Stars: Old Age, Death, and New Life

BC Science Probe 9Section 13.4

Pages 431-436

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!

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.

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.

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.

Hertzsprung-Russell

• Our Sun has 1 solar mass, and it is the star to which all others are compared.

Hertzsprung-Russell

• In the lower right of the diagram are the cooler, reddish stars that are small and dim.

Hertzsprung-Russell

• In the upper left are the massive, very bright, hot, bluish stars.– The blue giants.

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.

Hertzsprung-Russell

• The white dwarfs which are very hot and about 1/3 of a solar mass are in the lower left.

Hertzsprung-Russell

• The stars in the H-R diagram represent different stages in the lives of stars as their fuel is consumed.

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.

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!

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.

Red Giant to White Dwarf

• The Sun will expand for millions of years.• It will engulf Mercury, Venus, and maybe even

the Earth!

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.

Red Giant to White Dwarf

• Helium fusion will continue the expansion.• It will also produce heavier elements like

Carbon and Oxygen.

Red Giant to White Dwarf

• Now it is a fully formed red giant.• It will have several thousand times its original

brightness!

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.

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.

Red Giant to White Dwarf

• A star that has a mass over 10 solar masses will explode as a supernova.

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.

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?

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.

Supernovas

• The explosion of a star in which the star may reach a maximum intrinsic luminosity one billion times that of the Sun.

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.

Neutron Stars

• If a star starts out at 10-50 solar masses, the supernova will produce a neutron star.

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!

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.

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.

Black Holes

• If the mass left behind is more than 4 solar masses, the core will collapse in on itself.

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.

Black Holes

• A black hole of 10 solar masses would only be 60 km in diameter!

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.

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.