Question

• Choose one of the following two topics and write up a brief summary. You can assume solar metal content for the star. (5 points):

  • Describe the evolution of a one solar mass star after the core He-burning phase of evolution. Include the nature of the remnant and what supports it against collapse.
  • Describe the evolution of a ten solar mass star after the core He-burning phase of evolution. Include the nature of the remnant and what supports it against collapse.

Answer 1

a) Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

Stars that are ten times more massive than the Sun are over a thousand times more luminous than the Sun. Once a star exhausts its core hydrogen supply, the star becomes redder, larger, and more luminous: it becomes a red giant star. When a star exhausts the hydrogen in its core, it becomes a giant or supergiant. Once a star has used up all the hydrogen in its core, fusion of hydrogen into helium stops. The core starts to contract again ,just as it contracted as a protostar, before hydrogen fusion began.

STAR BIRTHS are started when the interstellar matter in gas clouds, such as the Eagle Nebula shown here, compresses and fuses. When the core reaches a temperature of about 2,000 degrees Kelvin, the molecules of hydrogen gas break apart into hydrogen atoms.For low-mass stars (left hand side), after the helium has fused into carbon, the core collapses again. As the core collapses, the outer layers of the star are expelled. A planetary nebula is formed by the outer layers. The core remains as a white dwarf and eventually cools to become a black dwarf.When helium in the core runs out it leaves an inert carbon core, which contracts heat. Helium ignites in a shell outside the core, and hydrogen burns in a shell outside that (double shell burning.) The star becomes a red giant again only even larger and more luminous.The stars above the main sequence are red giant stars, and these are stars which have finished burning hydrogen in their cores and have started burning it in a shell about a helium core. When a star does this it swells up, becoming cooler and much brighter.