Supernovae are not just beautiful cosmic fireworks—many of the elements that are essential for life, such as calcium and iron, are believed to have been produced by supernovae. But what causes these explosions? There are currently two explanations involving theoretical mechanisms that are related to the two ways in which stars die.
Stars cannot shine forever for the simple reason that their energy supply—nuclear burning—is finite. What happens to stars once they exhaust their nuclear fuel mainly hydrogen is believed to depend crucially on their mass.
One of the most important theoretical discoveries in astrophysics is that a critical mass exists above which stars cannot sustain themselves against their own gravitational pull without a continuous supply of energy.
The two types of star endings depend on whether their mass is above or below this critical mass, which is called the Chandrasekhar mass limit, named after Subrahmanyan Chandrasekhar Member, , , one of its discoverers. If, by the time a star exhausts its fuel, it has a mass greater than this limit, the core of the star cannot sustain itself and collapses.
A huge amount of energy is released when the core collapses to the tiny size of a few kilometers, becoming a black hole or a neutron star. While most of this energy is emitted in invisible neutrinos, a small fraction of this energy ejects the outer parts of the star, creating an explosion sufficient to produce a supernova.
Such a theoretical event is called a core-collapse supernova. Stars that are less massive than the Chandrasekhar mass limit, or lose enough mass during their life to become so, are able to resist gravity once their nuclear fuel is exhausted. Gravity does manage to shrink them considerably, however, and they settle at a radius of a few thousand kilometers. Such dense stars, with a mass comparable to that of the Sun one million times the mass of Earth and a size comparable to that of Earth, are called white dwarfs.
Without their previous nuclear energy supply, white dwarfs slowly become dimmer and eventually become unobservable. If properly ignited, white dwarfs are capable of powerful thermonuclear explosions with a sufficient energy release to account for a supernova. Such theoretical events are called thermonuclear supernovae. An appealing aspect of theories involving nuclear energy is that they naturally explain why the energy-per-unit mass released by supernovae is comparable to that of H-bombs and our Sun.
Yet neither core collapse in massive stars nor thermonuclear explosions of white dwarfs have been theoretically established to account for the supernovae that we see. While core collapse is likely inevitable, it has not been shown that the outer parts can be ejected successfully.
While white dwarfs can explode if ignited, a robust ignition mechanism has not been identified. Yet the two explosion mechanisms are widely believed to occur, based on several successes for explaining supernovae observations and given that we simply do not have better ideas. Perhaps the best clues for identifying an explosion mechanism have come from a few nearby supernovae where a star, located in pre-explosion images at the precise position of the supernovae, had disappeared in images taken after the supernova had faded away.
In all of these cases, the exploding star was massive and at a late stage of its life in accordance with the core-collapse theory. In all, eight supernovae in the Milky Way have been identified thanks to written testimony through the years.
You might be lucky enough to see one! NASA encourages citizens to search the night skies for them. After sifting through images for months, Moore found what turned out to be SN ha, one of the dimmest supernovae on record.
Gray looked through photos taken at the Abbey Ridge Observatory taken by a family friend. In them, she discovered SN It. Finally, here are some gorgeous pictures of supernovae remnants, captured by multiple space telescopes. A good night's sleep is crucial for a full day of space exploration.
Find and compare great local hotels with our search tool. What is a supernova? March 22, What is it? Thus, a supernova is a part of the circle of celestial life.
Put another way, a star explodes every second or so somewhere in the universe, and some of those aren't too far from Earth. Exactly how a star dies depends in part on its mass. Our sun, for example, doesn't have enough mass to explode as a supernova though the news for Earth still isn't good, because once the sun runs out of its nuclear fuel, perhaps in a couple billion years, it will swell into a red giant that will likely vaporize our world, before gradually cooling into a white dwarf.
But with the right amount of mass, a star can burn out in a fiery explosion. Let's look at the more exciting Type II first. For a star to explode as a Type II supernova , it must be at several times more massive than the sun estimates run from eight to 15 solar masses. Like the sun, it will eventually run out of hydrogen and then helium fuel at its core. However, it will have enough mass and pressure to fuse carbon. Here's what happens next:.
What's left is an ultra-dense object called a neutron star , a city-sized object that can pack the mass of the sun in a small space. There are sub-categories of Type II supernovas, classified based on their light curves. Both types have the signature of hydrogen in their spectra. This heat produces pressure that pushes outward against the forces of gravity that pull inward on the star. For most of the life of a star, inward gravity and outward pressure are in balance and the star is stable.
But as a star burns through its fuel and begins to cool, the outward forces of pressure drop. When the pressure drops low enough in a massive star, gravity suddenly takes over and the star collapses in just seconds. This collapse produces the explosion we call a supernova. Supernovae are so powerful they create new atomic nuclei.
These fusion reactions create new atomic nuclei in a process called nucleosynthesis.
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