Caoimhín's Content

As the title says, we are going to be looking at fast moving stars, specifically stars with high proper motion. This means stars that look like they’re moving quickly in our sky, the ones we can tell are moving fast. Usually, when we’re looking a the sky, we treat the stars as a fixed background, and for most purposes that works. Over longer timescales, the subtle differences in the motion of different stars is revealed, but it takes a long time for things like constellations to appreciably change.

However, all stars are moving, and not necessarily slowly. The Sun itself is moving, orbiting around the center of the Milky Way at about 792,000 kilometers an hour. Every star visible to the naked eye is within our galaxy, all roughly orbiting around the core of the Milky Way as well. Some groups of stars have very little difference in their relative motions, all moving together in a similar direction at a similar speed. Clusters of stars, such as the open cluster of the Pleiades, are often moving together through space. In the case of open clusters, all of the stars in them formed together in the same cloud of gas. As young stars, they do have time to be influenced by surrounding objects and their interactions with each other and end up travelling in different directions. This can also be true for older, more spread out groups of stars, such as the Hyades in Taurus, the kind of V shape at the front of the constellation. It’s true for most of the Plough as well, which is clearly a lot more spread out then the Pleiades. The Big Dipper or the Plough is only part of Ursa Major, and not all of the stars in it appear to be moving together, Alpha Ursa Majoris and Eta Ursa Majoris don’t see to be part of the group. These arrangements of comoving stars are known as moving groups. We can find these groups because there is a slight difference in the motions of different stars, some stars are moving faster or moving in slightly different directions then others, which lets groups that are all moving the same way stand out over long timescales. However, a few individual stars also stand out as moving very differently from the stars around them. Very few of them are easy to spot, with most requiring a telescope to see at all.

We will start with a star called 61 Cygni, in the constellation of Cygnus the Swan. It is a faint star, so Stellarium’s handy search function proves useful once again. Stars are often listed with a shortened name, in this case 61 Cyg, with Cyg being the shortened form of Cygnus. This one is pretty transparent, but others, such as UMa for Ursa Major and CVn for Canis Venatici, take some getting used to. Searching for the star that way reveals that this is really two stars, 61 Cygni A and 61 Cygni B. These are Latin A and B, not Greek Alpha and Beta, any Greek letters in a stars designation come before the constellation name, just like the numbers do. Having tow capital letters like this means this is a binary star, two stars known as 61 Cygni when being considered as a pair. 61 Cygni B is labelled as having it’s own common name, Bessel’s Star or Piazzi’s Flying Star, but these names are usually used when referring to the binary pair. Being in the constellation Cygnus, and not too far from the tail end with the star Deneb, this pair are circumpolar stars, from Ireland they never sets, so you can see them any night of the year at any time. By jumping through years at a time, 61 Cygni can be seen to change position compared to the background stars, something that we might normally associate with planets, especially slow moving ones like Neptune or Pluto. To get a significantly different position from 2024, I need to jump back until 1955, but even going by years at a time, the differences here aren’t going to be as dramatic as with planets. This visible motion of the star in the sky is high proper motion. This means how they appear to move very quickly in the sky. It is important to remember that we’re moving as well, the Sun, and all of the stars, are orbiting around the galactic core. Stars with high proper motion are the stars moving quicker or in a different direction to the other stars. This is very much limited by our perspective, we can see how much this star is moving across the sky, but the star could also be receding away from us or coming towards us. There are three dimensions to play with, so any star can be moving in a few different ways. Stellarium tells you the proper motion of a star. For 61 Cygni it’s about 5170 mas/yr or milli-arcseconds per year, with arcseconds being a way of measuring the curved area of the sky. This high proper motion was noticed by Giuseppe Piazzi, who named it his Flying Star due to it’s speed. It’s only 11 light-years away which is pretty close as far as interstellar distances go. Stars that are nearer to us are easier to see moving quickly, and with this star we know how far away it is quite accurately. Stars are often listed with a range of error on their distance, such as 100 light years plus or minus 5 or 100ly±5ly. This star is listed as 11.40±0.00ly, and how close it is is partly down to how close it is as well.

Jumping along year by year means that we are looking at the star at the same date each year, and the same position in our orbit. Parallax, which is listed next to proper motion inn Stellarium when a star is selected. If you were to jump in sets of six months, from one side of our orbit to the other, you would be looking at the star from two different location, a whole 2 AU apart. This causes stars to appear to shift back and forth compared to more distant stars. Stars that are closer to us will appear to move more, and this can be used to figure out how far away the star is. An astronomer named Friedrich Bessel measure the distance to 61 Cygni used this method along with estimations of the binary pairs orbital period to estimate how far away the star was, eventually leading to the first reliable measurement of the distance to a star other than the Sun, leading to the name Bessel’s Star. 61 Cygnus is just about 5th magnitude, making it visible to the naked eye when you’re in the countryside. It is in this rich region of the sky with the Milky Way, but you can just about see it with the naked eye, at least once it’s pointed out. 61 Cygnus is one of the only stars with a high proper motion that is visible to the naked eye, and although being one of the first stars noticed to move like this, other faster examples have been discovered. Even then, with detailed enough observation, differences in it’s position can be noticed in only 5 years or so.

Moving onto the star with the highest proper motion, we will be looking for a star invisible to the naked eye. It is in the constellation Ophiuchus the 13th sign of the zodiac, we’re looking for Barnard’s Star. This star is best known by this name, though it does have a designation of V2500 Ophiuchi. Similar to the name Bessel’s Star for 61 Cygni, Barnard’s Star gets its name from edward Barnard, the first person to record its high proper motion. It’s changing position at twice the speed of the last star, 10,308 mas/yr. Unfortunately, it’s magnitude is just 9.5 so it’s very faint, impossible to see with the naked eye. It is also a red dwarf, which are a particularly faint and small class of stars. It is a variable star so it has flares, moments where it brightens before dimming again. By hopping through time year by year, we get to see it moving across the sky, just like 61 Cygni, but moving even faster. Barnard’s Star is just 5.96 light years away, roughly 6 light years away, making it one of the closest stars to the Earth. Barnard’s Star is the third closest star to Earth after the Sun and Alpha Centauri at 4.25ly, the second not counting the Sun. Wolf 359 was the next closest star after that, but a pair of brown dwarfs discovered in 2013 have edged it out, something I only learned about recently. WISE 1049-5319 or Luhman 16 is just 6.5ly away from us, compared to Wolf 359’s 7.8ly. Thanks to the speed and closeness of Barnard’s Start, it can be seen to move across the sky even with just a few years of observations.

All of these stars are still being effected by Sagittarius A* (star), the big supermassive black hole in the center of the Milky Way, some of the stars move differently. This can be caused by a few different things. Some stars in our galaxy may actually be from a globular cluster or a dwarf galaxy that the Milky Way has sucked in. These stars didn’t necessarily form orbiting our black hole, but were pulled in, which can cause them to orbit at a different angle, causing them to move in a different way. This may be the reason for certain moving groups of stars or star streams. Stars can also be kicked around by other stars, especially when we have lots of stars in close proximity to each other. In the Pleiades for example and other young groups of stars, the density population of stars in these regions increase the interactions between stars, they’re more likely to end up pulling on passing stars or even orbiting each other. Let’s say for example that two stars were orbiting around each other in a binary pair, and some other star got too close to them. Disturbing the orbit of a binary pair could cause them to fly apart. The stars would go from circling around a common barycenter somewhere between them to flying straight through space. The angular momentum they would have in their orbit would need to be conserved, it would have to become, linear momentum, normal momentum sending the star forward. A star that has been flung out of a binary orbit with another star could end up streaming through space at a much higher speed. That probably didn’t happen for 61 Cygni, given that that’s still a binary star, but it may be the cause for Barnard’s Star. We do still only have theories to explain high proper motion in most cases as we’ve never seen a star getting accelerated up to this high speed.

Barnard’s Star in particular is quite famous, and it’s a quite an interesting star, as are some of the other nearby stars like Alpha Centuri and Wolf 359. Some of these stars have been mentioned in fiction quite a lot, particularly science fiction, and that is what I’m going to be taking a look at next time. I’ll be going through some of the real life stars referenced or used as settings for sci-fi and finding them in the sky. If you don’t wan to miss that article you can subscribe to this website or to my YouTube so you are more likely to be notified when it comes. Also coming up, on the 23rd of June, I will be doing a live show in The Friary in Cork City, so if you’re interested in seeing me discuss these topics in person you can come along to that. Hopefully I’ll see you back here for the next article between now and then.