# How well do we know galactic geography? **URL:** \ **Category:** Factual Questions\ **Created:** [September 11, 2009, 7:05am UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658 "2009-09-11T07:05:14Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [September 12, 2009, 5:56pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/41 "2009-09-12T17:56:23Z")
> [@Chronos](#): > > OK, now we’re into territory that’s meaningful, but even if you’re defining your terms that way, it’s still wrong. The only meaningful definition for “the edge of the Universe” is precisely that distance at which the recession speed is the speed of light. So by that standard, the Universe is expanding at exactly the speed of light, and always has and always will, including during inflation and right now. Fair enough. I’ll watch my terminology. ---
**Author:** ![hobscrk777](https://avatars.discourse-cdn.com/v4/letter/h/a6a055/32.png) [@hobscrk777](https://boards.straightdope.com/u/hobscrk777)\ **Post date:** [September 12, 2009, 7:50pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/42 "2009-09-12T19:50:48Z")
> [@adhay](#): > > WTF? > > Per second, per se, says nothing. Hertz commonly denotes the number of sinusoidal cycles per second although it could be confusingly to measure speed as in the frequency mileposts are encountered per unit time. Sorry, but I think you’re wrong. The units of frequency are [1/s], which is exactly 1 Hertz. By definition. ---
**Author:** ![ivan\_astikov](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ivan\_astikov](https://boards.straightdope.com/u/ivan_astikov)\ **Post date:** [September 12, 2009, 8:14pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/43 "2009-09-12T20:14:11Z")
> [@Chronos](#): > > The _nearest_ star to the Sun is still far enough away that the amount of its shift in apparent position due to our orbit is less than a single arc-second, and other galaxies are incomprehensibly further than the nearest star. For very nearby stars, it is (just barely) possible to measure the change in apparent position, and this is how we measure distances to them (this is called the method of parallax). In fact, this defines one of the units commonly used for astronomical distances, the parsec: A parsec is the distance at which an object would have a parallax of one arcsecond (so alpha Centauri, with a parallax of a little less than an arcsecond, is a little more than a parsec away). Yes, but aren’t we also on the end of an “arm” of a galaxy, which I presume is revolving and is not parked up in space somewhere, but is actually moving as fast as all the other galaxies are moving? So, in a week, we have moved as far as a galaxy can travel in that time, yes? So why is it all these stars remain in the same position, unless they are also moving along relative to us, which sounds weird, to say the least? ---
**Author:** ![ivan\_astikov](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ivan\_astikov](https://boards.straightdope.com/u/ivan_astikov)\ **Post date:** [September 12, 2009, 8:20pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/44 "2009-09-12T20:20:57Z")
missed the edit window Does anyone ever get the feeling that we are giving these astronomers too much credit? ---
**Author:** ![adhay](https://avatars.discourse-cdn.com/v4/letter/a/ecd19e/32.png) [@adhay](https://boards.straightdope.com/u/adhay)\ **Post date:** [September 12, 2009, 8:34pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/45 "2009-09-12T20:34:29Z")
> [@hobscrk777](#): > > Sorry, but I think you’re wrong. The units of frequency are [1/s], which is exactly 1 Hertz. By definition. [From the WIKI.](http://en.wikipedia.org/wiki/Hertz) > [@](#): > > ]The hertz (symbol: Hz) is a unit of frequency. It is defined as the number of complete cycles per second. The def of 1 Hertz is 1 cycle/sec. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 12, 2009, 8:50pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/46 "2009-09-12T20:50:41Z")
Ivan, not sure if I understand your points, but the stars within a galaxy are practically moving with each other, held together by orbiting the same effective mass in the center of the galaxy, and usually also orbiting other closer objects. For example, most stars are part of binary or more complex star systems. Also, some stars are in clusters that swarm around their common local center of mass. That great orbiting cloud, as a galaxy, is floating in space, and galaxies themselves are typically floating in some gravitationally bound way with other local galaxies. On a scale large compared to the typical distance between neighboring galaxies, they are all mutually fleeing one another due to the expansion of space. The arms of the galaxy aren’t made of stuff, exactly, they are regions in which star formation is happening rapidly and there are many new massive stars that shine brightly. It’s like that thing where all the people in a stadium jump up and sit back down in a loosely organized way that forms a wave traveling around the stadium. The wave is made of people, but it travels much faster than the people in it do. At least, this is as I remember it from quite a few years ago… ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [September 12, 2009, 9:09pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/47 "2009-09-12T21:09:54Z")
> [@ivan\_astikov](#): > > So, in a week, we have moved as far as a galaxy can travel in that time, yes? So why is it all these stars remain in the same position, unless they are also moving along relative to us, which sounds weird, to say the least? The stars in our galaxy ARE moving along relative to us. It’s like riding a carousel with your friends. You’re all going round and round but your positions relative to each other stay the same. Objects outside the carousel WILL appear to shift position. But objects that are very far away won’t shift very much. If I’m riding a carousel and look off toward a distant mountain range, it won’t appear to change very much at all even though I’m going round and round. Looking at distant galaxies from inside our rotating galaxy is like looking at a distant mountain range from a carousel. The change in our angle of view from the rotation is so tiny compared to the distance of the thing we’re looking at, it appears completely stationary. ---
**Author:** ![Sam\_Stone](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Sam\_Stone](https://boards.straightdope.com/u/Sam_Stone)\ **Post date:** [September 12, 2009, 9:48pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/48 "2009-09-12T21:48:23Z")
A couple of interesting points: The size of the universe: There are two pieces of data which suggest the universe is MUCH larger than our observable ‘light bubble’. The first is that the measured curvature of space is very flat. There are two ways that can happen - one is that the energy density of the universe is exactly enough to keep space flat through sheer chance, which seems unlikely. The other is that the universe is so huge that measurements of curvature in our ‘light sphere’ approximate flatness - just as measuring a distance of a few feet on earth will make you think the Earth is flat even though it’s spherical. Some theories suggest that the expansionary phase after the big bang lasted until the universe grew so large that it became essentially flat - it was driven into ‘flatness’. That would mean it is much, much bigger than what we can see. Maybe even infinite. The fate of our ‘light bubble’: Over time, as the unvierse expands more and more objects that we can see today will wind up outside our ‘light bubble’. Eventually, all we’ll be able to see from earth is our local group of galaxies, which are gravitationally bound together. Everything else will be outside our light bubble, and astronomers in the very distant future will have no physical evidence of the big bang at all. They’ll just see a collection of galaxies floating through what appears to be an empty void. Of course, the sun will be long gone by then, so maybe this isn’t a problem for us. But other civilizations that may come along hundreds of billions of years from now will have no way of seeing the big bang. Given the evidence we have today, the best guess is that the universe is either infinite or incredibly large - a googol of our light bubbles in size, maybe. That leads to all sorts of interesting philosophical questions. ---
**Author:** ![Spatial\_Rift\_47](https://avatars.discourse-cdn.com/v4/letter/s/a8b319/32.png) [@Spatial\_Rift\_47](https://boards.straightdope.com/u/Spatial_Rift_47)\ **Post date:** [September 12, 2009, 9:52pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/49 "2009-09-12T21:52:09Z")
**adhay** , you and **hobscrk777** are both right. The two definitions are interchangeable as far as SI units are concerned. Look no further than the fact that “cycle” is not an officially defined unit in the SI or any other system. A cycle is whatever we define it to be relevant to the system in question. This has been your friendly neighborhood physics grad student, at your service. ---
**Author:** ![adhay](https://avatars.discourse-cdn.com/v4/letter/a/ecd19e/32.png) [@adhay](https://boards.straightdope.com/u/adhay)\ **Post date:** [September 12, 2009, 10:03pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/50 "2009-09-12T22:03:21Z")
“SI”? ---
**Author:** ![Spatial\_Rift\_47](https://avatars.discourse-cdn.com/v4/letter/s/a8b319/32.png) [@Spatial\_Rift\_47](https://boards.straightdope.com/u/Spatial_Rift_47)\ **Post date:** [September 12, 2009, 10:26pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/51 "2009-09-12T22:26:46Z")
The International System of units, known as SI from the French nomenclature. It’s what scientists use 90% of the time. More info here: [International System of Units - Wikipedia](http://en.wikipedia.org/wiki/SI_units) ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [September 12, 2009, 10:27pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/52 "2009-09-12T22:27:37Z")
> [@adhay](#): > > “SI”? Système International, which is more often referred to as the metric system in the US. There are technically some differences between SI and metric. ---
**Author:** ![ivan\_astikov](https://avatars.discourse-cdn.com/v4/letter/i/58f4c7/32.png) [@ivan\_astikov](https://boards.straightdope.com/u/ivan_astikov)\ **Post date:** [September 12, 2009, 10:32pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/53 "2009-09-12T22:32:35Z")
All the representational models of the Big Bang I’ve seen, seem to show the universe to be expanding like the ripples on a pond surface… I take it this is just a simplification and that the universe didn’t expand along a flat plane? ---
**Author:** ![adhay](https://avatars.discourse-cdn.com/v4/letter/a/ecd19e/32.png) [@adhay](https://boards.straightdope.com/u/adhay)\ **Post date:** [September 12, 2009, 10:37pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/54 "2009-09-12T22:37:43Z")
> [@Spatial\_Rift\_47](#): > > **adhay** , you and **hobscrk777** are both right. The two definitions are interchangeable as far as SI units are concerned. Look no further than the fact that “cycle” is not an officially defined unit in the SI or any other system. A cycle is whatever we define it to be relevant to the system in question. > > This has been your friendly neighborhood physics grad student, at your service. Cycle could of course be anything but Hertz is defined (WIKI again) > [@](#): > > The unit hertz is defined by the International System of Units (SI) such that the hyperfine splitting in the ground state of the cesium 133 atom is exactly 9,192,631,770 hertz, ν (hfs Cs) = 9,192,631,770 Hz.[1] Equivalently, 1 Hz = 1⁄9,192,631,770 ν (hfs Cs). This definition is derived from the SI definition of the second. Hertz are inverse, s−1. In practice, the hertz simply replaced the older cycle per second. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [September 12, 2009, 10:56pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/55 "2009-09-12T22:56:56Z")
> [@ivan\_astikov](#): > > I take it this is just a simplification and that the universe didn’t expand along a flat plane? No. It expanded in all three dimensions. ---
**Author:** ![Spatial\_Rift\_47](https://avatars.discourse-cdn.com/v4/letter/s/a8b319/32.png) [@Spatial\_Rift\_47](https://boards.straightdope.com/u/Spatial_Rift_47)\ **Post date:** [September 12, 2009, 10:58pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/56 "2009-09-12T22:58:02Z")
Again, it says it right there: “Hertz are inverse, s[sup]-1[/sup].” If you want to attach a specific cycle to that, you can, but you don’t need to. You can use Hertz with or without referring to an established oscillatory cycle. ---
**Author:** ![hobscrk777](https://avatars.discourse-cdn.com/v4/letter/h/a6a055/32.png) [@hobscrk777](https://boards.straightdope.com/u/hobscrk777)\ **Post date:** [September 12, 2009, 11:32pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/57 "2009-09-12T23:32:54Z")
> [@adhay](#): > > This definition is derived from the SI definition of the second. _ **Hertz are inverse, s−1** _. In practice, the hertz simply replaced the older cycle per second. At the risk of getting into a semantics argument… ---
**Author:** ![adhay](https://avatars.discourse-cdn.com/v4/letter/a/ecd19e/32.png) [@adhay](https://boards.straightdope.com/u/adhay)\ **Post date:** [September 12, 2009, 11:37pm UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/58 "2009-09-12T23:37:55Z")
> [@hobscrk777](#): > > At the risk of getting into a semantics argument… 🙂 ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [September 13, 2009, 12:39am UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/59 "2009-09-13T00:39:04Z")
> [@Spatial\_Rift\_47](#): > > The International System of units, known as SI from the French nomenclature. It’s what scientists use 90% of the time. More info here: [International System of Units - Wikipedia](http://en.wikipedia.org/wiki/SI_units) And the other 10 percent of the time they just make shit up 🙂 I am not sure exactly where the OP is going, but let me say this. I get the impression the OP thinks that we may be able measure the position and velocity of most/many/all galaxies, then use this to run time forward or backwards and learn sometime specific. Chaos theory comes in play here. We know the movements of the planets WAY WAY WAY WAY more accurately than we do for even the nearest galaxy, much less the ones that arent close. Even then, astronomers have no idea what our solar system will look like billions of years from now. Predictions range from planetary collisions to planets being ejected to nothing at all happening. Its way worse for knowing how all these galaxies are going to interact in the future. If that point is irrelevant to the OP’s interest…carry on. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 13, 2009, 1:09am UTC](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658/60 "2009-09-13T01:09:25Z")
> [@AllFreedomUnlessDefyingScience](#): > > […] Conclusion: Current physics, with the aid of computers, can develop substantially accurate maps of the current location of Galaxies by extrapolation. […] Billfish’s interest in considering what the OP’s interest was seems a breath of fresh air. Is the point of the OP to get around the problem that we see distant galaxies as, and where, they were billions of years ago? That is, does all this discussion really facilitate getting at a current map, as opposed to one that is current at its center and very obsolete at its periphery? If so, the more interesting tidbit might be that there are actually several different versions of what “current” should mean. That is, it seems easy and obvious to think about “right now”, but the concept of “now” breaks down for locations that are not “here”. Things being simultaneous really only has physical meaning if they are at the same location, surprising though this may be. In human experience, the distinction is trivial, but in cosmology it is very big. A distant galaxy is in several different locations currently, depending on how we define “currently”. And, it’s not just the issue of how long it took the light to reach us. Of course we presume the light travel time issue will be managed. The problem I am discussing is beyond the light travel time issue. [Previous page](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658.md?page=2) [Next page](https://boards.straightdope.com/t/how-well-do-we-know-galactic-geography/509658.md?page=4)