Solar radius in the context of "Parker Solar Probe"

The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light and infrared radiation with 10% at ultraviolet energies. It is the main source of energy for life on Earth. The Sun has been an object of veneration in many cultures and a central subject for astronomical research since antiquity.

The Sun orbits the Galactic Center at a distance of 24,000 to 28,000 light-years. Its mean distance from Earth is about 1.496×10 kilometres or about 8 light-minutes. The distance between the Sun and the Earth was used to define a unit of length called the astronomical unit, now defined to be 149.5978707×10 kilometres. Its diameter is about 1,391,400 km (864,600 mi), 109 times that of Earth. The Sun's mass is about 330,000 times that of Earth, making up about 99.86% of the total mass of the Solar System. The mass of the Sun's surface layer, its photosphere, consists mostly of hydrogen (~73%) and helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron.

The solar core is the hot, dense region at the center of the Sun where energy is generated by nuclear fusion. It is considered to extend from the Sun's center to about 0.2 of the solar radius (139,000 km; 86,000 mi). It is the hottest part of the Sun and of the Solar System. It has a density of 150,000 kg/m (150 g/cm) at the center, and a temperature of 15,000,000 K (27,000,000 °F; 15,000,000 °C).

The core is made of hot, dense plasma (ions and electrons), at a pressure estimated at 26.5 million gigapascals (3.84×10 psi) at the center. Due to fusion, the composition of the solar plasma drops from about 70% hydrogen by mass at the outer core, to 34% hydrogen at the center.

A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are not easily observed. Not one star that fits the stricter definitions of a red dwarf is visible to the naked eye. Proxima Centauri, the star nearest to the Sun, is a red dwarf, as are fifty of the sixty nearest stars. According to some estimates, red dwarfs make up three-quarters of the fusing stars in the Milky Way.

The coolest red dwarfs near the Sun have a surface temperature of about 2,000 K and the smallest have radii about 9% that of the Sun, with masses about 7.5% that of the Sun. These red dwarfs have spectral types of L0 to L2. There is some overlap with the properties of brown dwarfs, since the most massive brown dwarfs at lower metallicity can be as hot as 3,600 K and have late M spectral types.

Beta Fornacis (Beta For, β Fornacis, β For) is solitary star in the southern constellation of Fornax. It is visible to the naked eye with an apparent visual magnitude of 4.46. Based upon an annual parallax shift of 18.46 mas, it is located around 177 light years away from the Sun. At that distance, the visual magnitude is reduced by an interstellar extinction factor of 0.1.

This is an evolved, G-type giant star with a stellar classification of G8 III. It is a red clump giant, which means it has undergone helium flash and is currently generating energy through the fusion of helium at its core. Beta Fornacis has 1.33 times the mass of the Sun and, at an age of 3.3 billion years, has expanded to 10.5 times the Sun's radius. It is radiating 51 times the solar luminosity from its outer atmosphere at an effective temperature of 4,790 K.

Rigel is a blue supergiant star in the equatorial constellation of Orion. It has the Bayer designation β Orionis, which is Latinized to Beta Orionis and abbreviated Beta Ori or β Ori. Rigel is the brightest and most massive component – and the eponym – of a star system of at least four stars that appear as a single blue-white point of light to the naked eye. This system is located at a distance of approximately 850 light-years (260 pc).

A star of spectral type B8Ia, Rigel is calculated to be anywhere from 61,500 to 363,000 times as luminous as the Sun, and 18 to 24 times as massive, depending on the method and assumptions used. Its radius is more than seventy times that of the Sun, and its surface temperature is 12,100 K. Due to its stellar wind, Rigel's mass-loss is estimated to be ten million times that of the Sun. With an estimated age of seven to nine million years, Rigel has exhausted its core hydrogen fuel, expanded, and cooled to become a supergiant. It is expected to end its life as a type II supernova, leaving a neutron star or a black hole as a final remnant, depending on the initial mass of the star.

The solar luminosity (L_☉) is a unit of radiant flux (power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun.

One nominal solar luminosity is defined by the International Astronomical Union to be 3.828×10 W. This corresponds almost exactly to a bolometric absolute magnitude of +4.74.

Helmet streamers, also known as coronal streamers, are elongated cusp-like structures in the Sun's corona which are often visible in white-light coronagraphs and during solar eclipses. They are closed magnetic loops which lie above divisions between regions of opposite magnetic polarity on the Sun's surface. The solar wind elongates these loops to pointed tips which can extend a solar radius or more into the corona.

During solar minimum, helmet streamers are found closer to the heliographic equator, whereas during solar maximum they are found more symmetrically distributed around the Sun.