Supernova

A supernova (pl.: supernovae) is a bright and powerful explosion of a star. It happens when a massive star ends its life or when a white dwarf starts a fast nuclear fusion. After the explosion, the star may turn into a neutron star or black hole, or it may vanish and leave behind a diffuse nebula. At its brightest, a supernova can shine as brightly as an entire galaxy before slowly fading away.

SN 1994D (bright spot on the lower left), a Type Ia supernova within its host galaxy, NGC 4526

Supernovae are rare but important. In our galaxy, they happen about once every 61 years. The last one seen was Kepler's Supernova in 1604. In 1987, SN 1987A was seen in the Large Magellanic Cloud, a small galaxy near ours.

Most supernovae happen in two ways: a white dwarf restarts nuclear fusion, or a core of a massive star collapses. These explosions send out lots of material and energy. They help make new elements and form stars. They also create cosmic rays and may make gravitational waves.

Occurrence

The first supernovae studied by astronomers were Tycho's Supernova in 1572 and Kepler's Supernova in 1604. Both happened in the Milky Way and could be seen without a telescope. Over the past 2,000 years, fewer than 10 supernovae have been visible to the naked eye.

Today, we see supernovae in other galaxies too. These explosions happen in our galaxy about once every 300 years. In 1987, SN 1987A appeared in the Large Magellanic Cloud, a small galaxy near the Milky Way. Scientists studied this event closely and measured special particles called neutrinos from it for the first time. The supernova was caused by a blue supergiant star exploding.

Etymology

The word supernova can be pluralized as supernovae or supernovas. It is sometimes shortened to SN or SNe. It comes from the Latin word nova, meaning "new." This name describes what looks like a bright new star appearing temporarily in the sky. The prefix "super-" helps set supernovae apart from ordinary novae, which are not as bright. The term supernova was first used by scientists Walter Baade and Fritz Zwicky in 1931, and it appeared in a scientific paper the next year by Knut Lundmark.

Observation history

Main article: History of supernova observation

Jades Deep Field. A team of astronomers studying JADES data identified about 80 objects (circled in green) that changed in brightness over time. Most of these objects, known as transients, are the result of exploding stars or supernovae.

Supernovae are bright explosions of stars. Because they are so bright, people have seen them for thousands of years. Only a few stars in a galaxy can become supernovae, usually very big stars or stars near a white dwarf star.

People have written about supernovae since ancient times. One of the earliest known supernovae was seen in the year 1006 AD in China, Japan, Iraq, Egypt, and Europe. Another famous one happened in 1054 AD and made what we now call the Crab Nebula. These events showed scientists that the universe changes. With telescopes, we can now find supernovae far away. These distant supernovae help us learn how fast the universe is growing. Today, both amateur and professional astronomers look for new supernovae using telescopes and special tools.

Historical novae
year	observed in	maximum apparent magnitude	certainty of the SN's identification
185	constellation of Centaurus	−6	possible SN, but may be a comet
386	constellation of Sagittarius	+1.5	uncertain whether SN or classical nova
393	constellation of Scorpius	−3	possible SN
1006	constellation of Lupus	−7.5±0.4	certain
1054	constellation of Taurus	−6	certain; remnant and pulsar known
1181	constellation of Cassiopeia	−2	likely Type Iax SN associated with the remnant Pa30
1572	constellation of Cassiopeia	−4	certain; remnant known
1604	constellation of Ophiuchus	−2	certain; remnant known

Naming convention

Multi-wavelength X-ray, infrared, and optical compilation image of Kepler's supernova remnant, SN 1604

When scientists find a supernova, they tell the International Astronomical Union. This group gives each supernova a special name. The name starts with "SN," short for SuperNova, and then the year it was found. For example, the third supernova found in 2003 is named SN 2003C.

If many supernovae are found in one year, letters are used to tell them apart. The first 26 supernovae get capital letters from A to Z. After that, small letters are used in pairs, like aa and ab. This helps each supernova have its own unique name.

Classification

Astronomers classify supernovae based on their light patterns and the elements they find. If a supernova shows hydrogen, it is called Type II. If it does not show hydrogen, it is called Type I.

Type I supernovae have groups like Type Ia, which shows a specific sign of silicon. Type Ib and Ic do not show hydrogen. Some rare Type I supernovae show strong lines of calcium and are called calcium-rich. Type II supernovae can have subgroups, such as Type IIn and Type IIb, which show helium lines. There are also unusual supernovae that do not fit into these main groups and are called peculiar.

Supernova taxonomy
Type I No hydrogen	Type Ia Presents a singly ionised silicon (Si II) line at 615.0 nm (nanometers), near peak light			Thermal runaway
	Type Ib/c Weak or no silicon absorption feature	Type Ib Shows a non-ionised helium (He I) line at 587.6 nm		Core collapse
		Type Ic Weak or no helium
Type II Shows hydrogen	Type II-P/-L/n Type II spectrum throughout	Type II-P/L No narrow lines	Type II-P Reaches a "plateau" in its light curve
			Type II-L Displays a "linear" decrease in its light curve (linear in magnitude versus time)
		Type IIn Some narrow lines
	Type IIb Spectrum changes to become like Type Ib

Current models

In the galaxy NGC 1365 a supernova (the bright dot slightly above the galactic center) rapidly brightens, then fades more slowly.

Supernovae are big explosions of stars that mark the end of a star’s life. They happen in two main ways: when a big star runs out of fuel and falls apart, or when a white dwarf star gets extra mass from a nearby star and blows up. After the explosion, the star either becomes a tiny, heavy object called a neutron star or black hole, or it breaks apart completely.

There are different kinds of supernovae, mainly grouped by what we see in their light and spectra. For example, Type Ia supernovae happen when a white dwarf explodes, while other kinds come from big stars falling apart. These explosions send heavy elements into space and can be so bright that we can see them from far away in the universe.

Core collapse scenarios by mass and metallicity
Cause of collapse	Progenitor star approximate initial mass (solar masses)	Supernova type	Remnant
Electron capture in a degenerate O+Ne+Mg core	9–10	Faint II-P	Neutron star
Iron core collapse	10–25	Faint II-P	Neutron star
	25–40 with low or solar metallicity	Normal II-P	Black hole after fallback of material onto an initial neutron star
	25–40 with very high metallicity	II-L or II-b	Neutron star
	40–90 with low metallicity	None	Black hole
	≥ 40 with near-solar metallicity	Faint Ib/c, or hypernova with gamma-ray burst (GRB)	Black hole after fallback of material onto an initial neutron star
	≥ 40 with very high metallicity	Ib/c	Neutron star
	≥ 90 with low metallicity	None, possible GRB	Black hole
Pair instability	140–250 with low metallicity	II-P, sometimes a hypernova, possible GRB	No remnant
Photodisintegration	≥ 250 with low metallicity	None (or luminous supernova?), possible GRB	Massive black hole

Physical properties of supernovae by type
Type^a	Average peak absolute magnitude^b	Approximate energy (foe)^c	Days to peak luminosity	Days from peak to 10% luminosity
Ia	−19	1	approx. 19	around 60
Ib/c (faint)	around −15	0.1	15–25	unknown
Ib	around −17	1	15–25	40–100
Ic	around −16	1	15–25	40–100
Ic (bright)	to −22	above 5	roughly 25	roughly 100
II-b	around −17	1	around 20	around 100
II-L	around −17	1	around 13	around 150
II-P (faint)	around −14	0.1	roughly 15	unknown
II-P	around −16	1	around 15	Plateau then around 50
IIn^d	around −17	1	12–30 or more	50–150
IIn (bright)	to −22	above 5	above 50	above 100

Energetics of supernovae
Supernova	Approximate total energy x10⁴⁴ joules (foe)^c	Ejected Ni (solar masses)	Neutrino energy (foe)	Kinetic energy (foe)	Electromagnetic radiation (foe)
Type Ia	1.5	0.4 – 0.8	0.1	1.3 – 1.4	~0.01
Core collapse	100	(0.01) – 1	100	1	0.001 – 0.01
Hypernova	100	~1	1–100	1–100	~0.1
Pair instability	5–100	0.5 – 50	low?	1–100	0.01 – 0.1

Fraction of core collapse supernovae types by progenitor
Type	Progenitor star	Fraction
Ib	WC Wolf–Rayet or helium star	9.0%
Ic	WO Wolf–Rayet	17.0%
II-P	Supergiant	55.5%
II-L	Supergiant with a depleted hydrogen shell	3.0%
IIn	Supergiant in a dense cloud of expelled material (such as LBV)	2.4%
IIb	Supergiant with highly depleted hydrogen (stripped by companion?)	12.1%
IIpec	Blue supergiant	1.0%

External impact

Supernovae help make heavier elements that spread through space. When a star explodes, it sends out a shock wave. This can help new stars to form. These explosions also make cosmic rays, which are fast-moving particles in space.

Supernovae are important because they make many of the elements in the universe, from oxygen to metals. They scatter these elements into space. They can become part of new stars, planets, and even living things. The elements from supernovae affect the life cycles of stars and can help planets form.

The energy from a supernova can also push on nearby clouds of gas and dust, helping new stars to form. Scientists believe that a supernova might have helped form our Solar System about 4.5 billion years ago.

Milky Way candidates

Main article: List of supernova candidates

The next supernova in the Milky Way might be bright enough to see, even if it happens far away. It could come from a red supergiant star, a huge star that will explode someday. Other big stars, like yellow hypergiants or Wolf-Rayet stars, might explode too. Another type, called a Type Ia supernova, happens when a white dwarf star explodes. These are harder to see before they explode.

Some well-known stars that might become supernovae include Betelgeuse, Antares, and Spica. Scientists are still learning which of these stars might explode next. The Milky Way has about two to twelve supernovae every hundred years, though we haven’t seen one for a long time.