How a Supernova Works

        Science | Stars

In this image, a white dwarf's strong gravitational pull siphons matter from a nearby star. If the white dwarf becomes too massive to support its own weight, it will collapse on itself and then explode in a supernova.

Humans are born, then we grow and die. Our life cycles are basically the same as those of the massive stars twinkling in the night sky -- if we exploded in a blaze of glory at the end of our time, that is.

When the cosmos' most colossal stars go out with a bang, the immense interstellar explosion is known as a supernova. While smaller stars simply fizzle out, the death of an astronomical heavyweight is a showstopper. It has spent its life cannibalizing its own innards -- and sometimes the innards of a solar neighbor -- for fuel. When there is nothing left for it to consume, it collapses in on itself and then explodes outward in a death knell that outshines other huge stars -- and sometimes entire galaxies -- for days, weeks or even months [source: Lemonick].

A supernova should, statistically, detonate once every 50 years or so in a galaxy the size of our Milky Way. However, until 2006, scientists believed the Milky Way's most recent supernova occurred in the late 1600s [source: Goddard Space Flight Center]. They then realized that a clump of interstellar debris they'd been tracking for 23 years was actually the remnants of a supernova just 140 years old [source: NASA]. Astronomers couldn't view the flare-up because of cosmic dust, which also blocks most of the 1 billion supernovae estimated to occur outside our galaxy every year [source: Odenwald].

In contrast, some supernovae are so bright that they can be seen with a simple set of binoculars. In September 2011, earthlings in the northern hemisphere could peer into the Pinwheel Galaxy -- which appears above the Big Dipper's handle but isn't visible from most of the southern hemisphere -- and see a supernova that detonated 21 million years ago [source: Perlman].

So how do you spot one? Identifying a new point of light as a supernova (as opposed to a high-flying aircraft or a comet) may be easier than you think.

How to Find a Supernova

It's easy to use a star chart to identify constellations on a cloudless night. After all, the positions of these celestial objects have been mapped for centuries. But what happens when a guest star suddenly appears among its well-documented peers? It's probably the remains of a star that exploded hundreds or millions of years ago, and whose light is only now reaching our skies.

It doesn't take a professional degree to make an astronomical discovery. In January 2011, a 10-year-old girl found a supernova in a galaxy 240 million light-years away [source: Vincent]. Scientists often rely on backyard astronomers to patrol the skies for newly appearing pinpoints that are brighter and clearer than the objects around them. Stars about to go supernova change color from red to blue due to their increasing temperatures [source: Minkel]. And supernovae maintain some blue color due to the Doppler effect: The light from their explosions moves toward us so fast that it appears blue [source: Murdin]. Plus, unlike a comet or commercial airplane, a supernova won't waver from its position.

If you spot a supernova that isn't on record, you can report it to the IAU Central Bureau for Astronomical Telegrams. From there, astronomers will study any electromagnetic radiation that the potential supernova is giving off -- that is, any gamma rays, x-rays, ultraviolet waves, visible light, infrared waves, microwaves and radio waves. This spectrum of visible and invisible radiation will help them learn about what the celestial object is composed of, how hot it is, how dense it is and how fast it's moving.

Astronomers living in ancient China made the first record of a supernova some 2,000 years ago. They didn't understand what they were seeing and were convinced that the point of light was a new one. However, after chronicling the "new" star for eight months, the object suddenly disappeared. Although this hide-and-seek star could have become a forgotten footnote, the discovery experienced a revival in 2006. That's when astronomers realized they were looking at remnants of the same supernova that had been documented in ancient China [source: Zielinski].

Supernovae like this have been found all over the cosmos, in our galaxy and other galaxies millions of light-years away. In 1987, we discovered a supernova so close to Earth that it could be seen without looking through a telescope. This supernova was located in the Large Magellanic Cloud, neighbor to our Milky Way galaxy [source: Space Telescope Science Institute]. It made history again in 2011, when scientists discovered its debris glowing brighter as it entered a new stage of decay. The light of this supernova remnant became more visible because its leftover mass of debris expanded and bumped into a ring of debris that had been discharged from the supernova before it exploded. When the matter collided, it produced x-rays and heat, which caused the remnant to look brighter [source: Beck].

But how did this star begin to self-destruct in the first place? Learn about the life cycle of giant stars on the next page.

Life Cycle of a Giant Star

This is the remnant of the supernova Tycho Brahe observed in 1572. The image is a colorized composite of low-energy x-rays (red) showing debris and high-energy x-rays (blue) showing the blast wave, plus the visible field of stars around it.
This is the remnant of the supernova Tycho Brahe observed in 1572. The image is a colorized composite of low-energy x-rays (red) showing debris and high-energy x-rays (blue) showing the blast wave, plus the visible field of stars around it.
NASA/CXC/Rutgers/K. Eriksen et al.

A giant star starts its life innocently, when gas and dust buckle under an assertive gravitational pull to form a baby star.

As the material at the center of a fledgling star heats, it attracts more interstellar gas and dust. This growth phase can take up to 50 million years, followed by another 10 billion years of shiny adulthood. What's the source of all that twinkling? Stars are fueled by the nuclear fusion of hydrogen into the slightly denser and heavier element helium. The fusion takes place in the star's core, and the energy it produces flows outward, creating the star's observable glow and preventing the heavy core from collapsing in on itself [source: NASA]. You can read more about the process in How Stars Work.

When a star starts running out of hydrogen to fuse into helium, it's the beginning of the end. With less energy radiating outward, the core begins to collapse, causing its temperature to spike. Hydrogen fusion continues only in the star's outer layers, which causes it to expand. It becomes a red giant.

A red giant will lose its outer layers to become a white dwarf. (If it's massive enough, the star will consume those layers by fusing them into heavier and heavier elements. If the star doesn't have enough gravity to do that, it will release its cooling outer layers into space.) A white dwarf with enough mass will eventually go supernova. Its core will collapse, resulting in an explosion that can't compare to any we might experience on Earth -- unless we were to bundle a few octillion nuclear warheads and detonate them all at the same time [source: Thompson, NASA]. Since that scenario is unlikely, we'll never experience a supernova-sized explosion -- despite sci-fi movie plots like "The Book of Eli," our sun isn't big enough to go out with such a bang.

Why such destructive collapses occur, what happens afterward and how a supernova will affect the rest of its galaxy all depend on several factors that we'll discuss on the next page.

Types of Supernovae

Stars that have enough heft to go out with a bang are separated into two supernova classes -- Type I and Type II. Astronomer Rudolph Minkowski laid out these classifications in 1941. Astronomers learn a lot about stars from the colors of light that they emit. Using a device called a spectrograph, they can get a clear picture of exactly what elements are burning inside a star.

By using a spectrograph, Minkowski noticed that some supernovae (Type I) don't contain hydrogen, but the others (Type II) do. In the 1980s, as observational technology improved, scientists further divided Type I supernovae into three subcategories: Type Ia (which contain silicon in their spectra), Type Ib (which contain helium) and Type Ic (which contain neither) [source: Swisburne University of Technology]. Stars lose elements when stellar winds rip their outer layers away long before they go supernova.

Type Ia supernovae work differently than all other types. A Type Ia supernova results from a white dwarf that's part of a binary system (that is, one that shares an orbit with another star) and was about twice the size of our sun during its life. This white dwarf's mass allows it to fuse elements slightly heavier than hydrogen, so it has a stable core of carbon and oxygen.

Left to its own devices, this white dwarf would eventually decay into a black dwarf. But since it's not alone, it has access to resources that other stars don't. The more massive of the two stars acts like an opportunistic sibling, using its gravitational pull to steal matter from the other star. This gluttonous star grows until it exceeds the Chandrasekhar limit -- a mass 1.4 times that of our sun, otherwise known as 1.4 solar masses. At this size, the white dwarf suddenly has enough heat and pressure in its core to fuse carbon, and all of that carbon fuses at once like a thermonuclear bomb going off, blowing the star to bits [source: Atkinson]. It leaves behind a gaseous remnant that's symmetrical in shape and contains a great a deal of iron created in the heat of the explosion [source: Chandra X-ray Observatory].

Because Type Ia supernovae all explode at the same point in their stellar deaths, they all peak at almost exactly the same brightness. It's so consistent that Type Ia supernovae are also called standard candles: Once astronomers find one in a region of space, they can use it as a baseline with which to compare other objects around it.

Type Ib, Type Ic and Type II supernovae, despite showing different elements in their spectra, all explode the same way. Find out how they work on the next page.

Core Collapse Supernovae

This is Cassiopeia A, a core collapse supernova remnant with a neutron star in its center. Located in the Milky Way only some 11,000 light-years away from Earth, it originally exploded about 330 years ago.
This is Cassiopeia A, a core collapse supernova remnant with a neutron star in its center. Located in the Milky Way only some 11,000 light-years away from Earth, it originally exploded about 330 years ago.
NASA/CXC/UNAM/Ioffe/D. Page, P. Shternin et al

Type Ib, Type Ic and Type II supernovae stars start out so huge -- possibly 8 times the size of our sun -- that they cannibalize themselves to the point of collapse [source: NASA]. A white dwarf eventually created from a star that massive has so much heat and pressure inside its core that lighter elements keep fusing into increasingly heavy elements instead of flying off into space. This produces enough radiating energy to support the star's increasing weight -- until iron forms. The fusion of iron into heavier elements actually uses energy rather than giving it off, so when iron begins to fuse, the star's outer layers lose their support and begin to fall inward [source: Nave]. To understand the huge explosion that results, you have to know what's going on with the star's tiniest particles.

If a white dwarf is massive enough to fuse the iron in its core, those iron atoms are incredibly hot and densely packed, squashed together like sweaty clowns stuck in a circus car. Their subatomic particles collide, and the iron atoms' nuclei split, leaving behind helium nuclei plus a few leftover neutrons and absorbing a lot of energy in the process.

Without that energy left to hold it up, the star's core starts shrinking rapidly. It goes from a diameter of some 5,000 miles (8,000 kilometers) to about 12 miles (19 kilometers) suddenly, creating temperatures somewhere in the region of 180 billion degrees Fahrenheit (100 billion degrees Celsius) [source: NASA]. That heat causes protons and electrons to fuse together, canceling each other out to become neutrons and expelling a bunch of neutrinos in the process. The neutrinos can escape, so they do, leaving the core with even less energy to hold itself up. The core contracts as much as it physically can, but star's outer layers keep falling inward, even after there's no more room. That's when they rebound in an enormous explosion.

All of that took a lot of words to explain, but it may happen in as little as a quarter of a second.

The explosion is hot enough to fuse elements far heavier than iron, and it releases these elements in a gaseous cloud that will become an asymmetrical remnant around the remaining, solid core [source: Chandra X-ray Observatory].

On the next page, we'll share more about what the destruction of a star can create.

What We Learn from Supernovae

Spiral galaxy NGC 4151 has a supermassive black hole in its center (shown here in white) that's actively growing. Its nickname is "The Eye of Sauron," and we promise we didn't choose this image just for the "Lord of the Rings" reference.
Spiral galaxy NGC 4151 has a supermassive black hole in its center (shown here in white) that's actively growing. Its nickname is "The Eye of Sauron," and we promise we didn't choose this image just for the "Lord of the Rings" reference.
X-ray: NASA/CXC/CfA/J. Wang et al.; Optical: Isaac Newton Group of Telescopes, La Palma/Jacobus Kapteyn Telescope; Radio: NSF/NRAO/VLA

British pop band Oasis' hit song "Champagne Supernova" is now fodder for retro radio stations -- or the occasional ringtone. But when it was first released in 1995, it burned up the charts, going on to sell 3.9 million copies [source: Gundersen].

Even with such a record of success, "Champagne Supernova" pales in comparison to actual supernova SNLS-03C3bb. Astronomers discovered the supernova in 2006 and promptly nicknamed it the "champagne" supernova because it rocked their expectations (and what better way to celebrate than with a little Britpop?). The supernova equaled 2 solar masses before it exploded. This far exceeded the 1.4 solar masses -- the Chandrekhar limit -- that astronomers would have expected [source: CBC, Jeffery].

So why celebrate the spotting of a really, really gigantic star's death? Not only was SNLS-03C3bb a game-changer, but understanding how different stars die allows scientists to predict how future supernovae will impact the rest of the universe.

Type Ia supernovae completely destroy the core of a star, but the other three types leave a super-dense core behind. When a Type Ib, Type Ic or Type II supernova results from a star with an inner core of less than 3 solar masses, it creates a neutron star with a core about as dense as an atom's nucleus and a powerful magnetic field. If its magnetic field creates lighthouse-style beams of radiation that flash toward Earth as the star rotates, it's called a pulsar.

When a star with a core equal to 3 solar masses or more explodes, the aftermath of its explosion can result in a black hole. Scientists hypothesize that black holes form when gravity causes a star's compressed inner core to continually sink into itself. A black hole has such a powerful gravitational force that it can drag surrounding matter -- even planets, stars and light itself -- into its maw [source: NASA]. You can learn more about them in How Black Holes Work.

All of their powers of destruction aside, a lot of good can come of a supernova. By tracking the demise of particular stars, scientists have uncovered ancient astronomical events and predicted future changes in the universe [source: NASA]. And by using Type Ia supernovae as standard candles, researchers have been able to map entire galaxies' distances from us and determine that the universe is expanding ever more rapidly [source: Cal Tech].

But stars leave more than an electromagnetic signature behind. When a star explodes, it produces cosmic debris and dust [source: NASA]. Type Ia supernovae are thought to be responsible for the large amount of iron in the universe. And all of the elements in the universe that are heavier than iron, from cobalt to roentgenium, are thought to be created during core collapse supernovae explosions. After millions of years, these remnants comingle with space gas to form new interstellar life: Baby stars that mature, age and may eventually complete the circle of life by becoming supernovae themselves.


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