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©2018 Reaching Space Science. 

The Death of Stars: Supernova, Black Holes and White Dwarfs

July 30, 2018

It is clear that stars are very active balls of gas, but they cannot burn forever. Very large stars burn their fuel quicker compared to smaller stars so they may only last for a few hundred thousand years. Smaller stars can last for several billion years because they burn their fuel much more slowly. Average stars will burn for the main period of their lives for around ten billion years. Our Sun is roughly halfway through this stage at 4.6 billion years old.

 

This is a false-color image of the Sun observed in the extreme ultraviolet region of the spectrum (NASA).

 

All stars will eventually run through its hydrogen, which means the nuclear fusion reactions (that made helium from hydrogen) the fuelled the star's energy stops. This outward-pushing force of fusion was balanced with the inward pull of gravity but without fusion, gravity takes over to compress the star. This causes the star to shrink and its temperature to rise, eventually getting high enough to spark an entirely new fusion process, one that fuses helium into carbon. This new source of energy causes the star to expand, eventually becoming a large “red giant”. Now, the temperature has spread throughout the larger size of the star and is generally cooler. This causes its shine to change to a redder colour. Stars will normally spend around one billion years as a red giant. When our Sun becomes a red giant, its atmosphere will grow so much that it will swallow the solar system’s inner planets (likely including the Earth). Once a star has reached this point, its fate is decided by its size and mass.

 

Small to average-sized stars, including our Sun, will have a generally quiet end. The newly formed carbon in its core cannot produce any more energy, but this core has become extremely dense (so much so that an area the size of a grape contains a full ton of matter in here). Meanwhile, its outer layers of gas continue to expand, eventually spreading into space. This cloud, or “nebula”, helps to fill interstellar space with the material that is needed to help form new stars. What remains is the small carbon core of the star, which is now roughly the size of the Earth. These stars are no longer fuelled by fusion, but shine from stored heat. They continue to cool and fade away forever and are often then called “black dwarfs” because they are now cold, small, burned-out embers in empty space. We have never actually seen a black dwarf because scientists believe that the universe is not old enough yet (despite being nearly 14 billions years old) for any to have even formed. This is because it takes billions of billions of years for a white dwarf to fade away into a black one.

 

Larger stars, those that are at least around 8 times as massive our Sun, will experience a more violent end that can even be seen from Earth. These biggers stars may only burn for a hundred million years before evolving to its final stage. Unlike the low-mass stars that never become hot enough to burn carbon in their cores, these massive stars can fuse even heavier elements when their inner cores contract and their temperatures rise. These temperatures are sufficient to burn carbon to oxygen, neon, silicon, sulphur, and eventually to iron. The stars’ interiors become much like an onion, where the shells of increasingly heavier elements burn at higher temperatures. Ultimately, the star reaches a stage where it can no longer burn through new elements to produce energy. Once the inner core begins to change into iron, the star is in trouble. This is because iron is the most stable form of nuclear matter, which simply means that no energy can be gained by burning it into any of the heavier elements. At this point, gravity overwhelms the pressure of the hot gas and the star implodes. The star can no longer support its own mass, which is why the star collapses in on itself. When this implosion bounces off the inner dense and hot core, the rest of the star’s material is violently thrown into space, forming what is called a “supernova”. This particular scenario is known as a Type II supernova. What remains is a small, ultra-dense, “neutron star” that no longer generates heat, and cools over time. Today it is believed that neutron stars are made up of very tightly packed neutrons (which are subatomic particles with no electrical charge) and take up only the size of an average city. A neutron star’s gravity is extremely powerful because of this high density, which would flatten anyone to a thin sheet of paper if they were to stand on its surface.

 

Another form of supernova (Type I) happens in a binary system with two stars. In this scenario, one of the stars takes matter from its companion and eventually accumulates too much material. This runaway nuclear reaction causes an explosive death of the larger star, resulting in a supernova.

Supernovae are a dramatic burst of light that can outshine galaxies and burn for just a few short weeks or even months. In our Milky Way galaxy, supernovae occur approximately two to three times every 100 years. These explosions take place regularly throughout the cosmos, but they are usually too far to be observed.

 

Despite their brief lifetime, supernovae are also important storytellers about the nature of the cosmos. For example, these events have informed us that our universe is growing at an increasing rate and that these explosions contribute to the distribution of elements throughout the universe. When supernovae occur, elements and debris are spread violently into space at around 30,000 kilometres per second - about 300 times as fast as a bolt of lightning. This spread of material helps to enrich the interstellar medium by laying the grounds for the birth of new stars. Supernovae are also where most of the elements heavier than iron are created, which are needed to form new planets. In other words, these fiery deaths of stars create the elements needed for sustaining life, including carbon, oxygen. It is likely that the atoms that make up our bodies were once created in a violent supernova explosion.

 

The bright leftovers from a supernova explosion called SNR 0519-69.0. Here, multimillion degree gas is seen in X-rays from Chandra (blue). The outer edge of the explosion (red) and stars in the field of view are seen in visible light from the Hubble Space Telescope (NASA).

 

Records have indicated that ancient civilizations have witnessed supernovae long before the invention of the telescope, as far back as 185 A.D. in China. The term “nova” was inspired by Danish astronomer Tycho Brahe, who called the observation a “de nova stella”, or “new star”. Americans Fritza Zwicky and Walter Baade coined the modern term “supernova” in the 1930’s, who named the explosive event they saw in the nearby Andromeda galaxy.

Recent efforts to better understand supernova explosions have also revealed a variety of intriguing findings. For example, before exploding, supernovae appear to vibrate and hum. Supernovae that happen as far as 3000 light years away may also have impacts on Earth. Research suggests that the explosion’s gamma rays may cause a chemical reaction in the Earth’s atmosphere that would severely harm the planet’s ozone layer. These effects have lead to theories about a possible connection between these astronomical events and historical mass extinctions of species on Earth.

 

Type II supernovae do not happen to all large stars however. It is believed that stars 20-30 times as massive as our Sun do not die in a supernova, but instead collapse to form a black hole.

 

 

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