A star is a massive,
luminous ball of plasma that is held together by
gravity. The nearest star to Earth is the Sun, which is
the source of most of the energy on Earth. Other stars
are visible in the night sky, when they are not outshone
by the Sun. Historically, the most prominent stars on
the celestial sphere were grouped together into
constellations, and the brightest stars gained proper
names. Extensive catalogues of stars have been assembled
by astronomers, which provide standardized star
designations.
For most of its life a star shines
due to thermonuclear fusion in its core, releasing
energy that traverses the star's interior and then
radiates into outer space. Almost all elements heavier
than hydrogen and helium were created by fusion
processes in stars. Astronomers can determine the mass,
age, chemical composition and many other properties of a
star by observing its spectrum, luminosity and motion
through space.
The main sequence is a continuous
and distinctive band of stars that appear 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. Stars on this band are known as main-sequence
stars or "dwarf" stars.
After a star has formed,
it creates energy at the hot, dense core region through
the nuclear fusion of hydrogen atoms into helium. During
this stage of the star's lifetime, it is located along
the main sequence at a position determined primarily by
its mass, but also based upon its chemical composition
and other factors. All main sequence stars are in
hydrostatic equilibrium, where outward thermal pressure
from the hot core is balanced by the inward
gravitational pressure from the overlying layers. The
strong dependence of the rate of energy generation in
the core on the temperature and pressure helps to
sustain this balance. The main sequence is sometimes
divided into upper and lower parts, based on the
dominant process that a star uses to generate energy.
Stars below about 1.5 times the mass of the Sun (or 1.5
solar masses) primarily fuse hydrogen atoms together in
a series of stages to form helium, a sequence called the
proton-proton chain. Above this mass, in the upper main
sequence, the nuclear fusion process mainly uses atoms
of carbon, nitrogen and oxygen as intermediaries in the
CNO cycle that produces helium from hydrogen atoms.
Energy generated at the core makes its way to the
surface and is radiated away at the photosphere. The
energy is carried by either radiation or convection,
with the latter occurring in regions with steeper
temperature gradients, higher opacity or both.
Main sequence stars with more than ten solar masses
undergo convection in the core region, which acts to
stir up the newly created helium and maintain the
proportion of fuel needed for fusion to occur. When core
convection does not occur, a helium-rich core develops
surrounded by an outer layer of hydrogen. For stars with
lower masses, this convective core is progressively
smaller until it disappears at about 2 solar masses.
Below this mass, stars have cores that are radiative but
are convective near the surface. With decreasing stellar
mass the convective envelope increases, and main
sequence stars below 0.4 solar masses undergo convection
throughout their mass.
Information
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