1 R☉ = | Units |
---|---|
6.95700×108 | metres |
695,700 | kilometres |
0.00465047 | astronomical unit |
432,288 | miles |
7.35355×10−8 | light-year |
2.25461×10−8 | parsec |
2.32061 | light-seconds |
Solar radius is a unit of distance used to express the size of stars in astronomy relative to the Sun. The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3: [1]
695,700 kilometres (432,300 miles) is approximately 10 times the average radius of Jupiter, 109 times the radius of the Earth, and 1/215th of an astronomical unit, the approximate distance between Earth and the Sun. The solar radius to either pole and that to the equator differ slightly due to the Sun's rotation, which induces an oblateness in the order of 10 parts per million. [2]
The uncrewed SOHO spacecraft was used to measure the radius of the Sun by timing transits of Mercury across the surface during 2003 and 2006. The result was a measured radius of 696,342 ± 65 kilometres (432,687 ± 40 miles). [4]
Haberreiter, Schmutz & Kosovichev (2008) [1] determined the radius corresponding to the solar photosphere to be 695,660 ± 140 kilometres (432,263 ± 87 miles). This new value is consistent with helioseismic estimates; the same study showed that previous estimates using inflection point methods had been overestimated by approximately 300 km (190 mi).
In 2015, the International Astronomical Union passed Resolution B3, which defined a set of nominal conversion constants for stellar and planetary astronomy. Resolution B3 defined the nominal solar radius (symbol ) to be equal to exactly695700 km. [5] The nominal value, which is the rounded value, within the uncertainty, given by Haberreiter, Schmutz & Kosovichev (2008), was adopted to help astronomers avoid confusion when quoting stellar radii in units of the Sun's radius, even when future observations will likely refine the Sun's actual photospheric radius (which is currently [6] only known to about an accuracy of ±100–200 km).
Solar radii as a unit are common when describing spacecraft moving close to the sun. Two spacecraft in the 2010s include:
Name | Radius (Solar radius) | Radius (kilometers) |
---|---|---|
Milky Way | 5.94×1011 | 4.134×1017 [7] |
WOH G64 (largest known star) | 1,540 [8] | 1,071,378,000 |
UY Scuti | 909 [9] | 632,400,000 |
Betelgeuse | 764 [10] | 531,500,000 |
Antares A | 680 [11] | 473,076,000 |
Rigel A | 74.1 [12] | 51,550,000 |
Aldebaran | 45.1 [13] | 31,375,000 |
Arcturus | 25.4 [14] | 17,670,000 |
Pollux | 9.06 [15] | 6,300,000 |
Sirius A | 1.711 [16] | 1,190,350 |
Sun | 1 | 695,700 |
Proxima Centauri | 0.1542 [17] | 107,275 |
Jupiter | 0.1028 | 71,492 [18] |
Saturn | 0.0866 | 60,268 [18] |
Uranus | 0.03673 | 25,559 [18] |
Neptune | 0.03559 | 24,764 [18] |
Earth | 0.009168 | 6,378 [18] |
Venus | 0.00869 | 6,051.8 [18] |
Mars | 0.00488 | 3,396.19 [18] |
Mercury | 0.0035 | 2,440.53 [18] |
Moon | 0.0025 | 1,738.1 [19] |
Pluto | 0.0017 | 1,188.3 [18] |
Zeta Cephei is a red supergiant star, located about 1000 light-years away in the constellation of Cepheus. Zeta Cephei marks the left shoulder of Cepheus, the King of Ethiopia. It is one of the fundamental stars of the MK spectral sequence, defined as type K1.5 Ib.
Eta Cephei is a star in the northern circumpolar constellation of Cepheus. With an apparent visual magnitude of 3.4, this is a third magnitude star that, according to the Bortle Dark-Sky Scale, is readily visible to the naked eye. Parallax measurements put it at a distance of 14.37 parsecs from Earth.
HD 115337 is a binary star located in the northern circumpolar constellation Camelopardalis. The pair have a combined apparent magnitude of 6.25, placing it near the limit for naked eye visibility. Parallax measurements place the system at a distance of 698 light years. It has a heliocentric radial velocity of −9.4 km/s, indicating that it is drifting towards the Solar System.
HD 165185 is the Henry Draper Catalogue designation for a star in the southern zodiac constellation of Sagittarius. It has an apparent visual magnitude of 5.94, which indicates it is a sixth magnitude star that is faintly visible to the naked eye. Parallax measurements give an estimated distance of 55.8 light years from the Sun. It is drifting further away with a heliocentric radial velocity of +15.4 km/s.
NGC 4302 is an edge-on spiral galaxy located about 55 million light-years away in the constellation Coma Berenices. It was discovered by astronomer William Herschel on April 8, 1784 and is a member of the Virgo Cluster.
HD 70514, also known as HR 3280, is a solitary, orange hued star located in the southern circumpolar constellation Volans, the flying fish. It has an apparent magnitude of 5.06, making it one of the brighter members of this generally faint constellation. Based on parallax measurements from the Gaia spacecraft, the star is estimated to be 298 light years distant. It appears that its distance to the Solar System isn't changing, having a heliocentric radial velocity of 0 km/s. Eggen (1994) lists it as a member of the thin disk population.
HD 193472 is a solitary star in the equatorial constellation Delphinus. It has an apparent magnitude of 5.94, making it visible with the naked eye if viewed under ideal conditions. Parallax measurements put it at a distance of 282 light years and has a radial velocity of −8 km/s, indicating that the object drifting towards the Solar System.
HD 193556 is a solitary star in the equatorial constellation Delphinus. It has an apparent magnitude of 6.17, making it visible to the naked eye under ideal conditions. Parallax measurements place the object at a distance of 467 light years and it is currently receding with a heliocentric radial velocity of 11.7 km/s.
19 Leonis Minoris is a spectroscopic binary located in the northern constellation Leo Minor. It has an apparent magnitude of 5.1, making it one of the brighter members of the constellation. The system is relatively close at a distance of 94 light years but is drifitng closer with a heliocentric radial velocity of 8.6 km/s.
HD 22764, also known as HR 1112, is an orange hued star located in the northern circumpolar constellation Camelopardalis. It has an apparent magnitude of 5.78, allowing it to be faintly visible to the naked eye. The object is located relatively far at a distance of approximately 1,770 light years based on Gaia DR3 parallax measurements but is approaching the Solar System with a heliocentric radial velocity of −12.5 km/s. At its current distance, HD 22764's brightness is diminished by 0.66 magnitudes due to interstellar dust.