Download The Life Cycle of Stars Introduction Stars are huge spheres of very

The Life Cycle of Stars
Introduction
Stars are huge spheres of very hot gas that emit light and other radiation. They are formed from clouds
of dust and gas, or nebulas, and go through different stages as they age. They are located various
distances from Earth. We use a light-year to describe the distance from Earth to far away objects such
as stars and galaxies. A light-year is the distance that light travels in one year, or about 9.5 x 1012 km.
Stars are powered by nuclear fusion reactions.
A star is held together by the enormous gravitational forces that result from its own mass. Inside the
pressure is more than a billion times the atmospheric pressure on Earth. The temperature is hotter than
15 million kelvins.
Fusion takes place in the core. Fusion combines the nuclei of hydrogen atoms into helium nuclei. When
hydrogen nuclei collide, they fuse to form deuterons, which have one proton and one neutron. Another
proton collides with a deuteron to form a helium isotope. Each time that two particles fuse, energy is
released. The core of the star is the hottest which can be around 10,000,000 K.
We can tell how hot a star is based on its color. For example, the sun appears yellow because the peak
wavelength of the sun’s energy is near yellow on the spectrum. Yellow also corresponds to a
temperature near 6,000 K. Hot stars emit more energy at every wavelength than cool stars do. Hot
stars emit the most energy at blue wavelengths. A red star will have the lowest temperature.
The Life Cycle of Stars
Stars are born, go through stages of development, and eventually die. About 90% of all stars in our
galaxy, including the sun, are in midlife, or converting hydrogen to helium in their interiors.
About 5 billion years ago, a thin cloud of dust and gas, or nebula, collapsed inward as it was pulled by
the force of its own gravity. As the nebula collapsed, it began to spin. This small, spinning cloud formed
a protostar. About 30 million years after the nebula started to collapse, the center of the nebula
reached a temperature of 15 million kelvins.
Electrons were stripped from hydrogen atoms to leave hydrogen nuclei, which are protons. At very high
pressures, protons may get as close to each other as 10-15 m. At such a small distance, the strong
nuclear force overpowers electrical repulsion and the protons fuse. The protons combine to form
helium through this process of nuclear fusion. The onset of fusion marks the birth of a star such as our
sun.
Fusion reactions in the sun’s core generate energy that produces an outward pressure that balances the
inward force due to gravity. Because of these balanced forces, the sun has maintained a stable size for 5
billion years.
The sun is currently converting hydrogen into helium. Over time, the percentage of the core that is
helium becomes larger. Eventually, the core will exhaust about 25% of its hydrogen, and the number of
fusion reactions will decrease. When this happens, the sun will begin to die. Scientists estimate,
however, that the sun will exist for another 5 billion years.
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The sun will become a red giant before it dies. A red giant is a large, reddish star late in its life cycle. As
the number of fusion reactions decreases, the pressure from the release of energy in the core of the sun
drops, and the core will contract causing its temperature to rise. The outer layers will expand, and the
sun will become a red giant. The star is red because its surface is relatively cool. But the core is hot
enough to convert helium into carbon and oxygen.
When the core of a red giant sun depletes most of its helium, it will contract further, which will cause
the outer layers to expand again. At this point, the temperature at the core is not high enough to fuse
heavier elements. The outer layers will expand out from the core and will eventually leave the star. The
remnants will become a white dwarf, a small, dim, and very dense star about the size of Earth. White
dwarfs no longer fuse elements, so they slowly cool. All stars that have a mass of 1.4 solar masses or
smaller, or a mass of less than 1.4 times the mass of our sun, will have a similar life cycle. In fact, most
stars in our galaxy will end as white dwarfs.
Massive stars evolve faster than smaller stars do. They also develop hotter cores that create heavier
elements through fusion. The formation of an iron core signals the beginning of a supergiant star’s
death because, unlike fusing lighter elements, fusing iron atoms to make heavier elements requires
adding energy rather than releasing it. When fusion requires more energy than it can produce, there is
no longer any outward pressure to balance the gravitational force. The core collapses because of its
own gravity and then rebounds with a shock wave that violently blows the star’s outer layers away from
the core. The resulting huge, bright explosion is called a supernova. A supernova is defined as a gigantic
explosion in which a massive star collapses and throws its outer layers into space.
The remnant of a supernova can become a neutron star. Neutron stars are only a few dozen kilometers
in diameter, but they are very massive. A neutron star is as dense as matter in the nucleus of an atom,
about 1017 kg/m3. Just a teaspoon of matter from a neutron star would weigh more than 100 million
tons on Earth. Neutron stars can be detected as pulsars, or spinning neutron stars that are sources of
pulsating radio waves.
If the leftover core of a supernova has a mass that is greater than 3 solar masses, it will collapse to form
something else—a black hole, which consists of matter so massive and compressed that nothing, not
even light, can escape its gravitational pull. Because no light can escape a black hole, black holes cannot
be seen directly. Black holes can, however; be detected indirectly by observing the radiation of light and
X rays from objects that revolve rapidly around them.
The Hertzsprung - Russell diagram (H-R diagram) is a tool that astronomers use to help them
understand how stars change over time. The vertical axis shows the luminosity (the total energy output
at every wavelength) or the brightness of the stars. The horizontal axis shows the surface temperature
of the stars, with hotter temperatures on the left side of the diagram.
Once a star is stable—fusing hydrogen into helium in its core—it appears on a diagonal line in the H-R
diagram called the main sequence. Most stars are main-sequence stars. The position of the star on the
main sequence depends on the initial mass of the star. Red giant stars are both cool and bright, so they
appear in the upper right. White dwarf stars are both faint and hot, so they appear in the lower left.
Our sun which is in the middle of the H-R diagram will eventually become a white dwarf and will be
located with the other white dwarfs on the diagram.
Taken from: Physical Science with Earth and Space Science, Holt, Rinehart and Winston, NewYork, 2008.
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Life Cycle of Stars Questionnaire
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Name _____________________________
Date _____________________
Period ____
List the phases of the birth of a star (like our sun) in order:
_________________________________________________________________________________
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Our sun has a _____________________ color spectrum which corresponds to what temperature?
__________________________
Around 90% of the stars in our galaxy mainly undergo what fusion reactions?
___________________________________________________________
The temperature required for fusion to occur in a star is? ______________________________
What forces etc. cause a nebula to ultimately become a star?
_________________________________________________________________________________
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What is special about 10 million kelvin with respect to the strong force?
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When will our sun start dying?
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Our sun will evolve or change to become what after this process occurs in #7? Describe it.
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After this, our sun will undergo what changes and become what at the end of its life?
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If our sun only makes helium, where is the higher numbered elements made? (Explain)
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What is a neutron star?
_________________________________________________________________________________
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12. What is a pulsar?
_________________________________________________________________________________
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13. On the graph called the HR diagram astronomers use show how stars change over time? Explain
the relationship the graph is making.
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14. Define black hole
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15. ON the HR diagram, our sun is listed as what type of star?
________________________________________________
16. The coolest stars appear where on the H-R diagram?
_______________________________________________________
17. These cool stars include what type of stars? _______________________________
18. Where do white dwarfs appear on the H-R diagram?
___________________________________________________
19. A red star will have what type of temperature according to the article?
_______________________________________
20. Define light year _________________________________________________________________
21. What is a nebula (plural nebulae) ?
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22. Why is it correct to say we are made of star stuff?
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