A Question Regarding the Milky Way

PCH (Paul) · #1 10-09-2009, 12:35 PM

Hi All,

this sounds like it could be such a profoundly dumb question that I hardly dare ask. And yet, for the life of me I can't think of the answer, so here goes anyway. If it is a really simple answer, please be gentle

Ok, so this is the build up. A couple of facts to start with.

I imagine it's no secret that we live in the Milky Way galaxy, which is a spiral galaxy about 100,000 LY across and about 1000 LY thick. And we are about 26,000 LY from the centre.

At the centre of this, and all, spiral galaxies is a gigantic beyond belief, seething cauldron of star activity about a trillion times as bright as our sun.

And this is the question ...

... when we here on the ground are facing the correct way (as any spot on the planet must be at some time during the year, month, day), meaning when we are looking out into space, and happen to be looking in the direction of the centre of the galaxy, how come we don't see a blindingly bright light filling part of the sky. I mean, this thing is so unbelievably bright, and it is definitely there, so how come we don't get to see it - ever?

I know it's a long way away, but we easily see other stuff that's not as bright, and is much further away.

All clues gratefully received

[1ponders] (Paul) · #2 10-09-2009, 12:42 PM

One reason is the amount of dust in between here and there. If you look at the Sagittarius star cloud that is looking through a bit of a window towards the central area.

jjjnettie (Jeanette) · #3 10-09-2009, 12:47 PM

There's a simple explanation, there is this huge dust cloud obscuring most of the light from our galactic centre.
Thank goodness for that, otherwise all the goodies in Scorpius and Sagitarious would be washed out with the brightness of it all.

jjjnettie (Jeanette) · #4 10-09-2009, 12:48 PM

Beat me by THAT much.

PCH (Paul) · #5 10-09-2009, 01:33 PM

Thanks Paul & JJJ,

so it's all down to a bit of dust. No wonder swmbo is always scraping it off the tele if that's what a build up does for you

But equally, shouldn't the same dust clouds stop us seeing in other directions too? Or is it that there's a particularly dense dust cloud blocking off our view of the centre, and not much blocking the other views? Is that how it works?

Thanks again for your replies guys, I'd really like to get this sorted in my mind.

Cheers

DavidU (Dave) · #6 10-09-2009, 02:12 PM

This pic shows the edge on view and the amount of dust obscuring the core
http://helensguidetothegalaxy.files..../milky-way.jpg

PCH (Paul) · #7 10-09-2009, 02:19 PM

David,

jeepers that is dense. I suppose I've seen pics like this before, but hadn't really considered their effect. Actually, I've never really wondered why we don't see the great big bright centre before. It just occured to me this morning I'm embarrassed to admit.

Hey, in the pic you attached David, do you or anyone know what the bright patch is that seems to be behind the dust cloud? Would it be daft to ask if that's a partial view of the central area?

Cheers,

[1ponders] (Paul) · #8 10-09-2009, 02:48 PM

See post #2

Quote:

If you look at the Sagittarius star cloud that is looking through a bit of a window towards the central area.

That is the Sagittarius Star Cloud

[1ponders] (Paul) · #9 10-09-2009, 02:52 PM

the other thing to remember is that the arms you see in spiral galaxies are actually optical illusions brought about by the density waves moving through the galaxy. We see the arms because of the amount of star formation going on there, but after the star formation finishes and the wave moves on the dust and gas is still there. So in effect the areas between the arms is full of dust as well. And stars, but they are cooler yellow and red stars that live longer than the hot OB and A stars and not as easily visible to us.

At least that's my understanding.

[1ponders] (Paul) · #10 10-09-2009, 02:54 PM

This might give you an idea of what I'm talking about
http://www.ozone3d.net/demos_project...xy_500x374.jpg

[1ponders] (Paul) · #11 10-09-2009, 02:58 PM

And these
http://www.freewebs.com/stargateente...xy-NGC1300.jpg
http://www.spacetoday.org/images/Hub...iralGalaxy.jpg
http://www.awitness.org/unified/page...ral-galaxy.jpg

btw if you manage to find the 3D animation of the jpeg from the previous post, the animation is not correct. Galaxies do not spiral like water going down a plug hole.

erick (Eric) · #12 10-09-2009, 03:21 PM

I read that:-

"Astronomers have been puzzled, however, by a shortage of young supernova remnants in our galaxy. Only half a dozen have been found, as opposed to the more than 30—roughly two a century—predicted to exist"

Are supernova (or remnants) detectable if they are well across the galaxy behind the galactic core? I have often wondered whether we are just not detecting them for that reason?

PCH (Paul) · #13 10-09-2009, 04:48 PM

Quote:

Originally Posted by [1ponders]

the other thing to remember is that the arms you see in spiral galaxies are actually optical illusions brought about by the density waves moving through the galaxy. We see the arms because of the amount of star formation going on there, but after the star formation finishes and the wave moves on the dust and gas is still there. So in effect the areas between the arms is full of dust as well. And stars, but they are cooler yellow and red stars that live longer than the hot OB and A stars and not as easily visible to us.

At least that's my understanding.

Paul,

thanks for your replies. I do believe the principal you've outlined here is what a section in this weeks 'Build a Model Solar System' was trying to describe. But I could 'get it' at all. Your description seems clearer - so thanks for that. All in all, I'm glad I asked my question, as with your various answers, and the others, plus a bit of thinking on my part, I now understand much more about this scenario than I did this morning. I knew you guys would be able to clear this up for me.

Cheers,

PCH (Paul) · #14 10-09-2009, 04:51 PM

Quote:

Originally Posted by erick

I read that:-

"Astronomers have been puzzled, however, by a shortage of young supernova remnants in our galaxy. Only half a dozen have been found, as opposed to the more than 30—roughly two a century—predicted to exist"

Are supernova (or remnants) detectable if they are well across the galaxy behind the galactic core? I have often wondered whether we are just not detecting them for that reason?

Hi Erick,

clearly I'm no expert, but how could we detect anything over that side? Surely everything would be washed out by the light that is there, even though we don't see all of it - as discussed in this thread.

Cheers,

erick (Eric) · #15 10-09-2009, 09:15 PM

Visible light would be out of the question, but I'm sure I've read that the galactic centre can be probed better in infrared wavelengths. I expect a supernova pours out energy across a huge range of the EM spectrum as well as high energy particles. Maybe other EM or these particles are detectable.

PCH (Paul) · #16 10-09-2009, 09:33 PM

Hmmm, yep - i can understand that.

GrahamL · #17 10-09-2009, 11:15 PM

You do get a fair hint of it though when you look towards Sagittarius
from a dark sky , the sheer density of stars and endless number of globs in this bit of sky is pretty noticable compared to say when galaxy season rises a little higher in the east , still lots to see but that ink black background is in stark contrast to whats setting in the west .