h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I was waiting outside my brother's house last night, and happened to glance upward to examine heaven. As usual my eyes were drawn to the Southern Cross, the most readily identifiable group of stars here in Australia (and, I suppose, throughout the southern hemisphere).

I got to wondering, why do the stars remain in exactly the same relative places? They always maintain their groups, clusters and constellations. But are they not all moving throughout heaven? How is it that certain stars always move in regard to other stars? How is it that the Southern Cross is always the same, never changing? Why do the stars never move away from one another?

There are several answers to this one:

1) If you viewed the galaxy not so much as a collection of entities but as one solid object, you would see that it rotates on its axis. All the stars are moving... however, they maintain their relative positions because all these objects are moving in roughly the same direction at roughly the same speed.

2) The distances involved would require that, in order for a star to significantly change its apparent position relative to us, it would have to travel light years' worth of distance. They don't travel *that* fast.

I'm sure someone else will pop along shortly, going on about quantum mechanics and fifth dimensional movement or something, but this is all I know on the subject off the top of my head.

Hi Colonel Sellers,

thanks for the quick response

I understand your point about the stars maintaining their relative positions, and your point about significant changes, but there is an underlying question that I was trying to get at, which I now may be able to frame more clearly...

What *causes* the stars to maintain their positions? Why don't they wildly fly about without any warning? Are they obeying some physical law?

Does this make more sense? I hope so!

They obey the same mysterious law that all bodies observe: gravity. Just as the moon orbits the earth, and the earth orbits the sun, so the sun orbits the center of the galaxy. Is there some massive gravity well that it orbits? Does the galaxy, in turn, orbit some massively dense body, like maybe Dan Quayle? Better minds than mine are required for that one.

Despite the best efforts of NASA, the presence of Dan Quayle in intergalactic space has not yet been confirmed. Our Galaxy is one of about 30 galaxies that form the Local Group. The Local Group is one of many groups of galaxies that make up the Local Supercluster. A supercluster can contain hundreds of thousands of galaxies, making it the largest structure we know of in the Universe.

The stars do indeed move. And they actually move quite fast: sometimes at speeds (relative to us) of hundreds of thousands of miles per hour and above. The whole galaxy that we are part of (the Milky Way) rotates around at a tremendous speed (we are currently moving around the Milky Way at 600,000 miles per hour), and all the stars within it are also moving relative to each other, following paths dictated by gravity. Beyond the stars in this galaxy, there are many many other galaxies, the vast majority of which cannot be seen by the naked eye. The speeds that these galaxies are moving relative to us is enormous, many times the speeds outlined above.

So why is it that relative to us, the stars do not seem to move? Simply because they are a long way away from us. The closest star to us is 25 thousand billion miles away, and most of the stars in the sky are over a 100,000 billion miles away. If a star is moving at 100,000 miles per hour relative to the sun, then it will move almost a billion miles in one year, a fantastic distance by our standards. However, a movement of 1 billion miles when it is 100,000 billion miles away doesn't even register. You would need to wait a long long time to perceive any significant movement in even the closest stars to us. Astronomers publish star maps every 40 years or so to register the tiny changes in the apparent movement of the stars, but even then the changes in relative position are insignificant.

I hope this helps.

CR

Colonel Sellers

It's been known for some time that galaxies in our local group are moving in the same direction towards something. For this to happen you need a truly huge gravitational field, big enough to move 30 galaxies. This was known as the Great Attractor. It's been found out that a supercluster beyond the Virgo (or is it Coma) cluster is the culprit.

Dave "greatly attracted to KFC's" Barlow

OK. As someone else said, the stars do move. However, all the stars we see in constalations are in our own galaxy. All the systems in the Milky way rotate about the center like one solid object. It takes the stars near the center the same time to arrive back in the same place as it does for the stars on the outside. So, our image of the heavens from the earth stays the same because of the way the galaxy rotates.
I certinly hope that I remembered the correct information and havn't put my foot in my mouth.

The stars on the inside of the Milky Way rotate a lot faster than the stars on the outside, so this isn't strictly true. However the stars that we see in the sky with the naked eye are "relatively" close to us, so they are moving at "relatively" the same speed and direction around the milky way as our own sun. However there are local effects, so some stars and star groups will be moving in different directions, just like whirlpools and eddys in a rapidly flowing river.

Nevertheless, the main reason we can't see the stars moving is because they are all a long, long, long, long way away from us and even if they were moving at a million miles per hour you would still not notice any change in their positions for hundreds of years.

PS when I say "relatively close" I mean, a long, long, long, long way away, ie. hundreds of trillions of miles.

CR

Excelent, hello.
Hope you don't mind me poping in for a bit of rather anoying advertising, but it is on the subject.

A month or so ago I set up a site, the H2G2 Astronomy Society which is a place to learn about the latest astronomy things (I update it 3 times a week or so) and ask/answer questions etc. Anyone can become a member and pop a little logo on their page. (I'll be linking this site cos it's great)The address is
http://www.h2g2.com/A413876
so do chack it out, and especially anyone from the Southern Hemisphere, as there is only one member so far.

Cheers,
Nick