Helium star
Encyclopedia
A helium star or helium strong star is a class
O or B star
(blue), which has extraordinarily strong helium
lines
and weaker than normal hydrogen
lines, indicating strong stellar winds and a mass loss of the outer envelope. Extreme helium star
s (EHe) entirely lack hydrogen in their spectra.
Previously, a helium star was a synonym for a B class star, but this usage is considered obsolete.
A helium star is also a term for a hypothetical star
that could occur if two helium
white dwarf
s with a combined mass of at least 0.5 solar masses merge and subsequently start nuclear fusion of helium, with a lifetime of a few hundred million years. It is believed this is the origin of the extreme helium stars.
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...
O or B star
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...
(blue), which has extraordinarily strong helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
lines
Fraunhofer lines
In physics and optics, the Fraunhofer lines are a set of spectral lines named for the German physicist Joseph von Fraunhofer . The lines were originally observed as dark features in the optical spectrum of the Sun....
and weaker than normal 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...
lines, indicating strong stellar winds and a mass loss of the outer envelope. Extreme helium star
Extreme helium star
An extreme helium star is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the Universe...
s (EHe) entirely lack hydrogen in their spectra.
Previously, a helium star was a synonym for a B class star, but this usage is considered obsolete.
A helium star is also a term for a hypothetical star
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 could occur if two helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
s with a combined mass of at least 0.5 solar masses merge and subsequently start nuclear fusion of helium, with a lifetime of a few hundred million years. It is believed this is the origin of the extreme helium stars.