Burst of Radiation that Often Briefly Outshines an Entire Galaxy
A supernova is a stellar explosion that is more energetic than a nova. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months.
Because supernovae are relatively rare events within a galaxy, occurring about once every 50 years in the Milky Way, obtaining a good sample of supernovae to study requires regular monitoring of many galaxies.
Supernovae in other galaxies cannot be predicted with any meaningful accuracy. Normally, when they are discovered, they are already in progress.
During this short interval a supernova can radiate as much energy as the Sun is expected to emit over its entire life span.
The explosion expels much or all of a star's material at a velocity of up to 30,000 km/s (10% of the speed of light), driving a shock wave into the surrounding interstellar medium.
This shock wave sweeps up an expanding shell of gas and dust called a supernova remnant.
Nova means "new" in Latin, referring to what appears to be a very bright new star shining in the celestial sphere; the prefix "super-" distinguishes supernovae from ordinary novae, which also involve a star increasing in brightness, though to a lesser extent and through a different mechanism.
The word supernova was coined by Swiss astrophysicist and astronomer Fritz Zwicky, and was first used in print in 1926.
Several types of supernovae exist. Types I and II can be triggered in one of two ways, either turning off or suddenly turning on the production of energy through nuclear fusion.
After the core of an aging massive star ceases generating energy from nuclear fusion, it may undergo sudden gravitational collapse into a neutron star or black hole, releasing gravitational potential energy that heats and expels the star's outer layers.
Alternatively a white dwarf star may accumulate sufficient material from a stellar companion (either through accretion or via a merger) to raise its core temperature enough to ignite carbon fusion, at which point it undergoes runaway nuclear fusion, completely disrupting it.
Stellar cores whose furnaces have permanently gone out collapse when their masses exceed the Chandrasekhar limit, while accreting white dwarfs ignite as they approach this limit (roughly 1.38 times the solar mass).
White dwarfs are also subject to a different, much smaller type of thermonuclear explosion fueled by hydrogen on their surfaces called a nova. Solitary stars with a mass below approximately 9 solar masses, such as the Sun, evolve into white dwarfs without ever becoming supernovae.
Although no supernova has been observed in the Milky Way since 1604, supernovae remnants indicate on average the event occurs about once every 50 years in the Milky Way. They play a significant role in enriching the interstellar medium with higher mass elements.
Furthermore, the expanding shock waves from supernova explosions can trigger the formation of new stars.
Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies.
They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical burst lasts 20–40 seconds.
The initial burst is usually followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, micro and radio).
Most observed GRBs are believed to consist of a narrow beam of intense radiation released during a supernova event, as a rapidly rotating, high-mass star collapses to form a neutron star, quark star, or black hole.
A subclass of GRBs (the "short" bursts) appear to originate from a different process, possibly the merger of binary neutron stars.
The sources of most GRBs are billions of light years away from Earth, implying that the explosions are both extremely energetic (a typical burst releases as much energy in a few seconds as the Sun will in its entire 10-billion-year lifetime) and extremely rare (a few per galaxy per million years).
All observed GRBs have originated from outside the Milky Way galaxy, although a related class of phenomena, soft gamma repeater flares, are associated with magnetars within the Milky Way. It has been hypothesized that a gamma-ray burst in the Milky Way, pointing directly towards the Earth, could cause a mass extinction event.
GRBs were first detected in 1967 by the Vela satellites, a series of satellites designed to detect covert nuclear weapons tests. Hundreds of theoretical models were proposed to explain these bursts in the years following their discovery, such as collisions between comets and neutron stars.
Little information was available to verify these models until the 1997 detection of the first X-ray and optical afterglows and direct measurement of their redshifts using optical spectroscopy.
These discoveries, and subsequent studies of the galaxies and supernovae associated with the bursts, clarified the distance and luminosity of GRBs, definitively placing them in distant galaxies and connecting long GRBs with the deaths of massive stars.
WR 104 is a Wolf-Rayet star discovered in 1998, located 8,000 light years from Earth. It is a binary star with a class OB companion.
The stars have an orbital period of 220 days and the interaction between their stellar winds produce a spiral "pinwheel" outflow pattern over 200 astronomical units long.
The spiral is composed of dust that would normally be prevented from forming by WR 104's intense radiation were it not for the star's companion.
The region where the stellar wind from the two massive stars interacts compresses the material enough for the dust to form, and the rotation of the system causes the spiral-shaped pattern.
Some optical measurements indicate that WR 104's rotational axis is aligned within 16° of Earth. This could have potential implications to the effects of WR 104's eventual supernova, since these explosions often produce jets from their rotational poles.
It is possible that WR 104 may even produce a gamma-ray burst, though it is not possible to predict with certainty at this time. Newer spectroscopic data suggest that WR 104's rotational axis is more likely angled 30–40° from Earth.