The Death of Stars Stellar Recycling

The fate of the Sun Eventually fusion will exhaust the hydrogen supply from the center of the Sun. Internal pressure decreases. Gravity pulls outer layers in. Hydrogen burns in a shell around the core. New energy released will cause the sun to expand in size. The Sun will swallow up Mercury and Venus. The surface will be cooler, so it will look red The Sun has become a red giant.

Planetary Nebulae The core becomes so hot that helium fuses to make carbon. The Sun becomes fainter and smaller. As the carbon core contracts, it heats up and generates more energy. Hydrogen to Helium fusion around core increases again. Outer layers of the Sun drift away forming a planetary nebulae. This is the Helix nebula.

Ring Nebula in Lyra

Planetary Nebulae, Keck Telescope

M 27 The Dumbbell Nebula

Double Vortex Nebula

Spirograph nebula

Stingray Nebula

Hourglass Nebula

NGC6826

NGC7027

NGC7009

NGC6578

White Dwarfs Stars less than 1.4 times the mass of the sun continue to shrink. Our Sun will shrink until it becomes the size of the earth. It becomes very dense (1 tsp. = 5 tons) Small and faint White Dwarfs are hard to see. They slowly fizzle out over billions of years.

White dwarf size compared with the Earth

Nova A nova is a star that has increased 10 times or so in brightness. Nova occur when a white dwarf draws off stellar material from a Red Giant companion star. This new material may heat up enough to cause the star to brighten dramatically. This can happen many times.

Red Supergiant Stars A star 15 times the mass of the Sun burns up faster than the sun and ends its life in an abrupt way. The core contracts as the outer layer expands. This causes Helium to fuse to form carbon By the time the Helium is exhausted the outer layers have expanded even further This creates a huge red supergiant star. Betelgeuse is an example

Supernovae The carbon core of a supergiant contracts, heats up, and begins fusing into even heavier elements. Eventually iron builds up in the core. When iron fuses to make heaver elements energy is absorbed not given off. Without this energy the strong gravitational pull of the star causes it to collapse on itself in seconds. The star rebounds in a tremendous explosions called a supernovae.

Supernovae cont. Type II supernovae are a result of the previous slides process. The Crab Nebula is the result of a supernova first seen by the Chinese in 1054 AD. Pictures taken of this nebula over the years prove that the cloud is still expanding.

Veil Nebula

Type II Supernovae Sometimes so much stellar material is absorbed by a white dwarf from its companion red giant star that the white dwarf blows up. Type II supernova take longer to happen and still have Hydrogen when they explode. This is a picture of Supernova 1987a seen in the Large Magellanic Cloud, Feb. 24, 1987

Neutron Stars The collapse of the core after the explosion causes the iron in the core to decay to Helium nuclei. Density becomes so high that electrons are squeezed into the nuclei. What’s left is neutrons. The neutron star is only 20 km across. 1 tsp. Would weigh a billion tons. They have a very strong magnetic field.

Pulsars Pulsars are neutron stars, spinning rapidly and giving off radio energy at their poles. They were first detected in 1967 by Jocelyn Bell. The rate of emissions can be timed and therefore people can calculate the rotation rate of the pulsar. Pulsars spin very rapidly.

Jocelyn Bell She was born in While at Cambridge, she helped her advisor, Antony Hewish, to create a large radio telescope. She was the first to hear the regular pulses from what would later be identified as pulsars. At first it was thought that these radio signals came from an alien civilization, so they were termed LGM’s (little green men!) She did this while she was working on her Ph.D. at Cambridge University in She received her Ph.D. in radio astronomy in She is currently a Visiting Professor of Astrophysics at the University of Oxford and a Fellow of Mansfield College. Dr. Bell Burnell is the current President of the Institute of Physics.

Pulsars cont.

Black Holes A dying star that retains more than three times the Sun’s mass will collapse past the neutron star stage. When the mass has been compressed to a certain size, its gravitational radius, radiation from the star can no longer escape into space. A black hole is formed and from its surface nothing escapes not even light.

Detecting Black Holes Black Holes attract matter and matter attracted to a black hole accelerates as it is pulled toward the hole. Some of the matter is pulled directly into the black hole and is never seen again. Some of the matter goes into a high speed orbit around the black hole. This matter forms an accretion disk around the black hole. This matter, in the disk, will be heated to a very high temperature by friction. It will radiate x-rays as the material spins into the black hole.

Black Holes in Galaxies Many people now believe that black holes can be found at the centers of most galaxies. Many people feel that a black hole’s gravity is necessary to hold the galaxy together.

Detecting Black Holes When people search the position of x-ray sources, they look for a star that is warped. People than look for an invisible companion. People than check for the right mass. Cygnus X- 1 was one of the first Black Holes discovered this way.