#proper motion

Constellations (specifically the stars that make them) can move and shift, but it takes thousands of years -- proper motion to be exact. So, what is proper motion? Proper motion is the idea/theory that stars travel through space over many many many thousands of years. Blitzo and Stolas are hellborn sinners that can and will live for thousands and thousands of years (most likely). They'll move and change and eventually the stars that make their constellations now will slowly die and soon be replaced by new stars that change their look. In this case, their look isn't necessarily literal; it could be (and most likely will be) their personalities shifting and changing as they grow.

Stolas' rush for Blitzo to "just look his way" shows he knows that things are changing. He knows things will soon draw to an end and that the stars that combine to make Blitzo are going through proper motion.

One of my favourite and closest open clusters to Earth, at 670 light years in the constellation of Cancer, it holds around 1000 stars making it one of the most populated clusters near to us.

It's thought to be around 600 million years old, and the age and proper motion compares similarly with the open cluster Hyades both containing similar %'s of red giants and white dwarfs which suggests a common origin for both.

Proper Motion

Proper Motion describes the small changes in the positions of stars due to constant motion of the universe. Some stars are moving towards Earth and some are moving away. Some move at an angle from or towards Earth and result in a change to a star’s position in the night sky (essentially, North/South: Declination and East/West: Right Ascension). These changes are almost always so small that they only become apparently over very large periods of time, usually thousands if not hundreds of thousands of year

Precession of the Equinoxes

To be more accurate however, a star’s position is both the result of the star’s own movement (proper motion) as well as the changing perspective of an observer. Most stars barely move in ten thousand years, but will appear to have move dramatically since the entire Earth’s axial tilt shifts under our feet. Precession gradually turns the sky in a circle and will have a profound impact over its 26 thousand year cycle. Combined together, stars will shift and crawl across the sky as well as appear to be rapidly flung and rotated in a circle around the changing celestial pole

Edmond Halley – Scientist of the Day

Edmond Halley, an English astronomer, was born Nov. 8, 1656.  

When Messier looked at these stars in 1780, he described nebulosity around them, however many decades later John Herschel could find no such nebula, and doubted even that this was a small open cluster as Messier had assumed. Despite that, he still included M73 in his NGC catalogue, designated NGC 6994.

An open cluster are stars gravitationally bound together and born within the same nebula, while an asterism are totally unrelated stars, that just happen to look like something to our human brains.

The debate rumbled on, until around year 2000 when one study found some connection in their colours with similar stars in globular clusters, although this didn't last long, as it was followed in the same year by another study that found quite the opposite to be true.

2 years later a much more detailed study of the spectrum's from the stars found them to not be connected, and in fact, moving at different paces around the galaxy, what is known as Proper Motion.

So it's confirmed, it is indeed an asterism, a group of stars unconnected to one another, that from Earth's perspective look like they are part of a structure. In distance they are indeed closer, with a few hundred light years between them all.

But while that may feel like a letdown, M73 has served a very useful purpose, there are many instances of open clusters that have been ripped apart of left with only a tiny collection of stars compared to their former glory, and learning how to measure and detect that they do belong together, can take M73 as a perfect example of when they are not, and how that was proven.

A Real Shooting Star

What we generally call shooting stars are not stars of course, they are mostly grains of dust that smash into our atmosphere and briefly burn up, but there is such a thing as a real shooting star !

It may look like stars sit in our night sky motionless, but they and we are moving at incredible speeds, and not all in the same direction or at the same speed.

Zeta Ophiuchus is one such high proper motion stars, but what happens when these stars hurtle through interstellar banks of dust and gas ? A bow shockwave irradiated by the stars UV light and glows red.

The star itself is a mag 2 O type star sitting near the ecliptic, the path of the Sun around the night sky, otherwise known as the Zodiac.

But wait .. Who here was born under Ophiuchus ? ;)

The Rogue White Dwarf Heading This Way ?

Back in 2022, a team of astronomers pouring over old Gaia data found something interesting.

Gaia has been slowly mapping the Milky Way, creating a 3D map and cataloguing each stars spectrum and motion.

What the scientists were looking for, was stars heading directly for Earth ! As the Sun hurtles around our galaxy, so do all the other stars, but not always at the same speed or even in the same direction, so the stars near to us today, will slowly move away or get closer.

What the team were looking for, were yet unknown examples of stars heading towards us, what could be classed as rogue stars, not following the normal precession around the galaxy (at least, what is normal from our perspective).

They stumbled upon a white dwarf WD 0810–353 currently 36 light years from us in the constellation of Puppis, but with a very dim magnitude of 14.4, only visible to large telescopes, at least currently.

What they found was, according to Gaia, the star was heading for a very close visit in 29,000 years from now, so close, it would begin to impact on the Oort clouds sending huge amounts of material inwards towards Earth.

But the good news is, It seems the Gaia data wasn't quite showing what the scientists thought it was. Some white dwarfs have unusually strong magnetic fields, and a recent survey of the white dwarf suggests this caused Gaia to believe it's motion was straight at us, when in fact, the magnetic field was messing up the spectral emissions and it turns out, it may even be heading away from us.

Disaster averted, at least in 29,000 years time, but this situation isn't unusual to Earth, we've had stars on our doorstep many times before, usually red dwarfs, but such stars will return again.

Looking 80,000 years into the future, Alpha Centauri will no longer be as close to us as it currently is, with Ross 128 gaining the expected crown, at least briefly.

Ursa Major

For those of you in the northern hemisphere, this is probably one of the most recognisable constellations in the night sky, although, today we think of it as The Big Dipper, The Frying Pan, it is as it's constellation name suggests in Latin, the Great Bear, and so the connecting lines may surprise you a little.

As you may know, most stars are binary pairs, unlike our star, they have other stars that orbit with them in their system, and in Ursa Major, you can actually see the other star with the naked eye.

Look at the photograph at the top, and towards the 2nd star in the handle, and you'll see clearly another star right next to it, even though that image is a little blurred due to the effects they were using for this photo.

One of my own personal interests about this constellation is, almost all of the stars in the big dipper part of the constellation have identical proper motion, meaning they are all moving around the galaxy together, and suggests that they were all born in the same nebula together, they also have very similar distances from Earth between 80 and 90 light years from us.

You can use the last two stars in the pan part to find the north star Polaris.

and finally, due to that proper motion, over the next 200,000 years the constellation will change, not every star belongs to this common group, and you'll spot which ones by watching this lovely animation of the past and future of Ursa Major.