Apocalypse Now!

by Nicholas Mee on August 16, 2013

Stars are huge balls of hydrogen and helium. But despite their almost identical composition their ultimate fates can be remarkably different.

The real action within a star takes place in its core where conditions are so intense that atomic nuclei merge to form heavier nuclei. These nuclear fusion reactions release vast amounts of energy that supports the star against its tendency to collapse under gravity.

The most important feature of a star is its mass, as this property determines almost everything else about the star. Stars of greater mass have higher temperatures in their core, which means that the nuclear reactions proceed faster. Very massive stars burn their nuclear fuel at a prodigious rate and race through their life much faster than lower mass stars.

A Cosmic Blink of the Eye

The Sun will continue to shine for around 10 billion years. A megastar with around 20 times the mass of the Sun would use up its nuclear fuel in around 10 million years, a cosmic blink of the eye. 20 times as much fuel is burnt in one thousandth of the time, which means that energy is being released at 20,000 times the rate of the Sun and therefore the star will shine 20,000 times as bright as the Sun. Fortunately, these stars are all much further away than the Sun, so we only see them as tiny pinpricks in the night sky.

What happens when the fuel runs out?

When a star has converted most of the hydrogen in its core into helium its outer layers swell up to form a bloated red giant. In five billion years time the Earth will be engulfed by the outer layers of the Sun as it approaches the end of its life. In a star such as the Sun these outer layers will eventually disperse into space to reveal the star’s core as an extremely dense glowing ember about the size of the Earth. The nuclear reactions in the core will have ceased and the core will gradually cool as it radiates its heat into the depths of space. The name for such a stellar remnant is a white dwarf. These stars are extremely hot, but very faint as they are so small. The nearest white dwarf is in orbit with Sirius – the brightest star in the night sky, but it is not visible without a large telescope.

Galactic Megastars

The really massive megastars have a more spectacular future to look forward to. When the hydrogen runs out the temperature rises and new nuclear processes begin. Helium is converted into carbon and oxygen, then even heavier atoms are cooked up. But eventually no new nuclear reactions are possible and the final collapse begins. At this point so much energy is released that the star blasts itself apart in a supernova explosion which may be as bright as an entire galaxy of 100 billion stars. When the smoke clears the core of the star may have been transformed into an object around 30 kilometres across – the size of a major city – but with the density of an atomic nucleus. These remarkable objects are known as neutron stars.

If the mass of the collapsing core is more than two or three times the mass of the Sun, its destiny is even stranger as its collapse cannot be stopped. The result is a black hole. In short, the end point of a star may be a white dwarf, a neutron star or a black hole.

The extremely massive star system known as eta Carinae which is belching gas into space as it approaches its terminal crisis. (copyright Hubble Space Telescope, NASA)

The maximum mass of a white dwarf is known as the Chandrasekhar limit after the Indian astrophysicist Subrahmanyan Chandrasekhar who first derived this result. It is about 1.4 times the mass of the Sun. If its mass exceeded this limit the white dwarf would collapse and form a neutron star. There is also a maximum mass for a neutron star, but as the physics of these weird objects is so exotic this limit is not known with the same certainty. However, it must be in the range of two to three times the mass of the Sun. A neutron star whose mass exceeded this value would inevitably collapse to form a black hole.

The Red Giant and the White Dwarf

A large proportion of stars live in binary or multiple star systems in which two or more stars are bound together and orbit around each other. When a white dwarf and a red giant are held in a gravitational embrace the result can be very interesting. The white dwarf is in a sense a dead star, as it is no longer undergoing nuclear fusion reactions. But as it travels around the red giant it can accumulate material from the outer layers of the red giant. This material is drawn to the surface of the white dwarf and compressed by its intense gravity to form a shell around the white dwarf. Eventually, a critical density is reached and this shell detonates in a huge nuclear fusion explosion that is visible from the other side of the galaxy.

An artist’s impression of a binary system in which material from the larger star is falling onto a white dwarf. (copyright NASA)

These events are seen quite regularly by astronomers. Suddenly a star appears as if from nowhere. It is known as a nova, meaning a new star. The nova will gradually fade and eventually disappear again. About 10 novae are seen in the Milky Way galaxy each year. (Another 30 or so are thought to be hidden from our view by dust and gas clouds.)

There is currently a nova in the constellation Delphinus that is visible to the naked eye. For details of how to find this nova click the link at the end of this article. If you find it you will be able to say you have seen a nuclear explosion on a white dwarf star.

A Ticking Time Bomb

The process that led to the nova will repeat as the white dwarf continues to attract material from its companion star. The period between eruptions is typically several thousand years, but it may be as short as a decade or two. For instance, the star RS Ophiuchi lit up in 1898, 1933, 1958, 1967, 1985, and 2006.

Over time the mass of the white dwarf will increase. Eventually, it will reach the Chandrasekhar limit, the point at which its mass can no longer be supported. The white dwarf will then undergo its terminal collapse into a neutron star. This cataclysmic event produces a catastrophic conflagration that may be as much as 100,000 times brighter than the earlier novae – the star lights up as a supernova.

Two Types of Supernovae

We have now described novae and their big brothers – supernovae. In fact, we have seen two ways in which a supernova may be produced.

When very massive stars have used up all their nuclear fuel they end their lives with a bang. These explosions are known as Type II supernovae.

Also, greedy white dwarfs in orbit with another star may accumulate so much extra mass that they collapse to form neutron stars. These explosions are known as Type Ia supernovae.

The bright star in the bottom left of the picture is a type Ia supernova that erupted in 1994 in the outskirts of a galaxy lying 100 million light years away known as NGC 4526.
(copyright Hubble Space Telescope, NASA)

These two processes are quite different and the characteristics of the resulting supernova explosions are different. They were classified before their origin was understood.

Further Cosmic Blasts

But this is not the whole of the story when it comes to cosmic blasts. I will have more to say about another types of stellar explosion in a follow-up article in the near future. In the mean time, why not see if you can find the nova in the constellation of the Dolphin. I will be taking a look on the next clear night.

Further Information

There is a lot more information about stellar evolution and supernovae in my book Higgs Force: Cosmic Symmetry Shattered.

To find out more about the nova take a look at this article from Sky and Telescope magazine: Bright Nova in Delphinus


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