Stellar Evolution Questions

Here are a few questions to get you started. Additional questions will be added over shortly so please check back soon. Click on the blue answer tags to get answers (and worked solutions for some questions).

  1. Which single property is most responsible in determining the evolutionary stages of a star?

    The mass of the star is the single most important property in determining its life cycle and fate.
  2. Two stars, A and B form in the same GMC. Star A is 5 solar masses, star B one solar mass.
    a) Which star would reach the main sequence first? Justify your choice.
    b) What can you infer about the metallicities of the two stars?
    c) At what wavebands would you best be able to observe the early stages of the formation of each of them?

    a) Star A would reach the main sequence first as it is more massive. The greater the mass of the stars, the stronger the inward gravitational pull. This in turn produces more heat which allows fusion ignition temperatures of > 107 K to be reached sooner. Once core fusion of hydrogen starts a star is on the main sequence.
    b) Stars that form in the same giant molecular cloud are likely to have much the same metallicity, that is abundance of metals, or elements heavier than helium, present.
    c) The early stages of star formation are obscured from visible wavelengths by the clouds of gas and dust. As the gas warms it becomes observable in the infrared. Electron transitions within molecules in cold clouds emit characteristic radiation in the millimetre part of the radio waveband hence can be observed using high-frequency radio telescopes. It is only when the protostar becomes exposed as with T Tauri stars that it is easily observed in visible wavebands.
  3. Why do stars need core temperatures in excess of 10 million K before fusion commences?

    Fusion is where two light nuclei come together to form a heavier nucleus and release energy in the process. As all nuclei are positively charged due to their protons, they experience an electrostatic or coulombic repulsion. At temperatures such as we experience here on Earth, nuclei have insufficient kinetic energy to overcome this repulsion hence do not undergo fusion. At v10 million K, hydrogen nuclei (that is protons) have sufficient energy to overcome repulsion and fuse together.
  4. A one solar mass star goes through several stages in its life. Some of the key stages are listed here but are out of order. Rewrite them in the correct sequence: planetary nebula, main sequence, white dwarf, asymptotic giant branch, giant molecular cloud, T Tauri star, red giant branch, protostar, helium flash, horizontal branch.

    Giant molecular cloud → protostar → T Tauri star → main sequence → red giant branch → helium flash → horizontal branch → asymptotic giant branch → planetary nebula→ white dwarf.