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Stars
A star is a huge, shining ball in space that produces a tremendous
amount of light and other forms of energy. The sun is a star,
and it supplies Earth with light and heat energy. The stars look
like twinkling points of light -- except for the sun. The sun
looks like a ball because it is much closer to Earth than any
other star.
The sun and most other stars are made of gas and a hot, gaslike
substance known as plasma. But some stars, called white dwarfs
and neutron stars, consist of tightly packed atoms or subatomic
particles. These stars are therefore much more dense than anything
on Earth.
Stars come in many sizes. The sun's radius (distance from its
center to its surface) is about 432,000 miles (695,500 kilometers).
But astronomers classify the sun as a dwarf because other kinds
of stars are much bigger. Some of the stars known as supergiants
have a radius about 1,000 times that of the sun. The smallest
stars are the neutron stars, some of which have a radius of only
about 6 miles (10 kilometers).
About 75 percent of all stars are members of a binary system,
a pair of closely spaced stars that orbit each other. The sun
is not a member of a binary system. However, its nearest known
stellar neighbor, Proxima Centauri, is part of a multiple-star
system that also includes Alpha Centauri A and Alpha Centauri
B.
The distance from the sun to Proxima Centauri is more than 25
trillion miles (40 trillion kilometers). This distance is so great
that light takes 4.2 years to travel between the two stars. Scientists
say that Proxima Centauri is 4.2 light-years from the sun. One
light-year, the distance that light travels in a vacuum in a year,
equals about 5.88 trillion miles (9.46 trillion kilometers).
Stars are grouped in huge structures called galaxies. Telescopes
have revealed galaxies throughout the universe at distances of
12 billion to 16 billion light-years. The sun is in a galaxy called
the Milky Way that contains more than 100 billion stars. There
are more than 100 billion galaxies in the universe, and the average
number of stars per galaxy may be 100 billion. Thus, more than
10 billion trillion stars may exist. But if you look at the night
sky far from city lights, you can see only about 3,000 of them
without using binoculars or a telescope.
Stars, like people, have life cycles -- they are born, pass through
several phases, and eventually die. The sun was born about 4.6
billion years ago and will remain much as it is for another 5
billion years. Then it will grow to become a red giant. Late in
the sun's lifetime, it will cast off its outer layers. The remaining
core, called a white dwarf, will slowly fade to become a black
dwarf.
Other stars will end their lives in different ways. Some will
not go through a red giant stage. Instead, they will merely cool
to become white dwarfs, then black dwarfs. A small percentage
of stars will die in spectacular explosions called supernovae.
This article discusses Star (The stars at night) (Names of stars)
(Characteristics of stars) (Fusion in stars) (Evolution of stars).
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