Am I right?

[Samphire]

We are looking at stars at night ?

 

We all know the velocity of light and stars are very far away from us.

 

So whether stars are in that point when we are looking at that instant at the same point?

[Xyph]

The stars will have moved slightly since the light you see began it's journey towards us, so no, they won't be in the exact same position. I think they generally move very slowly, though, so what you can see might not be too far off.

[danny8522003]

They wouldn't be at that same point because it would take billions of years for the light to reach us and things move a lot in billions of years.

[Xyph]

The stars aren't that far away. It's very hard to see anything billions of light years away with the naked eye. I'm not sure of the specific limits of what can be seen and what can't, but it takes only four years for light from the nearest star to reach us, in which time it wouldn't have moved very much at all. Stars on the other side of the galaxy would have around 80 millenia to move around, which might be time for some significant changes, but nowhere near what would change in billions of years.

[mezarashi]

Most of the stars you can see in the night sky with your naked eye are probably within our galaxy. Our galaxy has a diameter of only less than a million light years if I'm not mistaken. It is true that when you look at the stars, you are looking into the past. What you see now is not how they are right now. They might have burned out or exploded in a supernova, but we can only know this at the speed of light.

 

Other note: Since most of the stars we see are within our own galaxy, they move together so there may not be much of a displacement. Even if there is a displacement. If you do a little trigonometry you will see that they may need to be displaced by billions of kilometers for us to observe a change in the order of seconds (1/3600 degrees).

[danny8522003]

Actually it only takes 8 minutes for light from our nearest star to reach us . He didnt specify that he was viewing with the naked eye so i assumed he just meant all stars, some of which could have been extinguished long ago.

[Martin]

Actually it only takes 8 minutes for light from our nearest star to reach us ..

 

right

so if you wanted to point your finger at where the sun actually is (not where it appears to be) then you would have to point at the spot in the sky where it WILL BE in 8 minutes from now

 

but about Sam's question, I guess I agree with what Xyph and Mezarashi said. Maybe that takes care of it.

 

It seemed to me from Xyph answer that he knows the dimensions of our galaxy----it has been measured by radio astronomy (mapping the vast clouds of hydrogen from which stars could in some cases still be forming and which radiate 21 cm radiowaves)

 

We can't see the stars at the other side from us because there is too much dust. But radiowaves from the other side get thru the dust and reach us and give us a kind of skeleton picture of the spriral arms over there.

 

And Xyph is right, the other edge (altho not sharply defined) is roughly 80,000 LY away.

Because the diameter is roughly 100,000

so the radius is 50,000

and our distance from center has been measured at about 30,000.

So he did some arithmetic and said it would take 80,000 years for light from a star on the far edge to reach us.

 

(but we cant see those suckers even with a telescope because of all the dust)

 

 

we know that the speed of the sun (taking the planets with it in a trip around the center) is roughly 250 km/second.

We are traveling in a grand orbit around the center of Milky. I forget how long it takes to do the full circle. Maybe Xyph or somebody else knows.

 

Most of the other stars near us, say within 500 LY, are also going pretty much the same speed of 250, in pretty much parallel direction, because everybody is slowly orbiting the center just like we are

 

but each star also has a little bit of RANDOM velocity, doing its own peculiar thing, like besides just flying in perfect formation they have a bit extra----say 5 km/s or 10, or 30 km/s in a kind of HELTER-SKELTER so-called "proper" motion ("proper" meaning individual as distinct from collective). in the tables that can be broken down into a transverse part (across the line of sight) and a radial motion part (towards/or away, along the line of sight)

 

so when you look up tables of stars in the handbook it will give a distance like 100 LY and it may give a figure for the transverse proper motion like 30 km/second.

 

from this you can figure out, if you are curious, how far the star has moved in the time it took (e.g. 100 years) for the light to get here.

[Xyph]

Oh yeah, haha, sorry, I forgot about that other star.

 

A full rotation of the galaxy takes about 225 million years, as far as I remember.

[Janus]

As an example to waht Martin said: Arcturus has one of the largest proper motions of all the stars at 2.3'' a year. it is 36 ly away so in the time it take the light to reach here from there it will have moved 1' 22''.

 

The Moon has an angular width of 30', so this means that Arcturus will have moved about 1/22 the width of the moon in the time it takes its light to get here.