Wonders of the Universe

THE BEGINNING AND THE END

This computer-generated sequence of images shows what will happen when Betelgeuse goes supernova. Deep in the heart of the star, the core will succumb to gravity and fall in on itself, then rebound with colossal force. The blast wave emitted generates the highest temperatures in the Universe. Over millions of years the scattered elements of the exploded star will become a nebula, at the heart of which is a super-dense core that is Betelgeuse the neutron star.

After a few million years of life, the destiny of the largest stars in our universe is a dramatic one. Having run out of hydrogen and burnt through the elements all the way to iron, giant stars teeter on the edge of collapse. Yet even in this dilapidated state these stars have one last violent act, and it is a generous one. It occurs with such intensity that it allows for the creation of the heavy elements.

If we could gaze deep into the heart of one of these dying giants, we would see the core finally succumb to gravity. As fusion grinds to a halt, this giant ball of iron falls in on itself with enormous speed, contracting at up to a quarter of the speed of light. This dramatic collapse causes a rapid increase in temperature and density as the core shrinks to a fraction of its original size. The inner core may eventually shrink to 30 kilometres (19 miles) in diameter. At this point, with temperatures nearing 100 billion Kelvin and densities comparable to those inside an atomic nucleus, quantum mechanics steps in to abruptly halt the collapse. By now most of the electrons and protons in the core have been literally forced to merge together into neutrons. Neutrons, in common with protons and electrons, obey something called the Pauli exclusion principle, which effectively prevents them from getting too close to one another (in more technical terms, no two neutrons can be in the same quantum state). This has the effect of making a ball of neutrons the most rigid material in the Universe – 100 million million million times as hard as a diamond. When the neutrons can be compressed no more, the contraction must stop and all the superheated collapsing matter rebounds with colossal force. A shockwave shoots out through the star and as this blast wave runs into the outer layers of the star it generates the highest temperatures in the Universe – 100 billion degrees. The precise mechanism for this rapid heating is not fully understood, but it is known that for a matter of seconds the conditions are intense enough to form all the heaviest elements we see in our universe, from gold to plutonium. This is a Type II supernova – the most powerful explosion we know of.

Supernovae are so rare that since the birth of modern science we have never had the chance to see one close up. The last supernova explosion seen from Earth in our galaxy was in 1604, a few years before the invention of the astronomical telescope. On average, it is expected there should be around one supernova explosion in the Milky Way per century, but for the last 400 years we’ve had no luck. It’s long overdue and astronomers are always searching the skies for stars which they think might be the most likely candidate to go supernova.

One of the prime candidates is Orion’s shining red jewel, Betelgeuse. With so many telescopes trained on this nearby star, we have been able to follow its every move for decades. Charting its brightness, we have discovered that it is extremely unstable; it has dimmed by about 15 per cent in the past decade. As supernova candidates go, Betelgeuse is top of the list. It is generally thought that Betelgeuse could go supernova at any time. It is a relatively young star, perhaps only ten million years old, and has sped through its life cycle so rapidly because it is so massive. However, when you’re ten million years old, the end of your life can be quite drawn out and a phrase like ‘any time soon’ in stellar terms is not quite what you might expect. It means that Betelgeuse should go supernova at some point in the next million years, but equally it could explode tomorrow. What we do know is that when it does go it will provide us with quite a show. Betelgeuse is only 500 light years away, almost uncomfortably close, which means that the explosion will be incredibly bright. It will be by far the brightest star in the sky and it may even shine as brightly as a full moon at night and fill the sky as a second sun during the day.

The giant Orion Molecular Cloud is an extensive area of star formation about 1,500 light years from us, centred on the impressive Orion Nebula. This infrared image of it, from NASA’s Spitzer Space Telescope, shows light from newborn stars within the Orion Nebula. The nebula can be seen from Earth with the naked eye as a hazy ‘star’ in Orion’s sword.

NASA

This computer-generated image shows just how bright scientists believe the heavens will be once Betelguese has gone supernova; it will flood the skies with light – day and night.

When stars are more massive than about eight times the Sun, they end their lives in a spectacular explosion. The outer layers of the star are hurtled out into space at thousands of miles an hour, leaving a debris field of gas and dust. Where the star once was, a small, dense object called a neutron star is often found. While around only 16 kilometres (10 miles) across, the tightly packed neutrons it contains have more mass than the entire Sun. The bright blue dot in the centre of this X-ray image of RCW 103 is believed to show the neutron star that formed when the star exploded in a supernova 2,000 years ago.

NASA

In a single instant, Betelgeuse will release more energy than our sun will produce in its entire lifetime. As the explosion tears the star apart, it will fling out into space all the elements the star has created through its life.

Over millions of years these newly minted elements will spread out to become a nebula, a rich chemical cloud drifting in space. At the heart of it, all that will remain will be the super-dense core of neutrons; the remnants of the star that was once a billion miles across will have been squashed out of all recognition by gravity. This is a neutron star, the ultimate destiny of Betelgeuse; a dense, hot ball of matter which is the same mass as our Sun but only 30 kilometres (19 miles) across.

We may not have seen neutron stars close up, but we have seen them from afar. X-ray images have been taken that give us vital information about these stars, in particular recent pictures of RCW 103, the two-thousand-year-old remnant of a supernova explosion that occurred about 10,000 light years from Earth (see left).

When Betelgeuse explodes it will be incredibly bright. It will be by far the brightest star in the sky and it may even shine as brightly as a full moon at night and fill the sky as a second sun during the day.

This may sound like a cosmic graveyard, but it is in the deaths of old stars that new stars are born. This is the Earthly cycle of death and rebirth played out on a cosmic scale. We can see that beautiful cycle happening today in the constellation of Orion. In an area known as the sword handle lies the Orion Nebula. To the naked eye it appears to be a misty patch of light in the night sky, but through a telescope it is a majestic wonder of the Universe. Hidden in its clouds are bright points of light, new stars forming from the clouds of elements blown out by supernova explosions; the new born from the deaths of the old.

It is from such a cycle that we emerged – within a nebula just like this, five billion years ago, our sun was formed. Around that star a network of planets condensed from the ashes, and amongst them was Earth; a planet whose ingredients originated from the nebula, a cloud of elements formed in the deaths of stars, drifting through space.

But that’s not quite the end of the story, because it is now thought that the chemical elements themselves are not the most complex pieces of ‘us’ that were assembled in the depths of space