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Type-Casting Supernovas

by WKT

October 30, 2006 ::
In a landmark 1934 Paper, Fritz Zwicky and Walter Baade presented evidence for a previously unrecognized celestial phenomenon, which they called a supernova. These events, which were much more energetic than ordinary novas, clearly represented an explosive event which involved an entire star, not just its outer layers.

Crab Nebula
Composite Image of Crab Nebula
Credit: X-ray: NASA/CXC/ASU/J.Hester et al.; Optical: NASA/ESA/ASU/J.Hester & A.Loll; Infrared: NASA/JPL-Caltech/Univ. Minn./R.Gehrz
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Zwicky was a brilliant, unconventional and controversial astronomer who made many friends and enemies. As one of his colleagues said, "He always saw the Universe in his own original way; he loved the extraordinary objects it contained, and he explained them in his own fashion, sometimes wrong but never dull."

Tycho's Supernova Remnant
Chandra Image of Tycho's Supernova Remnant
Credit: NASA/CXC/Rutgers/J.Warren & J.Hughes et al.
A feud with his Caltech colleague, the great Edwin Hubble, led to Zwicky's being effectively barred from using the 100-inch telescope at Mt. Wilson to conduct a survey of galaxies, and search for more examples of supernovas. Undaunted, Zwicky acquired funds to build an 18-inch telescope on Palomar Mountain, which he used to discover dozens of supernovas.

In 1941, Rudolph Minkowski, another one of Zwicky's Caltech colleagues, showed that supernovas could be divided into two types based on their optical properties. Type II supernovas show evidence for hydrogen in the expanding debris ejected in the explosion, whereas Type I explosions do not.
Cassiopeia A
Composite Image of Cassiopeia A
Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech

Type II - Hot Young Stars

As other astronomers joined in the hunt for supernovas and the number of discovered supernovas increased exponentially, it became evident that Type II supernovas occurred in regions with lots of bright, young stars, such as the spiral arms of galaxies. They apparently do not occur in elliptical galaxies, which are dominated by old, low-mass stars. Since bright young stars are typically stars with masses greater than about 10 times the mass of the sun, this and other evidence led to the conclusion that Type II supernovas are produced by massive stars.

Cassiopeia A
G292.0+1.8
Credit: NASA/CXC/Rutgers/J.Hughes et al.
Type II supernovas are caused by the collapse of the core of a massive star to form a neutron star. This happens after an accelerating sequence of thermonuclear fusion reactions have culminated in the production of an iron stellar core. Further fusion reactions are not possible, so heat production stops, the core collapses catastrophically to form a neutron star. The formation of a neutron star releases an enormous amount of energy in the form of neutrinos and heat, which blows away all the remaining stellar matter.

Type Ib and Ic - Playing Against Type

The ejecta contains the hydrogen-rich outer layers of the massive star, accounting for the hydrogen observed in Type II supernovas. By the 1980's evidence had accumulated that, except for the absence of hydrogen in their spectra, some Type I supernovas showed many of the characteristics of Type II supernovas. These supernovas, called Type Ib and Type Ic, apparently differ from Type II because they lost their outer hydrogen envelope prior to the explosion. The hydrogen envelope could have been lost by a vigorous outflow of matter prior to the explosion, or because it was pulled away by a companion star. Both scenarios seem possible.

Type Ia - Strong, Silent Type

Type Ia supernovas, in contrast, are observed in all kinds of galaxies, and are produced by white dwarf stars, the condensed remnant of what used to be sun-like stars. A white dwarf star, a dense ball primarily composed of carbon and oxygen atoms, is intrinsically the most stable of stars, as long as its mass remains below the so-called Chandrasekhar limit of 1.4 solar masses.

If, however, accretion of matter from a companion star or the merger with another white dwarf, pushes a white dwarf star over the Chandrasekhar limit, it will collapse, heat up and explode like a thermonuclear bomb, leaving nothing behind. The expanding cloud of ejecta glows brightly for many weeks as radioactive nickel produced in the explosion decays into cobalt and then iron.

Because Type Ia supernovas all occur in a star that has a mass of about 1.4 solar masses, they produce about the same amount of light. This property makes them extremely useful as a distance indicator - if one Type Ia supernova is dimmer than another one, it must be further away by an amount that can be calculated. In recent years Type Ia supernovas have been used in this way to determine the rate of expansion of the universe. This research has led to the astounding discovery that the expansion of the universe is accelerating, possibly because the universe is filled with a mysterious substance called dark energy.

Further Reading: W. Hillebrandt, H. Janka, and E. Muller, Scientific. American. Oct 2006, p42

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