Star Movements

[dirty_g]

My question......

When you see a star in the sky you see it normally how it was hundreds of years ago as of course it takes years for the light to get to us. So If a star moves as Arcturus does (apparently it will dissapear from sight in 300 years) we see it where it was. My question is do we see it where it was if it moves towards us??? Am I making sense??? Or do we see it where it is now as its moving towards us. Do I make sense????

[Maksutov]

No matter what, we're seeing everything after its light was emitted and when received by our eyes. The delay is a function of distance. So if, as Uncle Jimbo might say, "It's coming right at us!", just before he shoots it, we'll not see any apparent motion, but the aspect of the object will be as it was when the light left it.

Of course, relativistically, everything is "now" for a particular frame of reference.

[antoniseb]

Arcturus will not be disappearing in 300 years.

However, your broader question is this: When a star is moving, do we see where it is, or where it was when it emitted the light that we are seeing now?

The answer is that we see light that the star gave off, and we see that light coming from where the star was.

Similarly, when I was a boy, I used to see low flying military jets flying over head on a regular basis, and I could hear their noise, but the noise was always seemed to be coming from about a mile behind the aircraft. This is because I heard the plane where it was when it made the noise, but by the time the noise got to me (about 5 to 10 seconds later) the plane had moved on.

[dirty_g]

with relation to Arcturus I ment 3000 years time. As it is moving very quickly.

I know we see stars where they were when the light left it but if a Star in the sky was moving directly towards us then would we still see it where it was?? Not where it is?

[antoniseb]

if a Star in the sky was moving directly towards us then would we still see it where it was?? Not where it is?

Yes. Where it was. But if it was coming directly toward us, you would need to do some very accurate parallax measurements to know the difference.

with relation to Arcturus I ment 3000 years time. As it is moving very quickly.

I am certain that it is not moving THAT quickly. The fastest star in the sky is Barnards Star, and that star will only be a little closer 3000 years from now.
Arcturus moves about 2.5 arcseconds a year. This means it moves about one degree in the sky every 1500 years. Two degrees is not enough to make it disappear.

[MG1962A]

with relation to Arcturus I ment 3000 years time. As it is moving very quickly.

I know we see stars where they were when the light left it but if a Star in the sky was moving directly towards us then would we still see it where it was?? Not where it is?

I see what you are getting at. No we will only see where the star was. The star is moving far slower than light, so new light cant over take old light.

However the doppler effect would make the star look a touch bluer than would be normally expected, though I doubt this could be noted with the naked eye

[mickal555]

What would be interesting if hypotheticly the star was moving faster then the speed of light... :k

[Maksutov]

What would be interesting if hypotheticly the star was moving faster then the speed of light... :k

Then it would get here before we knew about it, sort of the way things are run in Washington, DC.

Do you have any examples?

[dirty_g]

Yes. Where it was. But if it was coming directly toward us, you would need to do some very accurate parallax measurements to know the difference.

I am certain that it is not moving THAT quickly. The fastest star in the sky is Barnards Star, and that star will only be a little closer 3000 years from now.
Arcturus moves about 2.5 arcseconds a year. This means it moves about one degree in the sky every 1500 years. Two degrees is not enough to make it disappear.

I read in astronomy now that it would dissapear in 3000 years time. I will have to re-check though. Thanks for the answers guys. If a star moved at the speed of light we wouldnt even see it coming then?? Don't tell the planet X guys that as they will use that as an excuse as to why we can't see it!

[Maksutov]

I read in astronomy now that it would dissapear in 3000 years time. I will have to re-check though. Thanks for the answers guys. If a star moved at the speed of light we wouldnt even see it coming then?? Don't tell the planet X guys that as they will use that as an excuse as to why we can't see it!

We would see it coming, since the speed of light is independent of any reference frame. If it were coming straight at us at 300,000 km per second, from some distance greater than zero, we'd still see it.

But then all hope would be lost since the star's mass would be equal to or greater than all the mass in the universe.

[ozark1]

Can I add some facts to the recipe?

Arcturus is a star with one of the largest proper motions in the sky - 2.28 arc seconds per year. It is passing the Sun and is near closest approach.

It is moving at about the diameter of the full moon every 900 years and in about 50,000 years will be 4 degrees from where Spica is now. However even in 50,000 years it will only have faded to magnitude +0.20 - still one of the brightest stars.

Estimates suggest that Arcturus will become invisible to the naked eye in about 500,000 years. Somewhat symmetrically it became visible about 500,000 years ago.

[antoniseb]

I read in astronomy now that it would dissapear in 3000 years time.

An important lesson here is that "Astronomy" and all other astronomy magazines, blogs, and journals make mistakes, and sometimes pick up and pass along mistakes. Most of what they say is as right as you could hope for, but if you see a fact that seems out of line, you should apply a common sense check to it.

[dirty_g]

it didnt seem out of line to me. If a magazine that specialises in it tells me so I belieive it. Am hunting for my back catalouge of magazines now. If though weekly world news printed it I would have second thoughts as to its validity.

[antoniseb]

it didnt seem out of line to me. If a magazine that specialises in it tells me so I belieive it.

Just as another example, recently Universe Today, and New Scientist both ran a story about giant bubbles in a galaxy cluster a few billion light years away. The story said that the bubbles were 640 light years across, which made them seven times the size of the Milky Way. The Milky Way is more than 90 light years across... in fact it is a thousand times that size - and the bubbles were 640,000 lightyears across. The blurb from NASA's PR person for the Chandra observatory is the source of the mistake. Two publications I like and trust published the error. This sort of thing shows up pretty often.

[BISMARCK]

This is a perfect example:

[Romanus]

Arcturus's relative speed compared to the Sun is fairly high, and it indeed will become too faint to see with the naked eye some day; "Burnham's Celestial Handbook" gives a figure of 500,000 years for this, in line with others I've seen on the topic.