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A corona is an aura of plasma that surrounds the Sun and other stars. The Sun's corona extends millions of kilometres into outer space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. Spectroscopy measurements indicate strong ionization in the corona and a plasma temperature in excess of 1000000 kelvin, much hotter than the surface of the Sun.
The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as visible light, ultraviolet light, and infrared radiation. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometres, or 109 times that of Earth. Its mass is about 330,000 times that of Earth; it accounts for about 99.86% of the total mass of the Solar System. Roughly three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon and iron.
A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but due to their immense distance from Earth they appear as fixed points of light in the sky. The most prominent stars are grouped into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars, but most are invisible to the naked eye from Earth, including all individual stars outside our galaxy, the Milky Way.
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.
The photosphere is a star's outer shell from which light is radiated.
In physical cosmology, Big Bang nucleosynthesis is the production of nuclei other than those of the lightest isotope of hydrogen during the early phases of the Universe. Primordial nucleosynthesis is believed by most cosmologists to have taken place in the interval from roughly 10 seconds to 20 minutes after the Big Bang, and is calculated to be responsible for the formation of most of the universe's helium as the isotope helium-4 (4He), along with small amounts of the hydrogen isotope deuterium, the helium isotope helium-3 (3He), and a very small amount of the lithium isotope lithium-7 (7Li). In addition to these stable nuclei, two unstable or radioactive isotopes were also produced: the heavy hydrogen isotope tritium ; and the beryllium isotope beryllium-7 (7Be); but these unstable isotopes later decayed into 3He and 7Li, respectively, as above.
The Eddington luminosity, also referred to as the Eddington limit, is the maximum luminosity a body can achieve when there is balance between the force of radiation acting outward and the gravitational force acting inward. The state of balance is called hydrostatic equilibrium. When a star exceeds the Eddington luminosity, it will initiate a very intense radiation-driven stellar wind from its outer layers. Since most massive stars have luminosities far below the Eddington luminosity, their winds are mostly driven by the less intense line absorption. The Eddington limit is invoked to explain the observed luminosity of accreting black holes such as quasars.
The carbon-burning process or carbon fusion is a set of nuclear fusion reactions that take place in the cores of massive stars (at least 8 at birth) that combines carbon into other elements. It requires high temperatures (> 5×108 K or 50 keV) and densities (> 3×109 kg/m3).
The Cat's Eye Nebula is a planetary nebula in the northern constellation of Draco, discovered by William Herschel on February 15, 1786. It was the first planetary nebula whose spectrum was investigated by the English amateur astronomer William Huggins, demonstrating that planetary nebulae were gaseous and not stellar in nature. Structurally, the object has had high-resolution images by the Hubble Space Telescope revealing knots, jets, bubbles and complex arcs, being illuminated by the central hot planetary nebula nucleus (PNN). It is a well-studied object that has been observed from radio to X-ray wavelengths.
In nuclear astrophysics, the rapid neutron-capture process, also known as the r-process, is a set of nuclear reactions that is responsible for the creation of approximately half of the atomic nuclei heavier than iron; the "heavy elements", with the other half produced by the p-process and s-process. The r-process usually synthesizes the most neutron-rich stable isotopes of each heavy element. The r-process can typically synthesize the heaviest four isotopes of every heavy element, and the two heaviest isotopes, which are referred to as r-only nuclei, can be created via the r-process only. Abundance peaks for the r-process occur near mass numbers A = 82, A = 130 and A = 196.
The abundance of the chemical elements is a measure of the occurrence of the chemical elements relative to all other elements in a given environment. Abundance is measured in one of three ways: by the mass-fraction ; by the mole-fraction ; or by the volume-fraction. Volume-fraction is a common abundance measure in mixed gases such as planetary atmospheres, and is similar in value to molecular mole-fraction for gas mixtures at relatively low densities and pressures, and ideal gas mixtures. Most abundance values in this article are given as mass-fractions.
HD 330075 is a star in the southern constellation of Norma. It has a yellow hue and an apparent visual magnitude of 9.36, which makes it too faint to be seen with the naked eye – it is visible only with telescope or powerful binoculars. Parallax measurements provide a distance estimate of 148 light years from the Sun, and it is drifting further away with a radial velocity of 62 km/s. The star is estimated to have come as close as 111.5 light-years some 409 million years ago.
Supernova nucleosynthesis is the nucleosynthesis of chemical elements in supernova explosions.
An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune.
The core of the Sun is considered to extend from the center to about 0.2 to 0.25 of solar radius. It is the hottest part of the Sun and of the Solar System. It has a density of 150 g/cm3 at the center, and a temperature of 15 million kelvins.
The standard solar model (SSM) is a mathematical treatment of the Sun as a spherical ball of gas. This model, technically the spherically symmetric quasi-static model of a star, has stellar structure described by several differential equations derived from basic physical principles. The model is constrained by boundary conditions, namely the luminosity, radius, age and composition of the Sun, which are well determined. The age of the Sun cannot be measured directly; one way to estimate it is from the age of the oldest meteorites, and models of the evolution of the Solar System. The composition in the photosphere of the modern-day Sun, by mass, is 74.9% hydrogen and 23.8% helium. All heavier elements, called metals in astronomy, account for less than 2 percent of the mass. The SSM is used to test the validity of stellar evolution theory. In fact, the only way to determine the two free parameters of the stellar evolution model, the helium abundance and the mixing length parameter, are to adjust the SSM to "fit" the observed Sun.
Internal heat is the heat source from the interior of celestial objects, such as stars, brown dwarfs, planets, moons, dwarf planets, and even asteroids such as Vesta, resulting from contraction caused by gravity, nuclear fusion, tidal heating, core solidification, and radioactive decay. The amount of internal heating depends on mass; the more massive the object, the more internal heat it has; also, for a given density, the more massive the object, the greater the ratio of mass to surface area, and thus the greater the retention of internal heat. The internal heating keeps celestial objects warm and active.
HD 215497 is a single star in the southern constellation of Tucana. It has an orange hue with an apparent visual magnitude of 8.96, which is too dim to be viewed with the naked eye. A 2015 survey ruled out the existence of any stellar companions at projected distances from 26 to 300 astronomical units. Based on parallax measurements, it is located at a distance of 132 light years from the Sun. The star is drifting further away with a radial velocity of +49 km/s, having come as close as 45 light-years some 774,000 years ago. The absolute magnitude of this star is 5.77.
In astronomy and in astrophysics, for radiative losses of the solar corona, it is meant the energy flux radiated from the external atmosphere of the Sun, and, in particular, the processes of production of the radiation coming from the solar corona and transition region, where the plasma is optically-thin. On the contrary, in the chromosphere, where the temperature decreases from the photospheric value of 6000 K to the minimum of 4400 K, the optical depth is about 1, and the radiation is thermal.
This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.