Download Space The Life of a Star

Space
The Life of a Star
The life cycle of a star is controlled by its mass. Mass refers to how much matter, or
material, is in something. A star’s mass is determined by how much matter is in the
cloud of gas and dust that came together to form the star. As time passes, hydrogen,
which is one of the gases in the cloud, is pulled tighter together by the power of gravity.
The moment this begins to happen, the cloud of gas and space dust begins to spin. The
faster the cloud spins, the hotter it gets. When it gets hot enough, a protostar is formed.
A protostar is a baby star! Even though a protostar is very hot when compared to adult
stars, it is actually very cold and has a long way to go before it can be called an adult.
Adult Stars
As a protostar spins faster and faster, it heats up. The hotter it gets, the more mature
it becomes. After millions of years, the temperature in the protostar reaches 15 million
degrees Celsius (27 million degrees Fahrenheit). At this point, something important in the core, or center, of the
young star happens. This is called nuclear fusion. Nuclear fusion occurs when atoms combine together to make
stronger and heavier atoms. This process creates an incredible amount of energy and heat.
Because of the process of nuclear fusion, the protostar glows brighter and shrinks a little. At this point, it won’t
change for millions, or even billions, of years. It is an adult star. An adult star is called a main sequence star. You
probably see this type of adult star every day when you go outside, unless the weather is bad. Our huge, yellow
sun is a main sequence star.
The Red Giant
When you take a look at our sun, remember that it is an adult star. In fact, it is an old adult star. You don’t have
to worry that the sun will die in your lifetime, but if you could see into the future and watch our sun die, you would
see something amazing.
As an adult star shines brightly, the hydrogen gas in its center, or core, transforms. It transforms because of
the nuclear fusion that is taking place with the hydrogen atoms. When the transformation of the hydrogen is
finished, it becomes helium, which is another gas. Now, imagine that a star is a car and that the hydrogen in it
is the fuel that makes the car work. What happens to the gas in a car after it has been driven for a long time? It
gets used up.
This is what makes a star begin to die. As the hydrogen in its core is changed to helium, the hydrogen begins to
run out, like the gas in a car. At this point, the star is not making as much heat as it used to, so the core starts
having trouble holding its shape, and it begins to get smaller. While this happens to the core, the outside layers
of the star begin to expand, or grow larger. As these layers expand, they start cooling down, and they glow red.
At this point in the star’s life, it is called a red giant. It is red because it is not as hot as it was before, and it is
larger because the outside layers have expanded. When a star becomes a red giant, the helium in its core
changes into another gas called carbon. All stars follow the process of going from a protostar to an adult star and
then to a red giant. However, at this point, the mass that a star has controls what happens next.
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Reading Library
Level: 7.2
Lexile®: 950L
Word Count: 799
Space
The Life of a Star (continued)
Low-Mass and High-Mass
If a star, like our sun, does not have much mass in it, it is called
a low-mass star. For low-mass stars, when the helium becomes
carbon, the center of the star shrinks even more, and the outer
layers are pushed far away. The core of a low-mass star becomes a
white dwarf, which means it is much smaller than it ever was before,
and it is colored white. Once it cools down further, the star becomes
a black dwarf. Now, the star is finished with its life cycle.
High-mass stars explode after their red giant stage. If the star is
massive enough, it will eventually become a black hole. Other high-mass red giants may become neutron
stars. A neutron star is usually only 12 miles (20 kilometers) across, but its gravity is thousands of times
stronger than Earth’s. This means that it is almost perfectly smooth and that atoms and molecules that have
never been seen on Earth can be found there. Wouldn’t it be an amazing thing to be able to visit a neutron star
or a black hole?
Time_________
Level: 7.2
Lexile®: 950L
Word Count: 799
Answer comprehension questions on page 104.
Reading Library
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