EARTH & SPACE SCIENCE

EARTH & SPACE EARTH & SPACE SCIENCESCIENCE

Chapter 30 Stars, Galaxies, and Chapter 30 Stars, Galaxies, and the Universethe Universe

30.2 Stellar Evolution30.2 Stellar Evolution

30.2 Stellar Evolution30.2 Stellar EvolutionObjectivesObjectives

Describe how a protostar becomes a star.Describe how a protostar becomes a star.

Explain how a main-sequence star Explain how a main-sequence star generates energy.generates energy.

Describe the evolution of a star after its Describe the evolution of a star after its main-sequence stage.main-sequence stage.

Classifying StarsClassifying StarsMain sequence is the location on the H-R diagram Main sequence is the location on the H-R diagram where most stars lie; it has a diagonal pattern from where most stars lie; it has a diagonal pattern from the lower right to the upper left.the lower right to the upper left.One way scientists classify stars is by plotting the One way scientists classify stars is by plotting the surface temperatures of stars against their surface temperatures of stars against their luminosity. luminosity. The H-R diagram is the graph that illustrates the The H-R diagram is the graph that illustrates the resulting pattern.resulting pattern.Astronomers use the H-R diagram to describe the Astronomers use the H-R diagram to describe the life cycles of stars. life cycles of stars. Most stars fall within a band that runs diagonally Most stars fall within a band that runs diagonally through the middle of the H-R diagram. through the middle of the H-R diagram. These stars are main sequence stars.These stars are main sequence stars.

Hertzsprung-Russell DiagramHertzsprung-Russell Diagram

Star FormationStar Formation

A nebula is a large cloud of gas and dust in A nebula is a large cloud of gas and dust in interstellar space; a region in space where stars interstellar space; a region in space where stars are born.are born.A star beings in a nebula. A star beings in a nebula. When the nebula is compressed, some of the When the nebula is compressed, some of the particles move close to each other and are pulled particles move close to each other and are pulled together by gravity. together by gravity. As described in Newton’s law of universal As described in Newton’s law of universal gravitation, as gravity pulls particles of the nebula gravitation, as gravity pulls particles of the nebula closer together, the gravitational pull of the closer together, the gravitational pull of the particles on each other increases. particles on each other increases. As more particles come together, regions of dense As more particles come together, regions of dense matter begin to build up within the cloud.matter begin to build up within the cloud.


As gravity makes dense regions within a nebula As gravity makes dense regions within a nebula more compact, these regions spin and shrink and more compact, these regions spin and shrink and begin to form a flattened disk.begin to form a flattened disk.The disk has a central concentration of matter The disk has a central concentration of matter called a protostar.called a protostar.The protostar continues to contract and increase The protostar continues to contract and increase in temperature for several million years. in temperature for several million years. Eventually the gas in the region becomes so hot Eventually the gas in the region becomes so hot that its electrons are stripped from their parent that its electrons are stripped from their parent atoms.atoms.The nuclei and free electrons move independently, The nuclei and free electrons move independently, and the gas is then considered a separate state of and the gas is then considered a separate state of matter called plasma.matter called plasma.


A protostar’s temperature continually increases A protostar’s temperature continually increases until it reaches about 10,000,000°C.until it reaches about 10,000,000°C.At this temperature, nuclear fusion begins. At this temperature, nuclear fusion begins. Nuclear fusion is a process in which less-massive Nuclear fusion is a process in which less-massive atomic nuclei combine to form more-massive atomic nuclei combine to form more-massive nuclei. nuclei. The process releases enormous amounts of The process releases enormous amounts of energy.energy.The onset of nuclear fusion marks the birth of a The onset of nuclear fusion marks the birth of a star. star. Once this process begins, it can continue for Once this process begins, it can continue for billions of years.billions of years.


As gravity increases the pressure on the As gravity increases the pressure on the matter within the star, the rate of fusion matter within the star, the rate of fusion increase. increase.

In turn, the energy radiated from fusion In turn, the energy radiated from fusion reactions heats the gas inside the star. reactions heats the gas inside the star.

The outward pressures of the radiation and The outward pressures of the radiation and the hot gas resist the inward pull of gravity. the hot gas resist the inward pull of gravity.

This equilibrium makes the star stable in size.This equilibrium makes the star stable in size.

http://www.physicsoftheuniverse.com/topics_bigbang_timeline.html

http://www.gps.caltech.edu/~gab/astrophysics/astrophysics.html

The Main-Sequence StageThe Main-Sequence Stage

The second and longest stage in the life of a The second and longest stage in the life of a star is the main-sequence stage. star is the main-sequence stage. During this stage, energy continues to be During this stage, energy continues to be generated in the core of the star as hydrogen generated in the core of the star as hydrogen fuses into helium. fuses into helium. A star that has a mass about the same as the A star that has a mass about the same as the sun’s mass stays on the main sequence for sun’s mass stays on the main sequence for about 10 billion years. about 10 billion years. Scientists estimate that over a period of Scientists estimate that over a period of almost 5 billion years, the sun has converted almost 5 billion years, the sun has converted only 5% of its original hydrogen nuclei into only 5% of its original hydrogen nuclei into helium nuclei.helium nuclei.

Leaving the Main-SequenceLeaving the Main-Sequence

Giant StarsGiant Stars A giant a very large and bright star whose hot A giant a very large and bright star whose hot

core has used most of its hydrogen.core has used most of its hydrogen. A star enters its third stage when almost all of A star enters its third stage when almost all of

the hydrogen atoms within its core have fused the hydrogen atoms within its core have fused into helium atoms. into helium atoms.

A star’s shell of gases grows cooler as it A star’s shell of gases grows cooler as it expands. expands.

As the gases in the outer shell become As the gases in the outer shell become cooler, they begin to glow with a reddish cooler, they begin to glow with a reddish color. These stars are known as giants.color. These stars are known as giants.

Leaving the Main-SequenceLeaving the Main-Sequence

SupergiantsSupergiants Main-sequence stars that are more massive Main-sequence stars that are more massive

than the sun will become larger than giants in than the sun will become larger than giants in their third stage. their third stage.

These highly luminous stars are called These highly luminous stars are called supergiants. supergiants.

These stars appear along the top of the H-R These stars appear along the top of the H-R diagram. diagram.

The Final Stages of a Sunlike The Final Stages of a Sunlike StarStar

Planetary NebulasPlanetary Nebulas As the star’s outer As the star’s outer

gases drift away, the gases drift away, the remaining core heats remaining core heats these expanding these expanding gases. gases.

The gases appear as The gases appear as a planetary nebula, a a planetary nebula, a cloud of gas that forms cloud of gas that forms around a sunlike star around a sunlike star that is dying.that is dying.

Planetary Nebula ngc 6751

http://www.chiro.org/LINKS/WALLPAPER/Selection_of_Wallpaper.shtml


White DwarfsWhite Dwarfs As a planetary nebula disperses, gravity causes As a planetary nebula disperses, gravity causes

the remaining matter in the star to collapse the remaining matter in the star to collapse inward.inward.

The matter collapses until it cannot be pressed The matter collapses until it cannot be pressed further together. further together.

A hot, extremely dense core of matter - a white A hot, extremely dense core of matter - a white dwarf - is left. White dwarfs shine for billions of dwarf - is left. White dwarfs shine for billions of years before they cool completely. years before they cool completely.

The gases appear as a planetary nebula, a cloud The gases appear as a planetary nebula, a cloud of gas that forms around a sunlike star that is of gas that forms around a sunlike star that is dying.dying.


Novas and SupernovasNovas and Supernovas A nova is a star that suddenly becomes A nova is a star that suddenly becomes

brighter.brighter. Some white dwarfs revolve around red giants. Some white dwarfs revolve around red giants. When this happens, the gravity of the whit dwarf When this happens, the gravity of the whit dwarf

may capture gases from the red giant.may capture gases from the red giant. As these gases accumulate on the surface of As these gases accumulate on the surface of

the white dwarf, pressure begins to build up. the white dwarf, pressure begins to build up. This pressure may cause large explosions. This pressure may cause large explosions.

These explosions are called novas.These explosions are called novas.


Novas and Novas and Supernovas Supernovas (continued)(continued)

A white dwarf may also A white dwarf may also become a supernova, become a supernova, which is a star that has which is a star that has such a tremendous such a tremendous explosion that it blows explosion that it blows itself apart.itself apart.

The explosions of The explosions of supernovas completely supernovas completely destroy the white dwarf destroy the white dwarf star and may destroy star and may destroy much of the red giant.much of the red giant.

http://chandra.harvard.edu/edu/formal/stellar_ev/story/index8.html

Mira Red Giant with White Dwarf Companion (Chandra)

The Final Stages of Massive The Final Stages of Massive StarsStars

Supernovas in Massive StarsSupernovas in Massive Stars Massive stars become supernovas as part of Massive stars become supernovas as part of

their life cycle.their life cycle. After the supergiant stage, the star collapses, After the supergiant stage, the star collapses,

producing such high temperatures that producing such high temperatures that nuclear fusion begins again.nuclear fusion begins again.

When nuclear fusion stops, the star’s core When nuclear fusion stops, the star’s core begins to collapse under its own gravity. begins to collapse under its own gravity.

This causes the outer layers to explode This causes the outer layers to explode outward with tremendous force.outward with tremendous force.


Neutron StarsNeutron Stars A neutron star is a star that has collapsed A neutron star is a star that has collapsed

under gravity to the point that the electrons under gravity to the point that the electrons and protons have smashed together to form and protons have smashed together to form neutrons.neutrons.

Stars more massive than the sun do not Stars more massive than the sun do not become white dwarfs. become white dwarfs.

After a star explodes as a supernova, the core After a star explodes as a supernova, the core may contract into a neutron star.may contract into a neutron star.


http://essayweb.net/astronomy/blackhole.shtml


PulsarsPulsars A pulsar is a rapidly spinning neutron star that A pulsar is a rapidly spinning neutron star that

emits pulses of radio and optical energy.emits pulses of radio and optical energy. Some neutron stars emit a beam of radio Some neutron stars emit a beam of radio

waves that sweeps across space and are waves that sweeps across space and are detectable here on Earth. detectable here on Earth.

These stars are called pulsars. These stars are called pulsars. For each pulse detected on Earth, we know For each pulse detected on Earth, we know

that the star has rotated within that period.that the star has rotated within that period.


Black HolesBlack Holes A black hole is an object so massive and A black hole is an object so massive and

dense that even light cannot escape its dense that even light cannot escape its gravity.gravity.

Some massive stars produce leftovers too Some massive stars produce leftovers too massive to become a stable neutron star. massive to become a stable neutron star.

These stars contract, and the force of the These stars contract, and the force of the contraction leaves a black hole. contraction leaves a black hole.

Video LinksVideo Links

Spore - Protostar (1:32) - http://youtu.be/-ukzjQ_VdKkSpore - Protostar (1:32) - http://youtu.be/-ukzjQ_VdKkStar Size Comparison HD (2:34) - Star Size Comparison HD (2:34) - http://youtu.be/HEheh1BH34Qhttp://youtu.be/HEheh1BH34QSupernova Explosion (0:21) - Supernova Explosion (0:21) - http://youtu.be/RgfbjHz_UTohttp://youtu.be/RgfbjHz_UToPulsar Planets (3:12) - http://youtu.be/z2L7aNkL01oPulsar Planets (3:12) - http://youtu.be/z2L7aNkL01oWhat's Inside a Black Hole? (2:44) - What's Inside a Black Hole? (2:44) - http://youtu.be/GYKyt3C0oT4http://youtu.be/GYKyt3C0oT4The Beauty Of Stars Being Born (4:35) - The Beauty Of Stars Being Born (4:35) - http://youtu.be/wnb20chqbxMhttp://youtu.be/wnb20chqbxM

Video LinksVideo Links

Cosmic Journeys : The Largest Black Cosmic Journeys : The Largest Black Holes in the Universe (24:59) - Holes in the Universe (24:59) - http://youtu.be/xp-8HysWkxwhttp://youtu.be/xp-8HysWkxw

Through The Wormhole: The Riddle Of Through The Wormhole: The Riddle Of Black Holes (44:34) - Black Holes (44:34) - http://youtu.be/bOtuJ3nM_qIhttp://youtu.be/bOtuJ3nM_qI

Too Many People - Earth 101 (58:43) - Too Many People - Earth 101 (58:43) - http://youtu.be/dxQfMcnl6DYhttp://youtu.be/dxQfMcnl6DY

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