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Forecasting the real state of the galaxy



 
 
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  #1  
Old September 3rd 03, 11:23 PM
John Honan
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Default Forecasting the real state of the galaxy

Since the light from deep sky objects takes thousands, and sometimes
millions of years to reach our eyes I was wondering if anyone is doing any
research into forecasting what the actual sky looks like right now (imagine
the speed of light was infinite - how would the sky actually look?)

Taking the current position and structure of say, the Andromeda Galaxy,
attempt to create a forecasted image of what it actually looks like at the
moment in terms of its structure, and what its 'real' position in the sky is
(how that has changed over the course of 2 million years)

This could be quite a difficult task, since every object is at a different
distance from Earth and therefore must be forecasted ahead by different
amounts.

Is there a name for this area of research?

Okay, question two. Kind of related to the above. Andromeda is 200,000 light
years in diameter. We are viewing it almost 'edge on'. Therefore the light
from the edge closest to us is 200,000 years 'ahead' of the light from the
furthest edge. Therefore, how can what we see in the sky be an accurate
representation of the real structure of Andromeda? - It's as if the image
gets more 'lagged' as we move further from the leading edge.

The edge furthest away shows us what it looked like 2,200,000 years ago, the
light from the leading edge showed what it looked like 2,000,000 years ago
(approx) Could this actually be distorting our view of the galaxy to make it
look more spiral-shaped than it actually is?

John.


  #2  
Old September 4th 03, 03:28 AM
eyelessgame
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"John Honan" wrote in message ...
Since the light from deep sky objects takes thousands, and sometimes
millions of years to reach our eyes I was wondering if anyone is doing any
research into forecasting what the actual sky looks like right now (imagine
the speed of light was infinite - how would the sky actually look?)


Not much different than it does now. The speed of light, while
finite, is much much faster than the relative velocity of most of what
we can see (until you get very far away -- it turns out that much of
what we can see at the edge of the universe is by now past our event
horizon, i.e. they'll never see the light that's currently coming from
us).

Taking the current position and structure of say, the Andromeda Galaxy,
attempt to create a forecasted image of what it actually looks like at the
moment in terms of its structure, and what its 'real' position in the sky is
(how that has changed over the course of 2 million years)


Again, not much. It's a hundred thousand light years across, but the
stars in it only move at hundreds of miles per second, whereas light
travels at almost two hundred thousand miles per second. So it
doesn't change much at the scale we're talking about.


This could be quite a difficult task, since every object is at a different
distance from Earth and therefore must be forecasted ahead by different
amounts.

Is there a name for this area of research?


Other that "cosmology", I'm not aware of any. But it's essentially
not a big enough difference to matter at any but the most distant
scales, and even then it's only that the most distant (and
fastest-receding) stuff has actually left our "universe" by now.

Okay, question two. Kind of related to the above. Andromeda is 200,000 light
years in diameter. We are viewing it almost 'edge on'. Therefore the light
from the edge closest to us is 200,000 years 'ahead' of the light from the
furthest edge. Therefore, how can what we see in the sky be an accurate
representation of the real structure of Andromeda? - It's as if the image
gets more 'lagged' as we move further from the leading edge.


Yes, and the part moving toward us is blueshifted, which is one of the
ways we can tell the direction it rotates. But again, it's not moving
fast enough for this to distort the image in any really significant
way. The difference is less than one part in a thousand -- and we
know the direction it's moving, anyway.

The edge furthest away shows us what it looked like 2,200,000 years ago, the
light from the leading edge showed what it looked like 2,000,000 years ago
(approx) Could this actually be distorting our view of the galaxy to make it
look more spiral-shaped than it actually is?


No; it doesn't actually work that way. There are lots of spiral
galaxies that are face-on to us (the Whirlpool, for example, as the
brightest and clearest one).


John.


Good thinking; unfortunately it doesn't really have much of an effect.
  #3  
Old September 4th 03, 10:26 PM
John Honan
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"eyelessgame" wrote in message
om...
"John Honan" wrote in message

...
Taking the current position and structure of say, the Andromeda Galaxy,
attempt to create a forecasted image of what it actually looks like at

the
moment in terms of its structure, and what its 'real' position in the

sky is
(how that has changed over the course of 2 million years)


Again, not much. It's a hundred thousand light years across, but the
stars in it only move at hundreds of miles per second, whereas light
travels at almost two hundred thousand miles per second. So it
doesn't change much at the scale we're talking about.


You're right. I did some rough calculations today basing the speed of light
on 300,000Km/s, and the speed of the stars moving at 100km/s, the diameter
of Andromeda is 200,000 light years, and Amdromeda is 2,000,000 light years
away from us.

In the course of 2,000,000 years, a star in Andromeda will only have moved
1/300th of the way across the galaxy. (As you pointed out, not much change
at the scale we're talking about)

Thanks for the feedback.

John.


 




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