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These shining arks of stars come in all shapes and sizes, many the result of cosmic collisions

Andromeda Galaxy by Steve Peters

Andromeda Galaxy by Steve Peters

Galaxies are concentrations of millions or billions of stars, gravitationally bound together along with gas clouds and pockets of dust. There are probably over 100 billion of them in the Universe. Some of the largest nearby galaxies appear in the night sky as faint smudges of light, but it was only in the early 20th century that astronomer Edwin Hubble proved that they actually exist well beyond the Milky Way. Before then, they were thought to be spiral-shaped nebulae on the outskirts of our own Galaxy.

Hubble also established that galaxies vary in shape and size. Two-thirds have distinctive spiral patterns, while the rest range from neat ellipticals to irregular blobs. They can be dwarves containing millions of stars or giants harbouring trillions. Astronomers are still piecing together why this is the case, but collisions and mergers seem to be important in determining how a galaxy evolves. Central black holes also seem to govern how gas is consumed and when stars are formed within these cosmic conurbations.

Hidden Mass

Galaxies are much more massive than they look. Around 90 per cent of their mass is not in luminous stars and gas, but in unseen 'dark matter'. It's arranged in a spherical halo, which governs the motions of the stars within. This invisible cocoon explains why the outskirts of spiral galaxies spin faster than if they were influenced by the quantity of stars and gas alone. Dark matter also governs how galaxies clump together under gravity to form filaments and clusters. Yet dark matter remains an enigma, and astronomers are still trying to discern what it is. It must be exotic as it does not absorb or emit light.

Spiral galaxies such as the Milky Way are named for the arcs of bright stars that corkscrew into their centres. The spiral is a density wave embedded in a flattened disc of stars and gas that is arranged around a central bulge. Bright stars form where gas clouds are compressed. The disc is full of young stars and gas, and tends to be blue; the bulge appears redder. Discs form when a cloud of gas collapses under its own gravity, spinning faster as it shrinks vertically. Spirals are common across space, apart from in the centres of galaxy clusters, where discs are easily destroyed by collisions.

Shaped like rugby balls, elliptical galaxies are much like the bulges of spirals, but lack any disc. They contain little gas, and few stars are being formed within them. Old, red stars are the norm, travelling on inclined elliptical orbits. Groups of elliptical galaxies are often found in the centres of galaxy clusters.

Lenticular galaxies are lens shaped, their classification falling between spirals and ellipticals. Many are similar to spiral galaxies, containing a relatively small disc and large bulge, but lacking the spiral arms. These may be faded spirals, in which star formation has ceased. Others are likely to be the result of galaxy collisions, which could have ripped off part of a larger disc, or shut down star formation after a vigorous burst.

Irregular galaxies do not fall into any of the other main classification categories — they have no distinctive shape. This may be because they have been distorted in a collision or they may have formed that way. Some dwarf galaxies condensed in a haphazard manner from gas clouds and haven't settled into an ordered state.

What To See: Deep Sky — Glimpsing Galaxies

The Andromeda Galaxy, M31
Constellation: Andromeda
RA 00h 42m 42s, dec. +41 16' 00"
The magnificent Andromeda Galaxy is the nearest large galaxy to the Milky Way, and it is possible to see it with the naked eye. Under dark, Moon-free skies, you should be able to find this spiral galaxy as a faint misty patch a short distance from the band of the Milky Way without optical aids. Using binoculars, you'll find it with little or no difficulty. It will be oval in appearance — although you won't be able to make out any of the individual stars within it. Through a 6-inch telescope the galaxy appears as a larger, elongated oval shape with a core that shows up as a slightly brighter area.

The Whirlpool Galaxy, M51
Constellation: Canes Venatici
RA 13h 30m 00s, dec. +47 16' 00"
The Whirlpool Galaxy is a magnificent face-on spiral located in Canes Venatici. It can be found not far from mag. +1.9 Alkaid (Eta (—) Ursae Majoris). You'll need a large telescope to see its spiral arms clearly.

The Triangulum Galaxy, M33
Constellation: Triangulum
RA 01h 33m 54s, dec. +30 39' 00"
M33 can just be seen with the naked eye under pristine dark skies, but light pollution means binoculars at least. It sits between mag. +2.2 Hamal (Alpha (a) Arietis) and mag. +2.1 Mirach (Beta (b) Andromedae).

The Sombrero Galaxy, M104
Constellation: Virgo
RA 12h 40m 00s, dec. -11 37' 23"
Located just within Virgo, this spiral galaxy is easy to see in any scope. A 6-inch instrument shows an elongated glow, but its defining characteristic is a dark dust lane that cuts across the south of the central halo.

M81 and M82
Constellation: Ursa Major
RA 09h 55m 33s, dec. +69 03' 55"
These galaxies in Ursa Major, M81 or Bode's Galaxy (co-ordinates above)and M82 the Cigar Galaxy, are close to each other in the sky, so we're treating them as one sight here. With a small telescope and a low magnification eyepiece, you'll be able to see them in the same field of view.

The Leo Triplet
Constellation: Leo
RA 11h 18m 55s, dec. +13 05' 32"
The Leo Triplet is comprised of the spiral galaxies M65 (co-ordinates above), M66 and NGC 3628, and lies about halfway between mag. +3.3 Chertan (Theta (θ)Leonis) and mag. +6.6 Iota (ι) Leonis. Larger telescopes will show them clearly. Another group, M95 and M96, is nearby.

The Pinwheel Galaxy M101
Constellation: Ursa Major
RA 14h 03m 12s, dec. +54 20' 57"
This face-on spiral galaxy is comparable in size to the Milky Way, and while it can be spotted in binoculars its magnitude of +7.9 means you'll need dark skies and a 6-inch telescope to see its spiral arms.

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