| I | INTRODUCTION |
Supernova, violent explosion that occurs when a large
star uses up its supply of fuel, collapses under its own weight, and explodes. A
shock wave from this catastrophic event expands into space, followed by a shell
of material from the star’s atmosphere. The material blown off contains chemical
elements created throughout the star’s lifetime. Debris from supernovas enriches
the chemistry of interstellar space with material that becomes part of new stars
and planets. See also Astronomy; Interstellar Matter.
Supernovas are rare phenomena—fewer than five
supernovas in our Milky Way galaxy have been visible from Earth in the last
1,000 years. Some supernovas can be bright enough to see with the naked eye
during the day. They may continue glowing for several weeks or even months after
the explosion. Thick clouds of interstellar dust hide some supernovas, but
astronomers can detect those by the radio waves that the supernova emits. See
also Radio Astronomy.
Supernovas occur in all galaxies, not just the
Milky Way. Supernovas that occur outside the Milky Way are bright enough to
stand out against the other stars in the galaxy. However, they are usually not
bright enough to pick out without a telescope. A typical supernova can produce
as much light and other forms of electromagnetic radiation as billions of stars.
Electromagnetic radiation is energy carried through space by electric and
magnetic waves. The length of these waves determines the properties of the
radiation. In addition to the radiation energy a supernova produces, the force
of the explosion releases ten times more energy into the motion of the particles
that the explosion blows outward. These tiny particles, called neutrinos, carry
away a hundred times more energy than the electromagnetic radiation. Astronomers
discover about ten supernovas in distant galaxies each year.
| II | FORMATION OF A SUPERNOVA |
The mass of a star determines whether it will
end its life in a supernova explosion. During the courses of their lifetimes,
all stars convert hydrogen to helium in thermonuclear fusion
reactions in their cores (see Nuclear Energy: Nuclear
Fusion). Thermonuclear fusion reactions occur when the intense heat and
gravitational force in a star’s nucleus force hydrogen atoms together. The atoms
merge, or fuse together, creating helium atoms and releasing large amounts of
energy in the form of electromagnetic radiation and heat. Massive stars have
faster rates of fusion than smaller stars, so large stars may use up their fuel
faster. After most of the hydrogen is used up, a star goes into a
carbon-building phase, in which nuclear fusion turns the helium into carbon.
After the helium is exhausted, most stars gradually cool until they no longer
emit radiation.
When a star eight or ten times more massive
than the Sun exhausts its helium, however, the nuclear burning cycle is far from
complete. In these stars, the carbon core shrinks under its own weight, and its
temperature rises high enough to fuse carbon into oxygen, neon, silicon, sulfur,
and finally, iron.
Iron is the most stable element formed in
stars, and even the intense heat and pressure of a stellar nucleus cannot force
iron atoms to fuse into heavier elements. The thermonuclear process at the
star’s core is essentially complete. At this point, the outward pressure
produced by the reactions can no longer balance the inward gravitational
attraction between atoms. As a result, all the core can do is collapse under its
own weight. As it does so, the star implodes, transforming gravitational energy
into kinetic energy, or energy of motion. The core of the star collapses in on
itself, but as it does so, it transfers to the star’s atmosphere kinetic energy
that sends the atmosphere exploding outward from the star’s core. The particles
of the star’s atmosphere begin moving rapidly away from the star, tearing apart
the star’s atmosphere.
Astronomers know of several variations of
supernovas, but they all fall into one of two main types. The two kinds of
supernovas are called Type I and Type II and are differentiated mostly by the
presence of hydrogen in their debris. Type I supernovas tend to be older stars
that have completely exhausted their hydrogen. Type II supernovas come from
younger stars that have used up the hydrogen in their nucleus but have large
amounts of hydrogen in their atmospheres. Astronomers can measure what elements
exist in a star by examining its light because atoms of different elements emit
and absorb electromagnetic radiation at different wavelengths. By separating a
star’s light into its wavelengths, astronomers can tell which wavelengths are
missing or especially bright, and therefore what elements are present in the
star (see Spectroscopy).
| III | AFTER THE SUPERNOVA |
All supernova explosions produce clouds of
debris and release huge amounts of energy, but Type I supernovas typically
completely destroy their parent stars, while Type II explosions usually leave
the stellar core behind. The stellar atmosphere of both types expands into space
and appears as luminous clouds years, or even centuries, later. These clouds are
called supernova remnants. The Crab Nebula is one of the most spectacular
supernova remnants.
The fate of the stellar core left behind by
a Type II supernova depends on the mass of the original star. Normal atoms are
made up of positively charged particles called protons, particles with no
electric charge called neutrons, and much smaller, negatively charged particles
called electrons. If the original star had a mass about ten times that of the
Sun, the core collapses with such force that its protons and electrons combine
to form neutrons. The resulting body is composed entirely of neutrons, so
astronomers call it a neutron star. Most neutron stars created by supernovas are
pulsars. Pulsars are neutron stars that spin rapidly as they emit powerful
beacons of radio waves. From Earth, these spinning beacons appear as pulses of
radiation.
If the mass of the original star is greater
than about ten solar masses, the nuclear forces that hold up a neutron star are
too weak to resist the core’s gravitational pull. The core continues to collapse
past the neutron star stage. It crushes itself until its mass is concentrated
into a volume of space smaller than a typical city on Earth. At this point, the
speed at which an object would have to travel to escape the core’s gravitation,
like a space probe leaving Earth, is greater than the speed of light. No kind of
matter, or even radiation, can reach this speed and escape, so these
astronomical objects are invisible to the eye or to normal telescopes. Not only
can matter not escape, but the collapsed core pulls in any matter or radiation
that comes too close. Astronomers call this kind of an object a black hole
because no light can escape its gravitational pull.
| IV | STUDYING SUPERNOVAS |
Chinese astronomers recorded supernovas
visible from Earth as far back as ad 185. Probably the most well-known
ancient supernova is the one that created the Crab Nebula in 1054. From Chinese
and Japanese records, astronomers estimate that it was about 20 times as bright
as any other star in the night sky. It was visible even during the day for
several weeks after it first appeared.
The last time a supernova in the Milky Way
galaxy became visible from Earth was October 1604. It was bright enough to be
seen at night with the naked eye for more than a year. German mathematician
Johannes Kepler made detailed observations of the supernova and carefully
measured its position. Since then, astronomers have not seen any supernovas in
the Milky Way. A number of supernovas have appeared in other galaxies,
however.
One of the most important supernovas of the
20th century, and the brightest in the sky of the northern hemisphere since
1937, burst into view on March 28, 1993, in the galaxy M 81. Astronomers noticed
the strange behavior of the parent star—a huge red-colored star called a red
giant—before it exploded and were able to track its changes as it became a
supernova.
On February 24, 1987, one of the closest
supernovas in centuries occurred in the Large Magellanic Cloud (see
Magellanic Clouds), only 160,000 light-years from Earth. A light-year is a
measure of distance equal to the distance that light travels in a year, or 9.5
trillion km (5.9 trillion mi). This supernova was visible from the southern
hemisphere. Since this eruption, scientists have learned that its parent star
may have once been a hot blue star with a mass about 20 times that of the Sun.
The star probably swelled into a red giant star before it exploded.
Scientists are continually searching for and
studying supernovas. Astronomers learn about the final evolutionary paths of
massive stars from supernovas. In addition, supernovas give clues to the origin
of the chemical elements that make up stars, planets, and even life. A supernova
in a distant galaxy can even help astronomers measure the distance to the
galaxy. To do this, astronomers examine the radiation emitted by the shell of
material from the star’s atmosphere and use the information they gain to develop
models of how wide the shell is. They then compare the width of their model to
the apparent width of the shell as viewed from Earth to estimate the distance to
the supernova remnant and to its parent galaxy.
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