how to…ob­serve the an­dromeda Galaxy

Star clus­ters galore rest in Cas­siopeia’s ‘W’ shape

All About Space - - Contents -

How­ever much you dress it up with tech­ni­cal terms and scrawled equa­tions, as­tron­omy is ba­si­cally the study of things that are very far away. Even the clos­est ob­jects astronomers study – shoot­ing stars, noc­tilu­cent clouds and the north­ern lights – are tens of kilo­me­tres above our heads. The Moon is al­most 385,000 kilo­me­tres (239,000 miles) away, the Sun 146 mil­lion kilo­me­tres (91 mil­lion miles) away and the clos­est star is so many kilo­me­tres away that its light – trav­el­ling at 300,000 kilo­me­tres (186,411 miles) ev­ery sec­ond – takes over four years to reach us…

It stands to rea­son that out of all the bil­lions of ce­les­tial ob­jects out there, one must be the most dis­tant the naked eye can see. Luck­ily for sky-watch­ers, this ob­ject, which is 2 mil­lion light years away, is some­thing spec­tac­u­lar, and not just a faint star: M31, the great An­dromeda Galaxy.

M31 is a huge spi­ral galaxy, larger than our own and con­tain­ing hun­dreds of mil­lions of stars. In around 5 bil­lion years’ time our Milky Way will col­lide with M31, and the two gal­ax­ies will merge into one.

De­spite shin­ing at mag­ni­tude 3.3, M31's large ap­par­ent size of 3 x 1 de­grees – six times the size of the full Moon – means its light is spread over a wide area, giv­ing it a low sur­face bright­ness. Con­se­quently, any trace of light pol­lu­tion will wash M31 from the sky; you will only see it if you're look­ing at a truly dark sky.

Once you’ve let your eyes ad­just to the dark­ness you just need to do a bit of ‘star hop­ping’. First find Cas­siopeia, the dis­tinc­tive W-shaped con­stel­la­tion of the Queen, then drop down to the up­side-down ‘Y’ of Perseus (the Hunter) be­low it. That con­stel­la­tion’s cen­tral star, Mir­fak, is the start­ing point for a ce­les­tial hop­scotch to the east along the line of stars form­ing An­dromeda. Hav­ing reached Mirach, the third starry step­ping stone along, you should see a small smudge above it – that’s M31.

If it isn’t im­me­di­ately ob­vi­ous try us­ing the astronomers’ trick of ‘averted vi­sion’, which means look­ing just off to one side. This means the galaxy’s faint light will fall on more sen­si­tive parts of your retina and should be­come vis­i­ble.

“In 5 bil­lion years our Milky Way will col­lide with M31”

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