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.


The Big Bang Theory

The Big Bang Theory

About 14 billion years ago, the Universe was much smaller and hotter. In the 1960s, Robert Dicke predicted a remnant “glow” from the Big Bang. In 1965, radio astronomers Penzias and Wilson discovered that glow, named the cosmic microwave background radiation. The CBR was seen in all directions in empty space, with a black body curve (temperature ~3K). About 1 second after the Big Bang, the Universe was very hot, at ~1 billion K. At 3 minutes, protons and neutrons combine to form the nuclei of atoms. The hydrogen/ helium ratio (3:1) found today is about the same as what’s expected after the Big Bang. Atoms were “ionized” with electrons roaming free without being bound. At 300,000 years after the Big Bang, the Universe becomes transparent with a temperature of 3,000K. Light red-shifted by a factor of 1000.

Big Bang: Timeline

*Recent measurements show the Big Bang at 13.75 billion years ago. Scientists recently discovered dark energy; the Universe is not only expanding, but accelerating in expansion. So, earlier estimates of the age of the Universe at 15 billion years have been reduced to 13.75 billion years.

The Universe: Main Points

  1. Expansion of the Universe
  2. Cosmic Microwave Background
  3. Primordial Nucleosynthesis
  4. Evolution of Galaxies and Large Scale Structure Over 14 Billion Years

The Universe: Composition

  • 0.03% heavy elements
  • 0.3% neutrinos
  • 4% stars and gas
  • 25% dark matter
  • 70% dark energy

Expansion of the Universe

Expansion of the Universe

Hubble’s Law, written by Edwin Hubble in the 1920s, describes the expansion of the Universe.

Edwin Hubble & Expansion of the Universe – Timeline

1917: Vesto Slipher discovered that the spectra of galaxies were almost always red shift (moving away). Infact, most galaxies are moving away and 2 out of 15 spirals moving at over 2 million miles per hour.

1929: Edwin Hubble derived distances to these galaxies and showed that implied recessional speed, v1, is proportional to its current distance from us

  • Hubble’s Law: V – H0d, where  H0 is Hubble’s constant (71 km·s –¹/Mpc), v is velocity, and d is distance
  • The value of Hubble’s constant is how fast the Universe is expanding now; if Hubble’s constant is bigger, the Universe is expanding faster

1927: Belgian astronomer, G. Lamaitre, had a similar result, proving that the Universe is expanding

  •  Combined Einstein’s theory of relativity with the redshifts of spiral galaxies
  • Published a paper on mathematical  super structure connecting redshifts and expanding Universe of general relativity, but nobody noticed since he was only an obscure Belgian priest and mathematician
  • Universe began as a single pinpoint, a primordial soup

1998: Acceleration of the expansion of the Universe is caused by “cosmic anti-gravity” or “dark energy” (still unexplained)

Measuring Velocities of Red Shift

  • Light of a galaxy moving away from us will be “red-shifted,” or the wavelength gets longer
  • Light of a galaxy moving toward us will be “blue-shifted,” or the wavelength gets shorter
  • The faster the speed galaxies travel, the more the “red-shift”
  • Objects at the edges of galaxies tend to move faster than objects in the centers

Understanding the Expansion

  • Galaxies are all moving away from us: Does that mean we are at the center of the Universe?
    • No. There is “no” center. All points in space claim to be the center
    • e.g. Raisin bread rising: raising don’t expand, the space between them expands

Olbers’ Paradox— Why is the night sky dark?

  • In the 19th century, astronomer Wilhelm Olbers asked: If the Universe is finite, why isn’t the sky bright from starlight?
  • The solution is not that stars are increasing far away, but that the apparent brightness of a star decreases (1/d²), the area of shells of stars surrounding the Earth increases like d², so the effects cancel out
  • Another solution was that the Universe has finite size, so that not all of the light from all the stars has had time to reach us (Universe expanding); the Universe is 14 billion years old, and we only see “out” 14 billion light years distance from us

Messier: The “M” in M31

What does the “M” in M31 or M11 stand for? Messier [Me-Si-Eh]

Charles Messier

Charles Messier (1758-1772), a French astronomer, identified about 110 diffuse fuzzy objects that he named “Messier objects.” Messier then cataloged these objects in his Messier Catalog. He also discovered 13 comets; finding comets was a way to make a name astronomers of the 18th century).

Messier Catalog

Orion Nebula

M42: Orion Nebula

  • local region in the Milky Way (~1,300 light years away) with new stars
  • appears mostly red due to hydrogen gas abundance

M82: galaxy

  • ~12 million light years away
  • clouds of glowing hydrogen blown out, released by recent star formation

M31: Andromeda Galaxy


  • hundreds of nebulae (discovered 20th century)
  • with George E. Hale’s idea and Hooker’s money –> the Hooker Telescope (100-inch in diameter, 11 years to build, $100 million)

Today, the Sloan Digital Sky Map holds 15 Terabytes of data on the Universe.

Nebulae and Star Formation

Orion Nebula

Nebulae: a cloud of dust and gas that we see in light

  1. Emission Nebulae or Bright Nebulae: a glowing gas (hydrogen); e.g. Great Nebula in Orion, heated by the Trapezium
  2. Absorption Nebulae or Dark Nebulae: dark dust clouds; e.g. Horsehead Nebula
  3. Reflecting Nebulae: reflecting dust cloud; e.g. Pleiades in Taurus
  4. Planetary Nebulae: excited by central star; e.g. Dumbbell Nebula
  5. Cirrus



Stars form in Giant Molecular Clouds about 100,000 to 1 million solar masses. A few thousand in the Milky Way Galaxy, Giant Molecular Clouds break into denser bits, contract, and eventually form stars. The Orion Molecular Cloud has about 500 stars. The Trapezium and the Orion Nebula have solar masses of matter with young stars.

  1. Non-stellar galactic objects reside in HII regions with molecular clouds of pre-main sequence stars and dense clumps of dust.
  2. Protostars and newborn stars about 1/2 to 1 solar mass reside in Molecular Clouds.

Interstellar Medium – The Material Between Stars


– Interstellar Medium

Interstellar Medium

Interstellar Medium is gas and dust between stars, nebulae, and giant molecular clouds (basic building blocks of galaxies in star formation). The four types of matter in interstellar medium are: interstellar dust, interstellar atoms, interstellar molecules, and interstellar snowballs.

Interstellar Dust

  • Interstellar Reddening: dust that scatters blue light and causes stars to look redder
  • Extinction of Obscuration: high dust content that diminishes the brightness of stars, by as much as 25 magnitudes
  • Can be smaller than smoke particles
  • Consists of graphite, silicates, or ices
  • In core of heavy elements (e.g. iron, magnesium), mantle of organic compounds (oxygen, carbon, nitrogen), and outer mantle of ice


  • Radio waves = longest wavelength of electromagnetic waves
  • Brightest optical objects not necessarily the brightest radio objects
  • e.g. Taurus A (Crab Nebula) and Sagittarius A (center of the Milky Way Galaxy)
  • Radio Spectral Line: the frequency or wavelength at which radio noise is slightly more or less intense
    • Hydrogen: 21 centimeter line
    • Radio spectra lines of molecules
      • OH (hydroxide): 1963
      • H20 (water): 1968
      • NH3 (ammonia): 1968
    • Over 50 molecules in interstellar space
    • Gives information on temperature, density, and motion
    • Molecular absorption line in UV

Interstellar Molecules

  • Molecules: two or more atoms bound together (e.g. H2O, CO, CH4, OH, H2, NH3)
  • Give absorption or emission bands
  • Observable in very cold, low density interstellar environments

Interstellar Snowballs

  • Between the sizes of  grains and comets
  • Composed of water, carbon, silicates, and other molecules

Interstellar Regions

  1. HI region: 200 K
  2. HII region: 10,000 K
  3. Molecular clouds: 50% gas in our galaxy
  4. Hot interstellar medium: 1 million K, super-heated gas from expanding supernova blasts (up to 90% of total volume)
  • HI Region
    • High density of neutral hydrogen atoms about a million atoms per cubic centimeter (e.g. Orion Nebula)
    • ~ 200 K
  • HII Region
    • Hydrogen with electron removed; e.g. ionized hydrogen gas (in emission nebulae)
      • Average density of hydrogen elsewhere is 1 atom per cubic centimeter
    • ~ 10,000 K