Supernova - Overview

Contents

A supernova is an exploding star that can become billions of times as bright as the sun before gradually fading from view. At its maximum brightness, the exploded star may outshine an entire galaxy. The explosion throws a large cloud of dust and gas into space. The mass of the expelled material may exceed 10 times the mass of the sun.

Types of Supernovae

Astronomers recognize two types of supernovae -- Type I and Type II.

1. Type I supernovae probably occur in certain binary stars. A binary star is a pair of stars that are close together and orbit about each other. A Type I probably occurs in binaries in which one of the stars is a small, dense star called a white dwarf. If the two stars are close enough to each other, the gravitational pull of the white dwarf draws mass from the larger companion. When the white dwarf reaches a mass about 1.4 times that of the sun, it collapses and then explodes.
    
2. A Type II supernova results from the death of a single star much more massive than the sun. When such a star begins to burn out, its core quickly collapses. Tremendous energy is suddenly released in the form of neutrinos (a type of subatomic particle) and electromagnetic radiation (electric and magnetic energy). This energy causes the star to erupt into a supernova.
    

Most supernovae reach maximum brightness a few days after they occur and shine intensely for several weeks. Some fade within months. Others fade over a period of years. Supernovae also differ in the amount and composition of the material that they expel.

Supernovae can also leave behind different types of objects. After some supernova explosions, there remains a small, dense star composed mainly of neutrons or perhaps of elementary particles called quarks. Such a star is called a neutron star. Rapidly rotating, highly magnetized neutron stars are called pulsars. After other explosions, an invisible object called a black hole may be left behind. A black hole has such powerful gravitational force that not even light can escape it. In some cases, no object of any kind remains after a supernova explosion.

Scientists believe that supernovae created all the heavier elements, such as iron, gold, and uranium, that are found on earth and have been detected in objects outside the solar system. Also, there is evidence that some high-energy cosmic rays originate in supernovae.

In 1054, Chinese astronomers recorded a supernova so bright that it was visible during the day. The explosion left behind a pulsar and a huge cloud of gas and dust known as the Crab Nebula, which still can be seen today.

In 1987, a Type II supernova became visible in the Large Magellanic Cloud, the galaxy closest to the Milky Way. It was the first supernova to be visible to the naked eye in almost 400 years. It could be viewed only from the Southern Hemisphere. ^[1]

Mug Shots of Supernovas Reveal Two Key Findings

Scientists using NASA's Swift satellite have observed two dozen recent star explosions, called supernovae, quickly after the event and have discovered never-before-seen properties, some of which run counter to prevailing theories.

In one observation, they have confirmed the origin of Type Ia supernovae, an important class of explosions used to measure distances and dark energy. In other observations they have found new mechanisms to produce X-rays and ultraviolet light.

The findings have emerged from a concerted effort by the Swift team to capture images of ordinary supernovae as rapidly as possible. Stefan Immler of NASA's Goddard Space Flight Center, Greenbelt, Md., led the analysis of these supernovae.

"For many supernovae, we are getting to the scene of the crime to investigate the explosion within hours to days, as opposed to the typical delay time of days to weeks," said Immler. "We are finding clues about how stars explode that would have disappeared had we turned our telescopes to the site just a few days later."

Swift's primary goal to study gamma-ray bursts, the most energetic explosions in the universe. Most gamma-ray bursts last no longer than about ten seconds, so speed is essential to detect and study the bursts. Swift has three telescopes: A gamma-ray telescope to detect the burst, and X-ray and ultraviolet/optical telescopes to provide rapid follow-up observations, all the while broadcasting the burst location to other observatories.

"These same properties---speed, agility and multi-wavelength capability---are proving to be crucial in unraveling the supernova mystery as well," said Swift Principal Investigator Neil Gehrels of Goddard.

There are several types of supernovae, that differ in their origins and light characteristics. In our galaxy, a supernova will occur only once or twice a century. In the nearby universe, however, dozens of stars will explode, and they are close enough to study in detail with ground- and space-based telescopes. Collectively, the Swift supernova project provides a comprehensive picture of supernovae in their host galaxies, which Immler has arranged in a massive poster reminiscent of police mug shots. Two supernovae stand out.

An explosion called SN 2005ke is the first Type Ia supernova detected in X-ray wavelengths, and it is much brighter in the ultraviolet than expected. Type Ia are called "standard candles" and are used by astronomers to measure distances in the universe, because each Type Ia shines with a known luminosity. Immler's team says it has the first observational evidence to support one theory about the origin of these supernovae.

The two theories for the origins of a Type Ia are: an explosion of a white dwarf in orbit around another white dwarf or, an explosion of a white dwarf in orbit around a red giant star. The dense white dwarf can accumulate gas donated from the companion. When the dwarf reaches the critical mass of 1.4 solar masses, a thermonuclear explosion ensues.

Image left: Supernova SN 2006bp is a Type II, which is the core collapse of a massive star once it runs out of fuel. X-rays are present directly after the explosion, and then fade within days. Click image to view animation (12.8 Mb) (Source: NASA/Swift/S. Immler.)

Immler's group has found direct evidence in the X-ray and ultraviolet light of SN 2005ke that the white dwarf, now obliterated, was indeed orbiting a red giant. The scientists detected shock waves from the explosion ramming into gas from a red giant and found no evidence of a second white dwarf. This observation may help astronomers understand the birthplaces and evolution of these supernovae, so crucial to the field of cosmology and dark energy.

The second supernova is SN 2006bp, a Type II, which is the core collapse of a massive star once it runs out of fuel. Here, Immler's team observed the explosion in detail less than a day after the event, a record for any space-based telescope. The team found that X-rays are present directly after the explosion, a surprise discovery, and that these X-rays fade within days. This means they have simply been missed in previous supernova observations because X-ray observatories don't turn to the explosion typically for at least a week.

Image right: Longer gamma ray bursts, those lasting more than four seconds are caused by massive star explosions. A common supernova explosion occurs when there is no longer enough fuel to maintain the fusion process in the core of a massive star. As it begins to burn faster, it quickly depletes its hydrogen and swells into a red super-giant. The core, however, continues shrinking and, when it contains just iron, the fusion ceases to give any more energy. In less than a second, the core implodes, crushing iron atoms together while the temperature rises to over 100 billion degrees. The internal pressure, plus neutrines emitted during the formation of the neutron star, overcomes the force of gravity causing gas to shoot out from the heart of the star in an explosive shock wave. When the shock encounters the material in the star's outer layers, they are heated and propelled into space producing the brilliant display of light called a supernova." Click image to view animation (6.2 Mb) (Source: NASA.)

The X-rays speak directly about the chemical content and immediate surroundings of the exploded star, and they show the presence of hot gas heated by the explosion in the direct vicinity of the star. The Swift observation implies that the star's wind did not blow a cavity around the star before the explosion, as is commonly thought.

Swift, launched in November 2004, is a NASA mission in partnership with the Italian Space Agency and the Particle Physics and Astronomy Research Council, United Kingdom, and is managed by Goddard Space Flight Center. Penn State University personnel control science and flight operations from the Mission Operations Center. Immler, a member of the Swift Science Center, receives funding through the Universities Space Research Association. ^[2]

Image left: NASA's Swift satellite successfully launched on November 20, 2004, from the Cape Canaveral Air Force Station, Fla. The satellite pinpoints the location of distant yet fleeting explosions that appear to signal the births of black holes. Click image to view animation (5.2 Mb) (Source: NASA.)

References

1. Supernova - World Book at NASA, World Book Online Reference Center. 2005. Contributor: Kenneth Brecher, Ph.D., Professor of Astronomy and Physics, Boston University.
2. Mug Shots of Supernovas Reveal Two Key Findings - Christopher Wanjek, Goddard Space Flight Center News, Oct. 4, 2006, NASA.

Related EoC Articles

• Supernova Remnants

External Links

• Animations & Video: Supernovas & Supernova Remnants - Chandra X-ray Observatory, NASA/Harvard-Smithsonian Center for Astrophysics.
• International Supernovae Network
• Latest Supernovae - International Supernovae Network
• Supernova - Wikipedia
• Supernova Taxonomy - Marcos J. Montes, U.S. Naval Research Laboratory.
• Supernovae - Imagine the Universe, Goddard Space Flight Center, NASA.
• Supernovae, Supernova Remnant Research - U.S. Naval Research Laboratory.
  
• Supernovas & Supernova Research - Chandra X-ray Observatory, NASA/Harvard-Smithsonian Center for Astrophysics.
• Texas Supernova Research Group - Astronomy Department, University of Texas.
• X-ray Astronomy: Supernovae and their remnants - Introduction - Imagine the Universe, Goddard Space Flight Center, NASA.