Sky Matters: Galactic Cannibalism is actually very common

As the summer months draw to a close the autumn stars make their first appearance. 

The sky gets darker earlier, and because the Sun sets farther and farther below the horizon the nights begin to take on a truly “dark” hue.

And with September in Ireland sometimes fortunate enough to give us an Indian Summer, this can be a very pleasant time of the year with a surprising number of clear nights.

The Moon is “new” on September 1, meaning it never rises above the horizon and so never interferes with our view of the night sky.

The autumn equinox occurs on September 22 (at 3:21 in the afternoon to be precise!) and this means the sun is directly over the equator and nights and days are of equal length.

On September 16 the moon is full and is also referred to as the Harvest Moon, being the full moon that occurs closest to the September equinox.

This year it also marks the date of a “penumbral” lunar eclipse, meaning the Moon passes into the Earth’s partial shadow as it rises and if you really want to test your eyesight you can try to see a slight dimming of the moon from moonrise until about 9:30 pm.

September is a great month to see the farthest object in the universe that can be seen with the unaided eye.

Located next to the Great Square of Pegasus, 45 degrees above the horizon towards the south-east at 11pm, is the Andromeda Galaxy (see the finder chart in the skymatters newsletter online or at www.bco.ie/skymatters).

Similar to our home Milky Way galaxy, the Andromeda Galaxy is made up of stars which are largely held together by the gravitational pull they exert on one another – in this case, up to a trillion stars (making Andromeda larger than the Milky Way, though by how much is hotly debated).

Seen with the unaided eye the galaxy appears as a faint smudge and you may have to look slightly to one side to see it more clearly, a trick well known to skywatchers called “averted vision”.

But that visible smudge consists of light of billions of stars which has been travelling for 2.5 million years across the vast emptiness of space between our two galaxies, so definitely worth more than a second glance.

If you look at Andromeda with binoculars or a telescope, you will see it is more extended than a smudge – in fact if you had super-sensitive eyes you would see the galaxy extend to more than six times the diameter of the full moon.

Of course Andromeda looks faint because it is so far away, but if you can wait about 3.7 billion years it will be a truly spectacular sight as it ploughs into the Milky Way.

This type of “galactic cannibalism” is actually very common in the universe and both Andromeda and our own Milky Way have already cannibalised, or are the process of cannibalising other galaxies.

Closer to home and looking towards the north-east after sunset you can catch the first glimpses of a cluster of young stars called the Pleiades, or Seven Sisters.

The name is derived from the average number of stars that people can see with their unaided eyes, but eagle-eyed skywatchers can see more.

The Pleiades have an intensely cold blue colour and photographs show the stars are still enshrouded in the gases from which they were born.

Did you know?

Only 5% of the universe is made up of normal matter. The remaining 95% is made up of dark energy (70%) and dark matter (25%), and we have almost no idea what these are.


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