Stellar Evolution Explained
Δημοσιεύθηκε στις 11 Δεκ 2013
A look at stellar evolution; how
gravity causes hydrogen gas to coalesce under friction and pressure to
ignite in a flash of nuclear fusion, the energy and glow lasting
billions of years, and then the ultimate demise in the largest and most
colorful explosions in the cosmos.
Stellar evolution is the
process by which a star undergoes a sequence of radical changes during
its lifetime. Depending on the mass of the star, this lifetime ranges
from only 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. All stars are born 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.
Nuclear fusion
powers a star for most of its life. Initially the energy is generated by
the fusion of hydrogen atoms at the core of the main-sequence star.
Later, as the preponderance of atoms at the core becomes helium, stars
like the Sun begin to fuse hydrogen along a spherical shell surrounding
the core. This process causes the star to gradually grow in size,
passing through the subgiant stage until it reaches the red giant phase.
Stars with at least half the mass of the Sun can also begin to generate
energy through the fusion of helium at their core, whereas more massive
stars can fuse heavier elements along a series of concentric shells.
Once a star like the Sun has exhausted its nuclear fuel, its core
collapses into a dense white dwarf and the outer layers are expelled as a
planetary nebula. Stars with around ten or more times the mass of the
Sun can explode in a supernova as their inert iron cores collapse into
an extremely dense neutron star or black hole. Although the universe is
not old enough for any of the smallest red dwarfs to have reached the
end of their lives, stellar models suggest they will slowly become
brighter and hotter before running out of hydrogen fuel and becoming
low-mass white dwarfs.
Stellar evolution is not studied by
observing the life of a single star, as most stellar changes occur too
slowly to be detected, even over many centuries. Instead,
astrophysicists come to understand how stars evolve by observing
numerous stars at various points in their lifetime, and by simulating
stellar structure using computer models.
Star formation is the
process by which dense regions within molecular clouds in interstellar
space, sometimes referred to as "stellar nurseries" or "star-forming
regions", collapse to form stars. As a branch of astronomy, star
formation includes the study of the interstellar medium and giant
molecular clouds (GMC) as precursors to the star formation process, and
the study of protostars and young stellar objects as its immediate
products. It is closely related to planet formation, another branch of
astronomy. Star formation theory, as well as accounting for the
formation of a single star, must also account for the statistics of
binary stars and the initial mass function.
gravity causes hydrogen gas to coalesce under friction and pressure to
ignite in a flash of nuclear fusion, the energy and glow lasting
billions of years, and then the ultimate demise in the largest and most
colorful explosions in the cosmos.
Stellar evolution is the
process by which a star undergoes a sequence of radical changes during
its lifetime. Depending on the mass of the star, this lifetime ranges
from only 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. All stars are born 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.
Nuclear fusion
powers a star for most of its life. Initially the energy is generated by
the fusion of hydrogen atoms at the core of the main-sequence star.
Later, as the preponderance of atoms at the core becomes helium, stars
like the Sun begin to fuse hydrogen along a spherical shell surrounding
the core. This process causes the star to gradually grow in size,
passing through the subgiant stage until it reaches the red giant phase.
Stars with at least half the mass of the Sun can also begin to generate
energy through the fusion of helium at their core, whereas more massive
stars can fuse heavier elements along a series of concentric shells.
Once a star like the Sun has exhausted its nuclear fuel, its core
collapses into a dense white dwarf and the outer layers are expelled as a
planetary nebula. Stars with around ten or more times the mass of the
Sun can explode in a supernova as their inert iron cores collapse into
an extremely dense neutron star or black hole. Although the universe is
not old enough for any of the smallest red dwarfs to have reached the
end of their lives, stellar models suggest they will slowly become
brighter and hotter before running out of hydrogen fuel and becoming
low-mass white dwarfs.
Stellar evolution is not studied by
observing the life of a single star, as most stellar changes occur too
slowly to be detected, even over many centuries. Instead,
astrophysicists come to understand how stars evolve by observing
numerous stars at various points in their lifetime, and by simulating
stellar structure using computer models.
Star formation is the
process by which dense regions within molecular clouds in interstellar
space, sometimes referred to as "stellar nurseries" or "star-forming
regions", collapse to form stars. As a branch of astronomy, star
formation includes the study of the interstellar medium and giant
molecular clouds (GMC) as precursors to the star formation process, and
the study of protostars and young stellar objects as its immediate
products. It is closely related to planet formation, another branch of
astronomy. Star formation theory, as well as accounting for the
formation of a single star, must also account for the statistics of
binary stars and the initial mass function.
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