St Helena's dark skies

The Milky Way and the Magellanic Clouds

04 July 2019 By Mark Westmoquette, St Helena Astronomy Club Photo by Paul Stewart, New Zealand

 

The Milky Way

The bright band of the Milky Way across the sky is unmistakable here in the dark skies of St Helena. It is one of the most spectacular sights of the night sky, sweeping from horizon to horizon. Its milkyness betrays the presence of billions and billions of stars whose light appears smudged together to the naked eye.

Our Milky Way galaxy is a flattened disk of billions of stars about 100,000 light-years (the distance light travels in one year) across, with a big bulge in the middle – looking something akin to a very large fried egg. The Sun and our solar system are about three quarters of the way out towards the edge.

When we look up at the Milky Way, we're seeing the galaxy edge-on, from the inside.

The northern hemisphere, including England, faces away from the Milky Way's centre, towards the outskirts of the disk – that's why the band of stars is relatively faint.

From the southern hemisphere, e.g. in South Africa, the view is mostly towards the centre of the galactic disk, making it much brighter and more interesting.

Here in St Helena, being not-so-far south of the equator, we get some of both views, depending on the season. As we move from June into July and August, the centre of the galaxy (located in Sagittarius) will be rising, bringing more of the beautiful southern skies into view.

If you get a chance on a dark night, go outside and gaze upwards. It'll take 5-10 minutes for your eyes to fully adjust to the dark, but it won't take that long for you to see the Milky Way (unlike in the northern hemisphere or in a more light-polluted location).

Many cultures of the world, including the Chinese and Japanese, Hindus and some Australian Aborigines, see the Milky Way as a river across the heavens that separates the two halves of the sky. In Greek mythology, the Milky Way was formed after the trickster god Hermes laid Zeus's illegitimate son Heracles at the breast of Hera, the queen of the gods, while she was asleep. If she suckled him he would become immortal. When Hera awoke, she tore Heracles away from her breast and splattered her breast milk across the heavens.

 

The Magellanic Clouds

The Large and Small Magellanic Clouds, visible only from the southern hemisphere, are actually small galaxies in themselves located just outside our own Milky Way. They are two of only a handful of galaxies that are visible to the naked eye.

 

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Officially named after the Portuguese explorer Ferdinand Magellan (1480-1521), they were of course known to indigenous peoples well before the Europeans came to the southern hemisphere. The first preserved mention of the Magellanic Clouds is thought to be in ancient petroglyphs and rock drawings found in Chile.

The Large Magellanic Cloud (LMC) appears as a faint cloud covering several degrees of the sky (more than 20 times the width of the full Moon). It is easy to find in the constellations of Dorado and Mensa.

Here in St Helena, with our fantastic dark skies, it's easy to spot; but if you're struggling, the easiest way to find it is by drawing an imaginary line from Sirius past the right side of Canopus and follow the line (hint: it's bigger than you might think).

The LMC is 163,000 light-years distant and contains around 30 billion stars, weighing around a hundredth the mass of the Milky Way. It was once considered to be an irregular galaxy (that's a particular classification of galaxy that doesn't show any obvious structure) until astronomers studied it more closely. We now know it to have a bar across its heart, indicating that it may once have been a spiral like the Milky Way. The disruption of its spiral pattern is almost certainly due to it interacting with the strong gravity of the Milky Way, and it's expected to collide with the Milky Way in approximately 2.4 billion years.

Meanwhile, the Small Magellanic Cloud (SMC) is separated from the LMC by about 21 degrees in the sky (and about 70,000 light years as the crow flies – if it could fly in space!).

The SMC will be visible in the early part of the night here in St Helena as it begins rising again from July onwards. It's located about 20 degrees from the South Celestial Pole, in the corner of the Tucana constellation – about 15 degrees below the bright star Achernar in the constellation Eridanus.

The SMC is about 200,000 light-years distant and is also on a collision course with the Milky Way.

Over the course of history, it's thought the Milky Way has swallowed many small galaxies like the SMC and LMC, gaining weight each time.

Over the past millennia, the gravitational interactions caused by the impending collisions (of the Milky Way with the LMC and SMC) have stirred up gas clouds in both galaxies, causing them to vigorously form new stars. So when you look at the clouds like the Tarantula Nebula (see Stargazing Tip No. 3), really you are seeing the effects of our own gravity on another galaxy.