Stages in the Life of a Star

This set of notes by Nick Strobel covers: stellar evolution and stellar remnants. Most of these notes will be in outline form to aid in distinguishing various concepts. As a way to condense the text a bit, I'll often use phrases instead of complete sentences. I have italicized the vocabulary terms.

Contents

Stellar Evolution

Index

Timescales

Evolution of stars 
depends on Mass (at start of fusion only) with a tiny bit of dependence on chemical 
composition

Stages

Index

Stellar Nucleosynthesis

Index

Main Sequence Turnoff

Stellar Remnants

Index

First I'll give a definition of ``degenerate matter'' as used in astronomy. Then I'll discuss the different types of core remnants. What I'll call ``remnants'' in this section is the compressed core. The spewed out gases in supernova explosions and planetary nebula are also sometimes referred to as ``supernova remnant'' or ``planetary remnant'', but I'll be focussing on the remaining core left after the spectacular mass loss has occurred. It is important to remember that what happens to the core depends on the mass of the core, rather than the original mass of the main sequence star from which it came, because the only thing left for gravity to really compress is the core.

Degenerate matter

  • Degenerate matter--very dense matter in a state where the pressure no longer depends on temperature due to quantum mechanical effects. Rules: a) Certain permitted energies in a given space; b) Only 2 electrons (for white dwarfs) or 2 neutrons (for neutron stars) can share the same energy level in a given volume at one time; c) Closest spacing depends inversely on mass of particle (electrons further apart than neutrons and protons). Features:
    1. Resist compression. Electrons or neutrons locked into place because all the lower energy shells are filled up and the only way they can move is to absorb enough energy to get to upper energy shells (hard to do!). To compress a degenerate gas means to change the degenerate particle motions. But that requires A LOT of energy. Degenerate particles have no ``elbow room'' and their jostling against each other strongly resists compression. Gas like hardened steel!

    2. Pressure only depends on speed of degenerate particles NOT the temperature. To change speed of degenerate particles requires A LOT of energy. Adding heat only causes the non-degenerate particles to move faster, but the degenerate ones supplying the pressure are unaffected.

    White Dwarfs

    Index

  • White Dwarfs--if core mass < 1.4 solar masses. Electrons are degenerate. Mass of Sun compressed to size of Earth. Higher mass compressed to smaller radius! Neutrons and protons have room to move around freely. The density is about 1,000,000 g/cm^3 (one sugarcube > 1 car!). White dwarf cools off from initial formation temperature of about 100,000 K. Form as outer layers of red giant star puff out to make planetary nebula.

    Neutron Stars

  • Neutron Stars--1.4 < core mass < 3 solar masses. Compression so great that protons fuse with electrons to form neutrons. Neutrons are degenerate. About 30 km across! Density about 2*10^14 g/cm^3 (one sugarcube = mass of humanity!). Formed in supernova explosion.

    Pulsars

    Index

  • Pulsars--rapidly rotating neutron stars with STRONG magnetic fields (10^9 - 10^12 times Sun's). Light flashes with period of milliseconds at start and lengthening to periods of about 4 minutes over time. Why would neutron stars be fast rotators? Conservation of angular momentum (remember spinning ice skater)! Slowly rotating Red Giant star has same angular momentum as tiny, fast rotating neutron star. Rotating neutron star only thing that could create that frequency of pulses. Lighthouse model-strong magnetic field creates electric field making charged particles (mostly electrons) flow out of the magnetic poles. As the charged particles spiral around the magnetic field lines, they produce a non-thermal radiation (synchrotron) beam at the magnetic poles. If the beam sweeps past Earth, we see a flash of light.

      Energy from Spin

    1. Radiation production robs energy from pulsar so pulsar rotation slows down (angular momentum does slowly decrease).

      Spin Glitch

    2. Every now and then, neutron star suddenly shrinks by about 1 mm. Spin rate suddenly increases to conserve angular momentum. See glitch in pulse rate.

    Black Holes

    Index

  • Black Holes--core mass > 3 solar masses. Gravity finally wins, compressing core to mathematical point at center. Formed in supernova explosion. Surface gravity so strong that nothing can escape (not even light!) within a certain distance from mass point. Boundary is called the event horizon (or Schwarzchild radius)-no messages of events happening within radius (= 3 * core mass [in solar masses] km) make it to the outside. Use Einstein's General Relativity to describe gravitational effects. Gravitational Redshift & Time Dilation. Signatures of a black hole:

      Mass of Companion in Binary

    1. For binary, look at how the black hole moves visible companion around. Use Kepler's 3rd law to find sum of masses. Guess mass of visible companion then find unseen companion's (black hole's) mass.

      X-rays from Accretion Disk

    2. For binary, look for X-rays produced in hot accretion disk--material pulled off visible companion spirals onto black hole. Disk formed. Friction increases inward causing increasing temperature as approach the event horizon-produce wide spectrum of radiation. To make rapidly varying X-rays, unseen companion must be small!

    Index

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    last update 29 Aug 95


    Nick Strobel -- Email: strobel@astro.washington.edu

    (206) 543-1979
    University of Washington
    Astronomy
    Box 351580
    Seattle, WA 98195-1580