Anti-Matter vs. Dark Matter

The Collison Annihlation of Matter and Anti-matter

The Collison Annihlation of Matter and Anti-matter

What is the difference between anti-matter and dark matter? Is there anything anti-matter and dark matter have in common?

Anti-matter is the idea of negative matter, or matter with the same mass but opposite an charge and quantum spin than that of normal matter. Anti-matter is just like normal matter with different properties. The antimatter of the electron (e-)  is the positron (e+); similarly, the antimatter of the proton is the anti-proton (p-). When normal matter and anti-matter collide, the two annihilate each other. Scientists speculate that anti-matter and matter existed in equal quantities in the early Universe.  The apparent asymmetry of high quantities of matter and very low quantities of anti-matter is a great unsolved problem in physics. Anti-matter is only found through radioactive decay, lightning, and cosmic rays (high-energy particles from supernovae) and very expensive to produce. Practical uses of anti-matter include the positron emission tomography (PET) used for medical imaging and as triggers to nuclear weapons.

Dark matter cannot be seen and is hard to detect, because dark matter interacts by gravity and weak atomic force, not with strong atomic forces (nuclear force: holds subatomic particles, electrons, neutrons, and protons, together in an atom) or electromagnetism. Dark matter constitutes about 22.7% of the Universe. On April 3, 2013, the International Space Station’s Alpha Magnetic Spectrometer (AMS) found the first evidence of dark matter. [AMS was carried out by the Endeavor in 2011 in one of NASA's last space shuttle flights.] Normally, detectors are blocked by Earth’s atmosphere, but by orbiting Earth above its atmosphere,  the AMS can monitor cosmos rays (have an excess of anti-matter, discovered two decades ago) without hindrance. The AMS will tell scientists whether the abundance of positrons signal the presence of dark matter.  One theory scientists are testing is supersymmetry, which speculates that the collision and annihilation of two dark matter particles could produce positrons. Another instrument that could help the dark matter hunt is the Large Underground Xenon Experiment (LUX).

References

Anderson, Natali. “Antimatter Hunter aboard International Space Station Detects Hints of Dark Matter.” Sci-News.com. Sci-News.com, 4 Apr 2013. Web. 4 Apr 2013.

Boyle, Alan. “Space station’s antimatter detector finds its first evidence of dark matter.” NBCnews.com. NBC News, 3 Apr 2013. Web. 4 Apr 2013.

Supernovae: Dying Stars

Star Death

Lifetime of a Star

It is true that all living things come from stardust. In about 5 billion years, our Sun will have swelled to a red giant and engulfed the inner planets, ready to explode in a supernova. Supernovae enrich the interstellar medium with high mass elements, like iron and calcium. The high energy from supernovae also triggers formation of new stars. On average, supernovae occur only about once every 50 years in the Milky Way Galaxy. They are rare events— so rare that the last one in the Milky Way was discovered in 1604 (SN 1604, or Kepler’s Supernova)— spectacularly luminous and extremely destructive. In fact, supernovae can cause bursts of radiation more luminous than entire galaxies and emit as much energy as the Sun will in its entire lifespan! In a supernova, most of the star’s material is expelled into space at speeds up to 30,000 m/s. The shock wave passes through the supernova remnant, a huge expanding shell of gas and dust. Supernova are caused either by the sudden gravitational collapse of a supergiant star (Type I Supernova) or a white dwarf accreting enough mass or merging with a binary companion to undergo nuclear fusion (Type II Supernova). White dwarfs are very dense stars that do not have enough mass to become a neutron star (formed from supernova remnant, stars comprising almost entirely of neutrons). Supernovae can be used as standard candles (objects with known luminosity). For instance, the dimming luminosity of distant supernovae supports the theory that the expansion of the universe is accelerating. Now, with powerful telescopes like Hubble, many supernovae are discovered each year. How perfectly supernovae represent the circle of life: from death comes life!

History of Supernova Observations (Milky Way)

  • SN185 by Chinese astronomers
  • SN1006 by Chinese and Islamic astronomers
  • SN1054 (caused Crab Nebula)
  • SN1572 by Tycho Brahe in Cassiopeia
  • SN1604 by Johannes Kepler

* Supernova (SN) are named by the year they are discovered; if more than one in one year, the name is followed by a capital letter (A, B, C, etc.), and if more than 26, lowercase paired letters (aa, ab, etc.) are used

Below is a video on supernovae! Enjoy.

Black Holes – Formation

Black Hole

Black holes form after supermassive stars (> 10 solar masses) explode as Type II supernovae. The remaining cores of these stars range from 2- 103 solar masses. Nothing is strong enough to hold the remaining mass against the force of gravity and the dying supermassive stars collapse into black holes. Nothing escapes from black holes, not even light; thus, the escape velocity of black holes is the speed of light. Since nothing can travel faster than the speed of light, nothing can escape. Matter that disappears from black holes loses contact with the rest of the Universe. Black holes are a consequence of Einstein’s theory of gravity, or General Relativity.

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