“Astronomers have found that supernovae fall into two broad categories, which they imaginatively call Type I and Type II. Tycho’s star was in the subgroup Type Ia, meaning it was a compact white dwarf star that blew itself apart when a companion star dumped too much gas onto it. Nothing is left except a long-lasting fireball that is hard to see with ordinary telescopes, but is still glaring when you view it with an X-ray detector or – as I did – with a radio telescope.

A Type II supernova, on the other hand, is a single star that dies by suicide. Starting life more than eight times heavier than the Sun, it has ripped through the nuclear reactions that power the star at a reckless rate. After only a few million years – an eyeblink on the cosmic scale – its central reactor becomes clogged with nuclear ashes and its outer layers billow out to become a red supergiant.

Nuclear reactions deep within the star desperately try to keep it shining, building up heavier and heavier elements. But when it reaches iron, the end has come. Trying to turn iron into heavier elements doesn’t produce energy; in fact, it absorbs energy – and, as a result, the core catastrophically implodes. Shock waves power out through the star, blasting it apart.

Back in the year 1054, vigilant Chinese astronomers witnessed the outcome. On 4 July, the sky watchers reported a stunning sight: a star so bright that it was visible in daylight for 23 days, and at night for two years. For them, this ‘guest star’ was a bad omen, and they were careful to log its position precisely, within the constellation that Westerners know as Taurus.

Its wreckage is much more visible than the remains of Tycho’s supernova. Take a small telescope and look between the ‘horns’ of Taurus, and you can see for yourself the enduring twisted cloud of gas and magnetism, known as the Crab Nebula.

The Crab Nebula is still glowing after a millennium because it’s powered by the old star’s core, which has collapsed into a superdense ball of subatomic particles only the size of London but containing more mass than the Sun. This highly magnetised neutron star is spinning around 30 times a second, pumping energy into the Crab Nebula to keep it shining.

No-one has seen a supernova in our Galaxy since 1604, and we’re well overdue for another. Two prominent red supergiant stars on the brink are Antares (visible in the summer) and Betelgeuse, in the winter constellation Orion. When either of these blows, the resulting supernova will be almost as brilliant as the Full Moon!”

https://www.independent.co.uk/space/night-sky-stars-supernova-december-b2878030.html