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Recipe for a Star
Several factors are important for star formation. All of these factors depend on the way
materials interact. That is what you have studied
in this unit, Matter Is Marvelous.
First, gravity pulls a cloud of hydrogen gas
together. Second, density increases within the
cloud. The center of the cloud gets extremely
hot. After a great deal of time, sometimes millions of years, the star finally “turns on.” What
does that mean? What has to happen for a star
to “turn on”? In this reading, you will follow a
recipe that turns hot, dense matter into a star.
Do you realize that virtually every element on
Earth was once part of a star? Processes in stars
formed the carbon in animals and plants, iron in
your blood, nickel in your pocket, and oxygen
you breathe. To understand how stars make
these elements and how these elements got to
Earth, you need to understand what makes a star.
Imagine that there is a recipe for making stars.
The recipe starts the same way for all stars.
However, the end of the recipe varies because
stars have a variety of masses. Look at the recipes
for stars of different masses in figure 5.17.
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Recipe cards for stars.
You can think of star
formation in terms of
recipes. The recipe for a
star’s stages of formation depends on the
original mass of gas in
the nebular cloud.
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Unit 1
Matter Is Marvelous
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C05_232-237_RecipeforStar 7/12/05 5:36 PM Page 233
The general categories are low-mass stars,
such as the Sun, with a mass of MSun, and
high-mass stars a lot more massive than the
Sun. As you will see, you can predict what elements a star can produce by noting the starting
mass of the star.
So making a star looks simple, doesn’t it? In
fact, recipe cards are easy to follow if you know
what each step means. For example, you might
have tried to follow a recipe for making a cake,
only to get to a step that did not make sense,
such as “fold in the egg whites.” Coming across
this step in a cake recipe, you might wonder how
you “fold in” egg whites.
As you read these recipes for stars, you
might have trouble following them without getting some explanation of each step. Make note of
these questions in your science notebook and
read on to learn more.
Recipe for a Low-Mass Star
Step 1: Collect hydrogen and dust.
A star starts out as a huge cloud of hydrogen
gas. Gravity pulls the cloud together. The cloud
begins to spin. This spinning cloud is called a
protostar. Compared with actual stars, protostars are very bright, but their atmospheres are
still relatively cool (3,000 Kelvin [K]).
Step 2: Start fusion reactions.
The protostar continues to contract. High
energy strips nuclei of their electrons. Hydrogen
nuclei collide constantly. As gravity continues to
pull the nuclei inward, the cloud becomes
denser. Hydrogen nuclei collide more frequently.
The collapse causes the temperature and pressure in the protostar to rise. When the temperature gets high enough, hydrogen nuclei have
enough energy to break through the electron
repulsion of other hydrogen nuclei. Nuclear reactions then take place. Hydrogen nuclei begin to
fuse. The name of this process is nuclear fusion.
When nuclear fusion occurs, the star “turns
on.” Hydrogen fusion results in the formation of
a heavier element, helium. Vast amounts of
nuclear energy are released, increasing the temperature of the star and giving off light.
Step 3: Continue fusion.
The energy of nuclear fusion causes the star
to shine as the radiation moves outside of the
core (see figure 5.18). The Sun is currently at
this stage.
Early stages of
H-Fusion
in a low-mass
star
Hydrogen fusion in star
core (4H
He + energy)
Outer
hydrogen
layer
Figure 5.18 Burning star. In the early
stages of hydrogen fusion in a low-mass star,
the core is surrounded by a thick outer layer
of nonburning (nonfusing) hydrogen gas,
which shines.
Step 4: Make a red giant.
Eventually, the amount of hydrogen decreases,
which decreases the amount of hydrogen fusion.
The star stops producing energy to shine. Without
the outward pressure created by nuclear fusion,
gravity causes the star to collapse. This initial collapse causes the core to get even more dense.
Particles in the core collide more frequently and
with greater energy. The core gets hotter.
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The higher temperature causes the outer
layer to expand and cool. Now the star has a
dense, hot core with an expanded outer layer.
The star is called a red giant because it is brightest in the red portion of the spectrum. Because
the star’s diameter is larger, it is much brighter
than it was as a normal star.
Red giants may vary in size depending on
their mass. An example of a red giant is the star
called Aldebaran, in the constellation Taurus. As
described in the next step, this type of star can
make carbon.
Step 5: Collapse core. Make nebula.
The core of a red giant is full of helium. This
core continues to collapse. The core becomes
hot and dense enough for helium fusion. When
this happens, helium nuclei fuse to form carbon
nuclei.
The periodic table shows that carbon is heavier than helium or hydrogen. As a result, gravity
pulls carbon to the star’s core. Without their
electrons, helium and hydrogen gradually concentrate in surrounding outer layers because
red giant
core
they are less dense than carbon. The core of the
star is nonfusing carbon, surrounded by a shell
of mostly helium, then another shell of mostly
hydrogen (see figure 5.19). The helium fusion in
the core continues until helium is all used up in
making carbon.
Most of the red giants’s mass remains concentrated in the dense carbon core. These red
giants are not hot enough to start carbon fusion.
This means that carbon does not change to
heavier elements. Red giants, therefore, are the
main source of carbon for life in the universe. In
fact, most of the carbon in your body was produced in red giants that shone long before Earth
was formed.
Time passes. The red giant’s layers expand.
These outer layers keep expanding. Eventually,
these layers escape the surface of the star. They
are literally blown into space. The matter from
these layers becomes a nebula. A nebula is a cloud
of gas and dust that floats around a former star.
Light from the star allows astronomers to see the
nebula. Figure 5.20 shows one such nebula.
carbon
core
He–fusing
shell
H–fusing
shell
nonfusing
H shell
Figure 5.19 Red giant. This figure shows a
four-layered star. It is large and luminous, making
it a red giant. The core consists of carbon surrounded by helium and then hydrogen shells
undergoing fusion. These shells are then surrounded by hydrogen gas in a thick outer layer
that is not undergoing fusion.
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Figure 5.20 Helix Nebula. The Helix Nebula
is a cloud of dust and gas left over from the red
giant stage. It is one of the closest and largest
planetary nebula. The nebula is about 450 lightyears (ly) from the Sun in the direction of the constellation Aquarius.
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Step 6: Allow core to cool to a white dwarf.
You might predict that the carbon atoms will
fuse. This does not happen. There is not enough
energy in a low-mass star to fuse carbon. The
low energy makes it impossible for carbon to
fuse. Much of the star’s mass is blown away in
the nebula; the remaining core becomes a white
dwarf. It is called a white dwarf because it is still
very hot (white hot), but also small (dwarf) and
not very luminous. White dwarfs are dim and hot
stars.
Step 7: Let core get cold and become a black
dwarf.
Eventually, white dwarfs cool completely.
They become cold black dwarfs. The life of a
low-mass star is over!
Recipe for a High-Mass Star
To review, the recipe for a low-mass star
begins with a protostar that becomes a lowmass star, usually less than eight times the mass
of the Sun. Then in a series of steps, the star
changes into a red giant, a nebula, a white dwarf,
and finally a black dwarf.
But not all stars begin with a low mass.
Some stars begin fusion with much more hydrogen in the initial cloud. How do you make a highmass star? To find out, follow these easy recipe
directions.
Step 1: Follow Steps 1–4 of low-mass star
recipe.
To make a high-mass star, follow the recipe
for a low-mass star through Step 4. Then switch
to the recipe card for high-mass stars and begin
with Step 2, which follows.
Step 2: Use nuclear fusion to make carbon in
core.
The core of a high-mass star collapses rapidly
through the force of gravity. It becomes dense
and hot enough to make three helium nuclei
fuse. This results in one carbon nucleus. Carbon
then can fuse with itself and lighter nuclei to
make heavier elements.
This is where things really get exciting! For
example, how can a star manufacture elements
with an atomic mass greater than carbon? The
star has to fuse carbon with hydrogen to form
nitrogen. You probably know that nitrogen is a
vital element for life. If carbon fuses with helium,
it forms another vital element for life, oxygen.
The core region of the star now consists of a
mixture of carbon, nitrogen, oxygen, helium, and
hydrogen. All of those elements are undergoing
nuclear fusion and beginning to separate into
distinctive layers.
Step 3: Form new layers as more fusion
reactions begin.
The number of fusion reactions increase,
making more and more nitrogen, carbon, and
oxygen. These reactions produce heavier and
heavier elements from the periodic table. Gravity
draws these elements toward the core of the
star. Lighter elements move outward to form a
layer. The heaviest elements in the core then
undergo more nuclear fusion, making heavier
and heavier elements. Heavier elements displace
lighter elements, forcing them outward from the
core. The star has a layered appearance, something like the concentric layers of an onion.
Fusion reactions continue in the core of a
high-mass star. Each new layer results from the
fusion deep in the core. The star continues to
produce layers corresponding to heavier elements such as oxygen, neon, magnesium, silicon, and sulfur. As you will see, this sequence of
reactions slows down when the core of the star
consists of greater and greater amounts of iron.
Iron is not able to begin fusion because the star
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Multilayered Fusing Star
nonfusing hydrogen
hydrogen fusion
helium fusion
carbon fusion
oxygen fusion
neon fusion
magnesium fusion
silicon fusion
iron core
Image of Betelgeuse
Betelgeuse
core
Size of Star
Size of Earth's Orbit
Size of Jupiter's Orbit
Orion Constellation
Figure 5.21 Multilayered fusing star. The diagram shows the core of a high-mass
star in the supergiant stage. The center of the core consists of iron (a nonfusion core).
Layers surrounding the core are undergoing nuclear fusion reactions and consist of shells
of Si, Mg, Ne, O, C, He, and H. The nine-layered core would be surrounded by a thick, nonfusing hydrogen gas shell. These layers mix some at boundaries between layers.
does not produce enough energy to make iron
nuclei fuse. The iron then forms a nonfusing
core. Astronomers say that this part of a star’s
life is the supergiant stage (see figure 5.21).
An example of a red supergiant is Betelgeuse.
Figure 5.21 shows Betelgeuse at Orion’s left
shoulder.
What is the pattern you see in the layers of
elements when you locate the elements in the
periodic table?
Step 4: Try to fuse iron nuclei. The star will
collapse.
Fusion continues until elements with atomic
numbers 1–26 form. On the periodic table, those
are all of the elements up to iron. When the star
reaches this stage, things get even more exciting
because, until now, fusion reactions produced
heat. In contrast, the fusing nuclei of iron absorb
heat.
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The iron core of the star absorbs energy
from the layer above it. The outward force to
expand the star decreases. Gravity pulls the
material inward from outer layers. The star collapses on itself extremely rapidly, and the core
temperature and pressure increase tremendously.
Soon the temperature becomes so hot that
nuclei fall apart into protons and neutrons. The
core becomes a soup of protons, neutrons, and
electrons.
Step 5: Explodes to a massive-star supernova.
(Stand back and wear your safety goggles!)
Still, the collapse continues. Eventually, it
squashes protons and electrons together, causing them to become neutrons. The star is now a
core made of neutrons surrounded by collapsing
outer layers. The star “bounces” against its own
neutron core, and all of the layers explode into
space, leaving the core behind.
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This explosion is known as a massive-star
supernova. The explosion is so powerful that it
can briefly outshine an entire galaxy. It is the
most violent and spectacular explosion in the
universe. While the core explosion occurs in
about one second, the massive-star supernova
may continue to shine for weeks or months.
Massive-star supernovae are incredibly important because a flurry of fusion reactions before
and during the explosions creates all elements in
the natural environment with atomic masses
greater than iron.
Look at the periodic table. How many elements
have you heard of with atomic masses greater than
iron? These less familiar elements are rare because
they form only during a massive-star supernova
explosion.
Step 6: Scatter remains of massive-star
supernova event.
Take everything from the supernova
explosion and scatter it in space. This will form a
blurry-looking nebula around the exploded core
of the star. This matter may eventually become
the building material for the other bodies in
space, such as planets like Earth, and your very
own body!
Step 7: Collapse core to a neutron star.
All that remains of the core after a massivestar supernova event is a very hot, small, and
dense core of neutrons. This is called a neutron
star. A neutron star is so dense that 1 tsp of it
would weigh 100 million tons on Earth!
Extra recipe steps for extra high-mass stars:
Make a black hole.
If the remaining mass at the core is great
enough, the core of a neutron star can collapse
even further. After blowing away much of the
star during the massive-star supernova, the
remaining mass of the neutron star collapses to
a black hole. These bizarre objects are areas in
space that pull in everything around them—even
light! Because they do not emit light, black holes
are invisible.
If black holes are invisible, what makes
astronomers think they exist? Astronomers
know black holes exist because they can watch
the behavior of stars and other material orbiting
around black holes. People also have observed
stars, matter, and even light being pulled into
regions of space that could be black holes. In
fact, it is a good thing that the solar system is
28,000 ly from the center of the Milky Way
galaxy—strong evidence indicates that the center of our Milky Way contains a black hole!
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237