta Pavonis [18] | 20h 08m 43.6s | −66° 10′ 55″ | 19.9 | G8IV | 5604 | +0.33 | ~7 | [19] |
V538 Aurigae [20] | 05h 41m 20.3s | +53° 28′ 51.8″ | 39.9 | K1V | 5257 | −0.20 | 3.7 | [17] |
HD 14412 [21] | 02h 18m 58.5s | −25° 56′ 45″ | 41.3 | G5V | 5432 | −0.46 | 9.6 | [17] |
HR 4587 [22] | 12h 00m 44.3s | −10° 26′ 45.7″ | 42.1 | G8IV | 5538 | +0.18 | 8.5 | [17] |
HD 172051 [23] | 18h 38m 53.4s | −21° 03′ 07″ | 42.7 | G5V | 5610 | −0.32 | 4.3 | [17] |
72 Herculis [24] | 17h 20m 39.6s | +32° 28′ 04″ | 46.9 | G0V | 5662 | −0.37 | 5 | [17] |
HD 196761 [25] | 20h 40m 11.8s | −23° 46′ 26″ | 46.9 | G8V | 5415 | −0.31 | 6.6 | [19] |
Nu² Lupi [26] | 15h 21m 48.1s | −48° 19′ 03″ | 47.5 | G4V | 5664 | −0.34 | 10.3 | [19] |
These stars are photometrically similar to the Sun, having the following qualities: [1]
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.):
Identifier | J2000 coordinates [6] | Distance [6] (ly) | Stellar class [6] | Temperature (K) | Metallicity (dex) | Age (Gyr) | Notes | |
---|---|---|---|---|---|---|---|---|
Right ascension | Declination | |||||||
Sun | — | — | 0.0000158 | G2V | 5,778 | +0.00 | 4.6 | [7] |
Sigma Draconis [27] | 19h 32m 21.6s | +69° 39′ 40″ | 18.8 | G9–K0 V | 5,297 | −0.20 | 4.7 | [28] |
Beta Canum Venaticorum [29] | 12h 33m 44.5s | +41° 21′ 27″ | 27.4 | G0V | 5,930 | −0.30 | 6.0 | [17] |
61 Virginis [30] | 13h 18m 24.3s | −18° 18′ 40″ | 27.8 | G5V | 5,558 | −0.02 | 6.3 | [19] |
Zeta Tucanae [31] | 00h 20m 04.3s | –64° 52′ 29″ | 28.0 | F9.5V | 5,956 | −0.14 | 2.5 | [15] |
Beta Comae Berenices [32] | 13h 11m 52.4s | +27° 52′ 41″ | 29.8 | G0V | 5,970 | −0.06 | 2.0 | [17] |
61 Ursae Majoris [33] | 11h 41m 03.0s | +34° 12′ 06″ | 31.1 | G8V | 5,483 | −0.12 | 1.0 | [17] |
HR 511 [34] | 01h 47m 44.8s | +63° 51′ 09″ | 32.8 | K0V | 5,333 | +0.05 | 3.0 | [17] |
Alpha Mensae [35] | 06h 10m 14.5s | –74° 45′ 11″ | 33.1 | G5V | 5,594 | +0.10 | 5.4 | [15] |
HD 69830 [36] | 08h 18m 23.9s | −12° 37′ 56″ | 40.6 | K0V | 5,410 | −0.03 | 10.6 | [15] |
HD 10307 [37] | 01h 41m 47.1s | +42° 36′ 48″ | 41.2 | G1.5V | 5,848 | −0.05 | 7.0 | [17] |
HD 147513 [38] | 16h 24m 01.3s | −39° 11′ 35″ | 42.0 | G1V | 5,858 | +0.03 | 0.4 | [19] |
58 Eridani [39] | 04h 47m 36.3s | −16° 56′ 04″ | 43.3 | G3V | 5,868 | +0.02 | 0.6 | [15] |
47 Ursae Majoris [40] | 10h 59m 28.0s | +40° 25′ 49″ | 45.9 | G1V | 5,954 | +0.06 | 6.0 | [15] |
Psi Serpentis [41] | 15h 44m 01.8s | +02° 30′ 54.6″ | 47.8 | G5V | 5,683 | 0.04 | 3.2 | [42] |
HD 84117 [43] | 09h 42m 14.4s | –23° 54′ 56″ | 48.5 | F8V | 6,167 | −0.03 | 3.1 | [15] |
HD 4391 [44] | 00h 45m 45.6s | –47° 33′ 07″ | 48.6 | G3V | 5,878 | −0.03 | 1.2 | [15] |
20 Leonis Minoris [45] | 10h 01m 00.7s | +31° 55′ 25″ | 49.1 | G3V | 5,741 | +0.20 | 6.5 | [17] |
Nu Phoenicis [46] | 01h 15m 11.1s | –45° 31′ 54″ | 49.3 | F8V | 6,140 | +0.18 | 5.7 | [15] |
Helvetios [47] | 22h 57m 28.0s | +20° 46′ 08″ | 50.9 | G2.5IVa | 5,804 | +0.20 | 7.0 | [15] |
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 5,778 K 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]
Other Sun parameters: [54]
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.
Identifier | J2000 coordinates [6] | Distance [6] (ly) | Stellar class [6] | Temperature (K) | Metallicity (dex) | Age (Gyr) | Notes | |
---|---|---|---|---|---|---|---|---|
Right ascension | Declination | |||||||
Sun | — | — | 0.0000158 | G2V | 5,778 | +0.00 | 4.6 | [7] |
18 Scorpii [56] | 16h 15m 37.3s | –08° 22′ 06″ | 45.1 | G2Va | 5,433 | −0.03 | 2.9 | [57] [58] |
HD 150248 [59] | 16h 41m 49.8s | –45° 22′ 07″ | 88 | G2 | 5,750 | −0.04 | 6.2 | [58] |
HD 164595 [60] | 18h 00m 38.9s | +29° 34′ 19″ | 91 | G2 | 5,810 | −0.06 | 4.5 | [58] |
HD 195034 [61] | 20h 28m 11.8s | +22° 07′ 44″ | 92 | G5 | 5,760 | −0.04 | 2.9 | [62] |
HD 117939 [63] | 13h 34m 32.6s | –38° 54′ 26″ | 98 | G3 | 5,730 | −0.10 | 6.1 | [58] |
HD 138573 [64] | 15h 32m 43.7s | +10° 58′ 06″ | 99 | G5IV–V | 5,757 | +0.00 | 7.1 | [65] |
HD 71334 [66] | 08h 25m 49.5s | −29° 55′ 50″ | 124 | G2 | 5,701 | −0.075 | 8.1 | [67] |
HD 98649 [68] | 11h 20m 51.769s | –23° 13′ 02″ | 135 | G4V | 5,759 | −0.02 | 2.3 | [58] |
HD 143436 [69] | 16h 00m 18.8s | +00° 08′ 13″ | 141 | G0 | 5,768 | +0.00 | 3.8 (±2.9) | [70] |
HD 129357 [71] | 14h 41m 22.4s | +29° 03′ 32″ | 154 | G2V | 5,749 | −0.02 | 8.2 | [70] |
HD 133600 [72] | 15h 05m 13.2s | +06° 17′ 24″ | 171 | G0 | 5,808 | +0.02 | 6.3 | [57] |
HD 186302 [73] | 19h 49m 6.43s | −70° 11′ 16.7″ | 184 | G3 | 5,675 | +0.00 | 4.5 | [74] |
HIP 11915 [75] | 02h 33m 49.02s | −19° 36′ 42.5″ | 190 | G5V | 5,760 | –0.059 | 4.1 | [76] |
HD 101364 [77] | 11h 40m 28.5s | +69° 00′ 31″ | 208 | G5V | 5,795 | +0.02 | 7.1 | [57] [78] |
HD 197027 [79] | 20h 41m 54.6s | –27° 12′ 57″ | 250 | G3V | 5,723 | −0.013 | 8.2 | [80] |
Kepler-452 [81] | 19h 44m 00.89s | +44° 16′ 39.2″ | 1400 | G2V | 5,757 | +0.21 | 6.0 | [82] |
YBP 1194 [83] | 08h 51m 00.8s | +11° 48′ 53″ | 2934 | G5V | 5,780 | +0.023 | ~ 4.2 (± 1.6) | [84] |
Some other stars are sometimes mentioned as solar-twin candidates such as: Beta Canum Venaticorum; however it has too low metallicities (−0.21) for solar twin. 16 Cygni B is sometimes noted as twin, but is part of a triple star system and is very old for a solar twin at 6.8 Ga. Two solar sibling candidates (similar age, metallicity (−0.113), and kinematics) are Gaia DR2 1927143514955658880 and 1966383465746413568. [85]
Another way of defining solar twin is as a "habstar"—a star with qualities believed to be particularly hospitable to a life-hosting planet. Qualities considered include variability, mass, age, metallicity, and close companions. [86] [lower-alpha 2]
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. [86] [89] 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. [lower-alpha 3]
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. [51] [52] [86] [53]
Terrestrial planets in multiple star systems, those containing three or more stars, are not likely to have stable orbits in the long term. Stable orbits in binary systems take one of two forms: S-Type (satellite or circumstellar) orbits around one of the stars, and P-Type (planetary or circumbinary) orbits around the entire binary pair. Eccentric Jupiters may also disrupt the orbits of planets in habitable zones. [86]
Metallicity of at least 40% solar ([Fe/H] = −0.4) is required for the formation of an Earth-like terrestrial planet. High metallicity strongly correlates to the formation of hot Jupiters, but these are not absolute bars to life, as some gas giants end up orbiting within the habitable zone themselves, and could potentially host Earth-like moons. [86]
One example of such a star is HD 70642 [90] , a G5V, at temperature of 5533 K, but is much younger than the Sun, at 1.9 billion years old. [91]
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. [92] 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. [93]
107 Piscium is a single star in the constellation of Pisces. 107 Piscium is the star's Flamsteed designation. John Flamsteed numbered the stars of Pisces from 1 to 113, publishing his Catalogus Britannicus in 1725. He accidentally numbered 107 Piscium twice, as he also allocated it the designation of 2 Arietis. This star is faintly visible to the naked eye with an apparent visual magnitude that has been measured varying between 5.14 and 5.26. However, that finding of variation was not confirmed by subsequent observations and is most likely spurious data. It is located at a distance of about 24.8 light years away from the Sun. 107 Piscium is drifting closer to the Sun with a radial velocity of −33.6, and is predicted to come as close as 15.4 light-years in around 135,800 years.
HD 192263 is a star with an orbiting exoplanet in the equatorial constellation of Aquila. The system is located at a distance of 64 light years from the Sun based on parallax measurements, and is drifting further away with a radial velocity of −10.7 km/s. It has an absolute magnitude of 6.36, but at that distance the apparent visual magnitude is 7.79. It is too faint to be viewed with the naked eye, but with good binoculars or small telescope it should be easy to spot.
HD 147513 is a star in the southern constellation of Scorpius. It was first catalogued by Italian astronomer Piazzi in his star catalogue as "XVI 55". With an apparent magnitude of 5.38, according to the Bortle scale it is visible to the naked eye from suburban skies. Based upon stellar parallax measurements by the Hipparcos spacecraft, HD 147513 lies some 42 light years from the Sun.
HD 28185 is a yellow dwarf star similar to the Sun located 128 light-years away from Earth in the constellation Eridanus. The designation HD 28185 refers to its entry in the Henry Draper catalogue. The star is known to possess one long-period extrasolar planet.
37 Geminorum is a solitary Sun-like star located at the northwest part of the northern constellation of Gemini, about three degrees to the east of the bright star Epsilon Geminorum. The apparent visual magnitude of 37 Geminorum is 5.74, which is just bright enough to be visible to the naked eye on a dark night. It is located at a distance of 57 light years from the Sun based on parallax. This star is drifting closer with a radial velocity of −15 km/s, and is predicted to come as near as 13.8 light-years in approximately a million years. It is positioned close enough to the ecliptic to be subject to lunar occultations, such as happened on April 8, 1984.
HD 172051 is a single, yellow-hued star in the southern constellation of Sagittarius. The star is barely bright enough to be seen with the naked eye, having an apparent visual magnitude of 5.85. Based upon an annual parallax shift of 76.64 mas, it is located some 43 light years from the Sun. It is moving away from the Sun with a radial velocity of +37 km/s.
HD 70642 is a star with an exoplanetary companion in the southern constellation of Puppis. It has an apparent visual magnitude of +7.17, which is too dim to be readily visible to the naked eye. The system is located at a distance of 95.5 light-years from the Sun based on parallax measurements, and is drifting further away with a radial velocity of +49.3 km/s. It came to within 55.3 ly of the Solar System some 329,000 years ago.
HD 222582 is a multiple star system in the equatorial constellation of Aquarius. It is invisible to the naked eye with an apparent visual magnitude of 7.7, but can be viewed with binoculars or a small telescope. The system is located at a distance of 137 light years from the Sun based on parallax, and it is drifting further away with a radial velocity of +12 km/s. It is located close enough to the ecliptic that it is subject to lunar occultations.
HD 85512 is a solitary K-type main-sequence star 36.8 light-years away in the constellation Vela. It is about 1 billion years older than the Sun. It is extremely chromospherically inactive, only slightly more active than Tau Ceti. It exhibits a long-term variability and was thought to host one low-mass planet, although this is now doubtful.
HD 131664 is an 8th magnitude star in the southern constellation of Apus with an orbiting brown dwarf or stellar companion. Parallax measurements by the Gaia space observatory provide an estimated distance of 172.5 light years from the Earth. The system is moving further away with a baseline heliocentric radial velocity of +35 km/s.
HD 148156 is a star with an orbiting exoplanet in the southern constellation of Norma. It is located at a distance of 186 light years away from the Sun, but is drifting closer with a radial velocity of –1.8 km/s. The star has an apparent visual magnitude of 7.69, which is too dim to be visible to the naked eye. A survey in 2015 ruled out the existence of any stellar companions at projected distances from 49 to 345 astronomical units.
Gliese 146 is a star with an orbiting exoplanet in the constellation Horologium. Gliese 146 is also catalogued as HD 22496, HIP 16711, SAO-216392, and LHS 1563. With an apparent visual magnitude of 8.64, it is too faint to be visible to the naked eye. Gliese 146 is located at a distance of 44.4 light years based on parallax measurements, and is drifting further away with a heliocentric radial velocity of +21 km/s.
HD 38858 is a G-type star, much like The Sun, with one detected planet. The planet, designated HD 38858 b, is about twice the mass of Uranus and orbits in the star's habitable zone.
HIP 100963 is a G-type star in the faint northern constellation of Vulpecula resembling the Sun. It has an apparent visual magnitude of approximately 7.1, making it generally too faint to be seen with the naked eye in most circumstances. The distance to this star, as determined using parallax measurements from the Gaia spacecraft, is around 92 light-years.
HD 45184 is a star in the southern constellation of Canis Major. It is a yellow-hued star near the lower limit of visibility to the naked eye with an apparent visual magnitude of 6.37. The star is located at a distance of 71.65 light years from the Sun based on parallax. It is drifting closer with a radial velocity of −3.8 km/s.
HIP 11915 is a G-type main-sequence star located about 170 light-years from Earth in the constellation Cetus. It is best known for its characteristics, which are very similar to those of the Sun, including the mass, radius, temperature, metallicity and age, which means that it is almost a solar twin, being just 500 million years younger than the Sun and with a lower metallicity. It is also known for its planetary companion, HIP 11915 b, which has a mass and orbital distance very similar to that of Jupiter, but probably with a slightly higher orbital eccentricity.
HD 150248 is a Sun-like star 87 light-years from the Sun. HD 150248 is a G-type star and a near solar twin. HD 150248's photometric color is also very close to that of the Sun; however, it has a lower abundance of metals, and has an apparent visual magnitude of 7.02. At 6.2 billion years old, this star is 1.6 billion years older than the Sun and has passed the stable burning stage. HD 150248 is found on the border between the constellations Scorpius and Ara.
HD 71334 is a Sun-like star 126.7 light years from the Sun. HD 150248 is a G-type star and an older solar analog. It is older than the sun at 8.1 billion years, compared to the sun at 4.6 billion years old. At 8.1 billion years old, HD 71334 has passed its stable burning stage. HD 71334 has a lower metallicity that the Sun. HD 71334 is found in the constellations of Puppis. Puppis is one of the 88 modern constellations recognized by the International Astronomical Union. HD 71334 has a brightness of 7.8.
HD 168009 is a star in the northern constellation of Lyra. It has an apparent visual magnitude of 6.3, placing it just above to below the normal limit of stars visible to the naked eye under good viewing conditions of 6-6.5. An annual parallax shift of 42.93 mas provides a distance estimate of 76 light years. It is moving closer to the Sun with a heliocentric radial velocity of −65 km/s. In about 328,000 years from now, the star will make its closest approach at a distance of around 17 ly (5.1 pc).
HD 189567 is a star with a pair of orbiting exoplanets, located in the southern constellation of Pavo. It is also known as Gliese 776, CD-67 2385, and HR 7644. The star has an apparent visual magnitude of 6.07, which is bright enough for it to be dimly visible to the naked eye. It lies at a distance of 58 light years from the Sun based on parallax measurements, but is drifting closer with a radial velocity of −10.5 km/s.
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