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Page 464
Stages in the Life Cycles of Stars
The diagram on page 465 shows the stages that stars go through in
their life cycles. Notice that the length of a cycle and the way a star
changes depend on the mass of the star at its formation.
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Learn more about life
cycles of stars.
The stage in which stars produce energy through
the fusion of hydrogen into helium is called the main sequence.
Because they use their fuel slowly, lower-mass stars can remain in the
main-sequence stage for billions of years. The Sun has been a mainsequence star for 4.6 billion years and will remain one for about another
5 billion years. When a lower-mass star runs out of hydrogen, it expands
into a giant star, in which helium fuses into carbon. Over time a giant
star sheds its outer layers and becomes a white dwarf. A white dwarf is
simply the dead core of a giant star. Although no fusion occurs in white
dwarfs, they remain hot for billions of years.
Lower-Mass Stars
Stars more than eight times as massive as our Sun
spend much less time in the main-sequence stage because they use
their fuel rapidly. After millions of years, a higher-mass star expands to
become a supergiant star. In the core of a supergiant, fusion produces
heavier and heavier elements. When an iron core forms, fusion stops
and gravity causes the core to collapse. Then part of the core bounces
outward, and the star erupts in an explosion called a supernova.
Higher-Mass Stars
For a brief period, a supernova can give off as much light as a galaxy.
The outer layers of the exploded star shoot out into space, carrying
with them heavy elements that formed inside the star. Eventually this
matter may become part of new stars and planets.
Neutron Stars and Black Holes
The collapsed core of a supergiant star may form an extremely dense
body called a neutron star. Neutron stars measure only about 20 kilometers (12 mi) in diameter, but their masses are one to three times
that of the Sun.
Neutron stars emit little visible light. However, they strongly emit
other forms of radiation, such as x-rays. Some neutron stars emit beams
of radio waves as they spin. These stars are called pulsars because they
seem to pulse as the beams rotate.
A pulsar emits beams of
radio waves as it spins
rapidly. The pulsar seems
to pulse as the beams
rotate toward and away
from Earth.
464 Unit 4: Space Science
Sometimes a supernova leaves behind a core with a mass more than
three times that of the Sun. In such a case, the core does not end up as
a neutron star. Instead, it collapses even further, forming an invisible
object called a black hole. The gravity of a black hole is so strong that
no form of radiation can escape from it.
Check Your Reading
How do lower-mass stars differ from higher-mass stars after the
main-sequence stage?
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