This Monday 10th September one of two bright-green comets will be making an appearance in the night sky.

A comet is a small icy body composed of frozen rock, dust and gases such as carbon dioxide, carbon monoxide, cyanogen, methane, and ammonia. These balls of ice – or dirty snowballs as they are sometimes known – orbit the sun in highly elliptical orbits.

When the comet reaches perihelion – that is when it reaches the closest point to the Sun in its orbit – the heat from the sun turns the ice into gas. The small, 10-30 km wide, icy nucleus is therefore expanded into a gaseous ‘coma’ with a tail extending millions of miles pointing in a direction away from the Sun. The UV light from the Sun interacts with the gaseous ‘coma’ causing the comprising atoms and molecules to be ionized and/or excited. As the atoms relax into their ground state they emit light, which is the glow that we see. The colour of this glowing light depends on the composition ratios, which for green is predominantly cyanogen CN2 and diatomic carbon C2.

This Monday 10th September at 6:40 GMT comet 21P will be reaching perihelion and will appear to travel across the night sky with an apparent magnitude of 6.5-7. The comet will thus be observable with a good pair of binoculars.

In astronomy the apparent magnitude (m) of an object is the brightness that it appears in the night sky, which depends on the objects intrinsic luminosity and the distance it is from the observers observing it on Earth. The scale is logarithmic, so that each scale of magnitude has a change in brightness of the order 2.5. The reference frame for the scale is set at the brightness of our closest star Alpha Centauri, with m=0. Stars brighter than Alpha Centauri therefore go into the negative and less bright stars continue into the higher numbers.

As the comet continues in its orbit it will leave behind a trail of gas and dust – which is what gives rise to the annual Draconid meteor shower, peaking next month on October 8th at 23:30 GMT.

Later in the year, on December 16th, comet 46P will be making an appearance. Another bright-green comet but this time with a magnitude of 3, which means no binoculars are necessary.

By Amira Val Baker