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The Independent UK

Nigel Henbest

Stargazing in December: A tale of two supernovae

Nigel Henbest Gemini Mercury NASA Jupiter (Topic) Sun SpaceX the Crab Nebula

The beginning of winter is a great time to be taking up astronomy, with long dark nights and a multitude of bright stars on display. But the Danish astronomer Tycho Brahe was treated to a much finer spectacular than any of us, when he ventured outside in November 1572 to view the sky after a spell of cloudy days. Looking up at the familiar W-shape of stars making up the constellation of Cassiopeia, he saw a flaming intruder.

“Amazed and as if astonished and stupefied,” he later recalled, “I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes.”

But countryfolk passing by confirmed that a new star had indeed erupted onto the celestial stage. And this strange object has fascinated astronomers ever since – including myself, who researched into the lingering fireball four centuries later.

We now know that Tycho’s star was five billion times brighter than the Sun – so luminous that it’s classified as a supernova. It’s one of a handful of supernovae in our Galaxy, the Milky Way, that have been visible to the naked eye. But telescopic surveys are showing up a couple of thousand supernovae every year in other galaxies; and the newly opened Vera C. Rubin telescope in Chile should push the total well into the millions.

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!

What’s Up

It’s a sky of two contrasting halves this month. To the west you’ll find the large dim constellations of autumn, gradually sinking to the horizon; while in the east the bright winter star-patterns are rising.

The night sky at around 10pm this month (Nigel Henbest)

Helping to liven up the western skies is the planet Saturn: the Moon lies near it on Boxing Day. Check out Saturn with a small telescope to see the famous rings almost edge-on to us.

Its bigger sibling, Jupiter, lies over among the winter constellations. You can’t miss the giant of the solar system, as it’s the most brilliant object on view in the night sky this month, bar the Moon. To the upper left of Jupiter lie the twin stars of Gemini – Castor and Pollux – and the Moon forms a striking tableau with the planet and the two stars on 7 December.

By late evening, the full panoply of winter constellations is visible in the east. Above Jupiter and Gemini lies the celestial charioteer, Auriga, crowned by bright Capella. To the right is Taurus, the bull, with orange-red Aldebaran marking its angry eye, and the pretty star cluster of the Pleiades (Seven Sisters) nearby. Closer to the horizon you’ll find the distinctive shape of Orion (the hunter), with his two dogs Canis Major – featuring the brilliant Sirius – and Canis Minor.

During the first half of December, try to spot elusive Mercury making its best morning appearance of the year. The innermost planet is rising around 6 am in the south-east – so you’ll need a clear horizon in that direction – and the brightening sky will hide the diminutive world within an hour. A date to highlight is the morning of 17 December, when Mercury lies just to the left of the crescent Moon.

On the night of 13/14 December, look out for slow bright shooting stars streaming out from the direction of Gemini. The Geminid meteors are dust grains from the asteroid Phaethon blazing as they burn up in our atmosphere in the most spectacular shooting star display of the year, though the show is spoilt by moonlight after the Moon rises at 1am.