A TOURIST GUIDE TO THE PLOUGH

It is one of the most recognisable patterns in the sky. As such, The Plough, is an excellent place from which to start navigating the starry heavens. Stuart Atkinson guides us through the myths.

Stuart Atkinson

There are many famous patterns of stars in the night sky, but probably the most famous is one that can be seen all night long and all year round from mid-northern latitudes and above. However, it is not a constellation but an asterism. An asterism is an eye-catching pattern of stars that stands out among the fainter siblings. To many of the UK’s skywatchers, it is known as The Plough, because its seven stars resembles an old horse-drawn plough used on farms in years gone by. But to most western observers it is known as The Big Dipper because it forms the shape of a giant starry spoon.

This fascinating asterism is known by other names across the world too. In Chinese astronomy, it is Běi Dou, the Northern Ladle, representing the seven Taoist gods who stir the cosmic pot of immortality. To Hindus, the Plough is Saptarishi, the Seven Sages, seven rishis who guide humanity from the heavens. Norse skywatchers, bathed in the red and green glow of the northern lights, know it as Karl’s Vagn, a mighty wagon drawn by oxen across the northern sky, carrying the dead to Valhalla.

In Finnish folklore, the Plough is Otava, a sacred wooden ladle used by the sky god Ukko to scoop grain from the Milky Way’s storehouse. To the ancient Egyptians, who were of course famously fascinated by the night sky, the Plough was Meskhetyu the foreleg of the bull-god Taurus, linked to Ursa Major as the thigh of Seth, god of chaos and deserts.

While some asterisms contain stars from a number of different, neighbouring constellations – for example, the famous Summer Triangle is made of the brightest stars in the constellations of Cygnus, Lyra and Aquila – the Plough lies entirely within the constellation of Ursa Major, the Great Bear. Because its seven stars are all roughly the same colour and the same brightness, and have no particularly bright neighbours, the Plough really does stand out very prominently in the sky.

It is not just visually striking, it is useful too. An imaginary line drawn between two of its stars very helpfully points directly to Polaris, the Pole Star, allowing millions of telescope users and astrophotographers around the world to align their equipment accurately enough to track objects as they move across the night sky. Other stars and constellations can be found by drawing other imaginary lines out of the constellation.

Although the stars of the Plough all look close together, they are not gravitationally bound like the stars in a star cluster; they are all at slightly different distances and are slowly drifting apart. However, modern analysis of their motions through space suggests that most of them formed in the same birth cluster long ago.

Their constant drift apart means that in the far future, any stargazers looking up at the sky will not see our beloved Plough because its shape will have been distorted beyond recognition. But since that will not happen for millions of years, we can confidently enjoy it for a long time to come, and use it as a gateway to finding our way around the night sky.

Stuart Atkinson is the author of 18 astronomy books, including a biography of Felicette, the first and only cat to go into space.

ALKAID

Marking the end of the Plough’s handle, Alkaid’s magnitude of 1.9 makes it the asterism’s 3rd brightest star. 104 light years from Earth, it is a hot, blue main-sequence star that’s over 100 times brighter than the Sun and spins at 200 km/s.

MIZAR

At magnitude 2.2, Mizar is the next star along the Plough’s kinked handle. If you are blessed with good eyesight, or look at it through binoculars or a small telescope, you will see it has a much fainter star, 4th magnitude Alcor, close to it. Mizar itself is a quadruple system when viewed telescopically.

ALIOTH

The next star along the handle, magnitude 1.8 Alioth is 83 light years from Earth. It is probably the asterism’s most unusual star. Its strong magnetic field causes its spectrum to vary, and it rotates every 4 days, making it one of the fastest-spinning bright stars.

MEGREZ

With a magnitude of 3.3, Megrez, which connects the Plough’s handle to its bowl, is noticeably the faintest of the asterism’s seven stars. Lying 80 light years away, it is a white, main-sequence variable star.

PHECDA

This magnitude 2.4 star is 83 light years from Earth. It is a main-sequence star similar to Vega, with a luminosity about 200 times the Sun’s.

MERAK

Magnitude 2.3 Merak is the fainter of the ‘Pointers’, a pair of stars which point directly to the Pole Star. Lying 80 light years away, it is a subgiant star that is spinning very rapidly (over 100 km/s at its equator).

DUBHE

The final star on our Tour, Dubhe is the brighter of the Pointers. A giant star about 30 times the Sun’s radius, Dubhe’s magnitude of 1.8 makes it the asterism’s brightest star. Noticeably whiter than the other stars in the Dipper, it is also the farthest away – more than 123 light years from Earth.