Evolution of High-Mass Stars

Convection in the core of high-mass Main Sequence star “mixes” material. As hydrogen burns, helium builds uniformly throughout the core.

Evolution of High-Mass Star

>8 Solar Masses (M☉) Evolution

Note: H = hydrogen, He = helium, C = carbon, O = oxygen, Mg = magnesium, Ne = neon, S = sulfur, Si = silicon, Na = sodium, Fe = iron,

Step 1: H –> He (core) via CNO Cycle

Step 2: He –> C (core); H –> He (shell)

*Star is now a super-giant

Step 3: C –> O, Ne, Mg (core); He –> C (shell); H–> He (shell)

Step 4: S, Si –> Fe (core); O –> S, Si (shell); Ne –> O, Mg (shell); C –> Na, Ne, Mg (shell); He –> C (shell); H –> He (shell)

  • As high-mass stars enter evolution, they swell to enormous sizes as shells form around the cores
  • High-mass stars move horizontally back and forth across H-R diagram after leaving the Main Sequence
  • As stars pass the “instability strip,” they become pulsating variable stars (change in luminosities)
  • The heavier the element that the star starts producing in its core, the higher the temperature of the core, and the shorter the burning stage of the element
  • Fusing less massive elements (less massive than Fe) releases energy, while fusing heavier elements (heavier than Fe) doesn’t release energy; instead, heavier elements require energy for fusion— heavier elements do not “burn”
  • Fe is the most stable element and the most tightly bound (strong attraction between nucleus and electrons)

Core Collapse

  • The final  stage of nuclear burning in the core of a massive star: sulfur and silicon –> iron and nickel
  • Once the core produces iron, no more energy can be extracted via nuclear fusion reactions
  • Even electron degeneracy pressure (electrical repulsion of atoms) can’t prevent the core from collapsing to a much smaller, denser state
  • With no further source of energy, gravity begins to compress the core to smaller sizes
  • When the temperature reaches 10 billion K, the core implodes at 1/4 the speed of light
  • Black-body radiation is so intense in the core that iron nuclei are broken apart
  • Electrons combine with protons to form neutrons and neutrinos (e + p –> n +ν)
  • Neutrinos carry energy out of core
  • Core collapses to a radius of about 10 km
  • Outer layers of star are blown off explosively, leading up to a Type II Supernova
  • 99% of energy in Type II Supernova comes sin the form of neutrinos (e.g. supernova 2011 dh in galaxy M51 – Whirlpool Galaxy)

Supernova

Milky Way Galaxy Supernovae

  • Usually 1-2 supernovae per century
  • In 1572 and 1604: supernova observed
  • The supernovae may have been hidden by dust clouds
    • Crab Nebula: remnant of a Type II Supernova in 1054 A.D.
    • Vela supernova remnant
      • Cygnus Loop
      • SN1987A: Type I supernova in the Large Magellanic Cloud
      • Blue Supergiant at 20 M☉
      • 19 neutrinos detected at the Irvine-Michigan-Brookhaven neutrino detector (underground mine in Ohio) three hours before light from the supernova was seen

Neutron Star

Neutron Stars

  • Remnant of cores of massive stars, left over after the core collapses in the supernova explosion
  • 1.5x more massive than the Sun, only 10 km in radius
  • 1 cm³ = 1 billion tons
  • How to Detect Them
    • Pulsars: spinning neutron stars that emit radio waves
    • X-Ray Binaries: in a binary star system, accretes matter from the other star
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