Supernovas

• Type of physical science: Astronomy; Astrophysics
• Field of study: Stars

Supernovas, or exploding stars, are spectacular events emitting light at rates up to hundreds of millions times greater than the sun. They have been recorded throughout history and are an important mechanism in the synthesis of new elements as well as triggering new star formation.

Overview

A supernova, an explosion of a star, is one of nature's most spectacular events. In the first ten seconds, as much energy radiates from a tiny core 30 kilometers across as from all of the other stars and galaxies in the entire visible universe. The energy of this short burst is one hundred times more than the sun will radiate during its 10-billion-year lifetime. The dramatic supernova of 1987 (Supernova 1987A) discovered in the large Magellanic Cloud, although one of the fainter supernovas, emitted light at 100 million times the rate of the sun. Supernovas of the more massive stars may emit light at up to 600 million times that of the sun.

Supernovas are very rare. The last one observed in Earth's galaxy was Johannes Kepler's in AD 1604. According to estimates, up to five supernovas may occur in the Milky Way galaxy each century but most escape detection because of obscuration by interstellar dust in the galactic plane. In other galaxies, the rate varies from a few each century in the largest spirals to one every few centuries in the smallest spirals.

Supernovas in the night skies were noticed in ancient times. Although these civilizations had little scientific knowledge of their observations, the records are still an important resource for modern astronomy. Supernovas are dying stars, and observations on them provide estimates of the death rates among stars. Data on the time at which the starlight reached Earth may be provided in the historical records, along with perhaps information on the brightness, color, and length of time visible.

If a bright stationary star was observed for several months, it was more likely a supernova rather than a less spectacular and more common nova. Chinese records have provided reliable reports of various celestial events. There were particularly noteworthy events called "guest stars" sighted in the years 185, 386, 393, 1006, 1054, 1181, 1572, and 1604. The supernova of 1006 was perhaps the brightest to have appeared in the past thousand years.

Because of its position in the sky far south and below the horizon from northern Europe, the best observations came from Arabic, Chinese, and Japanese sources. This supernova was described as round in shape, two and one-half to three times the size of the planet Venus, twinkling considerably, and illuminating the horizon with a brightness greater than one quarter of the Moon.

The guest star that was sighted in June 1054 was described as bright as the planet Jupiter, visible in daylight for three weeks and in the night sky for two years. Interestingly, this bright object, although surely visible, went unrecorded in European annals. The split in Christianity between the Roman Catholic church and the Greek Orthodox church in the east may have caused Church officials to have deleted such a "portent event" in the heavens from European historical records. The event was noticed in the Middle East and in North America by American Indians. The sighting is recorded as a petroglyph incised on sandstone at Navajo Canyon, Arizona. The date has been estimated from association with Pueblo Indian dwellings and pottery by archaeological methods.

Tycho Brahe, a Danish astronomer, won fame for his observations of the supernova of 1572. The supernova is said to have inspired him to devote a lifetime to collecting measurements of the positions of the stars and planets. Based on careful observations and measurements over an eighteen-month period, Brahe concluded that the phenomenon was not a kind of comet or meteor but a star that had never been observed before.

Kepler, who was formerly Brahe's assistant, sighted a supernova on October 17, 1604.

He described the star as similar to Jupiter in brightness, with the color of a diamond. Visibility continued through October 1605, although the star briefly disappeared behind the sun from November 1604 until January 1605. Sightings were also recorded in Chinese and Korean records; comparing their observations to those of the Europeans allowed scientists to determine the accuracy of the records.

As an aftermath of the star's explosion, hot gases expand outward with speeds approaching that of light, forming a cloud of luminous material called a supernova remnant.

Within the past forty years, astronomers have identified the remnants of almost all of the supernovas known from history and more than a hundred other remnants. The Crab nebula in the constellation Taurus was studied extensively since the 1920s, when photographs taken eleven years apart showed outward expansion of gaseous material. Knowing the expansion rate of the gases from Doppler shift studies, Edwin Powell Hubble, in 1928, concluded that to reach its present size would take nine hundred years. The prediction agreed with sightings of 1054 and established the Crab nebula as the first confirmed supernova remnant.

The remnant of the brilliant 1006 supernova was first detected as a radio source in the early 1970s, but it was not photographed until 1976. Kepler's supernova of 1604 was first photographed in 1947 by Walter Baade after determining its location from Kepler's position. The position of the remnant of Brahe's supernova was first confirmed as a strong radio source in 1952 and photographed shortly after with the George Ellery Hale 5-meter telescope.

Stars obtain their energy from hydrogen fusion for the greater portion of their life spans. The sun will continue to use its supply of hydrogen for about 5 billion more years, but stars more massive will burn their supplies more rapidly. A star equal to 20 solar masses will shine with a brightness ten thousand times that of the sun but will exhaust its fuel in only a few million years. Whatever its mass, a star will eventually run out of hydrogen.

The end of the first stage in the star's life cycle is reached when most of the hydrogen in the core has been fused into helium. The star was stable, generating energy with the pressure of hot gases outward and compensated for by the pull of gravity inward. When the rate of energy production slows down, outward radiation diminishes, and gravity compresses the core more compactly. Temperatures rise through the star, igniting unburned hydrogen in regions surrounding the core. The outer layers of the star expand to immense proportions, while the surface cools; the star now becomes a red giant. The core continues to contract until the temperatures reach 100 million Kelvins; then helium atoms fuse to form carbon atoms, releasing greater energy. The star becomes differentiated, with the heavier elements located in the centermost zones and the lighter elements fusing in shells farther out. Carbon fusion releases still more energy and fusion produces oxygen nuclei. The fusion may progress, creating the heavier elements, neon, magnesium, silicon, and iron in a stepwise process known as nucleosynthesis.

Most stars are not massive enough to reach the stage of carbon fusion and end up with a core of carbon, becoming white dwarf stars. The more massive stars can fuse heavier elements, but further fusion in any case cannot proceed past the isotope of iron, which has an atomic weight of fifty-six. If additional particles were added onto this iron nucleus, energy would be removed from the surrounding shells rather than released from the core as in the previous fusion process. A massive star that ends up with an iron core is surrounded by shells of progressively lighter elements in onionlike layers.

At this stage in its life cycle, the massive star can no longer provide sufficient outward radiation pressure to balance the crushing pull of gravity inward, because the iron core is no longer releasing energy. The next surrounding layer of silicon, however, continues to burn, adding more iron onto the core. If the mass of the core reaches a critical value called the Chandrasekhar limit, collapse of the star begins in a matter of minutes. The compression by the collapse increases the core temperature; however, the iron cannot fuse at the higher temperatures but instead starts to disintegrate. The process removes energy from the core and in turn lowers the pressure of the surrounding gas, allowing additional compression. Electrons are forced tightly into the nucleus, combining with protons, and release tiny particles called neutrinos. The neutrinos remove energy from the center of the star very quickly, accelerating the collapse process and permitting gravitational contraction to incredible densities, reaching that of an atomic nucleus. A shock wave is formed, but it takes several hours to reach the surface. Great numbers of neutrinos remove enormous energy from the exploding star and generally continue their journey outward uninterrupted.

There are two possible scenarios for the next stage of the star, depending upon its mass.

For stars of 8 to 10 solar masses, the explosion leads to the formation of a neutron star, which is merely the remnant of the collapsed core compressed to the density of an atomic nucleus, or 100 million times that of water. These stars are very tiny and hot, with core temperatures exceeding 1 billion Kelvins. Neutron stars rotate rapidly because of conservation of their original angular momentum, and their magnetic fields are strongly concentrated into two beams at the poles.

Neutron stars that blink on and off rapidly when this beam of radiation is observed are called pulsars. Very massive stars equal to 30 solar masses or more are so massive that the gravitational forces overcome the core strength of even a neutron star (known as neutron degeneracy), and the star collapses to a singularity point known as a black hole.

In late 2017, scientists revealed a discovery proving that there is still more to learn about supernovas and the way in which stars die. A student at University of California, Santa Barbara, after studying data, found in 2015 that a supernova that had been located near the Ursa Major constellation in 2014 and had seemed at the time to be dimming was not behaving in the same way as other supernovas. While experts had expected that the supernova would continue to get dimmer over time before fading to nothing, it had actually been brightening again over recent months. Ultimately, after further observation, it was reported that the supernova maintained this brightness for over six hundred days. At that point, scientists were still trying to understand the strange phenomenon.

In the 2020s, scientists continued to use the the Spitzer Space Telescope, the Hubble Space Telescope, and the Chandra X-ray Observatory to study the remains of the Kelper supernova.

Applications

Telescopic surveys of galaxies have expanded the number of supernovas discovered from 20 in 1934 to more than 650 by 1978. The number of discoveries increased dramatically after 1950, with the development of the Schmidt telescopic camera used for surveying large areas of the sky. Detailed analysis on smaller regions was performed with telescope apertures as large as 5 meters.

High-altitude observatories were able to study radiation emitted by Supernova 1987A that would otherwise be absorbed by the earth's atmosphere. Information on the size of the original star was obtained from the ultraviolet flash recorded by the International Ultraviolet Explorer satellite. Observations followed for several months, revealing emissions from a shell of gas surrounding the supernova. Radiation characteristics led to the conclusion that the initial light emitted from the supernova was from material at a temperature of a half million Kelvins.

The high temperature, combined with the strong ultraviolet radiation, is expected whenever a powerful shockwave breaks through the surface of the exploding star, ejecting gaseous material.

Measurements of the expansion velocity led to the determination of the total energy of the explosion.

Several months after the initial outburst of Supernova 1987A, an infrared telescope flown at 12,000 meters on a jet transport revealed many elements from the supernova core, including iron, nickel, cobalt, oxygen, neon, sodium, potassium, silicon, and magnesium. The intensity of the infrared lines proved that the quantities of these elements was larger than what could have been present initially in the star at birth.

Neutrino observation has been used in the analysis of the explosion of Supernova 1987A, as it has given astrophysicists the precise time of the explosion. Neutrinos, once emitted from the shocked core, travel virtually untouched outward through the remainder of the star.

Electromagnetic radiation or light, however, is absorbed and reradiated by matter while on its way to the star's surface, emerging several hours later with the shock wave. As a result, neutrinos released from the star arrived at Earth before the initial light burst after traveling for 160,000 years. By measuring the time difference from the detection of the neutrinos to the first observed light from the exploding star, it was concluded that the original star was relatively small, placing it in the blue rather than the red supergiant class. The neutrino detectors are typically large tanks of water placed in deep underground mines as a shield from cosmic background radiation and other undesired events. The tanks are surrounded by electronic amplifiers called photomultipliers that detect rare collisions of the neutrinos with electrons and positrons (which result from direct neutrino-proton collisions).

The light emitted by Supernova 1987A followed an unusual light variation called a light curve. Supernovas are categorized into either Type I or Type II based on their light curves.

Type I has a rapid brightening, followed by a quick decline of more than 3 magnitudes from maximum brightness in one hundred days, followed by a more gradual decline of 7 magnitudes after three hundred days. Type I supernovas evolve from less massive stars and are observed in both spiral and elliptical galaxies. Type II supernovas have a rapid decline of more than 4 magnitudes in the first one hundred days, followed by a more gradual decline of 7 magnitudes.

Type II supernovas evolve from the more massive stars and are observed only in spirals and irregular galaxies.

The light curve of Supernova 1987A was expected to follow that of the Type II Supernova but was observed approximately 2 magnitudes dimmer at maximum brightness. Since blue supergiant stars are smaller than Type II precursors, the explosion had to transport the gases farther to remove them from the surface of the star. Because of the greater amount of energy needed to break up the star, there was less energy to appear as visible light, making the supernova appear dimmer.

The triggering shock wave is thought to originate not at the center of the star but at the Chandrasekhar mass limit, or about halfway out of the iron core; inside this distance, only the pressure wave propagates. Computer-generated models on the propagation of the shock wave indicate a more complicated mechanism than previously believed. All the energy of the shock wave may be used up initially in dissociating nuclei, such as iron, into component nucleons.

Perhaps the shock wave may be revived because of the absorption of large amounts of neutrinos.

An additional process of convective transport may be involved in transferring the energy liberated by neutrino absorption to the shock wave.

Context

Supernovas, or massive explosions of stars, are very rare events; only one has appeared visible to the eye in the Milky Way since 1604. By studying supernovas and the synthesis of elements that they produce in their unimaginably high temperatures and pressures, astrophysicsts can model the final stages in the life cycle of a star. Theoretical models, for example, indicate that stars such as the sun do not end in a violent explosion but pass through a more quiescent phase in late stages. These stars can expect to end up as tiny white dwarfs, which are merely the burned out remnants of a carbon core.

Stars of greater size may have sufficient mass to exert greater gravitational force on the core and compress it to higher temperatures and progressive nucleosynthesis. The cores of these stars become so dense as to end up with pure neutron cores of incredible densities that rebound with shock waves that blow them apart. Stars of even greater solar mass literally collapse upon themselves to form singularity points called black holes.

Supernovas are significant in the universe for a number of reasons. Interstellar clouds, the birthgrounds of new stars, may get their thrust for gravitational contraction and star formation by the triggering effect of a nearby supernova explosion, sending a shock wave into the region.

Extraterrestrial events such as supernovas may be responsible for major extinctions of life-forms on earth. Such an event would also set the stage for the development and radiation of new life-forms. Gene changes or mutations may result from excess radiation striking Earth from a nearby supernova. It is the process of mutation that results from the imperfection of the replication process that moves evolution forward with the development of advanced species. If it were not for mutations, then all organisms today would be merely replicas of the initial simple forms of life.

Since supernovas are so rare, it is not known when or where the next one is likely to occur or what that star will look like. The star would be in its last stages before collapse and would be a red giant star. Concentrating on nearby red giants, there are several possible candidates, including Ras Algethi in the constellation Hercules, Antares in constellation Scorpius, and Betelgeuse in constellation Orion. Betelgeuse is unstable and pulsates, which results in a stellar wind. Another possibility is the star in the Eta Carina nebula, which has a stronger stellar wind than Betelgeuse and is losing significant mass.

Overall, supernovas are important in providing an enrichment of heavy element supply to instellar space; the formation of the sun and Earth would not have been possible without the explosion and death of other stars.

Principal terms

ANGULAR MOMENTUM: momentum caused by the rapid spinning of a star about its axis of rotation

CHANDRASEKHAR LIMIT: the limiting size of a white dwarf star that could support its own weight, which is about 1.4 times as massive as the sun

GRAVITATIONAL CONTRACTION: the inward pull on a star because of its mass

GUEST STAR: another term for a nova or new star

ISOTOPE: a variety of an element that has a different mass number because of variations in the number of neutrons

NEUTRINO: a tiny particle that has no apparent mass or electric charge, no effect on ordinary matter, and travels at a speed nearly that of light

NEUTRON STAR: a very dense star composed of neutrons, typically with a radius of 10 kilometers and a mass equal to or slightly greater than the sun

NUCLEOSYNTHESIS: the process leading to the formation of new elements

SHOCK WAVE: a zone of compression and heating of matter traveling faster than the speed of sound

SUPERNOVA REMNANT: gaseous material expelled by a supernova explosion

Bibliography

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