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Stars: Old Age, Death, and New Life
BC Science Probe 9Section 13.4
Pages 431-436
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Stars
• Stars age as they use up their hydrogen.• After millions or billions of years, a star will
enter the last stages of its life.• At the end, a star can either fade out of
existence, or explode in a life-renewing cycle!
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Hertzsprung-Russell
• A star’s mass determines its brightness, colour, size, and how long it will live.
• This information is organized in a diagram called the Hertzsprung-Russell (H-R) diagram which plots the lives of stars.
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Hertzsprung-Russell
• An H-R diagram shows the temperature and luminosity of stars.
• Luminosity is energy output and the Sun is assigned the value of 1.
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Hertzsprung-Russell
• Most stars fit into the main sequence of the H-R diagram.– This is the diagonal band on the diagram.
• Where it fits on the main sequence depends on its mass.
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Hertzsprung-Russell
• Our Sun has 1 solar mass, and it is the star to which all others are compared.
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Hertzsprung-Russell
• In the lower right of the diagram are the cooler, reddish stars that are small and dim.
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Hertzsprung-Russell
• In the upper left are the massive, very bright, hot, bluish stars.– The blue giants.
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Hertzsprung-Russell
• The cooler red giants (0.4-10 solar masses) and the super giants (10-70 solar masses) are found off the main sequence to the upper right.
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Hertzsprung-Russell
• The white dwarfs which are very hot and about 1/3 of a solar mass are in the lower left.
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Hertzsprung-Russell
• The stars in the H-R diagram represent different stages in the lives of stars as their fuel is consumed.
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Red Giant to White Dwarf
• After 10 billion years as a main sequence star, most of the Sun’s hydrogen will be converted to helium.
• The helium forms a core inside a shell of the remaining hydrogen.
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Red Giant to White Dwarf
• Less hydrogen = less energy = less outward flow of energy.
• This causes the core to shrink and the shrinking/contraction reheats the rest of the hydrogen and starts fusion again!
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Red Giant to White Dwarf
• Even though the core is shrinking, the outer layers will expand and then cool.
• This expanded, cooler Sun will eventually be a red giant.
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Red Giant to White Dwarf
• The Sun will expand for millions of years.• It will engulf Mercury, Venus, and maybe even
the Earth!
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Red Giant to White Dwarf
• Once the hydrogen is actually all used up, the core will heat to a temperature high enough to begin helium fusion.
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Red Giant to White Dwarf
• Helium fusion will continue the expansion.• It will also produce heavier elements like
Carbon and Oxygen.
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Red Giant to White Dwarf
• Now it is a fully formed red giant.• It will have several thousand times its original
brightness!
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Red Giant to White Dwarf
• Red giants give off dust and gas, so they begin to lose mass.
• Even a star that starts at a mass of up to10 solar masses will eventually lose enough mass to become a stable white dwarf.
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Red Giant to White Dwarf
• A white dwarf will have a mass no larger than 1.4 solar masses and it is compressed to the size of the Earth.
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Red Giant to White Dwarf
• A star that has a mass over 10 solar masses will explode as a supernova.
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Red Giant to White Dwarf
• The Sun is 1 solar mass, so it will become a white dwarf.
• It will release particles that will collide with the matter it shed during its last stages as a red giant.
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Red Giant to White Dwarf
• The energy from these collisions of particles will illuminate the clouds of gas and dust and create a nebula…
• What can a nebula form?
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Red Giant to White Dwarf
• The white dwarf, or the remains of the Sun, will keep its place in the Milky Way until it burns out and becomes a black dwarf.
• It will no longer be visible.
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Supernovas
• The explosion of a star in which the star may reach a maximum intrinsic luminosity one billion times that of the Sun.
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Supernovas
• They may only last a few months.• The energy that they generate can drive the
fusion and formation of all of the elements on the Periodic Table.
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Neutron Stars
• If a star starts out at 10-50 solar masses, the supernova will produce a neutron star.
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Neutron Stars
• The core of a neutron star is made mostly of neutrons.
• They are so tightly packed together that 250 ml (1 cup) of the core would have a mass of millions of kilograms!
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Neutron Stars
• A pulsar is a kind of neutron star that sends out light and a beam of very high-energy radio waves.
• It rotates giving off the beam of energy kind of like a light house.
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Black Holes
• If a star has an initial mass of over 50 solar masses, it will become a supernova and produce very heavy elements like iron and nickel.
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Black Holes
• If the mass left behind is more than 4 solar masses, the core will collapse in on itself.
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Black Holes
• The object’s mass is still immense, so its gravitational pull is also immense.
• Not even light can escape.• It has become a black hole.
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Black Holes
• A black hole of 10 solar masses would only be 60 km in diameter!
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Black Holes
• Black holes are very difficult to find because they are small and do not give off light.
• Many stars exist as binaries ( 2 stars circling each other), this was how the first black hole was discovered.– A blue giant and its invisible companion.
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Black Holes
• The first black hole was found in the constellation Cygnus.– The black hole pulls gas from the blue giant.– The gas heats up and emits X-rays which allowed
the black hole to be detected.– The blue giant has a solar mass of 27 and the black
hole (Cygnus X-1) has a solar mass of 14.
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