Subgiant star
Encyclopedia
A subgiant star is a star
that is slightly brighter than a normal main-sequence
(dwarf) star of the same spectral class
, but not as bright as true giant star
s. Although certain subgiants appear to be simply unusually bright metal-rich hydrogen-fusing stars (in the same way subdwarfs are unusually dim metal-poor hydrogen-fusing stars), they are generally believed to be stars that are ceasing or have already ceased fusing
hydrogen
in their cores. In stars of roughly a solar mass
, this causes the core to contract, which increases the star's central temperature enough to move hydrogen fusion into a shell surrounding the core. This swells the star on the way to becoming a true giant. At the start of the subgiant phase (such as a star like Procyon A) the diameter and brightness have increased, but the star has yet to cool down or change color significantly. Later subgiants that are closer to becoming true giants have larger diameters and lower temperature than stars of similar mass in the main sequence. Overall luminosity changes little during the subgiant stage, as shown by a more or less horizontal evolution off the immediate main sequence; this feature is prominent in Hertzsprung-Russell diagrams of globular clusters, as the lateral extension after the turnoff point
but before the "ascent" to red giant
status.http://www.seds.org/messier/xtra/leos/M5f4.html
Many subgiants are rich in metals, and commonly host orbiting planets. Partly for these reasons, and also because the subgiant phase can last up to a few billion years, subgiants are the only type of stars other than main-sequence stars believed capable of hosting life-bearing planets. In Yerkes spectral classification their luminosity class is IV.
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
that is slightly brighter than a normal main-sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
(dwarf) star of the same spectral class
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
, but not as bright as true giant star
Giant star
A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...
s. Although certain subgiants appear to be simply unusually bright metal-rich hydrogen-fusing stars (in the same way subdwarfs are unusually dim metal-poor hydrogen-fusing stars), they are generally believed to be stars that are ceasing or have already ceased fusing
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
in their cores. In stars of roughly a solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
, this causes the core to contract, which increases the star's central temperature enough to move hydrogen fusion into a shell surrounding the core. This swells the star on the way to becoming a true giant. At the start of the subgiant phase (such as a star like Procyon A) the diameter and brightness have increased, but the star has yet to cool down or change color significantly. Later subgiants that are closer to becoming true giants have larger diameters and lower temperature than stars of similar mass in the main sequence. Overall luminosity changes little during the subgiant stage, as shown by a more or less horizontal evolution off the immediate main sequence; this feature is prominent in Hertzsprung-Russell diagrams of globular clusters, as the lateral extension after the turnoff point
Turnoff point
The turnoff point for a star refers to the point on the Hertzsprung-Russell diagram where it leaves the main sequence after the exhaustion of its main fuel. By plotting the turnoff point of the stars in star clusters, one can estimate the clusters age....
but before the "ascent" to red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
status.http://www.seds.org/messier/xtra/leos/M5f4.html
Many subgiants are rich in metals, and commonly host orbiting planets. Partly for these reasons, and also because the subgiant phase can last up to a few billion years, subgiants are the only type of stars other than main-sequence stars believed capable of hosting life-bearing planets. In Yerkes spectral classification their luminosity class is IV.