What is the Sun?

ALAN-FRIEDMAN-SUN-HALPHAThe Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields. It has a diameter of about 1,392,684 km (865,374 mi), about 109 times that of Earth, and its mass (about 2×1030 kilograms (2×1027 tons) accounts for about 99.86% of the total mass of the Solar System. Chemically, about three quarters of the Sun’s mass consists of hydrogen, while the rest is mostly helium. The remainder (1.69%, which nonetheless equals 5,628 times the mass of Earth) consists of heavier elements, including oxygen, carbon, neon and iron, among others.

The interior of the Sun. Credit: Kelvin Song
The interior of the Sun. Credit: Kelvin Song

When did the Sun Form?

The Sun formed about 4.6 billion years ago from the gravitational collapse of a region within a large molecular cloud. Most of the matter gathered in the center, while the rest flattened into an orbiting disk that would become the Solar System. The central mass became increasingly hot and dense, eventually initiating thermonuclear fusion in its core. It is thought that almost all other stars form by this process. The Sun’s stellar classification, based on spectral class, is a G-type main-sequence star (G2V) and is informally designated as a yellow dwarf because its visible radiation is most intense in the yellow-green portion of the spectrum, and although its color is white, from the surface of the Earth it may appear yellow because of atmospheric scattering of blue light. In the spectral class label, G2 indicates its surface temperature of approximately 5778 K (5505 °C), and V indicates that the Sun, like most stars, is a main-sequence star, and thus generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen each second.

Absolute and Appatent Magnitude:

Once regarded by astronomers as a small and relatively insignificant star, the Sun is now thought to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs. The absolute magnitude of the Sun is +4.83; however, as the star closest to Earth, the Sun is the brightest object in the sky with an apparent magnitude of −26.74. The Sun’s hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.

The Sun and the Milky Way Galaxy

The Sun is currently traveling through the Local Interstellar Cloud (near to the G-cloud) in the Local Bubble zone, within the inner rim of the Orion Arm of the Milky Way galaxy. Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being a red dwarf named Proxima Centauri at approximately 4.2 light-years away), the Sun ranks fourth in mass. The Sun orbits the center of the Milky Way at a distance of approximately 24,000–26,000 light-years from the galactic center, completing one clockwise orbit, as viewed from the galactic north pole, in about 225–250 million years. Since the Milky Way is moving with respect to the cosmic microwave background radiation (CMB) in the direction of the constellation Hydra with a speed of 550 km/s, the Sun’s resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo.

Facts about our Neighbor

The mean distance of the Sun from the Earth is approximately 149.6 million kilometers (1 AU), though the distance varies as the Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth’s climate and weather. The enormous effect of the Sun on the Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th century prominent scientists had little knowledge of the Sun’s physical composition and source of energy. This understanding is still developing; there are a number of present day anomalies in the Sun’s behavior that remain unexplained.

Discovered By
Known by the Ancients
Date of Discovery
Equatorial Inclination to Orbit
7.25 with respect to the ecliptic
Mean Radius
Metric: 695,508 km
English: 432,168.6 miles
Scientific Notation: 6.9551 x 105 km
By Comparison: 109.2 x that of Earth
Equatorial Circumference
Metric: 4,370,005.6 km
English: 2,715,395.6 miles
Scientific Notation: 4.37001 x 106 km
By Comparison: 109.2 x that of Earth
Metric: 1,409,272,569,059,860,000 km3
English: 338,102,469,632,763,000 mi3
Scientific Notation: 1.40927 x 1018 km3
By Comparison: 1,301,018.805 Earths
Metric: 1,989,100,000,000,000,000,000,000,000,000 kg
English: 4,385,214,857,119,400,000,000,000,000,000 lbs
Scientific Notation: 1.989 x 1030 kg
By Comparison: 333,060.402 x Earth’s
Metric: 1.409 g/cm3
By Comparison: 0.256 that of Earth
Surface Area
Metric: 6,078,747,774,547 km2
English: 2,347,017,636,988 square miles
Scientific Notation: 6.07877 x 1012 km2
By Comparison: 11,917.607 Earths
Surface Gravity
Metric: 274.0 m/s2
English: 899.0 ft/s2
Scientific Notation: 2.740 x 102 m/s2
By Comparison: 27.96 x Earth’s surface gravity
Escape Velocity
Metric: 2,223,720 km/h
English: 1,381,756 mph
Scientific Notation: 6.177 x 105 m/s
By Comparison: 55.20 x Earth
Sidereal Rotation Period (Length of Day)
25.38 Earth days
609.12 hours
By Comparison: Rotation slows to about 35 days at the poles.
Minimum/Maximum Surface Temperature
Metric: 5,500 °C
English: 10,000 °F
Effective Temperature
Metric: 5504 °C
English: 9939 °F
Scientific Notation: 5777 K