Star
From Halbeeg, the open encyclopedia · Af-Soomaali
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A star is a celestial body composed of extremely hot gas, mostly hydrogen and helium, held together by its own gravitational force. Nuclear fusion occurs at the core of a star—a process in which small types of atoms combine and release energy. That energy emerges as light and heat.
The Sun is the star closest to Earth and serves as an example of a medium-sized star. Other stars visible at night are extremely distant, and therefore appear as small points of light. Stars gather together into large groups called galaxies.
How They Form
Stars begin as vast clouds of gas and dust. When the density of part of a cloud increases, gravity pulls it inward. Because of this inward contraction, the temperature at the core rises. When the temperature reaches a sufficient level, hydrogen fusion begins, and the star enters a stable state in which the pressure from within balances the gravitational force.
Structure and Energy
A star can be divided into a core (where fusion occurs), intermediate layers that transport energy, and a surface from which light emerges. Energy from the core travels through the layers—sometimes as radiation, sometimes as the convection of hot gas. The mode of transport depends on the density and temperature of the layer.
Variation
Stars vary greatly in mass, size, surface temperature, and color. Hot stars appear blue or white, while cooler ones appear red or orange. A star's mass determines how long it will live: the most massive stars exhaust their fuel quickly, while smaller ones can burn steadily for an extremely long time.
End of Life
When nuclear fuel runs out, internal pressure fails. Medium-sized stars like the Sun transform into a large red star and eventually shed their outer layers, leaving behind a dense core called a white dwarf. Very massive stars can end in a massive explosion (supernova), leaving behind a neutron star or black hole. The heavy elements forged in stars are then scattered through space between stars, becoming the material from which later generations of stars and planets form.