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Stars and Galaxies

Part 5: The Evolution of Stars

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Evolution

• Change over time• Astronomers refer to the evolution of a

star as “The Life Cycle of a Star.”• How a star evolves is mainly dependent

upon the mass that the star begins with.

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Gravity vs. Nuclear Fusion

Nuclear reactions in the cores of stars produce energy. The outflow of energy from the core provides the thermal pressure necessary to keep the star from collapsing under its own weight.

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The Effect of Mass

• More massive stars will actually burn out faster than less massive stars.

• Although they have more mass, that means more gravity compressing that mass, which causes them to burn hotter and use up their fuel faster.

• A star 60 times the size of our Sun will use all its fuel in as little as a million years, while our Sun is expected to burn as it is for about 10 billion years.

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Protostars

• A hot, contracting cloud of dust and gases

• Protostars are formed from the dust and gases in a nebula being acted on by gravity.

• When temperatures reach 10 million °C nuclear fusion begins and a star is born. The Pillars of Creation

within the Eagle Nebula

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Three Possibilities

• Depending upon the initial mass of the star, there are three possible ways it may evolve:

View QuickTime Video from NASA: Lifecycle of a Star

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The Fate of Medium-Sized StarsAfter billions of years, a medium-Sized star, such as our sun, will:1. Have converted most of the hydrogen in its core to

helium so that nuclear fusion begins to fail2. Gravity will then prevail and the core will begin to

collapse, heat up, and release energy.3. This causes the outer hydrogen layer to expand and

cool, turning the star red. The star is now a red giant.

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The Fate of Medium-Sized StarsAfter millions of years, a medium-Sized star, such as our sun, will:4. The red giant continues to burn the hydrogen in its shell

while its core gets hotter and hotter.5. At about 200 million °C the helium atoms begin to

undergo fusion to form carbon atoms.6. The remaining hydrogen in the shell drifts away to form

a planetary nebula, leaving behind a very dense, white dwarf star.

7. The white dwarf will eventually use all its remaining fuel, nuclear fusion will cease, and it will become an inert black dwarf.

The Fate of Medium-Sized Stars

9The Hubble Heritage Team (AURA/STScI/NASA)

The most famous of all planetary nebulae: the Ring Nebula, 2,000 light years from Earth

The Fine Ring Nebula, unusual because it is appears as an almost

perfect circular ring. Image from the ESO Faint Object Spectrograph and Camera mounted on the New Technology Telescope at the La Silla Observatory in Chile

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Stars More Than 5X the Mass of the Sun

1. Begins the same as a lower mass star, as a Main Sequence Star that eventually becomes a red giant or a supergiant.

2. Because of the greater mass, once carbon atoms are formed the greater force of gravity continues to compress the core, increasing the temperature to 600 million °C, at which point the carbon atoms begin fusing to form new and heavier elements.

3. Fusion to form new elements continues until iron atoms are formed.

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Stars More Than 5X the Mass of the Sun

4. When fusion ends, the core is mainly iron.5. The iron core, unable to sustain itself against

gravity, collapses in a matter of seconds. 6. The outer layers of the star collapse

downward onto the stellar core and bounce off the core in a supersonic shockwave observed as a gigantic explosion. This explosion is known as a supernova.

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Stars More Than 5X the Mass of the Sun

7. In less than one second, the explosion emits more than 100 times the total energy the Sun will emit over its entire lifetime of 10 billion years. Temperatures can rise to 1 billion °C causing iron atoms to fuse and resulting in the formation of elements even heavier than iron.

8. The resulting cloud of gases and dust form a new nebula containing many elements formed during the supernova.

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Stars More Than 5X the Mass of the Sun

• FYI: The most famous supernova ever recorded was observed by Chinese astronomers in 1054. It lit the day sky for 23 days and could be seen at night for more than 600 days.

• Its remains became the Crab Nebula (right).

This is a mosaic image of the Crab Nebula, a six-light-year-wide expanding remnant of a star's supernova explosion, taken by NASA's Hubble Space Telescope. From NASA, ESA, J. Hester and A. Loll (Arizona State University)

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After a Supernova

What happens to the core of the star after a supernova depends upon the initial mass of the star. • A star that begins at roughly 8 to 20 times* the

mass of the sun:**– the core collapses to a very dense neutron star– The neutron star rotates rapidly, sending out a narrow

beam of energy (like a lighthouse); appears as a regular pulse of energy on Earth so called a pulsar

*These size estimates are greater than those in the textbook, but most current. **It actually depends on the core size after the supernova. If the mass of the core is between 1.4 and 3.0 solar masses the core will become a neutron star.

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After a Supernova

What happens to the core of the star after a supernova depends upon the initial mass of the star.• A star greater than 25 times the mass of the sun:– Yields a remnant core with greater than 3 times the solar

mass– Gravity collapses the core– The resulting collapse never stops. The space remaining

from the ongoing collapse is known as a black hole.

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Black Holes

• Not a hole at all• The tiny, spherical remains of the stellar core

form a boundary between normal space and the space inside the black hole that scientists call a singularity.

• Has so much mass compressed into such a small volume that its gravity is enormous

• Nothing, not even light, can escape this gravity. • Difficult to detect

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Black Holes• If a black hole forms from one of the stars in a binary

star system, material from the remaining companion star can be pulled into the black hole.

• The material being piled on top of the black hole heats up and emits energy in the form of x-rays just before being sucked in and lost forever.

• So, a sign of a black hole is the emission of x-rays without a visible object.

This artist’s impression depicts the newly discovered stellar-mass black hole in the spiral galaxy NGC 300.

Credit: ESO/L. Calçada

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