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. 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.
What is a Supernova?
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.
A supernova is the catastrophic death of a star, characterized by a massive output of energy. In the Milky Way, supernovae are relatively rare, with a few notable incidences of historical supernovae recorded as far back as 185 CE. Many previous supernovae were probably topics of conversation and concern among the people who witnessed them. Around the universe, several hundred are observed and recorded each year, providing information about the formation of the universe and the objects within it.There are two basic types of supernova, although each type is broken up into subtypes. In the case of a Type One, an instability arises in the chemical makeup of the star, leading to a thermonuclear explosion of formidable power. The core temperature of the star rises as a result of pressure and the imbalance, ultimately igniting the star in an explosion which can sometimes be visible with the naked eye from Earth.
A Type Two supernova involves the collapse of the core of a star, triggering a chemical reaction which causes the center of the star to essentially implode. The core of the star compresses into a neutron star, while the outer layers of the star are blown away into the surrounding space. A neutron star is an extremely dense star, all that remains of the compressed core of a star which has exploded in a Type Two supernova. Neutron stars have a number of unusual properties which make them highly intriguing for astronomers.
Astronomers study supernovae because they can provide valuable information about the universe. When stars explode, they initially form a cloud of plasma, creating a shock wave which leaves behind a distinct signature. The star also distributes heavy metals throughout the universe, and the large amount of energy behind a supernova can make it very easy to spot for an astronomer. By identifying and studying supernovae, astronomers can learn more about the size of the universe and the bodies in it. Supernovae created the materials which later came to become the Solar System, and a supernova will probably ultimately destroy our solar system as well.
Historically, the appearance of a supernova within the Milky Way has sparked discussion and debate. Supernovae helped early scientists to learn about the world around them, but also stimulated a general response among the populace. A supernova can last for weeks, and a close supernova would burn brighter than the sun. Many cultures feared that the appearance of a supernova signaled the end of the world, or the wrath of an angry God.
