Chapter 26 Part 1 of Section 2: Evolution of Stars

How Do Stars Form? Formation begins with condensation of a large cloud of cold gas, ice, and dust called a nebula. The nebula contracts, breaks into fragments, and increases in temperature. When the center of the cloud reaches 2 million degrees F (1 million K), it becomes a protostar. When the temperature reaches 18 million degrees F (10 million K), the hydrogen fuses to become helium and a star is born.

H-R Diagram: Hertzsprung and Russell Early 1900’s chart to compare the temperature of a star to its luminosity (magnitude). Higher temperature stars have higher magnitudes (radiate more energy) 90% of all stars are found in the main sequence of the diagram.

How do Stars Change? A star like our Sun was probably 100 x’s bigger as a protostar. As it continues to form it shrinks, becomes more dense, and increases in temperature. The protostar has a large amount of hydrogen in it. The hydrogen begins to fuse together to form helium. The energy of hydrogen fusion is that of a bomb.

Stellar Equilibrium Equilibrium (balance) is attained when the outward force of fusion is = to the inward pull of gravity. When a star reaches equilibrium it will be found in the main sequence of the H-R diagram. Stars spend most of their lives in equilibrium (main sequence). Our Sun has been in equilibrium for approximately 5 billion years.

Losing Equilibrium When a star has fused all of its hydrogen into helium it begins to lose equilibrium because the outward force from fusion becomes less than the inward force of gravity. What a star becomes next depends on its mass. The bigger the mass of a star, the shorter its life and more dramatic its death will be.

Stars Smaller than our Sun 80% of stars in the universe are smaller than our Sun. These are called red dwarfs. Proxima Centauri is a red dwarf. Could remain on main sequence for 16 trillion years. Slowly fade away

Average Size Stars Like Our Sun Our sun is considered a medium sized yellow dwarf. As the Sun fuses hydrogen the outer layers will expand and our sun becomes a red giant. When the Sun runs out of hydrogen in approx. 5 billion years, gravity will begin to take over and crush the star inward. This makes the star hotter. Hot enough to fuse helium into heavier elements like carbon and stop the crushing force of gravity. The fusion of helium causes the outer layer to swell so much that it escapes the gravity of the star and is released into space. What’s left behind is a white dwarf.

The White Dwarf Is it the final stage for medium size stars? For our Sun- YES. For stars that are part of a binary system or star cluster- NO (on next slide) The white dwarf is the dense core left behind from the previous red giant. 1 tsp. of white dwarf matter would have a mass of several tons. This will continue to shine for billions of years.

If the Star is Part of a Binary System or Star Cluster… It’s not over for the dying white dwarf. More than ½ of the stars are part of a binary system. The dying white dwarf can steal hydrogen from its binary partner and grow in mass to an unstable limit 40% higher than our Sun. This leads to Type 1 A Supernova.

Stars 8-10 x’s More Massive Than Our Sun These are called supergiants. Example of a supergiant is Betelgeuse. Hydrogen fuses to become helium Higher temperatures allow helium to fuse to become carbon or oxygen. Carbon and Oxygen can further fuse to become even heavier elements which leads to elements like iron and nickel. It cannot fuse iron any further so gravity takes over and the star collapses leaving a core the size of the Earth. This causes a Type 2 Supernova releasing heavy elements into space.

After a Type 2 Supernova: The Neutron Star and Pulsar The remaining dense core is made up almost entirely of neutrons. This is called a neutron star. These are as small as 10 miles across. They are extremely dense. 1 tsp. of neutron star matter = 1 billion tons The Neutron star begins to spin rapidly and create a large magnetic field that begins to glow. This is called a pulsar. (Like a lighthouse)

Stars that are 25-40 x’s the Mass of the Sun Also called Supergiants So massive that even the neutron star cannot hold up under the pressure of gravity. When the neutron star collapses it becomes a black hole.

Black Holes Ultimate death of a VERY massive supergiant. A region of space with such high density and gravity that nothing can escape it.