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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.
|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
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."
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
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
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.
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