Caoimhín's Content

Today we will begin by revisiting an object that I have mentioned a few times before, the Ring Nebula in Lyra. The Ring Nebula in Lyra is a fantastic object to try and take a look at, as well as being a fantastic example of a planetary nebula. Medium to large telescopes will give you a view of this distant object, but not smaller telescopes as it isn’t even a particularly big object in space. It’s about two and a half light years across, much smaller than a galaxy and many other types of nebulae, but certainly bigger than a planet. The name planetary nebula is not the most accurate, but like the names of so many things, like the Crab Nebula for example, the reason its name is what it is, is that telescopes weren’t quite as good in the past. Getting a clear, sharp view of these very distant objects is tricky even now, and was simply impossible with early telescopes.

To find the Ring Nebula at this time of the year, we need to look South early in the evening. Over the next few weeks we’ll have the Summer Triangle nice and high in the sky as soon as the Sun goes down. We’re out of summer now so the Summer Triangle is not going to be sticking with us the whole night long anymore, Vega or Altair should set before the Sun rises. We’re looking up towards Vega, the brightest star in the Summer Triangle, one of the three corners. Vega is in the constellation of Lyra and Lyra is where we find the Ring Nebula. It is very difficult to see, it’s certainly impossible to see with the naked eye, but luckily Stellarium can highlight deep sky objects once you zoom in to roughly the right location. The Ring Nebula is also called the Annular Nebula, both of which are great names because it is a ring shaped circle. With the lovely high quality images we have available to us today, it definitely doesn’t look like a planet, but to early astronomers this looked like a round object in the sky. If you start zooming in on stars, they tend to remain point like no matter how closely you look, you’d need an incredibly big telescope to see a star as an actual sphere in space. Planets look like points of light to the naked eye, but even early telescopes could resolve them into discs. This round shape reminded astronomers of planets and so planetary nebulae became the name.

Charles Messier is credited as the discoverer of the Ring Nebula, but he probably thought he saw a comet. Charles Messier was a comet hunter who happened to find a bunch of other objects, the Ring Nebula being number 57 on the Messier list, a list that goes up to 110, quite a lot of objects. The Ring Nebula is probably my favourite example of a planetary nebula, partly because it’s up for the entire summer in Ireland and at the most times of the year it will be up for at least a couple of hours are in the evening or couple of hours in the morning. The constellation of Lyra itself may not be particularly famous, but Vega and the Summer triangle are certainly quite well known, unfortunately the Ring Nebula is rather small and difficult to see. Luckily there are other planetary nebula, some of which also show a lot more of the interesting internal structure that planetary nebulae can have.

The next nebula looked at in the video is most commonly known as the Eskimo Nebula. In the video, I do not mention the name, but I do not particularly discuss why, mostly because I am not an expert in the issues surrounding the usage of this name today. As far as I am aware, there are no groups of people who were once referred to by this name who are not preferentially referred to by a different name today. Broadly, the people who today live and have for generations lived inside the Arctic Circle in Northern Eurasia and America were once referred to by this name as a group or ethnicity. Today however, most of these people are referred to as Inuit, Yupik, Aleut and more, depending on where exactly they live and the language they speak. They are often also referred to as native, indigenous or First Nations people. Those terms include various other groups who were not referred to as Eskimo in the past, such as the First Nations people of the Pacific Northwest Coast or the are surrounding the Great Lakes.

Regardless of potential complications associated with the name, this is the name it is given, and the reason it is given this name is because this nebula looks like it’s wearing a parka. A parka is a particular type of jacket with a big hood and furry ring around the outside, and if you take a closer look at the nebula you may see why. The idea is that the central region is a face and the surrounding ring is the fluffy fur lining of the parka.. Clown Face Nebula is also an option that you could call it, however the first name mentioned is the more famous one. Over all, we can see that this planetary nebula has a similar structure, it’s got that lovely circular shape typical of a planetary nebula, a really perfect circle in this case. However we can also see that there’s definitely differences to the internal structure compared to what we saw with the Ring Nebula. There are a lot of these nebulae, and although they are all broadly similar and share common characteristics, so far it doesn’t seem like any two are exactly alike.

Planetary nebulae are the remains of a star similar to our Sun that has reached the end of its life. generally, these stars cool down as they get older, but once their core starts fusing different elements coming to the end of its life, the core of the star will heat up the outer layers, causing them to expand and eventually drift off into space. This is of course a simplified explanation, but I will leave the late stage processes of stellar fusion for another time.

Stellarium provides a list of various planetary nebulae, thought not all of them have images. For example, the Baby Dumbbell Nebula doesn’t seem to have a picture, but images can be found on the internet. The Dumbbell Nebula however does have an associated image. It clearly has a round shape over all, and through the glow of light pollution you probably won’t see why it’s called Dumbbell, with alternative names including Diablo and Apple Core nebula. In a sufficiently clear image, you can see that it has got a sort of lobed structure, much like the objects it is named after, especially the apple core in my opinion. For the sake of simplicity, I will refer to the stars that create planetary nebulae as stars like our Sun, but really I just mean small stars. A star doesn’t necessarily have to be very similar to our Sun in any way other then its size to produce these kind of remains. When a star like our Sun finally blows its outer layers into space, the magnetic field of the star, the rate at which it’s spinning and any wobble that the star had, all of those things will be factors that are going to effect how these layers blow off and the structure that the resulting cloud of gas has. This of course is going to influence how that nebula looks to us, and so we get to see a lot of variation and different internal structures depending on the nature of the star. Trying to figure out exactly what stellar characteristics give rise to what kinds of planetary nebula however, that’s a whole thing. Magneto-hydrodynamics, the study of how magnetic fields flow and interact, is as complicated as hydrodynamics but with electromagnetism added in, it is a tricky are of physics. You can see here from the names that a lot of these nebulae have lobed shapes. The Bowtie Nebula is a little hard to see in Stellarium, but from what I can see the alternative name of Scarab Nebula may be even more accurate, at least in less clear images. These shapes, like the Bowtie Nebula, the aforementioned Dumbbell Nebula, all relate to this lobed shape. The upcoming Calabash Nebula is named for the same reason, a calabash is a type of gourd that is usually swollen on two ends and narrower in the middle, it’s often used for the transport of water. Dumbbells of course are dumbbell shaped, a diablo in this instance is shaped like two cones joined at the tip and used to perform tricks using a length of string. Another example is the Hourglass Nebula, named for the same reason, that distinctive shape. These shapes come from how material is effected by the magnetic field of the star, which of course runs from North to South, so it creates this kind of lobed or double-ended shape that these planetary nebulae and to their precursors show.

Before a planetary nebula we get a protoplanetary nebula. The Calabash Nebula is one example, all though I thought it was planetary for the majority of the video. It is protoplanetary rather than planetary, and these are the precursors to a planetary nebula. A planetary nebula is the cloud of material, gas and plasma, that’s after emanating from a star that has fallen apart. A protoplanetary nebula is the star just as it’s falling apart, just as those materials are getting thrown out. Planetary nebula and protoplanetary nebula, protoplanetary nebula especially, they are transient stellar phenomena compared to the grand scale of stellar evolution. The Rotten Egg Nebula is a bit of a nasty name, but it is there as an alternative to the Calabash Nebula, which to me sounds a bit better. There are certainly some wacky colours visible in that nebula, at least in the presumably false colour images I’ve seen. Those bands and filaments of colour are the material as it’s getting shot out, it hasn’t formed that full circular ring yet. This means that we’re really seeing the material as directed by the magnetic field and the motion of the star is still having an ongoing effect. This kind of thing only last for a few tens of thousands of years. The protoplanetary stage itself arises and lasts for only around 10,000 years, and may fade away with out becoming a planetary nebula if the outer layers of gas drift too far to be ionized and lit up by the remaining white dwarf in the center. Once a planetary nebula forms, it will also only last about 10,000 years before it’s outer shell grows too cool and faint, either from distance or the cooling of the central white dwarf. All in all, from the beginning of the protoplanetary nebula to the disappearance of the planetary nebula, these objects likely won’t last even 100,000 years. Compared to the millions of years that even comparatively short lived stars burn through, this is nothing. Our Sun will last for billions of years yet, and these protoplanetary and planetary nebulae they only come from smaller stars which are generally, more long-lived than the larger ones. A small star like our Sun can be, for billions of years, burning away in it’s main phase. It will then spend a bit of time, about 100,000 to a million years total, moving off the main branch into the asymptotic giant branch, before then falling apart. For just tens to hundreds of thousands of years, you’ll have those planetary and protoplanetary nebulae as the remains of the star.

There are a few other nice protoplanetary nebulae, but many unfortunately suffer the same lack of inbuilt images as many of the planetary nebulae. With the names of course you can look them up, or use the information in Stellarium to find them in the real night sky. The Egg Nebula is probably favourite protoplanetary nebula, it is also up in the summer triangle. In the video I almost say “It’s not a million miles away from the Ring Nebula”, a common enough phrase for “not particularly distant”. I am of course referring to the apparent distance in our sky, from the Ring Nebula just under Vega in Lyra, one corner of the triangle, over to the Egg Nebula just under Deneb in Cygnus, the other corner. I believe it is actually a lot more than a million miles away, they are more likely trillions and trillions of kilometers away from each other, it’s definitely quite a few light years.. However, they aren’t too far away from each other in he sky. The Cygnus Egg is an alternative name, not quite The Egg of the Swan, but Cygnus is the constellation of the Swan. It is a nice protoplanetary nebulae, if you can get an image of it, as is the Boomerang Nebula. One of the important things about protoplanetary nebulae is that they are the very beginnings of a star falling apart, shedding its outer layers into space, rather than collapsing in on itself in a supernova. This means it must be a star too small for a supernova, like our Sun. This is what’s going to happen to our Sun. In billions of years of course, we’ve got plenty of time left. However, our Sun is eventually going to fall apart in this graceful, gradual way, leaving behind this brightly coloured transient, phenomenon. It will exist for just a brief period of time, brief for any part of stellar evolution at least. Due to the short lived nature of these things, we generally see fewer of them in space. We’re only looking at a snapshot of time here, the length of a few human lifespans, so there are going to be fewer of these short-lived phenomenon visible in the sky, and the protoplanetary stage of a planetary nebula lasts even less time than the planetary stage.

Again, despite the name, it is the remains of a star, there is nothing planetary about it and we do have planets in the sky at the moment if you’d like to compare what they look like through a telescope. I hope you do get to see some of the planetary nebula, they are a really fascinating niche part of stellar evolution. It’s also nice that they are something that only happens to smaller stars. Supernovas and black holes get a lot of the attention, but that’s not where our solar system is heading, this is.

I hope that you enjoyed this piece discussing planetary and protoplanetary nebulae. If you did, and if you’d like to see more of them, then please do subscribe to this website and my YouTube channel. Hopefully, I will see you back here again next time.