What are Brown Dwarfs?

Brown Dwarf

BROWN DWARFS: stars too small to perform nuclear fusion (no new energy) but too massive to be a planet

  • Masses range from 13 Jupiter-masses to 25 Jupiter-masses
  • Radius same as Jupiter but be up to 60-90 Jupiter masses
  • Some emit x-rays
  • All glow red in the infrared spectra until they cool off to 1,000 K

In 1995, the first brown dwarf, Teide 1 of the Pleiades cluster (M8 star), was discovered by the Spanish Observatory of Roque de los Muchachos and verified. Most brown dwarfs belong to spectral types L and T, which contain cooler stars than spectral type M. So far, more than 1,000 brown dwarfs have been discovered.

What are Neutron Stars?

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

Pulsar

Pulsars: In 1967, Jocelyn Bell discovered that a radio source emitted regular “pulses” of radio waves every 1,337 seconds, like clockwork. A pulsar is a rotating neutron star (e.g. Crab Pulsar). Its strong magnetic field generates radio emission. Since a beam of radio waves “sweeps” past us as the star rotates, the neutron star appears to change in luminosity.

X-Ray Binary

X- Ray Binaries: In a binary star system, a neutron star can accrete mass spilling off of a companion star. There may also be black holes in x-ray binaries (e.g. GS2000+25)

What are White Dwarfs?

White Dwarf

White dwarfs are the bare cores of low-mass stars such as the Sun. A low-mass Main Sequence star becomes a white dwarf when the star uses up all its hydrogen, swells, and ejects its outer layers.

If a white dwarf has a binary companion…

  • Mass-Transferring Binary Star System (e.g. white dwarf and red giant)
  • Gas “spills over” from red giant’s atmosphere and is gravitationally pulled into the white dwarf
  • Nova– explosion powered by fusion of hydrogen to helium on the surface of a white dwarf star; caused by matter spilling onto the star from its binary companion
    • Star brightens rapidly, then fades over weeks or months
    • Nova explosions can recur in the same binary system

Maximum Mass

  • S. Chandrasekhar(1930): calculated the maximum mass of white dwarf
    • Electron degeneracy pressure can only support a white dwarf less than 1.4 M☉
    • If a white dwarf accreted enough mass that overcomes this limit, gravity would win and something dramatic would happen
    • Chandrasekhar’s Limit: a white dwarf’s mass can only be less than 1.4M☉

After the Type Ia Supernova, the white dwarf is completely destroyed; no solid remnant is left, although the companion star might remain.

Type Ia Supernovae

  • Brightens over 2 weeks, reaches a peak, and then fades
  • At its peak, the supernova is 10 times more luminous than the Sun (e.g. 1994 D in galaxy NGC 4526)
  • Composed of mostly iron and other heavy metals
  • Core collapse supernova produces carbon, oxygen, neon, magnesium, silicon, and other lighter elements, and iron and other heavy elements
  • The ejected material is “recycled” into new generations of stars and planets

*Note: Without supernova explosions, earth-like planets, organic chemistry, and life wouldn’t exist.

  • All heavy elements were created inside stars or during supernova explosions, and then expelled into interstellar space
  • Heavier elements in supernova form by neutron capture: in an dense environment of  free neutrons, atoms absorb neutrons, beta (β) decays, and a proton forms — when an atom gains a proton, its identity changes and it moves one atomic number on the periodic table
  • Carbon, nitrogen, and oxygen are winners in the burning (release tons of energy)
  • Lithium, beryllium, boron are destroyed and not created in stars
  • Iron, the most stable element, is the end of nuclear burning
  • More massive elements formed by neutron capture followed by β-decay

Evolution/ Death 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