The document discusses stars and their life cycles. It begins by explaining that stars are massive spheres of hot gas, and our sun is the closest star to Earth. It then discusses how a star's color depends on its temperature, with red stars being coolest and blue stars being hottest. Stars produce energy through nuclear fusion, especially the fusion of hydrogen into helium. Over their lifetime, stars begin as gas clouds that collapse and ignite nuclear fusion, spending most of their life on the main sequence, and eventually ending as white dwarfs or supernovas.

2. Star (astronomy), massive shining sphere of
hot gas. Of all the stars in the universe, our
Sun is the nearest to Earth and the most
extensively studied. The stars visible to the
naked eye all belong to the Milky Way
Galaxy, the massive ensemble of stars that
contains our solar system (the Sun and its
nine planets).

3. The color of stars—ranging from the deepest red through all
intermediate shades of orange and yellow to an intense white-blue—
depends directly on their temperature. The coolest stars are red and
the hottest stars are blue. Most stars make light by several different
kinds of thermonuclear fusion, a process in which the nuclei of atoms
combine to form a heavier element and release energy (see Nuclear
Energy). One of the most common thermonuclear fusion processes
occurs in stars when four hydrogen atoms combine into a helium
atom, releasing energy that is transformed into light and heat.

4. About 5,000 stars can be seen
with the naked eye, although
not all of these stars are
visible at any given time or
from any given place. With a
small telescope, hundreds of
thousands of stars can be
seen. The largest telescopes
disclose millions of galaxies,
which may each contain over
200 billion stars.

5. Life of a Star
A star begins life as a large, relatively cool mass of gas in a nebula,
such as the Orion Nebula (left). As gravity causes the gas to contract,
the nebula’s temperature rises, eventually becoming hot enough to
trigger nuclear reactions in its atoms and form a star. A main
sequence star (middle) shines because of the massive, fairly steady
output of energy from the fusion of hydrogen nuclei to form helium.
The main sequence phase of a medium-sized star is believed to last
as long as 10 billion years. The Sun is just over halfway through this
phase. Stars eventually use up their energy supply, ending their lives
as white dwarfs, which are extremely small, dense globes, or in the
case of larger stars, as spectacular explosions called supernovas. A
supernova is shown within the Large Magellanic Cloud at the bottom
right of the rightmost photo.