We joke that death and taxes are the only certain things in life. While they may not have to pay taxes, stars do eventually die, and some stars go out with a cosmic blast. The explosion that occurs when a star dies is known as a supernova.
Scientists believe that by studying supernovae, we can learn about our universe, its expansion and other aspects. Like a lot of things in space, supernovae are fascinating, complex and not fully understood.
What makes a star a star?
When discussing supernovae, it's important to define what a star is — it's an immense mass of hydrogen and helium gasses held together by gravity. Stars are fueled by nuclear fusion reactions. When a star burns through all of its fuel, it dies.
Which stars become supernovae?
An artist's illustration of supernova 1993J, an exploding star in the galaxy M81. The supernova originated in a double-star system in which one member was a massive star that exploded after siphoning most of its hydrogen envelope to its companion star, which is seen in the center. (Photo: NASA, ESA, and G. Bacon (STScI)/Wikimedia Commons)
Not all stars become supernovae. It turns out that a star's mass determines its fate. The line between stars that will explode and those that will not is called the Chandrasekhar Limit. According to PBS, "the Chandrasekhar Limit is now accepted to be approximately 1.4 times the mass of the sun; any white dwarf with less than this mass will stay a white dwarf forever, while a star that exceeds this mass is destined to end its life in that most violent of explosions: a supernova."
So, our own sun will not explode into a supernova but instead will become a red giant. It will swallow the Earth in this process and eventually become a white dwarf.
Only stars with much greater mass than our sun will experience the spectacular fate of supernovae. However, some stars with even greater mass might explode into supernovae before possibly becoming stellar-mass black holes. These stellar-mass black holes could, in turn, transform into the phenomenon known as supermassive black holes.
Are there different kinds of supernovae?
An artist's illustration of a Type IA supernova explosion. Type IA supernovae are thought to be the result of the explosion of a small and dense star — a white dwarf — inside a binary system. (Photo: ESO/Wikimedia Commons)
There are two main ways in which a supernova may form, determining which type of supernovae the star will become. There are subcategories of supernovae, too, and that's based on their light curves.
The first kind of supernova, called a Type IA supernova, is created when a white dwarf in a binary star system attracts matter from its companion. White dwarf stars are extremely dense by nature, which explains how they can pull matter from their partner star. When the white dwarf’s mass exceeds the mass of the Chandrasekhar Limit, the star explodes in a nuclear reaction creating a Type Ia supernova.
A Type II supernova or core-collapse supernova occurs during the death of a massive star. Because of the star's gigantic mass, its core eventually exceeds the Chandrasekhar Limit and cannot withstand its own gravity and therefore the star collapses in on itself. These Type II supernovae often become neutron stars.
How long does a supernova last?
How long the supernova event lasts depends on various factors including whether it is a Type IA or Type II supernova.
In the video above, Universe Today's Fraser Cain explains that it takes, "A few million years for the star to die, less than a quarter of a second for its core to collapse, a few hours for the shockwave to reach the surface of the star, a few months to brighten and then just few years to fade away."
A composite image of Cassiopeia A from the Hubble and Spitzer space telescopes. Cassiopeia A is the remnant of a supernova. (Photo: NASA/JPL-Caltech/Wikimedia Commons)
After the supernova event, remnants of the reaction called "SNRs" are still present in space. The remnants are classified as crab-like remnants, shell-like remnants, or composite remnants. We can see these visualized in composite images created from telescopes. Cassiopeia A (above) is a prime example of a shell-like remnant.
How often do supernovae occur?
Scientists estimate that a supernova occurs in the Milky Way about once every 50 years. It's a good thing that our galaxy is a pretty large place, as a near-Earth supernova explosion would be catastrophic.
Can you see supernovae with the naked eye?
This image combines data from four space telescopes to create a multi-wavelength view of all that remains of RCW 86, the oldest documented example of a supernova. (Photo: NASA/CXC/SAO & ESA; NASA/JPL-Caltech)
Yes, but you have to be lucky. The oldest documented supernova, RCW 86 (above), was seen in 185 A.D. According to NASA, its remnants are about 8,000 light-years away.
In 1604, Johannes Kepler observed a brief, unexplained, bright light in the sky. Kepler hypothesized that it was a new star. However, scientists later determined that this event was actually the death of a star.
More recently, Supernova 1987A was witnessed in February 1987. It was visible to the naked eye in the Southern Hemisphere.
Today, scientists are able to study these supernova remnants thanks to the Hubble Space Telescope, the Spitzer Space Telescope and the Chandra X-Ray Observatory.
Supernovae may be discovered by scientists and amateur astronomers alike. A recent project between Zooniverse, BBC Stargazing Live and hopeful astronomers everywhere led to the discovery of five previously undocumented supernovae.
Who knows, you could follow in Kepler's footsteps too.