A version of this piece was originally broadcast on Radio Scilly in August 2019.
We’ve had some delightful evenings stargazing in the last couple of weeks, with relatively cloudfree skies – and a late-rising Moon – meaning that between 9.30pm and 11.30pm (ish) there’s been a nice window for observing. By that I mean before the very bright Moon pops up and wipes out all but the brighter stars up there.
At this time of year the Milky Way arcs brightly across our sky. It’s something many visitors to Scilly come to marvel at and appreciate, thanks to our naturally dark skies here. It must be stressed that you need dark minimal light pollution to stand a chance of seeing it clearly, which wipes out much of the mainland! Indeed a recent report by the Campaign for Rural England warned that many children in the UK risk growing up never seeing the Milky Way, because of the damaging effect of artificial light on the night sky. Unless they live in rural areas, and that’s a real deprivation.
One sad fact is that England has only 21.7% of pristine, unpolluted sky left (even sadder is that most of these areas are sparsely or unpopulated – so go figure what the cause is).
Anyway, we treasure our dark sky here in Scilly and long may our nights gazing at the Milky Way continue. But what is the Milky Way and what are we seeing when we look at it?
The Milky Way is the galaxy that our Sun, and therefore our solar system, belongs to. In fact, all the individual stars we can see in our night sky are part of the same galaxy, the same Milky Way.
A galaxy is a group of stars, gas, dust, remains of stars, and dark matter, bound together by gravity. There are several different types of galaxy, named by shape. The Milky Way is a spiral galaxy, which means it resembles…a spiral. Following so far? Our Milky Way is somewhere between 150,000 and 200,000 light years across. So, very big then. You might at this stage be wondering how on earth we know all this. And the answer is, very clever guess work, scientific estimations – based on other spiral galaxies in space and observations of patterns of behaviour of the stars we can see. If you’re still doubting, you’ll have to take my word for it.
We are inside the galaxy, and as such we’ll never be able to see ALL of the stars in it. What we can see is a disk of stars that forms the Milky Way from inside the disk, which tells us that our galaxy is basically flat… but not much more.
Several telescopes have taken detailed images of the bar of the Milky Way, from different directions, and all have found a concentration of stars in a band in the middle, which adds evidence that we’re a spiral galaxy. Put it this way, if we were an elliptical galaxy, we’d see stars of our galaxy spread around the sky, not just in a single band.
At the centre of our galaxy is a dense bar-shaped cluster of stars, a veritable bulge, where most of the activity happens. And all the stars in our galaxy revolve around this galactic centre. It takes our Sun about 250 million years to do one full orbit so in terms of whether we notice the difference in our position, I’d say the answer is no.
Even though all the stars we can see in the sky are part of the Milky Way, the galaxy gets its name – no, not from the chocolate bar, the stars came first – but from how it appears as a milky band of light in the sky. So what are we looking at?
In short, that bright, milky streak in the sky, is what I’ve talked about: the bright, star-packed centre of our galaxy. Are we alone in it? Who knows. Give it a wave next time we get a clear night, and most of all appreciate how lucky we are to be able to see this wonder at all.