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] |
A proplyd, short for ionized protoplanetary disk, is an externally illuminated photoevaporating protoplanetary disk around a young star. Nearly 180 proplyds have been discovered in the Orion Nebula. Images of proplyds in other star-forming regions are rare, while Orion is the only region with a large known sample due to its relative proximity to Earth.
Sigma Canis Majoris, also named Unurgunite, is a variable star in the southern constellation of Canis Major. It is approximately 1,290 light-years from the Sun and has an average apparent visual magnitude of +3.41.
La Superba is a strikingly red giant star in the constellation Canes Venatici. It is a carbon star and semiregular variable.
KW Sagittarii is a red supergiant star, located approximately 2,420 parsecs away from the Sun in the direction of the constellation Sagittarius. It is one of the largest known stars, with a diameter about 1,000 times larger than the Sun. If placed at the center of the Solar System, the star's surface would engulf Mars, coming close to Jupiter's orbit.
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.
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HD 111395 is a single, variable star in the northern constellation of Coma Berenices. It has the variable star designation LW Com, short for LW Comae Berenices; HD 111395 is the Henry Draper Catalogue designation. The star has a yellow hue and is just bright enough to be barely visible to the naked eye with an apparent visual magnitude that fluctuates around 6.29. Based upon parallax measurements, it is located at a distance of 55.8 light years from the Sun. The star is drifting closer with a radial velocity of −8.9 km/s. It is a member of the Eta Chamaeleontis stellar kinematic group.
Westerlund 1-20 (abbreviated to Wd 1-20 or just W20) is a red supergiant (RSG) located in the Westerlund 1 super star cluster. Its radius was calculated to be around 965 solar radii (6.72 × 108 km, 4.48 au), making it one of the largest stars discovered so far. This corresponds to a volume 899 million times bigger than the Sun. If placed at the center of the Solar System, the photosphere of Westerlund 1-20 would almost reach the orbit of Jupiter.
12 Pegasi is a K-type supergiant star in the constellation of Pegasus. It has a spectral type of K0Ib Hdel0.5, which indicates that it is a less luminous K-type supergiant with strong H-δ Balmer lines. The star has expanded to 81 times the radius of the Sun, and has an effective temperature of 4,185 K.
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.