Caoimhín's Content

Happy Halloween everybody! This piece should be coming out before the 31st but this is the closest to Halloween that I will be posting. We’re going to go on a bit of a journey through a few things today.

We’re starting by taking a look at the night sky for Halloween, moving all the way through to midnight and just about crossing over from Halloween onto the 1st of November. Taking a look at the night sky for midnight on Halloween, we’ve got the Moon, Saturn, Jupiter stretching across the sky and we’ve also got the constellation of Orion fully above the horizon. We’re going to move straight out into the countryside because we’re going to be focussing on some parts of Orion that are hard to see from the city. Orion’s Belt is nice and clear even from the city but in darker skies you can also see Orion’s sword just beneath the belt. We’re going to take a closer look at the region around Orion’s sword. It is a part of the sky that I’ve taken a closer look at a few times before. This whole region of the sky has a lot of hydrogen nebulae, a lot of star forming regions. Through a telescope or binoculars several nebulae are visible in this area. The light of stars illuminates the hydrogen clouds from within, and the dark shapes that those hydrogen clouds leave when they block out the light from the stars make for some pretty impressive shapes.

The different regions of gas have different names. A little above and east of the belt is the Casper the Friendly Ghost Nebula, which of course is quite appropriate for the time of year. Right around the eastern star of Orion’s belt is more nebulosity, including the Burning Bush Nebula or the Flame Nebula. It’s got many different names, including the Ghost of Alnitak another fantastically appropriate name. Alnitak is the easternmost star in Orion’s belt. In the same region we’ve got the Horsehead Nebula which is a really, famous one, and it does look like a kind of a dark figure of a horse’s head. There are a lot of bright stars just next to the dark cloud, with Sigma Orionis being one of the main stars in the Sigma Orionis Cluster. There are a lot of stars around this area and a lot of them are blue. The blue ones are quite young stars, stars that formed thanks to this dark cloud of star forming material.

I have talked about Orion’s Sword before and it is usually broken up into various features as well. The central region, where young stars are blowing away gas into a bubble, is really the Great Orion Nebula, with the darker cloud next to it being the De Mairan’s Nebula, another star forming region. The Lost Jewel of Orion is also part of this region. It’s the lower part of Orion’s sword, which may appear to be just one star, but on closer inspection it reveals itself to be many stars when you zoom in with a telescope or binoculars. These are regions where stars form, but they’re not just regions where stars form. When a star forms in these clouds of dust and gas, they very often have their own disks of dust and gas around them, and these disks of dust and gas are what eventually go on to form planets. These young stars have young planetary systems around them. The easier ones to find are those that have already formed a planet which has been discovered orbiting the star. One example is Mayrit 260182, a planet orbiting one of the Sigma Orionis Cluster stars. It doesn’t have a particularly catchy name, which is of course the case for many exoplanets, but exoplanets need to form. Exoplanets come from somewhere, they don’t pop up out of nowhere, they come from the disks of dust and gas that are around the star as the star is forming.

There are a couple of established exoplanets in this region, exoplanets that have already formed that we can detect because of the way they block out the light of the star as they orbit it around, or because they’re big enough or warm enough that they can be seen reflecting light in direct images. A lot of the ones in this region were discovered via direct imaging. The transit method is the kind of more old fashioned, but is also a very common way of discovering exoplanets. Most of the exoplanets tracked by Stellarium are ones that have already fully formed. Thanks to these regions being so rich in stars and so rich in dust and gas, they’re are also regions where new planets are still forming, just beginning to turn into solar systems. Stars and planets often form together. There are so many examples of these objects getting formed, some of them sit on the boundary between a star and planet. Some of the stars in these regions are multiple star systems, even Sigma Orionis itself. The simplest is a binary star system or a double star, two stars co-orbiting together, but they can form bigger groups. Not every disk of dust around a star is going to turn into a planet, some of them are big enough to form stars themselves, and that’s one of the way that binary stars can form. If there’s a very small star orbiting around a big star, it’s often the case that the smaller star actually formed around the larger star rather than just being captured because they were close together. The disk around a star can turn into planets or stars, or into objects that are right on the middle.

In our solar system, Jupiter is the biggest planet, but we have found planets bigger than that and we have found large stars with much smaller stars orbiting around them. For example, if we turn around towards the north we can find Cassiopeia. For midnight on Halloween, if we spin around a little bit again, almost towards the zenith is Andromeda and the Andromeda Galaxy. We’re going to look for the star Kappa Andromedae. As we zoom in, Stellarium provides names for the brighter stars, but Kappa is pretty far down the Greek alphabet, indicating that this star isn’t very bright. With Stellarium you can search for it by name, otherwise it is towards Cassiopeia from Alpheratz, the star the joins Andromeda to the Square of Pegasus. It is a double star, composed of Kappa Andromedae A and Kappa Andromedae B. Both of these stars are stars, but one of them is quite a large bluish white star and the other is a much smaller brown dwarf.

Brown dwarfs are barely stars. Brown dwarfs are right on the line between a star and a planet. They haven’t really undergone nuclear fusion, so they’re not producing their own light and heat, but very often they’re large enough and have taken on enough hydrogen that they’re able to retain enough heat from their formation. They may still glow, they’re still producing heat, some of it due to radioactive decay, some of it picked up from the heat of their formation. The friction and the forces of the material all coming together generates quite a lot of heat. Although they are almost stars, they’re not. They’re bigger than Jupiter, but they’re still smaller, too small to undergo nuclear fusion and truly become stars. In this interpretation, Kappa Andromedae has a planet, Kappa Andromedae b, that is just unusually large. Often these objects are found orbiting around other stars, but occasionally they will be found just floating through space. These independent objects are rogue planets. The rogue planets that we have found so far, assuming that they are planets, they very often sit at this boundary between what is a star and a planet. They need to be very large and producing some amount of heat for us to actually find them at all. There aren’t that many known rogue planets and certainly very few that are known to be what we would call a planet.

Technically, none of the rogue planets are what we would call a planet because a planet is an object that orbits a star. That’s part of the definition of a planet, that it goes around a star. How it forms doesn’t really matter, which means that Brown dwarfs going around much larger stars, given that brown dwarfs aren’t really stars, they do also technically count as planets. In systems like the Kappa Andromedae system, they do sit on a line between what is a binary star system and what is an exoplanetary solar system, because they are, in a way, planets, but they are also very similar to stars being big balls of hydrogen. While I’m discussing binary stars and binary objects, Albireo one of the best binary objects to actually see, right in the middle of the summer triangle. Both of the stars involved are quite large, bright stars. We’ve got a sort of a orangey-yellow giant star and then we’ve got a hotter, smaller blue star, still very bright and easy to see. This is one of the best options if you have a telescope or binoculars and you want to see a binary star, especially because it is visible as one dot to the naked eye. Both of those are definitely stars, two objects that are undergoing nuclear fusion.

As we discover more and more things in space, we’re discovering that we haven’t already described them, which, of course, should come as no surprise. When we first started studying planets and stars scientifically we had a very limited sample size, only a few planets in our solar system, and only thousands of stars that we could see with early telescopes and early astronomy. Now that we’ve discovered millions and billions of stars, other galaxies that are themselves filled with stars, and other solar systems that are filled with planets, we’ve discovered many more examples of what planets and what stars can be. That’s caused us to expand our definitions and run into areas where we don’t quite have adequate descriptions yet. For example, a brown dwarf going around a star, is that a solar system or a binary star system? Is a brown dwarf closer to a planet, closer to a star, do they deserve their own definition? When something that could have been a star if it was a little bit bigger is flying through space on its own, is that a rogue planet? Is anything that flies through space on its own a rogue planet? Are planets only those things which orbit stars? These are all great questions that we still have to answer, a lot of the subjects of these confusing questions are in very famous parts of the sky that we can go and take a look at, even on Halloween.

I hope that you enjoyed this piece, if you did, please do like it. If you like this kind of content, then please subscribe to this website and my YouTube channel. Once again, Happy Halloween and thank you very much for watching, hopefully I’ll see you back here next time.