# Thread: Galaxies and spiral arms

1. ## Galaxies and spiral arms

Please, oh please don't take this as anything more than a passing thought.
I'm sure there's a major flaw in this suggestion that someone will point out
and that's in part why I'm putting the idea out there.

Light travels at a finite speed. When we look at a galaxy, we are seeing it
at many different points in time, as the part of the galaxy that is closest
to us can be millions of light years closer than the far end, thus the light
from the far end takes millions more years to reach us. Basically what I am
saying is, in *some* cases, could the reason that some galaxies appear to
have spiral arms be because they are rotating, and the length of the arm is
thousands of light years long, therefore the closest part will appear
farthest in the direction of rotation and the farther back the arm you go,
the farther back in time you go causing it to look like a spiral arm?

Whew....I hope somebody understands that.....

- Jeff
http://www.planetary.org
\$30 a year and you're part of the team sending the first solar sail into
space......

2. ## Galaxies and spiral arms

In message <yzD5d.158175\$XP3.131357@edtnps84>, Jeff Hammersmark
<jdhammer@telus.net> writes

No, because we see galaxies face on and they show spiral arms - that's
when they are best seen.
Galaxies are a few thousand (not million) light years across.
Spiral arms are an illusion in the sense that they aren't permanent
things that go round a galaxy, but they aren't that sort of illusion.
--
What have they got to hide? Release the ESA Beagle 2 report.
Remove spam and invalid from address to reply.

3. ## Galaxies and spiral arms

I realize that spiral arms really do exist and I know that galxies that we
see face on prove it, I was just thinking of a situation where it isn't face
on - more at an angle. Could this situation create an illusion of spiral
arms or a spiral pattern which isn't actually there? Also, galaxies are of
course not millions of light years across (I guess I was confusing that with
light years away) but if you want to get technical, they are usually more
than just a few thousand light years across - Tens to hundreds of thousands
would probably be a more accurate description of their sizes.
"Jonathan Silverlight" <jsilverlight@spam.merseia.fsnet.co.uk.invalid> wrote
in message news:fG6XytBJ6wVBFwUM@merseia.fsnet.co.uk...

4. ## Galaxies and spiral arms

Jeff Hammersmark wrote:
For features to change their apparent shape because of the time-lag
between the arrival of the light from the 'foreground' parts of the
galaxy and the 'background', they'd have to be moving extremely fast
-- a substantial fraction of c -- which doesn't agree with other
observations, like the red- (or blue-) shift variation from one side
to the other. Our models of galactic motion already have trouble
explaining how galaxies manage to hold together with rotational
periods in the hundreds of millions of years (hence the posited "dark
matter" to give them more gravity); the discrepancy would be that
much worse if their rate of rotation were orders of magnitude faster.
Moreover most spiral galaxies' arms can be seen to 'wrap' all the way
around the hub, or even several times around, which I don't think can
be reconciled with a time-lag explanation. Finally, we see plenty of
spirals 'face-on' or nearly so, in which case all the light we see
must have been emitted at about the same time, give or take a few
thousand years.

--
Odysseus

5. ## Galaxies and spiral arms

"Jeff Hammersmark" <jdhammer@telus.net> wrote in message
news:ZME5d.116052\$KU5.112508@edtnps89...
that we
isn't face
spiral

Jeff,
For that to be the case, the radius of the galaxy (in light years)
would have to be a fairly high proportion of the rotation period (in
years).

I'm not saying its impossible, but I'm not aware that there have been
any such examples. For example, our Galaxy has a diameter of some
100,000 Ly, but its rotation period is something like 250 - 300 Million
years. In the time that light takes to travel across the radius of the
galaxy, the galaxy rotates something like one fiftieth of a degree.

It would be an interesting exercise to work out the mass of the galaxy
that would rotate with a period of (let us say) 1 million years.
However, I don't expect we'll ever find any which are the size of the
Galaxy, but can rotate so rapidly.

It's a good question though - don't be afraid to ask more like that.

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