The core continues to gain mass, contract, and increase in temperature, whereas there is some mass loss in the outer layers.
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If the star's mass, when on the main sequence, was below approximately 0. It will therefore remain a hydrogen-fusing red giant until it runs out of hydrogen, at which point it will become a helium white dwarf. According to stellar evolution theory, no star of such low mass can have evolved to that stage within the age of the Universe. In stars above about 0. When the core is degenerate helium fusion begins explosively , but most of the energy goes into lifting the degeneracy and the core becomes convective.
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The energy generated by helium fusion reduces the pressure in the surrounding hydrogen-burning shell, which reduces its energy-generation rate. The overall luminosity of the star decreases, its outer envelope contracts again, and the star moves from the red-giant branch to the horizontal branch. As with the earlier collapse of the helium core, this starts convection in the outer layers, triggers a second dredge-up, and causes a dramatic increase in size and luminosity.
This is the asymptotic giant branch AGB analogous to the red-giant branch but more luminous, with a hydrogen-burning shell contributing most of the energy. Stars only remain on the AGB for around a million years, becoming increasingly unstable until they exhaust their fuel, go through a planetary nebula phase, and then become a carbon—oxygen white dwarf.
They start core-helium burning before the core becomes degenerate and develop smoothly into red supergiants without a strong increase in luminosity. At this stage they have comparable luminosities to bright AGB stars although they have much higher masses, but will further increase in luminosity as they burn heavier elements and eventually become a supernova.
They form oxygen—magnesium—neon cores, which may collapse in an electron-capture supernova, or they may leave behind an oxygen—neon white dwarf. O class main sequence stars are already highly luminous. The giant phase for such stars is a brief phase of slightly increased size and luminosity before developing a supergiant spectral luminosity class. Type O giants may be more than a hundred thousand times as luminous as the sun, brighter than many supergiants.
Classification is complex and difficult with small differences between luminosity classes and a continuous range of intermediate forms. The most massive stars develop giant or supergiant spectral features while still burning hydrogen in their cores, due to mixing of heavy elements to the surface and high luminosity which produces a powerful stellar wind and causes the star's atmosphere to expand.
A star whose initial mass is less than approximately 0. For most of their lifetimes, such stars have their interior thoroughly mixed by convection and so they can continue fusing hydrogen for a time in excess of 10 12 years, much longer than the current age of the Universe.
They steadily become hotter and more luminous throughout this time.
Eventually they do develop a radiative core, subsequently exhausting hydrogen in the core and burning hydrogen in a shell surrounding the core. Stars with a mass in excess of 0. Shortly thereafter, the star's supply of hydrogen will be completely exhausted and it will become a helium white dwarf. There are a wide range of giant-class stars and several subdivisions are commonly used to identify smaller groups of stars.
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Subgiants are an entirely separate spectroscopic luminosity class IV from giants, but share many features with them. Although some subgiants are simply over-luminous main-sequence stars due to chemical variation or age, others are a distinct evolutionary track towards true giants. Another luminosity class is the bright giants class II , differentiated from normal giants class III simply by being a little larger and more luminous. Within any giant luminosity class, the cooler stars of spectral class K, M, S, and C, and sometimes some G-type stars  are called red giants.
Red giants include stars in a number of distinct evolutionary phases of their lives: a main red-giant branch RGB ; a red horizontal branch or red clump ; the asymptotic giant branch AGB , although AGB stars are often large enough and luminous enough to get classified as supergiants; and sometimes other large cool stars such as immediate post-AGB stars.
The RGB stars are by far the most common type of giant star due to their moderate mass, relatively long stable lives, and luminosity.
They are the most obvious grouping of stars after the main sequence on most HR diagrams, although white dwarfs are more numerous but far less luminous. Giant stars with intermediate temperatures spectral class G, F, and at least some A are called yellow giants. They are far less numerous than red giants, partly because they only form from stars with somewhat higher masses, and partly because they spend less time in that phase of their lives.
However, they include a number of important classes of variable stars. High-luminosity yellow stars are generally unstable, leading to the instability strip on the HR diagram where the majority of stars are pulsating variables. The instability strip reaches from the main sequence up to hypergiant luminosities, but at the luminosities of giants there are several classes of variable stars:.
The Size of Our Sun Compared to the Biggest Stars in the Milky Way Galaxy
Yellow giants may be moderate-mass stars evolving for the first time towards the red-giant branch, or they may be more evolved stars on the horizontal branch. Evolution towards the red-giant branch for the first time is very rapid, whereas stars can spend much longer on the horizontal branch. Below this mass and you get the failed star brown dwarfs. One fairly well known example of a red dwarf star is Proxima Centauri; the closest star to Earth.
Our own Sun is an example of an average star. It has a diameter of 1. But when our Sun nears the end of its life, it will bloat up as a red giant, and grow to times its original size. This will consume the orbits of the inner planets: Mercury, Venus, and yes, even Earth. An example of a larger star than our Sun is the blue supergiant Rigel in the constellation Orion. This is a star with 17 times the mass of the Sun, which puts out 66, times as much energy.
Rigel is estimated to be 62 times as big as the Sun. No problem. Betelgeuse has bloated out to more than 1, times the size of the Sun.