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Today, we are going to take a look at a couple of different things. We’re going to start by taking a look at the constellation Bootes or Boötes, (or Böötes or Böotes). It is a tricky one to pronounce, but it isn’t a tricky one to find, especially at this time of the year. From the northern hemisphere, we’re taking a look over in the west just after sunset. The very bright star that we can see, pretty much due west at about 9:30, is Arcturus. You can make sure that this is Arcturus by using the Plough or the Big Dipper. At this time of the year, we can see the Plough just over in the northwest. You follow the handle of the Plough or Big Dipper, and follow that arc to Arcturus.

I’ve mentioned the arc to Arcturus a couple of times before. Arcturus is the brightest star in the constellation Boötes, it is a tricky one to pronounce. As with many constellations, there is an English name, Boötes is the Herdsman, but the Latinized Greek name Boötes is the more common version. Boötes could also be the Ploughman, as the name comes from the Ancient Greek word for cow. In the past, oxen drew ploughs, and Boötes is very close to the Plough in the sky. The shape of the constellation is very similar to a kite, with the diamond shape rising above Arcturus. Arcturus is also called Alpha Boötes, or ɑ Boo for Boötes. What makes the pronunciation tricky is that you’re meant to pronounce these two O’s separately, so it’s bo-otes (bo.o.tiz) rather than boo-tes (bu.tiz). It’s not “boo” like a ghost, but “bo” like Bo Peep, or maybe more like Bo O’Peep for the two O’s. It is a tricky one,and it is occasionally spelled with two dots over one or both O’s to make it a little bit easier to tell that you’re supposed to pronounce them separately. These dots are called dieresis when they are meant to separate vowels, like in coöccur. You will see these dots in some languages, like German, called an umlaut, but those dots serve a different function. The image of the constellation is a picture of the herdsman with a spear, or goad, and a sickle. Arcturus is down at the bottom of the sort of a kilt or tunic that Boötes is wearing. The Boötes Void is in Boötes, that’s where it gets its name. The Boötes Void is in the direction of the constellation Bootes.

If we take away the atmosphere and any light pollution, we’ll see there’s quite a few stars out in this direction, so we’re not talking about a lack of stars. If we bring up our markers for extra-galactic objects, we’ll see that there isn’t too many out in this direction, but that’s true for several different parts of the sky. Even if we get rid of the stars, so there’s nothing else in the way, we can see that around and near Boötes there are several galaxies. We can see several galaxies in the direction of the Plough or the Big Dipper, and also in the direction of Corona Borealis just a little above Boötes, but we’re not seeing many in the direction of Boötes itself. This sort of empty gap, empty of galaxies, is what we are looking for. As we zoom in, we can see things appearing but sort of sliding off to the side because they’re not exactly where we’re looking. Stellarium has markers for galaxies, even those galaxies that don’t have any picture associated with them. There’s not as many galaxies in a particular area of the sky, a region of space, which is sort of behind the constellation of Boötes. The Boötes Void isn’t really something that we get to see and it isn’t completely empty. It’s a huge region of space with only about 60 galaxies in it, instead of thousands. We can see there are some galaxies in this direction, the Boötes Void is behind another group of galaxies, the Shapley Cluster.

I’ve mentioned galactic clusters before. We’re in the Virgo Supercluster of galaxies. After arcing to Arcturus, you can spike on to Spica, the brightest star in Virgo. Unfortunately, Virgo is under the horizon at the moment. With the ground out of the way, we can see the shape of Virgo and Leo. The Sun is in just the right place for these things to be completely blocked out, pretty much right between, though a little lower, than this pair of constellations. With all the inconveniences removed we can see a bunch of the galaxies in the Virgo Cluster. Along with the Virgo Cluster, we form the Virgo Supercluster. We here is the Milky Way, the Andromeda Galaxy and the Triangulum Galaxy, us 3 galaxies. Galaxies often come in clumps, they bundle together in space, so there are regions of the sky where we see lots of galaxies sort of clumped together. On the other hand, there are regions of the sky with very few galaxies, areas in space where there is a lack of galaxies. There is a huge region out behind Boötes which only has about 60 galaxies when you’d expect to find a couple of thousand in a region of space so large. That is the Boötes Void and it’s not really something we can see. As such, in order to talk about it, if somebody needs a picture to go with an article about The Boötes Void they often end up using a thing called a Bok Globule.

To find a Bok Globule, first off, we’re going to head out into the countryside. Of course, that will let us see many more stars, and it will let us see various nebulae as well. If we start zooming in on the core of the Milky Way, there’s already regions of space where there’s plenty of stars and dark things blocking out the stars behind. If we want to take a look for one in particular, we’ll need its name. The one we’re looking for is LDN 57, from Lynd’s catalogue of Dark Nebulae. It’s also called B68 for Barnard 68 from Barnards Catalogue. This does look like an empty hole in space, in a region that’s otherwise full of stars it looks like there are dark empty holes, but these are the opposite of empty. These dark nebulae are areas that are filled with hydrogen gas and dust, really dense clouds that are so full of stuff that they block out the light of the stars behind them.

There is a group of them with a slightly snappier name, Thackeray’s Globules. They’re under the horizon for us in Ireland at the moment but that’s okay. These globules are in a region full of red emission nebulae. This ends up looking like a big, bright red nebula, but within it there are these dark objects, these dark clouds. In this case, it seems a little bit more obvious that these are dark objects floating against a bright background, but because of how dark and how empty these objects look, in the cases where they’re directly in front of star fields, it looks like a void. They look like an empty hole in space, but they are actually the opposite of empty. So if you see an image of a star field with a dark cloud in front of it, that “looks like a void” isn’t. This is a thing that’s full of stuff, full of hydrogen, a dark cloud blocking out the light of the stars behind it. The Boötes Void is a genuinely empty region of space, and it’s in the constellation Boötes. It’s in a completely different direction, very different to the Barnard 68/LDN 57, which is down in the constellation of Ophiuchus, the Snake Bearer. If we bring up the English names we get Serpent Bearer, and if we bring up the lines, we can see the shape of Ophiuchus and the serpent that he’s carrying.

These are two very different things, the image that you often see associated with the Boötes Void, or Boötes Supervoid, is actually a completely different thing and it is the opposite of a void, it’s actually a part of space that’s filled with dust and gas rather than being empty. In the case of the Boötes Void, it’s empty of galaxies, and looking in this direction, we are still seeing galaxies. There’s a bunch of galaxies out in this direction, but the Boötes Void is behind them, it’s behind these galaxies that we can see sort of in front. Galaxies do group together in this way, and that’s how we have clusters (like, as I mentioned, the Virgo supercluster) but of course if there’s going to be clusters in some places, there has to be emptiness in other places. Galaxies aren’t evenly distributed through the universe, they tend to clump up together due to gravity. However, we can’t really see those things, those huge structures in space. We know that they exist, but we can’t actually see the whole structure from here on Earth. These are huge 3 dimensional structures that span across huge regions of space. We have to look past the stars in our galaxy to see these clusters and superclusters of galaxies. It’s very hard to get a picture of shapes like that, so Bok globules get shown instead. Those dark clouds that look like voids in space are a handy way of showing something, giving some sort of picture to a thing that we can’t actually see.

That’s the difference between a Bok Globule and the Boötes Void. The Boötes Void is out in the direction of Boötes, but of course, we’re not really going to see anything because it is a void. It is a galactic void, a place where there is fewer galaxies, about 60 galaxies instead of the 2000 we’d expect. Bok globules are something that we can see by taking a look into the Milky Way, loads of nebulae have Bok globules, dark clouds of gas floating through them. So the next time you see an article that shows a field of stars with a dark shape and then tells you it’s the Boötes void, you now know that that is not the case, that those are two very different things, a lovely picture of a Bok globule paired with an article about an area with not as many galaxies.

I hope that you enjoyed this piece. I hope that you don’t see too many misleading articles, but if you do, I hope that you now recognize how they’re misleading. If you enjoyed this piece then please do like it. If you like this kind of content, then you can see subscribe to this website and my YouTube channel. Thank you very much for reading and hopefully I’ll see you back here next time.