When a suitably massive star comes to the end of its life, it collapses in on itself in a massive supernova explosion. If that original star was between 10 and 29 solar masses it collapses to the point where the protons and electrons in its core are squashed together leaving behind a neutron star.

Neutron stars are incredibly small and incredibly dense. They cram roughly 1.3 to 2.5 solar masses into a sphere of roughly a few miles in diameter.

If we could extract just one sugar-cube sized piece of a neutron star, it would weigh more than one billion tons. That's equivalent to the weight of Mount Everest.

If the neutron star weighs more than three solar masses, it will continue to collapse in on itself to form a black hole.

Neutron stars also have very large magnetic fields (approximately one trillion times stronger than the Earth's magnetic field) and are the strongest magnets in the universe. They have a surface temperature of one million Kelvin (the Sun's surface temperature is 5,778 Kelvin).

Magnetars are another type of neutron star. The magnetic field of a neutron star is one trillion times that on Earth, but the magnetic field of a magnetar is another 1,000 times stronger.

In all neutron stars, the star's crust is linked to its magnetic field so that any change in one affects the other.

So, a small movement in the crust will cause ripples in the magnetic field. For a magnetar, such movement cause a huge burst of electromagnetic radiation. In December 2004, a massive magnetar blast blinded satellites and partially ionised the Earth's atmosphere. The cosmic blast only lasted one-tenth of a second but it released more energy than the Sun has emitted in 100,000 years.

* No Ironmen were harmed in the making of this Sunday Science.

When two neutron stars smashed into each other in August, we weren't quite sure what was left over. But it's looking increasingly likely that it could be the biggest neutron star we've ever detected. The unprecedented celestial collision produced a gamma-ray burst and astronomers at LIGO detected the resulting gravitational waves.

If you'd like to find out more about why pulsars spin so fast and so regularly, you can read more here.

The story of the discovery of pulsars is also one of my favourite astronomy anecdotes as it features a great female scientist, Jocelyn Burnell (nee Bell). I've copied and pasted this fantastic article from the American Society of Physics covering the discovery:

"In 1967, when Jocelyn Bell, then a graduate student in astronomy, noticed a strange “bit of scruff” in the data coming from her radio telescope, she and her advisor Anthony Hewish initially thought they might have detected a signal from an extraterrestrial civilization. It turned out not be aliens, but it was still quite exciting: they had discovered the first pulsar. They announced their discovery in February 1968.

Bell, who was born in Ireland in 1943, was inspired by her high school physics teacher to study science, and went to Cambridge to pursue her Ph.D. in astronomy. Bell’s project, with advisor Anthony Hewish, involved using a new technique, interplanetary scintillation, to observe quasars. Because quasars scintillate more than other objects, Hewish thought the technique would be a good way to study them, and he designed a radio telescope to do so.

Working at the Mullard Radio Astronomy Observatory, near Cambridge, starting in 1965 Bell spent about two years building the new telescope, with the help of several other students. Together they hammered over 1000 posts, strung over 2000 dipole antennas between them, and connected it all up with 120 miles of wire and cable. The finished telescope covered an area of about four and a half acres.

They started operating the telescope in July 1967, while construction was still going on. Bell had responsibility for operating the telescope and analyzing the data — nearly 100 feet of paper every day–by hand. She soon learned to recognize scintillating sources and interference.

Within a few weeks Bell noticed something odd in the data, what she called a bit of “scruff.” The signal didn’t look quite like a scintillating source or like manmade interference. She soon realized it was a regular signal, consistently coming from the same patch of sky.

No known natural sources would produce such a signal. Bell and Hewish began to rule out various sources of human interference, including other radio astronomers, radar reflected off the moon, television signals, orbiting satellites, and even possible effects from a large corrugated metal building near the telescope. None of those could explain the strange signal.

The signal, a series of sharp pulses that came every 1.3 seconds, seemed too fast to be coming from anything like a star. Bell and Hewish jokingly called the new source LGM-1, for “Little Green Men.” (It was later renamed.)

But soon they managed to rule out extraterrestrial life as the source of the signal, when Bell noticed another similar signal, this time a series of pulses arriving 1.2 seconds apart, coming from an entirely different area of the sky. It seemed quite unlikely that two separate groups of aliens were trying to communicate with them at the same time, from completely different locations. Over Christmas 1967, Bell noticed two more such bits of scruff, bringing the total to four.

By the end of January, Bell and Hewish submitted a paper to Nature describing the first pulsar. In February, a few days before the paper was published, Hewish gave a seminar in Cambridge to announce the discovery, though they still had not determined the nature of the source.

The announcement caused quite a stir. The press jumped on the story–the possible finding of extraterrestrial life was too hard to resist. They became even more excited when they learned that a woman was involved in the discovery. Bell later recalled the media attention in a speech about the discovery: “I had my photograph taken standing on a bank, sitting on a bank, standing on a bank examining bogus records, sitting on a bank examining bogus records. Meanwhile the journalists were asking relevant questions like was I taller than or not quite as tall as Princess Margaret, and how many boyfriends did I have at a time?”

Other astronomers were also energized by the finding, and joined in a race to discover more pulsars and to figure out what these strange sources were. By the end of 1968, dozens of pulsars had been detected. Soon Thomas Gold showed that pulsars are actually rapidly rotating neutron stars. Neutron stars were predicted in 1933, but not detected until the discovery of pulsars. These extremely dense stars, which form from the collapsed remnants of massive stars after a supernova, have strong magnetic fields that are not aligned with the star’s rotation axis. The strong field and rapid rotation produces a beam of radiation that sweeps around as the star spins. On Earth, we see this as a series of pulses as the neutron star rotates, like a beam of light from a lighthouse.

After discovering the first pulsars, Jocelyn Bell finished her analysis of radio sources, completed her PhD, got married and changed her name to Burnell. She left radio astronomy for gamma ray astronomy and then x-ray astronomy, though her career was hindered by her husband’s frequent moves and her decision to work part time while raising her son. Anthony Hewish won the Nobel Prize in 1974 for the discovery of the first pulsars. Over 1000 pulsars are now known.**

As for little green men, they haven’t been found yet, but projects such as the Search for Extra Terrestrial Intelligence (SETI) are still looking for them."
​
** Now, more than 2,000 pulsars have been discovered, but this figure was accurate in 2006 when the article was originally published.

If a star has enough mass, it will explode in a monumental supernova explosion at the end of its life.

When a supernova occurs, it burns brighter than an entire galaxy and radiates more energy than our Sun will in its entire lifetime, all within just over a minute. According to NASA, supernovae are “the largest explosion that takes place in space.”

​For a supernova to happen, the star must be between eight and 15 solar masses. One supernova also occurs roughly every second in our universe, but you can't see them all. In fact, a supernova occurs in our galaxy once roughly every 50 years.

Most of the universe's chemical elements are also made in a supernova thanks to the phenomenal heat and pressure in these explosions.

There are two ways a star can go supernova.

A star can pull matter from a nearby neighbour until it has sufficient mass that a runaway nuclear reaction starts and ends in a supernova explosion.

For a Type I supernovae, a White Dwarf usually pulls matter from its companion star in a binary system and builds up enough mass to go supernova.

Type 1 supernovae are thought to blaze with equal brightness at their peaks, so they are used by astronomers as "standard candles" to measure cosmic distances.

A large star runs out of nuclear fuel and collapses under its own gravity. But, instead of ballooning to a Red Giant and contracting to a White Dwarf, it explodes in a supernova. 

Before it reaches the supernova stage, heavier elements gradually build up in the centre of the star. The star forms layers, like an onion, with the heavier elements sat at the centre and the lighter ones towards the outside.

Once the star has built up enough mass (and passes something called the Chandrasekhar limit) the star implodes. The core heats up and becomes even denser before the outer layers are thrown off in a dramatic supernova explosion.

A neutron star is left behind. We'll look at neutron stars next week.

If the original star is 25 times more massive than our Sun, a black hole is left behind. And if the star is more than 100 times more massive than our Sun, nothing is left behind. The star just explodes and leaves nothing behind.

I've oversimplified here as there are three subsets of the type I supernovae: Ia, Ib and Ic. The Ia category supernovae begin as the binary systems we described earlier where the White Dwarf pulls matter from its companion.

Type Ib and Ic supernovae are caused by the implosion of massive stars that have already shed some of their outer layers. Type Ib has lost its hydrogen layer and Type Ic has lost its hydrogen and helium layers.

But supernovae don't like to conform. For example, a very bizarre supernova was recently detected that explodes again and again.

NASA also managed to catch a supernova explosion with the Kepler Space Telescope last year. This animation was produced from the telescope's data as it recorded the supernova:

The other week, I explained how a dying star balloons into a Red Giant when it runs out of fuel. The next stage in this evolution is a White Dwarf.

A Red Giant will eventually blow off the material contained in its outer layers. These expelled outer layers may go on to form planets (more on that later).

All that's left behind is a small and incredibly dense and hot core.

This is a White Dwarf. 

Sirius B (pictured above) is a White Dwarf. Its mass is 98 percent of our own Sun but it is only 12,000 kilometres in diameter, making it smaller than even the Earth and much denser. 

Because they are small and incredibly dense, the gravity on the surface of a typical White Dwarf is around 350,000 times stronger than the gravity on Earth.

No hydrogen fusion occurs to counteract the force of gravity. So, all the matter gets squashed together until all the electrons are pushed far further together than for normal matter.

When all the electrons are squashed together, the matter is now known as a "degenerate" gas. It can't be squashed any further as quantum mechanics dictates there is no more available space for the electrons.​

No. Smaller stars, such as Red Dwarfs, don't reach the Red Giant stage and just burn through all their hydrogen. They should then evolve into White Dwarfs - but Red Dwarfs take trillions of years to consume their hydrogen fuel (which is longer than the age of the universe) so none have gone through this transformation yet.

Massive stars, which are about eight times the mass of our Sun, will never be white dwarfs. Instead, they explode in a violent supernova and either leave behind a black hole or a neutron star. We'll look into the massive supernova explosions that form neutron stars next week...

Many White Dwarfs will fade and eventually become Black Dwarfs when all their energy is radiated away.

However, if a White Dwarf is part of a binary system (where two stars orbit around one another) then its gravitational pull is so large that it may start to pull material off its companion star. ​Adding more mass to the White Dwarf in this way may cause it to become a neutron star or cause a supernova explosion.

If it only pulls off a small amount of matter from the companion star, a smaller explosion called a "nova" may occur. This process can be repeated several times caused a small cosmic fireworks display.

But, if you have two White Dwarfs in a binary system then they may merge together and, again, cause a supernova explosion.

The first exoplanet to be discovered is believed to have been orbiting around a White Dwarf waaaaaay back in 1917. While White Dwarf orbiting exoplanets are highly unlikely to support life, they do provide us with a wealth of information on how to analyse potentially life-harbouring exoplanets.

White Dwarf is also a rather good magazine from British games manufacturer Games Workshop.

And here are five fascinating facts about White Dwarfs and a jolly good video about them:
​

No you smeghead, I'm not talking about the smash Brit comedy Red Dwarf.

Accounting for 80% of the stellar population, Red Dwarfs are the most common type of star in the universe.

​They are smaller than our Sun with between 7.5% and 40% of its mass. Their reduced mass means they have a cooler surface temperature of approximately 3,500K, compared to 5,750K for our Sun. ​A Red Dwarf generates energy in the same way as our Sun, fusing hydrogen into helium.

​Because they are small and cool stars, there's less nuclear fusion and so Red Dwarfs emit only around 10% of the Sun's light - some of the smallest examples have just one ten-thousandth of the Sun's luminosity.

​One of the major differences between Red Dwarfs and other stars is that they're "convective". What this means is that when hydrogen fuses to form helium, the helium does not accumulate in the core. Instead, the helium is continually being mixed throughout the star.

​This means that the nuclear reactions in the star are slowed down and Red Dwarfs have a very long lifetime. It's estimated that a Red Dwarf with a tenth of the Sun's mass will live for at least 10 trillion years. And the lighter a red dwarf is, the longer it lives. ​Because they have such a long life, Red Dwarfs are used to calculate the age of star clusters. 

​Computer modelling suggests Earth-type planets are more likely to form around Red Dwarf stars. One Red Dwarf hit the headlines in February when NASA announced it had seven Earth-sized planets, with two planets orbiting the star in its habitable zone. The TRAPPIST-1 Red Dwarf is 39 light years away from Earth.

Main sequence stars are classified by their temperature. The hottest are blue stars and the coolest are our Red Dwarfs.

Did you know that about 40% of Red Dwarfs host "super earth" planets? Click here to read 10 interesting facts about our universe's most popular stars. Here's a great little video explaining Red Dwarf stars:

And I couldn't live with myself if I didn't include one video from the Red Dwarf show, here are the top 10 highlights from the sci-fi comedy: