Caoimhín's Content

Today we are going to take another look at galaxies. We’ve looked at galaxies before, and you can take a look back through my archives for some examples, but thankfully they are a big topic, so there is plenty to talk about. We are going to be taking a look specifically at spiral galaxies like our Milky Way and our Milky Way in particular. It can be easy to forget the the fuzzy band of light that stretches overhead under dark conditions is fundamentally the same structure as the swirls of stars that we can see through our telescopes.

Of course we won’t see any galaxies from the city with our naked eyes, our own or any other. If you have a telescope you can spot some galaxies from the city, but it is difficult thanks to the extra light from light pollution. Faint objects like galaxies are especially obscured by extra light. Dark countryside skies are what we need to see any galaxy with the naked eye. Once it is dark enough, and the right time of year, our own Milky Way should be obvious. In truly dark countryside skies, it dominates the sky, and it is something well worth seeing if you haven’t yet. Even before our galaxy becomes visible, we can figure out where it will appear, thanks to the Summer Triangle and the constellations of Sagittarius and Scorpius. The Milky Way will stretch from the horizon between Sagittarius and Scorpius, low to the South from here in Ireland, up through the Summer Triangle, and down through Cassiopeia towards the North. Along the Southern horizon, we can see that the Milky Way is wider, and less milky white than the name indicates, it has some reddish and orange colours as well as darker patches. Moving up along the Milky Way, it takes on the pale colour it’s normally associated with, an almost bluish-white, as it narrows. It may seem redundant to describe, but for a long time, this is all any one limited to one hemisphere would see. It can certainly be divided into two portions, the faint bluey-white portion and the wider, brighter, redder area down here. Luckily we’re not limited to the northern hemisphere anymore, we can travel around the world. Combining our view from each hemisphere we’ll see that this wider more reddish section tails away again into almost a mirror image of the pale bluishy-white region we’re used to seeing. The Milky Way galaxy, as we see it from the inside, stretches the whole way around, even if we turn around to the North, there is a little bit of galaxy even there behind us, but it is much thinner and fades away substantially. From what we we can see, our galaxy does look like it has a middle (or core) region, the reddish-orange wider area and then it has these edges (or arms) stretching out to the side. This is what the Milky Way looks like from the inside, and that is the only view we have ever had.

Other galaxies are certainly visible, and the easiest one to see is the Andromeda Galaxy. I have been told that the Triangulum Galaxy is on edge of being visible in perfect conditions, but we don’t even need incredibly perfect conditions to see the glow of the Andromeda Galaxy, pretty good conditions are fine. If you can see the more northerly arms of the Milky Way, there’s a good chance you’ll be able to see the Andromeda Galaxy with just your eyes, as long as you know where to look. There are a couple of ways to find it, one of which uses the Square of Pegasus. As long as the Milky Way is far enough above the horizon, the Square of Pegasus just to the East of it. This puts it just above the horizon at around 11:30pm from here in Ireland, up above and a little North of Saturn. If you go through the Square of Pegasus from the Southern corner to the Northern corner, and a little out the other side, you will be in Andromeda the constellation. As you move into the Andromeda constellation, just before you hit the Milky Way, The Andromeda Galaxy should just a little higher than the line you are following, a little separated from the Milky Way. If the Square of Pegasus isn’t in the sky, you can also use the constellation of Cassiopeia. Cassiopeia is a W shape in the arm of the Milky Way, quite far to the North. It has a slightly more even triangle on one side which practically points to the Andromeda Galaxy. To find Cassiopeia, you can start by following the Plough to the North Star. Rather than going from the front of the Plough, go back and make a line from the base of the Handle of the Plough or Big Dipper, where it joins the bucket or pot like rectangle part. Go straight through the North Star, and through the more even triangle of the W. That will bring you down right next to the Andromeda Galaxy. If it is dark enough, you should be able to see it as a fuzzy object with just your eyes. The Andromeda galaxy is our closest neighbour, so it does give us an opportunity to study another galaxy, especially what’s inside it, in a little bit more detail. Through a telescope you can see that it has a Dwarf Galaxy orbiting around it, much like the Large and Small Magellanic Clouds orbiting around the Milky Way. You might just about see that it has a bright center with dark lines of dust, it has a bright and slightly warmer toned center and much paler blueish areas out towards the side. Unfortunately, we see the Andromeda Galaxy at a little bit of an angle, so we don’t quite get to see how its arms look, and were also not quite seeing how much the center bulges out from the rest. This is because we’re not looking at it edge on or face on.

Face on or edge on are the two of best ways to see a galaxy. Many galaxies are somewhere in between, which obscures some of the details. A galaxy viewed edge on can look a little bit odd, quite unlike what we usually image when we think of a galaxy. A face on galaxy is the familiar, they are the typical way a galaxy will be represented. One of the best examples, known as a grand design spiral galaxy due to its clear structure, is the Whirlpool Galaxy in Ursa Major. This brings us back to the Plough, just below the bottom of the handle, on the opposite side to the North Star. There a quite a few galaxies out in this direction, and the Whirlpool Galaxy is distant enough to look pretty fuzzy through low power telescopes, but fuzziness is a good clue. Stars are remain as points of light, even through quite powerful telescopes, but if something looks fuzzy, there’s something else going on. It could be a globular cluster, it could be a nebula or, as in this case, it could be a galaxy. Taking a closer look, it’s clear that we are looking straight down on this galaxy from above. You can really see the shape of the arms, the spiral of a spiral galaxy is much more visible here than in Andromeda. The Whirlpool Galaxy was the first galaxy to have its spiral structure noted, through the Leviathan telescope in Birr in County Offaly here in Ireland, once the largest telescope in world. You can also see that it’s definitely brighter in the center, there are dark lanes of dust and gas, and it definitely gets a bit fainter as you go out towards the edges. Interestingly, the Whirlpool Galaxy is swallowing up a smaller galaxy here, you can see it falling apart at the end of one arm, something that even our own Milky Way has done in the past,

The Whirlpool Galaxy has a very clearly perfect spiral shape, where as a nearby galaxy has a messier spiral pattern. Above the end of the Ploughs handle, roughly towards North Star, is another fuzzy shape, a little easier to spot than the Whirlpool. This is the Pinwheel Galaxy, and on close inspection, it clearly has a vaguely similar structure, but not exactly the same. The arms are less distinct and less even, making the galaxy look a little lopsided. All the same, it has a brighter, warmer coloured center and bluer arms that fade away towards the edge. For an edge on galaxy, one of the best example is the Sombrero Galaxy. It lies just in between Virgo and the constellation Corvus. Corvus the Crow is just below the middle of Virgo, and it has a distinct diamond or rhombus shape. the Sombrero Galaxy lies between them, just a little East of Spica. It is a difficult galaxy to spot due to its apparent shape. Looking at the galaxy edge on is almost the same as looking at any roughly flat object edge on, it will look almost like a flat line. However, the core of a galaxy is thicker than the edges, and that’s very apparent here. The brighter middle section bulges out from the flat disc around it. This is highlighted by the dust and gas around the edges of the galaxy. There is enough material obscuring the light of the stars, it appears like a dark band across the middle. In a good enough view, you can just about see the band sweeping back at the edges, as our view isn’t perfectly edge on, but still much more tilted than our view of the Andromeda Galaxy. Each of these extreme perspectives lets us see slightly different details. By looking at a face on galaxy we get to see how it gets fainter towards the edges, the arms of the galaxy stretching out from the center, as well as a view of the slightly different colour. With an edge on galaxy, we get to see the bulge in the center, and how the edges taper away, as well as seeing any dust and gas backlit by the galaxy itself.

We can then those characteristics of these other spiral galaxies to the details that we see here in our own galaxy. The wider center, the thinner edges, the differences in colour and dark patches of dust and gas, all of these things line up with characteristics of a spiral galaxy. Thanks to all this, we can be pretty confident in saying that our galaxy the Milky Way is a spiral, even though we haven’t seen it from the outside. It’s also very difficult to observe anything on the far side of the Milky Way, past the core in the center, it’s even difficult to see the other arms of the Milky Way through the arm that we are in. Our local region does have a name, the Orion Arm, or the Orion Spur, of the Milky Way galaxy. Our local region seems to be a branch of a bigger arm, known as the Sagittarius or Sagittarius-Carina Arm. By measuring the motions of the stars and looking at how far apart they are, scientists do have a reasonably good idea of what our Milky Way would look like from the outside. There should be another arm, the Perseus Arm, just outside our position, which seems to be one of the main arms, leading all the way around the far side of of the core as it curves in towards the center, where the Sagittarius arm may branch off of it. The other main arm, the Scutum-Centaurus Arm, lies inside the Sagittarius Arm, starting almost on the near side of the core, coming towards us in front of the core, and then leading all the way around behind it and out the other side. This is at least what we can see on our the side of the galaxy, as the dense core region blocks our view of the far side.

We have now gotten to know our galaxy, the Milky Way, pretty well, but a lot of our modern understanding started by looking at other galaxies. If you enjoyed this piece than feel free to like it, you can also subscribe to this website and my YouTube channel if you’d like to see more. However you find your way back, I hope you come back next time.