When stars collapse: what is a supernova?

What is a supernova? We explore what the Universe’s most sudden – and violent – events can tell us.

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An image of Cassiopeia A, captured by NASA’s Chandra X-ray Observatory. This object is a supernova remnant: the remains of an exploded star.
Credit: NASA/CXC/SAO

 

Written by Elizabeth Pearson

 

When large stars begin their death throes they explode in a colossal supernova, one of the most sudden and violent events in the sky. 

Large stars live fast and die young. 

They, like stars of all sizes, create light by converting hydrogen to helium in a process known as nuclear fusion.

But in the most massive stars this process is rapidly accelerated, meaning that they can burn through their stores of hydrogen gas within as little as a few million years.

Compare this to the Sun, which has a lifespan estimated to be 10 billion years. 

Stars live in a delicate balance between two opposing forces: the gravity trying to make them collapse inwards and the thermal pressure of nuclear fusion exerting an outward pressure.

As the hydrogen in the core gets used up, the star begins to convert helium into heavier and heavier elements; at first lithium and oxygen, before working all the way up the periodic table to iron.

Not only do these heavier elements cause the core to become more dense – increasing its gravitational pull – but the fusion reactions also release less energy to balance this out.

Eventually all that the star has to act against gravity is electron degeneracy pressure – the resistance to having more than one electron in the same place at the same time.

When the core reaches a critical density of 1.4 solar masses, known as the Chandrasekhar limit, even the plucky electrons can’t keep up the fight.

At that point the star’s core collapses in a matter of seconds, quickly followed by the outer layers of gas rushing in at as fast as 25 per cent the speed of light.

The core continues to collapse until the resistance between atomic particles stops it from collapsing any further.

All the atomic nuclei are tightly packed together by now, creating a solid surface.

Normally, this tightly packed ball remains as a neutron star, but if the core is massive enough then it may continue to collapse even further, creating a black hole. 

Meanwhile, the gas that was rushing in at incredible speeds strikes against the now solid surface of the neutron star and rebounds in a massive shockwave that ends in a huge explosion – the supernova.

 


This artist's impression shows a binary star system, one of which is a white dwarf. Over time, the white dwarf steals material from its neighbour and eventually explodes, causing a Type Ia supernova. These explosions always occur at the same brightness, so astronomers can measure the apparent brightness to work out how far away they are. Such calculations help astronomers measure distances in the Universe.
Credit: NASA

 

Looking for the light

These stellar detonations create a huge amount of light for a short amount of time. On Earth, researchers hoping to study such fleeting but fiery events have to look out for points of light that suddenly appear in the night sky.

Originally this was done by eye, and some amateur astronomers still search for supernovae at the eyepiece, but most professional supernova surveys nowadays use automated systems to image the sky, searching for ‘stars’ that weren’t there the night before.

Not everything they find is an exploding star, however.

Some explosions are dimmer nova, where the interaction between a pair of stars causes one to temporarily flare up.

And in August 2017, researchers observed a kilonova for the first time, a much brighter explosion caused by the collision between two neutron stars.

These are thought to be the origin of all naturally occurring elements heavier than iron.

To uncover what kind of nova an explosion might be, researchers have to watch how the light from the bright, new object changes.

But they need to be quick.

After the explosion the light quickly fades from view, so once a new supernova is found astronomers immediately notify all the telescopes in the world that might be able to observe the star.

Together they take brightness measurements across every possible wavelength and use spectroscopy to pick out which elements were present in the star when it exploded.

These elements don’t just disperse into the Universe, but instead form a type of nebula called a supernova remnant.

These nebulae are rich in hydrogen gas, which clumps together to form the next generation of stars.

Meanwhile, the heavy elements coalesce together, eventually forming planetary systems around the stars in the nebula. 

By studying supernovae, researchers not only grow to understand the life cycles of these massive stars, but also the origins of the planets too.

 


When a star’s core becomes so dense that it can no longer contract, the material still collapsing onto it rebounds and sets off the supernova.

 

Types of supernova

The state of a star before it collapses can change the supernova it creates.

There are two main categories depending on how much hydrogen is seen in the afterglow – Type I supernovae contain a small amount of hydrogen, Type II contain more.

Type II supernovae originate from the biggest stars, which have a very short lifespan so their outer layers of hydrogen gas are still intact when they explode.  

But slightly smaller stars might lose this layer over time, either through their own solar wind or because a neighbouring star strips away the gas.

In Type Ib supernovae, the outer layer of hydrogen has been lost, while Type Ic have shed their helium as well. 

Type Ia supernovae, meanwhile, are created by smaller stars that get locked in a tight binary pair with a smaller white dwarf. 

Over time, the white dwarf steals material away from its neighbour, until it reaches the critical mass to cause an explosion. 

As an explosion’s brightness is linked to how much mass the star had, Type Ia supernovae always have the same luminosity.

This means astronomers can work out how far away one happened, and so they can be used to trace out distances in the Universe.

 

Elizabeth Pearson is BBC Sky at Night Magazine’s news editor.

 


 

 

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