Chapter 17

Ten Things to Look for under a Dark Sky

In This Chapter

Counting the stars overhead

Finding the Milky Way and other galaxies

Looking for other faint fuzzies and lights in the sky

In this chapter, I describe ten things to look for the next time you’re lucky enough to be out stargazing under a dark sky. Most people live under a perpetual shroud of light pollution from towns and cities, so you may have to travel to see everything in this chapter. In fact, you’ll certainly have to travel to see them all, because some of them are only visible from certain parts of the Earth and at certain times of year. Chase down those dark skies; you won’t be disappointed.

The Number of Stars

The first thing you’ll notice as you move from city to countryside is the sheer number of stars overhead on a clear night. You should make a habit of counting how many stars you can see in a familiar constellation such as Orion, from different locations, and that number will help you work out just how good (or bad) your sky is.

In a city, you may see only a hundred or so of the brightest stars (those brighter than magnitude 4, say), while under darker conditions, you may see more than a thousand.

Once you’re out somewhere really dark, the only limit is your eyesight. Many thousands of stars are up there if you can catch them.

The Milky Way from Horizon to Horizon

The Earth’s galaxy, the Milky Way, is an impressive sight under a dark sky. You can catch glimpses of the Milky Way from the outskirts of towns and cities, from where it may appear as a faint patch overhead. Get away from light pollution, though, and you can see the band of the galaxy stretching across the sky.

If you ever get somewhere really dark, then you’ll begin to see structure in the Milky Way – dark bands and blobs that come from clouds of dust in the galaxy blocking out the starlight of the stars behind. One of the most breathtaking sights in the night sky is the Milky Way sitting overhead in an arch stretching from horizon to horizon.

The Andromeda Galaxy, M31

The next closest big galaxy to Earth’s is the Andromeda Galaxy or, to give it its catalogue number, M31. If you know where to look, you can catch glimpses of the Andromeda Galaxy from light-polluted skies, but you can see it best when it’s darker overhead.

The Andromeda Galaxy should look like a cigar-shaped smudge to your naked eyes. Through a good pair of binoculars or a telescope you can see more detail; perhaps you’ll catch a glimpse of the Andromeda Galaxy’s nearby companion galaxies, M31 and M110, or even see dark dust lanes within the main galaxy itself.

Remember that what you’re looking at is a galaxy that’s even bigger than the Milky Way, containing perhaps a trillion (1,000,000,000,000) stars. Aliens living on a planet orbiting a star in the Andromeda Galaxy would look up into their night sky and see a faint Andromeda-like smudge, too – our Milky Way galaxy seen from 2.5 million light years away.

The Triangulum Galaxy, M33

If Andromeda is an easy galaxy to find, its neighbour, the Triangulum Galaxy, or M33, is much more elusive. The Triangulum Galaxy is much fainter and therefore harder to find if any light pollution is around, but that makes it a great benchmark for dark skies. Try to chase down the Triangulum Galaxy to see whether your skies fall under the category of really dark.

You’ll have to travel out of the glow of suburbia to see M33, and even then it’s hard to spot. Even under clear rural skies you’ll have to use averted vision to catch it at all, but if you want to see the Triangulum Galaxy when you’re looking straight at it, then you have to head to a truly dark-sky site, many miles from the nearest town.

M33 looks like an even fainter version of the Andromeda Galaxy, almost impossibly faint, because it’s smaller, farther away, and has less than a tenth the number of stars than Earth’s nearby giant galaxy.

The Seven Sisters

A great test of your eyesight – and your dark skies – is the faint fuzzy known as the Pleiades, or the Seven Sisters (M45 to its friends). Under city skies, you see it looking like a distinct fuzzy patch. You may be able to pick out the brighter stars, but you need to be somewhere dark; you’ll have to wait for a long time to let your eyes adapt before you’ll see all seven.

Some people with excellent eyesight claim to be able to see more than seven stars in the Pleiades. Have a go, and after you’ve counted the stars with your naked eye, see how many you can spot through a pair of binoculars. Wow, right?

Aurorae

The elusive, shimmering colours of the northern or southern lights, the aurora borealis or aurora australis, are chased away by light pollution, and so you need to head out of town to see them. But your journey may not end there. In order to see the aurora best, you have to head to high latitudes.

In the northern hemisphere, you can see the aurorae best from the Arctic, northern Europe, Scandinavia, northern Russia, Canada and northern America states; in the southern hemisphere, you see them best from Antarctica, southern South America, New Zealand and southern Australia.

The aurorae don’t happen every night, only after strong solar storms, so keep an eye on that space weather!

Meteor Showers

Not strictly a dark-sky phenomenon, meteors are visible even in cities, but you can see lots more of them the darker your sky is. On any night, you can expect to see a few shooting stars under dark skies, but during meteor showers this rate increases dramatically. At the peak of an active meteor shower like the Perseids or the Geminids, you can expect the maximum rate to get up to 100 per hour – that’s 100 shooting stars an hour, more than one per minute! But light pollution will drown out the fainter ones, so in suburbia you may see only 30 per hour, and in a city centre perhaps only 10 an hour.

Zodiacal Light

If you’re lucky enough to be stargazing from a good dark site in spring or autumn, you may catch a glimpse of the elusive zodiacal light. Zodiacal light is light from the Sun scattering off particles of dust in the solar system and reflecting back to your eyes.

You can see zodiacal light only just after sunset twilight or just before sunrise twilight, and your window of opportunity is very narrow. Time your zodiacal light hunting for the end of evening astronomical twilight in spring or the beginning of morning astronomical twilight in autumn.

The zodiacal light looks light a faint cone of light – much less bright than the Milky Way – that stretches along the line of the zodiac, or ecliptic. Under exceptional dark skies, you can see a yellowish colour to the zodiacal light, which can stretch across the whole sky in an arc and can even get bright enough to cast a shadow.

Gegenschein

The elusive partner to the zodiacal light is called Gegenschein. The name is German, meaning counter-shine, and it’s light from the Sun reflecting off particles of dust that lie directly opposite the Earth from the Sun, in a part of the sky called the antisolar point. Because you see these dust particles lit face-on, the Gegenschein can be a bit brighter than the zodiacal light elsewhere in the sky.

You need to be somewhere really dark to see Gegenschein, and you need to look in the right direction. Follow the cone of the zodiacal light up, along the ecliptic, and look for a faint oval glow; this is Gegenschein, and not many people get to see it.

Airglow

If you’re somewhere with no manmade light pollution in the sky, you may notice that the sky still isn’t pitch black. That’s partly due to something called airglow, which is the emission of light by the molecules of gas that make up the atmosphere. Airglow is a perfectly natural phenomenon, but it means that the sky never gets truly dark. You may catch a glimpse of airglow low on the horizon, where you’re looking through more atmosphere, around one fist-height or 10 degrees above the horizon. It’ll look like a faint blue glow in the air.