Supernova

A supernova is a powerful and luminous explosion of a star.

These events mark the final evolutionary stages of certain stars. During the peak of the explosion, the optical luminosity can be comparable to an entire galaxy. This intense brightness eventually fades over several weeks or months. The original object that explodes is known as the progenitor. Depending on the type of explosion, the progenitor may collapse into a neutron star or a black hole. In other cases, the star is completely destroyed, leaving behind a diffuse nebula.

Theoretical studies indicate that most supernovae are triggered by two basic mechanisms. The first involves the sudden gravitational collapse of a massive star's core. This occurs when the core can no longer produce enough energy from nuclear fusion to counteract gravity. This process often begins when the star starts fusing iron. The second mechanism is the sudden re-ignition of nuclear fusion in a white dwarf. This can happen through the accretion of material from a binary companion star. A stellar merger can also trigger this runaway fusion. In a white dwarf explosion, the temperature rises enough to completely disrupt the star.

Scientists classify these explosions into different types based on their characteristics. One major category involves the core collapse of massive stars. Another involves white dwarfs in binary systems, often referred to as Type Ia.

These stars are not all the same. Some massive stars might undergo collapse during earlier stages of metal fusion. The resulting remnants can vary significantly. Some leave behind an isolated neutron star. Others result in the formation of a black hole. These distinct paths depend on the mass and history of the progenitor star.

Humanity has observed these stellar deaths for centuries. The first supernovae studied with astronomical methods were Tycho's Supernova in 1572 and Kepler's Supernova in 1604. Both were located in the Milky Way and were visible to the naked eye. Johannes Kepler began observing his namesake supernova on 17 October 1604. He tracked its brightness until it faded a year later. The term "supernova" was coined by Walter Baade and Fritz Zwicky in 1931. It appeared in a journal article by Knut Lundmark in 1933. The name is derived from the Latin word for "new star."

Supernovae occur at different rates depending on the galaxy. In the Milky Way, they are expected to occur on average once every 61 years. Recent studies suggest a rate of 1.6 to 4.6 times per century. In 1987, the supernova SN 1987A appeared in the Large Magellanic Cloud. This event allowed for the only measurements of astronomical neutrinos outside of the Sun.

In distant galaxies, astronomers typically see several thousand supernovae every year. Some recorded events are incredibly bright. For example, ASASSN-15lh peaked with twice the bolometric luminosity of any other known supernova.

These explosions play a vital role in the evolution of the universe. They can expel several solar masses of material at speeds reaching several percent of the speed of light. This process drives an expanding shock wave into the interstellar medium. This wave sweeps up gas and dust to form a supernova remnant.

Supernovae are a major source of elements ranging from oxygen to rubidium. These elements are distributed throughout space via the explosion. Furthermore, the expanding shock waves can actually trigger the formation of new stars. They are also a major source of cosmic rays and may produce gravitational waves.

Modern astronomy uses supernovae to understand the scale of the cosmos. During the 1960s, astronomers discovered that peak intensities could serve as "standard candles." This means they can be used as indicators of astronomical distances. Observations of distant supernovae in 2003 showed they were dimmer than expected. This finding supports the idea that the expansion of the universe is accelerating. Today, researchers use computer-controlled telescopes and neutrino detectors to hunt for these events. Systems like the Supernova Early Warning System help provide early notice of nearby explosions.