Supernova Remnants

FIELDS OF STUDY: Astronomy; Observational Astronomy; Stellar Astronomy

ABSTRACT: Supernova remnants are the material that is left after a massive star has exploded. The explosion of the supernova ejects material and rips a hole in the interstellar medium, causing a shockwave. The remaining material is the supernova remnant. Different types of supernova remnants include plerions, shell-type remnants, plerionic composites, and thermal composites. Supernova remnants are responsible for creating supercharged particles and cosmic rays. Scientists study supernova remnants in order to learn more about how stars and galaxies evolve.

Supernovae and Supernova Remnants

Supernova remnants are the materials that are left after a massive star collapses in on itself and explodes. Supernovae are the most powerful events in the universe. They are so bright that when they first explode, they can briefly outshine the galaxies they reside in.

When stars are born, they are made mostly of hydrogen and some helium, which are the lightest elements in the universe. The immense heat and pressure at the star’s core fuses the hydrogen atoms together to form helium. However, after billions years of nuclear fusion, a star uses up its hydrogen “fuel” and begins to fuse helium into heavier elements, such as carbon. As a star creates these heavier elements, its core becomes hotter, causing its outer atmosphere to expand. Stars about the size of our sun cannot fuse together elements heavier than carbon and will eventually run out of stellar fuel and shrink down to become a white dwarf.

However, more massive stars continue to produce energy and heat, eventually fusing its fuel into iron. At this point, a star cannot continue the fusion process. Gravity takes over and its core collapses, causing a massive explosion that blows the star apart.

When a star explodes into a supernova, material is ejected from the star’s center, ripping a hole through the interstellar medium that surrounds it. The explosion creates an expanding shock wave that forms a type of bubble. Inside the bubble, the material ejected from the star combines with the interstellar medium it displaces. This combined material is the supernova remnant. It can reach up to several million degrees Celsius.

Formation of Supernova Remnants

Scientists study the energy emitted from supernova remnants at different wavelengths of the electromagnetic spectrum.At first, the heat from the supernova explosion is so great that it creates thermal x-rays and radiation. As the remnants cool, they emit more ultraviolet radiation and less x-rays and synchrotron radiation. The remnants will continue to cool and disperse into the surrounding interstellar medium for up to about ten thousand years after the initial explosion.

Supernova remnants are classified into three main groups: shell-type remnants, crab-type remnants, and composite remnants. Shell-type remnants are surrounded by ring-like shells of material disturbed by the shock wave. They emit most of their radiation from these shells. Crab-type remnants, also called plerions, have pulsars at their centers. They emit most of their radiation from inside their expanding shock waves. Crab-type remnants get their name from a supernova that was observed by Chinese astronomers in the year 1054. Later astronomers saw the glowing remnants of gas and stellar material and thought it look like a crab, so they named it the Crab Nebula. Composite remnants have characteristics of both shell-type and crab-type remnants, depending on the wavelength at which they are observed. Thermal composites appear shell-like at radio wavelengths but crab-like at x-ray wavelengths. Plerionic composites appear crab-like at both radio and x-ray wavelengths, but they have a shell typical of shell-type remnants.

Discoveries from Supernova Remnants

In the 2010s, scientists discovered that cosmic rays are produced by supernova remnants. For nearly a century, scientists had tried to determine the source of cosmic rays. This was difficult because magnetic fields throughout the solar system cause the particles that make up cosmic rays to change course as they travel.

Even though scientists could not be sure, they had assumed that supernova remnants cause particles to accelerate and create cosmic rays. Eventually, they learned that the magnetic fields in the remnants cause these particles to be bounced back and forth. Sometimes the particles are pushed through the shock waves surrounding the remnants, which energizes them. They escape the remnants and travel through the universe as cosmic rays. Some of these particles eventually reach Earth.

Studying Supernovae and Supernova Remnants

Studying supernovae and supernova remnants is important for a number of reasons. One reason is that scientists believe supernovae seed the universe with many of the elements that help create planets and make life possible, including carbon, oxygen, nitrogen, and iron. Supernovae are responsible for creating all elements in the universe that are heavier than lead. Therefore, metals such as copper, gold, and iron all exist because of supernovae.

Scientists are also interested in studying supernova remnants because these objects give scientists more information about the Milky Way and the evolution of galaxies in general. Supernovae play an important role in the galaxy, and studying their remnants could help scientists better understand the formation of the Milky Way and its current composition.

PRINCIPAL TERMS

• cosmic rays: highly energized subatomic particles traveling at near the speed of light.
• electromagnetic spectrum: the range of all possible frequencies and wavelengths of electromagnetic radiation, including radio waves, microwaves, infrared, visible light, ultraviolet light, x-rays, and gamma rays.
• interstellar medium: gas and dust found in the spaces between stars; the matter from which new stars are formed.
• plerion: a type of supernova remnant, also called a crab-type remnant, that emits radiation from a pulsar in its center.
• plerionic composite: a type of supernova remnant that appears like a plerion at both x-ray and radio wavelengths but also exhibits the shell of a shell-type remnant.
• pulsar: a rapidly rotating neutron star that emits pulses of electromagnetic radiation at regular, extremely short intervals.
• supernova: the immensely forceful explosion produced when a massive star reaches the end of its life cycle and collapses under its own mass.
• synchrotron radiation: radiation produced when charged particles move in a curved path.
• thermal composite: a type of supernova remnant that appears shell-like at radio wavelengths but crab-like at x-ray wavelengths.

Bibliography

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