December Stargazing: Two Supernovae and Winter's Brightest Stars

As winter's long nights descend, they offer a perfect opportunity to gaze upwards. The season provides some of the clearest and most spectacular views of the cosmos, with a host of bright stars and planets on vivid display.

A Tale of Two Supernovae

In November 1572, the Danish astronomer Tycho Brahe witnessed a sight that stunned him. After cloudy days cleared, he looked at the constellation Cassiopeia and saw a brilliant, unfamiliar star. "Amazed and as if astonished and stupefied," he wrote, doubting his own eyes. This was a supernova, an exploding star now known to be five billion times brighter than our Sun.

This event, classified as a Type Ia supernova, occurred when a dense white dwarf star catastrophically exploded after absorbing too much material from a companion. Centuries later, its remnant remains a powerful source of X-rays and radio waves.

In stark contrast, a Type II supernova is the violent death of a single, massive star. Chinese astronomers witnessed such an event on 4 July 1054. They recorded a 'guest star' so bright it was visible in daylight for 23 days. Today, we know its wreckage as the Crab Nebula, a tangled cloud of gas visible with a small telescope between the horns of Taurus.

The Crab still glows a millennium later because it is powered by the star's collapsed core, now a city-sized, rapidly spinning neutron star containing more mass than the Sun.

Winter's Celestial Spectacle

The December sky is split between fading autumn constellations in the west and the rising brilliance of winter in the east. The planet Saturn lingers in the western sky, with the Moon passing nearby on Boxing Day. Its rings appear almost edge-on through a telescope.

To the east, Jupiter dominates as the brightest nighttime object after the Moon. On 7 December, the Moon forms a striking line with Jupiter and the twin stars of Gemini, Castor and Pollux.

By late evening, the full splendour of winter constellations is revealed. Look for Auriga with bright Capella, Taurus marked by orange Aldebaran, and the distinctive shape of Orion near the horizon, flanked by Sirius, the night's brightest star.

December's Astronomical Highlights

Mercury makes its best morning appearance of the year in the first half of December, rising around 6am in the southeast. Spot it near the crescent Moon on the morning of the 17th.

The peak of the annual Geminid meteor shower arrives on the night of 13/14 December. These 'shooting stars', caused by dust from asteroid Phaethon, offer the year's most spectacular display, though moonlight will interfere after 1am.

We are long overdue for a new galactic supernova. Two prime candidates are the red supergiants Betelgeuse in Orion and Antares. When either finally explodes, it will shine almost as brightly as the Full Moon.

December Diary