Black Holes and Supernovae – The End of the Stellar Life Cycle

Credit: ESO/H. Boffin

Fleming 1 planetary nebula, located in the Centaurus constellation. Main sequence stars, such as the Sun, leave behind spectacular planetary nebulae when they die.

Nothing is known to last forever, including even the Universe itself. Just like us, the Universe is finite and mortal. The end of the stellar life cycle varies depending mostly on its size. Larger stars tend to have much more dramatic deaths, whereas smaller stars, such as our own, have relatively peaceful endings.

The Fate of the Sun and Earth

The Sun is not a very special star when it comes to the greater scheme of things. It has a circumference 109 times that of Earth and a mass 333,000 times greater. It’s 4.57 billion years old and is approximately half way through its life-span as a main sequence star, after which is will start its journey into decline.

The ultimate natural fate of planet Earth is entirely down to the fate of the Sun. The Sun is constantly expanding and getting hotter as it burns through its hydrogen fuel. In 4.5 billion years from now, the Sun will reach the red giant stage, as it starts to cool and run out of fuel.

Before the Sun reaches its maximum size as a red giant, it will swallow Mercury and Venus. Life on Earth will have long since ceased to exist by then, and no trace of life or human civilization is likely to remain as the oceans boil away, the atmosphere gets sucked into space and the surface is bathed in lethal cosmic radiation.

At this point, our world will be a charred, lifeless rock with a surface entirely covered by molten lava baking in the intense heat of the Sun right next door. Eventually, the Sun will likely swallow the Earth as well, although there is a chance that its orbit might widen enough to escape complete destruction.

Since the edge of our star will be much closer to Jupiter and the other gas giant planets at this stage, they will also be profoundly affected. Saturn’s moon, Titan, for example, will be a hundred degrees warmer than it is now, perhaps warm enough for new life to develop. By this time, the Sun will extend far beyond the current orbit of Earth and will glow a deep red in the sky.

Once the Sun has reached the height of its red giant stage, it will become a planetary nebula. At this point, the Sun will look like some of the beautiful nebulae that we can see out in the depths of space. The term planetary nebula is rather misleading however, since this stage of a star’s life has nothing to do with planets. This part of the Sun’s life will be relatively short, lasting only a few tens of thousands of years as almost all its remaining matter is propelled out into space. However, out of this ejected matter from our dying star, new stars and solar systems may form.

By this time, all that will be left of the Sun is a white dwarf star. This is what will remain of the Sun’s core – a tiny celestial body about the size of the Earth. It will, however, by extremely dense and massive despite its size. In fact, a teaspoon worth of matter from such a star will weigh several tonnes. The white dwarf will be small and dim, a mere shadow of its former self.

White dwarfs are the oldest stars in the universe, but even they do not last forever. Eventually, white dwarfs will start to cool and it is expected that they will become black dwarfs. Black dwarfs are still hypothetical, however, since the universe simply isn’t old enough for them to have come into being yet.

Most of the stars in the universe have a similar life cycle to our own, but there are a few mind-blowing exceptions.

Supernovae and Hypernovae

ESO/J. Pérez

Located some 800 light years away, the Vela supernova remnant is what’s left of a star that went supernova some 11,000 years ago.

Larger stars have far more dramatic ends to their life cycles. Just like any other star, they do go through a red giant stage, albeit becoming red supergiants instead. These stars are highly unstable and extremely enormous. They are the largest individual entities in the universe in terms of volume. The best known red supergiants are Antares and Betelgeuse, which are around 880 and 1,100 times the radius of the Sun respectively.

Red supergiants have less chance of ending their lives peacefully than standard red giants, and instead are likely to go supernova. A supernova is by far the most destructive force of nature – a spectacular explosion of unimaginable proportions. As the red supergiant star depletes its nuclear fuel, it collapses before exploding in a supernova. They are among the most luminous objects in the universe, often shining more brightly in the sky than an entire galaxy for a brief time. During the explosion, the vast majority of the matter that the star is composed of is propelled out into space at more than 18,000 miles (29,000 km) per second (about 10% of the speed of light). The shockwave annihilates everything in its path.

Supernovae are rare, occurring only once every fifty years in the Milky Way galaxy. The last one to be viewable from Earth by the naked eye was Kepler’s Supernova, seen in 1604. For about three weeks, it glowed brighter than everything else in the night sky, except for the moon and Venus. The event actually happened some 20,000 years before but, due to the finite speed of light, it wasn’t witnessed until 1604.

A hypernova basically a supernova on steroids. The gamma-ray burst expelled by this monstrous force of nature is may have been what caused the Ordovician-Silurian mass extinction event on Earth 440 million years ago, wiping out sixty percent of maritime species.

A supernova occurring near Earth has profound effects on its atmosphere and, in the most extreme scenarios, even entirely deplete the ozone layer and bathe the planet in dangerously high amounts of cosmic radiation. If any star within about 100 light-years from Earth goes supernova, the effect on our solar system would be very noticeable and, in some cases, absolutely devastating. Fortunately, this does not tend to happen more than once every few hundred million years.

Neutron Stars and Black Holes

ESO/S. Guisard

A view of the heart of our Milky Way, a region of the galaxy chock full of dying stars with a supermassive black hole lurking at its centre.

When a star goes supernova, it creates a colourful nebula of matter which may one day form into a new star. What is left of the star itself is a small and extremely compact core, a little like the white dwarfs that regular stars leave behind after their red giant stages.

Neutron stars may weigh several times more than the Sun, but they are much, much smaller, having a radius of only a few dozen kilometres. This means that such stars are extremely dense, much more so than even white dwarfs. An amount of matter the size of a car from such a star weighs as much as the moon.

In some cases, a supernova leaves behind a black hole – the most bizarre and physics-defying entities in existence. A black hole is the product of gravitational collapse. The remaining core of the star, left behind after a supernova, has such an immense density that its gravitational pull becomes so strong that even light cannot escape it. A star that ultimately becomes a black hole becomes invisible and is only detectable by observing the effects on its surroundings. Black holes continue to grow by consuming the material around them, eventually becoming supermassive black holes. These are believed to exist in the centre of many galaxies including our own Milky Way.

The border of a black hole is known as the event horizon. This is the point of no return. Once something passes this point, it is not ever going to come back again. The centre of the black hole is known as the gravitational singularity. At this point, the laws of physics, time and space simply break down. Here, the curvature of spacetime becomes infinite. This region has no volume, yet is infinitely dense, a concept that the human mind simply is not built to understand.

Nothing can survive falling into a black hole. Since tidal forces become infinite past the event horizon, anything crossing this line is destroyed and stripped of many of its physical properties, such as volume. The only properties that survive are angular momentum, charge and mass. All other information is lost in what is called the (so far unexplained) information loss paradox.

Black holes are not widely understood due to their mysterious and physics-defying nature, but all sorts of exotic theories exist. Some believe, for example, that black holes may be connected to hypothetical white holes located in another area of spacetime entirely. These are known as wormholes and have long played a major part in science fiction space operas as tools of faster-than-light or even inter-dimensional travel. What ultimately happens to matter that falls into a black hole is, however, not entirely known, and it is likely that we will never be able to find out.

Conclusion

One thing that all stars have in common is that, when they die, they look particularly splendid and, with the right equipment, even you can enjoy the beauty of stellar remnants with your own eyes. Even with a beginner’s telescope, such as the Celestron Nexstar 130 SLT or the Meade ETX-90, you can expect to see famous planetary nebulae such as the Eskimo Nebula or the Dumbbell Nebula, as well as a whole lot more.