Reply
Wed 23 Aug, 2006 05:22 am
Galaxy distance is given in light years from earth and are reported to be 10's of light years across. My question is, do we not see the star closer to us sooner than we see the distance star that make up the galaxy. If this is true what is the distance given, an average of the stars or the nearest star?
The distance to other galaxies is much larger than their dimensions.
Brandon9000 wrote:The distance to other galaxies is much larger than their dimensions.
I understand that, but if the nearest star in the galaxy is 10 LY and the galaxy is 10 YR in diameter and the galaxy is slight edge on to us, then the distant start is really 20 LY from us and we could not be seeing the galaxy as it is at any given moment since the light takes an additional 10 LY to get to us, Correct?
captain ed wrote:Brandon9000 wrote:The distance to other galaxies is much larger than their dimensions.
I understand that, but
if the nearest star in the galaxy is 10 LY and the galaxy is 10 YR in diameter and the galaxy is slight edge on to us, then the distant start is really 20 LY from us and we could not be seeing the galaxy as it is at any given moment since the light takes an additional 10 LY to get to us, Correct?
I don't think you do understand it. The distances to the galaxies is much larger than their dimensions. You present an example in which the distance is the same as the dimensions.
captain ed is talking about the distance of stars within our galaxy.
Quote:. . . and we could not be seeing the galaxy as it is at any given moment . . .
That's right. The light of a star that we see on earth right now could well be a nova, black hole, white drawf or whatnot right now, but all we have to see is it's light that took all those years to get here.
InfraBlue wrote:captain ed is talking about the distance of stars within our galaxy.
I don't think so - not with the numbers he's using and the "slight edge on to us" comment.
markr wrote:InfraBlue wrote:captain ed is talking about the distance of stars within our galaxy.
I don't think so - not with the numbers he's using and the "slight edge on to us" comment.
I am talking about galaxies other than our own milky way. I am confused by the statement that the distance to the galaxy is more than their dimension. Our Milky way galaxy is how many light years in diameter? Even if the other far away galaxies are only 100 LY across it would still mean the nearest stars light would reach us before the other star located 180degrees away, right?
Perhaps this will explain your original question Captian Ed.
Let's say that there is a galaxy who's closest edge is 10 light years from us and it is 5 light years in diameter. This galaxy is lying so that it appears, to us, like a plate on a shelf that it tilted slightly (so you can see the far edge) out in space where we are looking at it horizontinally.
The light from the star at the closest edge would take 10 years to reach us.
The light from the farthest star would take 15 light years to reach us.
If you looked up at this galaxy you would be seeing light that emited form the closest star 10 years ago and and from the farthest star 15 years ago.
Yyou would never see a snapshot of the light being emitted from all of those stars at the exact same moment. You don't however,see the closer star "sooner" unless all of the stars came into existance after you've started looking. You aren't seeing it sooner, you are seeing "newer light".
The Milky way (our galaxy) is about 200,000 light years across.
All of the stars that you can see are in the Milky way (our galaxy). There are about 200-400 billion stars in the Milky way and we can only see a small fraction of these.
The nearest Galaxy to us (besides the Milky way) is the Andromeda Galaxy which is currently about 3,000,000 light years (that means it would take 15 of our galaxies arranged side to side to equal the distance between the two galaxies) (i.e. there is a whole lot of empty space between us). This is what Brandon was pointing out with dimensions (the distance between galaxies is larger than the size of galaxies).
The Andromeda Galaxy can be seen (as a whole) with the naked eye on a very clear night if there is no lights around (it is impossible to see much if you live near a city). It looks like a small smudge. You can not see any individual stars.
If I understand your original question, I think you are asking about the meaning of sentences like "The Andromeda Galaxy is 3,000,000 ly from Earth" (whether this is the center, or the outermost star).
The answer is because of the distances involved, it really doesnt matter. Think if I tell you that there is a basketball that is a mile away. The mile is probably not an exact measurement anyway, but certainly because the basketball is so small compared to the distance being measured, this question really doesn't matter. I am going to tell you a mile, even if you change what part of the basketball I am measuring to.
For the closest galaxies in our neighborhood (which we call the local group) it matters a small bit since the distances is only 15 times the width of the object. You probably should think about this being to the center (but to be honest I am not even sure). For anything other that the closest couple of galaxies, this question is completely irrelevant.
fishin wrote:Perhaps this will explain your original question Captian Ed.
Let's say that there is a galaxy who's closest edge is 10 light years from us and it is 5 light years in diameter. This galaxy is lying so that it appears, to us, like a plate on a shelf that it tilted slightly (so you can see the far edge) out in space where we are looking at it horizontinally.
The light from the star at the closest edge would take 10 years to reach us.
The light from the farthest star would take 15 light years to reach us.
If you looked up at this galaxy you would be seeing light that emited form the closest star 10 years ago and and from the farthest star 15 years ago.
Yyou would never see a snapshot of the light being emitted from all of those stars at the exact same moment. You don't however,see the closer star "sooner" unless all of the stars came into existance after you've started looking. You aren't seeing it sooner, you are seeing "newer light".
That helps. However, when scientist state this or that galaxy is xxLY away are they are talking the closest, or the average
captain ed wrote:fishin wrote:Perhaps this will explain your original question Captian Ed.
Let's say that there is a galaxy who's closest edge is 10 light years from us and it is 5 light years in diameter. This galaxy is lying so that it appears, to us, like a plate on a shelf that it tilted slightly (so you can see the far edge) out in space where we are looking at it horizontinally.
The light from the star at the closest edge would take 10 years to reach us.
The light from the farthest star would take 15 light years to reach us.
If you looked up at this galaxy you would be seeing light that emited form the closest star 10 years ago and and from the farthest star 15 years ago.
Yyou would never see a snapshot of the light being emitted from all of those stars at the exact same moment. You don't however,see the closer star "sooner" unless all of the stars came into existance after you've started looking. You aren't seeing it sooner, you are seeing "newer light".
That helps. However, when scientist state this or that galaxy is xxLY away are they are talking the closest, or the average
I beleive the standard is to use the center of the distant galaxy unless they note otherwise. If there is no mention of "the edge of galaxy X" then they mean the center (which may or may not be the average). But as ebrown mentioned, the exact measurement may be off by a few light years.
Thanks to all for in-LIGHTening me on this. I do understand there is still some dark matter surrounding the question.
Stumble It!
Tweet This
Bookmark on Delicious
Share on Facebook
Share on MySpace