The biggest 'atomic bombs' in the universe

A supernova is an explosion that releases so much energy and brightness that it outshines entire galaxies containing several hundred billion stars .

Astronomers Walter Baade and Fritz Zwicky coined the term supernova to describe the remnants of neutron stars in 1933, according to Popular Mechanic . Before that, astronomers called any ancient object that exploded in the sky a 'nova,' which means 'new' in Latin. Supernova explosions fall into two main categories, each with completely different conditions and mechanisms that drive each other. The first type is a core-collapse supernova, or Type II supernova. These occur when a massive star reaches the end of its life.

Simulation of the brightest supernova explosion SN2016aps in a galaxy 4.6 billion light years from Earth. (Photo: iStock)

Every star in the universe fuses elements at its core throughout its lifetime, from the smallest, which are just a tenth the mass of the Sun, to the most massive, which are 100 times more massive. They fuse hydrogen into helium for most of their lives. Sun-like stars can live for about 10 billion years before expanding into red giants. But the largest stars, with their intense gravity crushing their cores, will exhaust their hydrogen reserves in just a few million years.

After exhausting its hydrogen, the giant star begins fusing helium into carbon and oxygen, then silicon and magnesium. Finally, it creates a sphere of nickel and iron at its core. Just before its death throes, the star resembles a bloated monster. Its outer layers of atmosphere are completely stripped away. The interior of the star resembles an onion, with an iron core surrounded by layers of lighter elements.

Each stage in a star's life is shorter than the one before it. A star spends millions of years quietly burning through its hydrogen, and less than a million years fusing helium. It can sustain carbon fusion for 1,000 years. The products of the reaction with iron in the core last a total of 15 minutes. All stars between 8 and 200 times the mass of the Sun go through the same process.

The reaction with iron doesn't release any energy. Instead, the reaction expends energy to fuse iron into heavier elements. The rest of the star continues to collapse into its core, but there's no energy released from the fusion reaction to balance it out. All the mass of the star presses down so tightly on the core that the iron atoms rearrange themselves, electrons moving into protons, turning the entire mass of iron into a giant ball of neutrons.

When the neutron ball can no longer hold its weight, all the remaining matter of the star slams into it and rebounds, triggering a supernova explosion. In less than a second, the entire star explodes from the inside out, sending shock waves through its own matter at nearly the speed of light. The radiation that accompanies the explosion is extremely energetic. For example, the star Betelgeuse, about 650 light years from Earth, will go supernova in a few million years. When it does, it will be bright enough to be seen during the day, brighter than the full moon.

Earth is fortunately not near a giant star with an unstable outer atmosphere. The radiation and particles emitted from a core-collapse supernova would tear everything within 100 light years to pieces.

Another type of supernova, the type 1a, is equally deadly . This type of explosion does not come from a single star dying, but from a star cannibalizing its own kind. Most stars in the universe exist in pairs. The members of a binary system never have the same mass. In fact, they often have different masses. They cycle at different rates, with the more massive star dying first. If the star is about the same mass as the Sun, it leaves behind a white dwarf, a dense core of unfused carbon and oxygen.

Sometimes, the star in the same system swells to become a red dwarf. Some of its atmosphere spills over onto the surface of the white dwarf. When that atmosphere reaches a critical density, fusion reactions with hydrogen release a type of flare called a regular nova.

But when conditions are right, the red giant continuously pours its own atmosphere onto the white dwarf's surface, causing the pressure and temperature to slowly rise. The thick hydrogen atmosphere triggers a thermonuclear reaction that releases all of its energy at once. The explosion shocks the white dwarf, forcing the carbon and oxygen to undergo uncontrolled thermonuclear reactions.

Supernovae are the largest atomic bombs in the universe, objects the size of Earth but more massive than the Sun, transforming all of their mass into a nuclear fireball. When they explode, the supernova is brighter than the galaxy, which contains hundreds of billions of stars. But the light show doesn't last forever. Within weeks, the supernova fades and dies, leaving nothing but a remnant floating in interstellar space.

A galaxy like the Milky Way has only a few supernovae per century, about a third of which are type 1a explosions. If one were too close to Earth, the radiation and shockwaves of a supernova would wipe out the planet's atmosphere.