Stargazing in October: Dark secrets of the Local Group

How far can you see? Well, as I write this, the uplands of the Cotswolds appear in the hazy distance some 40km away. In terms of astronomy, of course, our horizons are much broader. Saturn, on view tonight, is 1,420 million km away, while the star Deneb clocks in at 25,000 trillion kilometres, or 2600 light years: the light we see from Deneb left it when the Babylonians were building their famed Hanging Gardens.

But look up towards the zenith on a dark moonless night, and you’ll see a faint fuzzy patch that puts all these objects to shame. The Andromeda galaxy lies 2.5 million light years away. We are seeing Andromeda as it was when the first humans began to evolve.

It’s a shame it lies so far off, because the Andromeda galaxy would be a stunning sight in close-up – a stately spiral shape comprising hundreds of billions of stars, along with glowing gas clouds and dark cobwebs of interstellar gas. It’s a close twin to our own Milky Way, the galaxy that’s home to the sun and all the stars we see at night.

And there’s a smaller sibling on view tonight, the Triangulum galaxy – though “small” is a relative term, as it is still host to over 10 billion stars. Seek out this dim glowing blur with binoculars near the sharp end of the constellation Triangulum (the Triangle), to the lower left of Andromeda. If you’re lucky enough to be out on a totally black night in the desert, you may spot this galaxy with your unaided eye – pushing your record for distance-vision to almost 3 million light years.

These three star-cities are not alone. Between and around them swarm dozens of other galaxies, making up the Local Group of galaxies. Most of these cosmic neighbours are far smaller, some containing only a million stars.

You may wonder why astronomers spend time studying these dwarf galaxies, when far away there are much more spectacular specimens, including starburst galaxies and quasars. It’s because our neighbourhood is just so ordinary: it’s likely to be typical of conditions throughout most of the universe. The Local Group is a microcosm of the cosmos.

The smallest galaxies on our doorstep are the most common kind of star-grouping in the cosmos, yet they’re so faint we can’t observe them outside the Local Group. And they are the building blocks of galaxies: in the distant past, millions of these galaxies coalesced to make galaxies like our Milky Way.

And the process continues to this day. From the southern hemisphere, you can easily spot two more members of our group, the large and small Magellanic Clouds. They look like star-clouds torn from the Milky Way. In fact, the opposite is true: our galaxy’s gravity is shredding the two Magellanic Clouds, and eventually the Milky Way will swallow them up.

The most intriguing of our neighbours are the invisible galaxies that are believed to throng around us. The normal matter in our universe is far outweighed by unseen “dark matter”, and our best theories of galaxy birth say that a lot of it ended up in small clumps. Some of these included stars, and we see these clumps as the dwarf galaxies near the Milky Way.

But in many cases, astronomers think, there wasn’t enough gas to make stars inside these dark condensations. So a lot of our near neighbours may be invisible, composed only of dark matter – and researchers are hot on their trail. The Local Group is the only place where these strange dark galaxies are close enough for us to study, providing us with unique insights into the secrets of the cosmos.

What’s up

You may think that Venus, hanging in the gloaming after sunset, is glorious enough – but this month it turns up the brightness control even more, as the planet heads towards the Earth. The evening star pairs up with the crescent moon on 9 October, making an unforgettable sight.

Second only to Venus in brilliance, the giant planet Jupiter lies to the south. To its right, you’ll find the fainter Saturn – though the ringworld still outshines most of the stars. The moon glides under the two giant planets on the nights of 13-15 October.

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Higher up in the southern sky, you can’t miss a large square of stars. To the ancient Greeks, this was the body of the flying horse Pegasus. Oddly enough, they saw him upside down in the heavens: the stars to the right of the square depict his neck and head, with his nose (the star Enif) pointing upwards.

Pegasus was the offspring of the sea-god Poseidon and the monstrous Medusa (so ugly that humans turned to stone just looking at her). And his birth was pretty unusual: the winged horse sprang from his mother’s neck when the superhero Perseus decapitated her.

You’ll find Perseus over in the east, next to princess Andromeda. He won her hand by killing the sea-monster Cetus (to the lower left of Pegasus), and using Medusa’s severed head to turn Andromeda’s other suitors to stone.

Meanwhile, back in the dawn sky, towards the end of October you have a chance to spot the innermost planet. This month, elusive Mercury is putting on its best morning appearance of the year, low in the east before sunrise.

Diary

9 October: crescent moon very near Venus

10 October: crescent moon near Antares

13 October, 4.25 am: first quarter moon near Saturn

14 October: moon between Jupiter and Saturn

15 October: moon near Jupiter

16 October: Venus near Antares

20 October, 3.56 pm: full moon

21 October: maximum of Orionid meteor shower

22 October: moon near the Pleiades

23 October: moon near Aldebaran

25 October: Mercury at greatest elongation west

27 October: moon near Castor and Pollux

28 October, 9.05 pm: last quarter moon

29 October: Venus at greatest elongation east

31 October, 2.00am: British Summer Time ends

Philip’s 2022 Stargazing (Philip’s £6.99) by Nigel Henbest is your guide to everything that’s going on in the sky next year