Solar analog

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"Solar twins" redirects here. For the musical group, see Solar Twins (band). For a star from the same stellar nursery as the Sun, see Solar sibling.

Solar-type star, solar analogs (also analogues), and solar twins are stars that are particularly similar to the Sun. The 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.

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—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

The Sun (left) compared to the similar but slightly smaller and less active Tau Ceti (right).

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[citation needed], 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)[citation needed]. 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)[2] have estimated the ages for the 108 solar-type (F8V–K2V) main-sequence stars within 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, based on current measurements.

Sample of solar-type stars
Identifier J2000 Coordinates[3] Distance[3]
(ly)		 Stellar
Class[3]		 Temperature
(K)		 Metallicity
(dex)		 Notes
Right ascension Declination
Epsilon Eridani [4] 03h 32m 55.8s −09° 27′ 29.7″ 10.5 K2V 5,153 −0.11 [5]
Tau Ceti [6] 01h 44m 04.1s −15° 56′ 15″ 11.9 G8V 5,344 –0.52 [7]
40 Eridani [8] A 04h 15m 16.3s −07° 39′ 10″ 16.5 K1V 5,126 –0.31 [7]
82 Eridani [9] 03h 19m 55.7s −43° 04′ 11.2″ 19.8 G8V 5,338 –0.54 [10]
Delta Pavonis [11] 20h 08m 43.6s −66° 10′ 55″ 19.9 G8IV 5,604 +0.33 [5]
HR 7722 [12] 20h 15m 17.4s −27° 01′ 59″ 28.8 K2V 5,166 −0.04 [5]
Gliese 86 [13] A 02h 10m 25.9s −50° 49′ 25″ 35.2 G9V 5,163 −0.24 [7]
54 Piscium [14] 00h 39m 21.8s +21° 15′ 02″ 36.1 K0V 5,129 +0.19 [10]
V538 Aurigae [15] 05h 41m 20.3s +53° 28′ 51.8″ 39.9 K1V 3,500–5,000 −0.20 [10]
HD 14412 [16] 02h 18m 58.5s −25° 56′ 45″ 41.3 G5V 5,432 −0.46 [10]
HR 4587 [17] 12h 00m 44.3s −10° 26′ 45.7″ 42.1 G8IV 5,538 0.18 [10]
HD 172051 [18] 18h 38m 53.4s −21° 03′ 07″ 42.7 G5V 5,610 −0.32 [10]
72 Herculis [19] 17h 20m 39.6s +32° 28′ 04″ 46.9 G0V 5,662 −0.37 [10]
HD 196761 [20] 20h 40m 11.8s −23° 46′ 26″ 46.9 G8V 5,415 −0.31 [5]
Nu² Lupi [21] 15h 21m 48.1s −48° 19′ 03″ 47.5 G4V 5,664 −0.34 [5]

Solar analog

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

  • Temperature within 500 K from that of the Sun (roughly 5200 to 6300 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:

Identifier J2000 Coordinates[3] Distance[3]
(ly)		 Stellar
Class[3]		 Temperature
(K)		 Metallicity
(dex)		 Notes
Right ascension Declination
Alpha Centauri [22] A 14h 39m 36.5s −60° 50′ 02″ 4.37 G2V 5,847 +0.24 [23]
Alpha Centauri [24] B 14h 39m 35.0s −60° 50′ 14″ 4.37 K1V 5,316 +0.25 [23]
70 Ophiuchi [25] A 18h 05m 27.3s +02° 30′ 00″ 16.6 K0V 5,314 −0.02 [26]
Sigma Draconis [27] 19h 32m 21.6s +69° 39′ 40″ 18.8 K0V 5,297 −0.20 [28]
Eta Cassiopeiae [29] A 00h 49m 06.3s +57° 48′ 55″ 19.4 G0V 5,941 −0.17 [30]
107 Piscium [31] 01h 42m 29.8s +20° 16′ 07″ 24.4 K1V 5,242 −0.04 [10][32]
Beta Canum Venaticorum [33] 12h 33m 44.5s +41° 21′ 27″ 27.4 G0V 5,930 −0.30 [10]
61 Virginis [34] 13h 18m 24.3s −18° 18′ 40″ 27.8 G5V 5,558 −0.02 [5]
Zeta Tucanae [35] 00h 20m 04.3s –64° 52′ 29″ 28.0 F9.5V 5,956 −0.14 [7]
Chi¹ Orionis [36] A 05h 54m 23.0s +20° 16′ 34″ 28.3 G0V 5,902 −0.16 [10]
Beta Comae Berenices [37] 13h 11m 52.4s +27° 52′ 41″ 29.8 G0V 5,970 −0.06 [10]
HR 4523 [38] A 11h 46m 31.1s –40° 30′ 01″ 30.1 G5V 5,629 −0.29 [5]
61 Ursae Majoris [39] 11h 41m 03.0s +34° 12′ 06″ 31.1 G8V 5,483 −0.12 [10]
HR 4458 [40] A 11h 34m 29.5s –32° 49′ 53″ 31.1 K0V 5,629 −0.29 [5]
HR 511 [41] 01h 47m 44.8s +63° 51′ 09″ 32.8 K0V 5,333 +0.05 [10]
Alpha Mensae [42] 06h 10m 14.5s –74° 45′ 11″ 33.1 G5V 5,594 +0.10 [7]
Zeta1 Reticuli [43] 03h 17m 46.2s −62° 34′ 31″ 39.5 G3–5V 5,733 −0.22 [7]
Zeta2 Reticuli [44] 03h 18m 12.8s −62° 30′ 23″ 39.5 G2V 5,843 −0.23 [7]
55 Cancri [45] 08h 52m 35.81s +28° 19′ 51″ 40.3 G8V 5,235 +0.25 [30]
HD 69830 [46] 08h 18m 23.9s −12° 37′ 56″ 40.6 K0V 5,410 −0.03 [7]
HD 10307 [47] 01h 41m 47.1s +42° 36′ 48″ 41.2 G1.5V 5,848 −0.05 [10]
HD 147513 [48] 16h 24m 01.3s −39° 11′ 35″ 42.0 G1V 5,858 +0.03 [5]
58 Eridani [49] 04h 47m 36.3s −16° 56′ 04″ 43.3 G3V 5,868 +0.02 [7]
Upsilon Andromedae A [50] 01h 36m 47.8s +41° 24′ 20″ 44.0 F8V 6,212 +0.13 [7]
HD 211415 [51] A 22h 18m 15.6s –53° 37′ 37″ 44.4 G1–3V 5,890 −0.17 [7]
47 Ursae Majoris [52] 10h 59m 28.0s +40° 25′ 49″ 45.9 G1V 5,954 +0.06 [7]
Alpha Fornacis [53] A 03h 12m 04.3s −28° 59′ 21″ 46.0 F6V 6,275 −0.19 [7]
Psi Serpentis [54] A 15h 44m 01.8s +02° 30′ 55″ 47.9 G5V 5,636 −0.03 [10]
HD 84117 [55] 09h 42m 14.4s –23° 54′ 56″ 48.5 F8V 6,167 −0.03 [7]
HD 4391 [56] 00h 45m 45.6s –47° 33′ 07″ 48.6 G3V 5,878 −0.03 [7]
20 Leonis Minoris [57] 10h 01m 00.7s +31° 55′ 25″ 49.1 G3V 5,741 +0.20 [10]
Nu Phoenicis [58] 01h 15m 11.1s –45° 31′ 54″ 49.3 F8V 6,140 +0.18 [7]
51 Pegasi [59] 22h 57m 28.0s +20° 46′ 08″ 50.9 G2.5IVa 5,804 +0.20 [7]

Solar twin

These stars are more similar to the Sun still, having the following qualities:[1]

  • Temperature within 50 K from that of the Sun (roughly 5720 to 5830 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 solitary
  • An age within 1 billion years from that of the Sun (roughly 3.5 to 5.6 Ga)

The following are the known stars that come closest to satisfying the criteria for a solar twin. (The Sun is listed for comparison.)

Identifier J2000 Coordinates[3] Distance[3]
(ly)		 Stellar
Class[3]		 Temperature
(K)		 Metallicity
(dex)		 Age
(Gyr)		 Notes
Right ascension Declination
Sun — — 0.00 G2V 5,778 +0.00 4.6 [60]
18 Scorpii [61] 16h 15m 37.3s –08° 22′ 06″ 45.1 G2Va 5,790 −0.03 5.0 [62][63]
HD 9986 [64] 01h 37m 40.9s +12° 04′ 42″ 84 G2V 5,785 +0.09 3.4 [63]
HD 150248 [65] 16h 41m 49.8s –45° 22′ 07″ 88 G2 5,750 −0.04 6.2 [63]
HD 164595 [66] 18h 00m 38.9s +29° 34′ 19″ 91 G2 5,810 −0.04 4.5 [63]
HD 195034 [67] 20h 28m 11.8s +22° 07′ 44″ 92 G5 5,760 −0.04 2.9 [68]
HD 117939 [69] 13h 34m 32.6s –38° 54′ 26″ 98 G3 5,730 −0.10 6.1 [63]
HD 138573 [70] 15h 32m 43.7s +10° 58′ 06″ 101 G5IV-V 5,760 +0.00 5.6 [71][63]
HD 71334 [72] 08h 25m 49.5s −29° 55′ 50″ 124 G2 5,770 −0.06 5.1 [63]
HD 98649 [73] 11h 20m 51.769s –23° 13′ 02″ 137 G3 5,770 −0.02 4.7 [63]
HD 134664 [74] 15h 12m 10.4s –30° 53′ 11″ 140 G3 5,810 +0.13 2.6 [63]
HD 143436 [75] 16h 00m 18.8s +00° 08′ 13″ 141 G0 5,768 +0.00 3.8 [71]
HD 129357 [76] 14h 41m 22.4s +29° 03′ 32″ 154 G2V 5,749 −0.02 8.2 [71]
HD 118598 [77] 13h 38m 13.7s −23° 41′ 20″ 160 G2 5,800 +0.02 4.3 [63]
HD 133600 [78] 15h 05m 13.2s +06° 17′ 24″ 171 G0 5,808 +0.02 6.3 [62]
HD 115382 [79] 13h 16m 48.4s +12° 24′ 56″ 176 G1 5,780 −0.08 6.1 [63]
HIP 11915 [80] 02h 33m 49.02s −19° 36′ 42.5″ 190 G5V 5,760 -0.059 4.0
HD 101364 [81] 11h 40m 28.5s +69° 00′ 31″ 208 G5V 5,795 +0.02 3.5 [62][82]
BD +15 3364 [83] 18h 07m 18.6s +15° 56′ 46″ 209 G2 5,785 0.07 3.8 [63]
HD 197027 [84] 20h 41m 54.6s –27° 12′ 57″ 250 G3V 5,723 −0.013 8.2 [85]
YBP 1194 [86] 08h 51m 00.8s +11° 48′ 53″ 2934 G5V 5,780 +0.023 4.2 [87]

Some other stars are sometimes mentioned as promising solar-twin candidates, particularly: Beta Canum Venaticorum (see references for Turnbull & Tarter), 37 Geminorum (see references for Turnbull & Tarter) and 16 Cygni B (Porto de Mello et al. 2000). However, all three have temperatures and/or luminosities that are too high for true solar twins. Furthermore, Beta Canum Venaticorum and 37 Geminorum have too low metallicities for solar twins. Finally, 16 Cygni B is part of a (very wide) binary system and is very old for a solar twin (at least 7 to 8 Gyr). Beta Canum Venaticorum is mentioned above as a nearby solar analog.

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.[88]

  • At least 3 billion years old
  • On the main sequence
  • Non-variable
  • Capable of harboring terrestrial planets
  • Support a dynamically stable habitable zone

The requirement that the star remain on the main sequence for at least 3 Ga sets an upper limit of approximately 1.5 solar masses, corresponding to a hottest spectral type of F5 V. Such stars can reach an absolute magnitude of 2.5, or 8.55 times as bright as the Sun, at the end of the main sequence.[88]

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.[88]

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.[88]

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.[88]

One example of such a star is HD 70642 [89].[90]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Epsilon Eridani at SIMBAD - Ids - Bibliography - Image.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 Lua error in package.lua at line 80: module 'strict' not found. See VizieR catalogue J/A+A/487/373.
  6. ↑ Tau Ceti at SIMBAD - Ids - Bibliography - Image.
  7. ↑ 7.00 7.01 7.02 7.03 7.04 7.05 7.06 7.07 7.08 7.09 7.10 7.11 7.12 7.13 7.14 7.15 7.16 Lua error in package.lua at line 80: module 'strict' not found.
  8. ↑ 40 Eridani at SIMBAD - Ids - Bibliography - Image.
  9. ↑ 82 Eridani at SIMBAD - Ids - Bibliography - Image.
  10. ↑ 10.00 10.01 10.02 10.03 10.04 10.05 10.06 10.07 10.08 10.09 10.10 10.11 10.12 10.13 10.14 10.15 Lua error in package.lua at line 80: module 'strict' not found. See Vizier catalogue V/130.
  11. ↑ Delta Pavonis at SIMBAD - Ids - Bibliography - Image.
  12. ↑ HR 7722 at SIMBAD - Ids - Bibliography - Image.
  13. ↑ Gliese 86 at SIMBAD - Ids - Bibliography - Image.
  14. ↑ 54 Piscium at SIMBAD - Ids - Bibliography - Image.
  15. ↑ V538 Aurigae at SIMBAD - Ids - Bibliography - Image.
  16. ↑ HD 14412 at SIMBAD - Ids - Bibliography - Image.
  17. ↑ HR 4587 at SIMBAD - Ids - Bibliography - Image.
  18. ↑ HD 172051 at SIMBAD - Ids - Bibliography - Image.
  19. ↑ 72 Herculis at SIMBAD - Ids - Bibliography - Image.
  20. ↑ HD 196761 at SIMBAD - Ids - Bibliography - Image.
  21. ↑ nu2 lupi at SIMBAD - Ids - Bibliography - Image.
  22. ↑ Alpha Centauri at SIMBAD - Ids - Bibliography - Image.
  23. ↑ 23.0 23.1 Lua error in package.lua at line 80: module 'strict' not found.
  24. ↑ Alpha Centauri B at SIMBAD - Ids - Bibliography - Image.
  25. ↑ 70 Ophiuchi at SIMBAD - Ids - Bibliography - Image.
  26. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  27. ↑ Sigma Draconis at SIMBAD - Ids - Bibliography - Image.
  28. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  29. ↑ Eta Cassiopeiae at SIMBAD - Ids - Bibliography - Image.
  30. ↑ 30.0 30.1 Lua error in package.lua at line 80: module 'strict' not found. See VizieR catalogue J/ApJS/159/141.
  31. ↑ 107 Piscium at SIMBAD - Ids - Bibliography - Image.
  32. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  33. ↑ Beta Canum Venaticorum at SIMBAD - Ids - Bibliography - Image.
  34. ↑ 61 Virginis at SIMBAD - Ids - Bibliography - Image.
  35. ↑ Zeta Tucanae at SIMBAD - Ids - Bibliography - Image.
  36. ↑ Chi1 Orionis at SIMBAD - Ids - Bibliography - Image.
  37. ↑ Beta Comae Berenices at SIMBAD - Ids - Bibliography - Image.
  38. ↑ HR 4523 at SIMBAD - Ids - Bibliography - Image.
  39. ↑ 61 Ursae Majoris at SIMBAD - Ids - Bibliography - Image.
  40. ↑ HR 4458 at SIMBAD - Ids - Bibliography - Image.
  41. ↑ HR 511 at SIMBAD - Ids - Bibliography - Image.
  42. ↑ Alpha Mensae at SIMBAD - Ids - Bibliography - Image.
  43. ↑ Zeta1 Reticuli at SIMBAD - Ids - Bibliography - Image.
  44. ↑ Zeta1 Reticuli at SIMBAD - Ids - Bibliography - Image.
  45. ↑ 55 Cancri at SIMBAD - Ids - Bibliography - Image.
  46. ↑ HD 69830 at SIMBAD - Ids - Bibliography - Image.
  47. ↑ HD 10307 at SIMBAD - Ids - Bibliography - Image.
  48. ↑ HD 147513 at SIMBAD - Ids - Bibliography - Image.
  49. ↑ 58 Eridani at SIMBAD - Ids - Bibliography - Image.
  50. ↑ Upsilon Andromedae A at SIMBAD - Ids - Bibliography - Image.
  51. ↑ HD 211415 at SIMBAD - Ids - Bibliography - Image.
  52. ↑ 47 Ursae Majoris at SIMBAD - Ids - Bibliography - Image.
  53. ↑ Alpha Fornacis at SIMBAD - Ids - Bibliography - Image.
  54. ↑ Psi Serpentis at SIMBAD - Ids - Bibliography - Image.
  55. ↑ HD 84117 at SIMBAD - Ids - Bibliography - Image.
  56. ↑ HD 4391 at SIMBAD - Ids - Bibliography - Image.
  57. ↑ 20 Leonis Minoris at SIMBAD - Ids - Bibliography - Image.
  58. ↑ Nu Phoenicis at SIMBAD - Ids - Bibliography - Image.
  59. ↑ 51 Pegasi at SIMBAD - Ids - Bibliography - Image.
  60. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  61. ↑ 18 Scorpii at SIMBAD - Ids - Bibliography - Image.
  62. ↑ 62.0 62.1 62.2 Lua error in package.lua at line 80: module 'strict' not found.
  63. ↑ 63.00 63.01 63.02 63.03 63.04 63.05 63.06 63.07 63.08 63.09 63.10 63.11 Lua error in package.lua at line 80: module 'strict' not found.
  64. ↑ HD 9986 at SIMBAD - Ids - Bibliography - Image.
  65. ↑ HD 150248 at SIMBAD - Ids - Bibliography - Image.
  66. ↑ HD 164595 at SIMBAD - Ids - Bibliography - Image.
  67. ↑ HD 195034 at SIMBAD - Ids - Bibliography - Image.
  68. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  69. ↑ HD 117939 at SIMBAD - Ids - Bibliography - Image.
  70. ↑ HD 138573 at SIMBAD - Ids - Bibliography - Image.
  71. ↑ 71.0 71.1 71.2 Lua error in package.lua at line 80: module 'strict' not found.
  72. ↑ HD 71334 at SIMBAD - Ids - Bibliography - Image.
  73. ↑ HD 98649 at SIMBAD - Ids - Bibliography - Image.
  74. ↑ HD 134664 at SIMBAD - Ids - Bibliography - Image.
  75. ↑ HD 143436 at SIMBAD - Ids - Bibliography - Image.
  76. ↑ HD 129357 at SIMBAD - Ids - Bibliography - Image.
  77. ↑ HD 118598 at SIMBAD - Ids - Bibliography - Image.
  78. ↑ HD 133600 at SIMBAD - Ids - Bibliography - Image.
  79. ↑ HD 115382 at SIMBAD - Ids - Bibliography - Image.
  80. ↑ HIP 11915 at SIMBAD - Ids - Bibliography - Image.
  81. ↑ HIP 56948 at SIMBAD - Ids - Bibliography - Image.
  82. ↑ Lua error in package.lua at line 80: module 'strict' not found. See table 9.1.
  83. ↑ BD +15 3364 at SIMBAD - Ids - Bibliography - Image.
  84. ↑ HIP 102152 at SIMBAD - Ids - Bibliography - Image.
  85. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  86. ↑ Cl* NGC 2682 YBP 1194 at SIMBAD - Ids - Bibliography - Image.
  87. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  88. ↑ 88.0 88.1 88.2 88.3 88.4 Lua error in package.lua at line 80: module 'strict' not found.
  89. ↑ HD 70642 at SIMBAD - Ids - Bibliography - Image.
  90. ↑ Lua error in package.lua at line 80: module 'strict' not found.

Further reading

  • Lua error in package.lua at line 80: module 'strict' not found.
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  • Lua error in package.lua at line 80: module 'strict' not found.
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