Astronomy question

[War Arrow]

I've asked this elsewhere to no avail, so as I'm hoping one of you might be more mathematically adept than I am...

Suppose, I've found a habitable planet out on the absolute furthest edge of the Milky Way. I'm standing there looking up into the night sky at the Sagittarius dwarf galaxy and the M54 globular cluster found somewhere within aforementioned SagDEG. Given that the Sagittarius dwarf galaxy is (I think) 10,000 light years across and seems to be actually nudging at the edge of the Milky Way, and M54 is 150 light years across at (I think) 16 light years distance from the Milky Way - what would the night sky look like from where I'm standing?

Pretty much as things look from Earth, or pretty much black but for a big central splodge of stars comprising above named stellar objects, or black, black, black with Sagittarius and M54 occupying an area equivalent to maybe the moon in our own night sky?

Before anyone asks, there is NO archaeological subtext to this question.

I'd ask this on google but as you may be able to appreciate, I'm not even quite sure what terms to enter into the search engine.

[Minimalist]

http://rst.gsfc.nasa.gov/Sect20/A2a.html




The Hubble telescope has captured pictures of distant galaxies. Since your eye will only pick up the light and not the blackness, my guess is it would like pretty much like the sky from earth.

[War Arrow]

The Hubble telescope has captured pictures of distant galaxies. Since your eye will only pick up the light and not the blackness, my guess is it would like pretty much like the sky from earth.

That was my feeling too, though an astronomy obssessive pal who has promised to call me back this afternoon once he's done the maths seems to think the view would be similar to our own view of the Milky Way but with noticably less starry patches of sky above and below the main er... strip (?) we'll see I guess, though probably not in my lifetime. Anyway thanks for that link. Some great images there and I've saved it for future reference.

[Forum Monk]

I'll take a look at the problem as well WA, but in the meantime, since you like to ponder these kinds of things, may I suggest a free program found in several places on the web, called Celestia. ( http://www.shatters.net/celestia/ ) It produces a 3D virtual environment for exploring the cosmos, and allows one to zoom out to the far reaches of the milky way and visualize what the sky would look like.

Of course in order to use it, you have to be willing to endure the download time, and a bit of time figuring out how to navigate in a three dimensional universe that is millions of light-years across.

I have the program on another computer but never bothered to learn it as I am more interested in the view from own backyard, than Delta Reticulae.

[Forum Monk]

Of all the extragalactic targets you could have picked, you have picked one of the most interesting. But in trying to determine the location of SagDEG in a 3D reference frame wrt to our solarsystem location, is not so easy, as SagDEG is now scattered into a ring, completely surrounding the milky-way. Checkout this site, as one of the very best to explain what SagDEG is and how it got to where it is today. It is an excellent site, not dumbed-down but yet understandable to the layman, with links to animations, etc.
http://www.solstation.com/x-objects/sag-deg.htm

As for M54 which may be part of SagDEG (depending on which source you consult) is at least a measurable target and so I will do a simple trigonometric relationship to detemine is apparant size based on its present apparent diameter of 12 arc minutes at a distance of 87,400 light-years. Since you do not specify in any detail, I will assume you move closer along the line of sight for a distance of 60,000 light years.

So here is my math so your astronomy friend can verify:

Given M54 has an apparent radius of 6 arc min at 87400LY, its true radius is 87400 * TAN(0.1 degrees) = 152.5 LY (305LY in diameter). So reducing the distance to the object to a mere 27400LY the new apparent radius is given by r = ATN(152.5 / 27400) = .32 degrees (diameter = .64 degrees). This math works because the distances are still large, but for more precision, use the formulae given here: http://en.wikipedia.org/wiki/Angular_diameter

This would make it about twice the apparent size of the full moon. Impressive, but I have not bothered to calculate its apparent magnitude because that's just more math than I care to attempt this weekend.

[Minimalist]

That's more math than I would care to attempt in a lifetime!

[Frank Harrist]

Just readin' that gave me a headache.

[War Arrow]

Just readin' that gave me a headache.

Me too, regardless of which, thanks a lot, FM. I'll take a look at what you've posted and linked as soon as my head's cleared a little. Maths is not my strong point so it looks daunting. My astronomy correspondent (Andy) now claims that so far as he is able to tell (emphasis on the lack of certainty there) M54 would appear in the night sky (and to a lesser extent the daytime sky assuming an earth-like atmosphere) of my imaginary planet roughly 30 times the arc diameter (?) of the full moon, in other words, if I was there looking up at it, I could stretch out my arm and hold up my fist and just about block it from view. That said, Andy is also the last man alive (to my knowledge) who refuses to budge from Hoyle's steady-state theory of universal expansion (I don't know myself, but I admire his resolve) so the jury is out, but I'll see what your links yield. As this is all going towards a piece of fiction, the need for accuracy is probably open to a broader interpretation than if it were otherwise, but I'd still like to avoid too much needless artistic license.
I must admit, it would be nice if I could find two matching descriptions of the where and why of SagDEG. I haven't yet come across a claim that the Sagittarius dwarf galaxy is in fact to be found just outside Heston service station as you approach Solihull, but I expect it's only a matter of time.
Thanks for the advice everyone. I've asked this question on the Richard Dawkins forum which has about 500 regular posters, all of them from a hard science background and not one of them has yet deigned to respond. Definitely a better class of poster around here.

[Forum Monk]

Your astronomy minded friend has obviously chosen a location much closer to the target, as I simply moved you along the line of sight for 60,000 light years.

Better class of poster? No. Just not afraid to stick my foot in my mouth.

[War Arrow]

Thanks again, FM. That first link was easily the most detailed I've come across and particularly useful for this illustration:

which (alongside the text) is the first place I've seen which attempts to describe the location and general shape of SagDEG beyond "over in that direction somewhere" - in other words, it was exactly what I've been looking for.
I think the discrepancy between your estimate of how large M54 would appear in the night sky and that of my Patrick Moore worshipping buddy (I swear, he hasn't yet built a shrine to the man, but it can only be a matter of time. He's even got CDs of Patrick Moore playing his glockenspiel) is due to his figure being at 71,000 ly hence, as you say, much nearer.
I'm very tempted by that virtual universe programme but I think I'm going to have to check to see whether my computer could cope with it first. I know it's top range on one sort of memory, but apparently not very good on the other sort (I think that being the sort you need if you want to fill it up with games where trolls invade castles and so on - never really quite seen the appeal) and the lengthy download time you mentioned sounds worrying.

[Digit]

So what happens when you reach the edge of the universe and turn your back on it?
What do you see? And what happens if you keep on going?
I know the official answer, but I have my doubts.

[Ishtar]

So what happens when you reach the edge of the universe and turn your back on it?
What do you see?

The Restaurant at the End of the Universe, of course.

Milliways, also known as The Restaurant at the End of the Universe, can only be visited practically by time travel, as it exists at the end of time and matter. Marvin the Paranoid Android is one character who manages to reach Milliways without the use of time travel, merely by being very patient. One of the restaurant's major attractions is that diners can watch the entire universe end around them as they eat. The terminal moment is followed by dessert. Reservations are easily obtained, since they can be booked once the patron returns to his or her original time after their meal, and the restaurant's bill can be paid by depositing a penny in any bank account of the present time: by the end of the universe, compound interest will be enough to pay the extremely high bill. Depending on when in Milliways' timeline they visited, the diners' vehicles may have been parked by none other than Marvin the Paranoid Android. Near-instant transportation to the restaurant can be achieved in certain rarefied circumstances, such as being next to an exploding hyperspatial field generator on the planet where Milliways will eventually be built several billion years after the explosion occurs.

Among the items on the menu are various cuts of meat from the very obliging Ameglian Major Cow and the slightly less obliging vegetables in a green salad. While water and Aldebaran liqueurs are in stock, tea is not.

Because of the special number of impossibilities surrounding all aspects of Milliways, their advertising firm penned the following slogan: "If you've done six impossible things this morning, why not round it off with breakfast at Milliway's—the Restaurant at the End of the Universe!" "Six impossible things" is a quote from Lewis Carroll's Through the Looking-Glass.

In the book form of the series, the visit to Milliways takes place in book two, The Restaurant at the End of the Universe. In the different versions of the story, Milliways is built on the ruins of either Magrathea or of Frogstar World B.

The end of the movie has a joke involving flying to Milliways, and the ship turning the wrong way. Marvin corrects the others, a squeal of brakes is heard, and the ship sets off in the other direction. This is somewhat peculiar because Milliways is at the end of the universe in time, not in space. However, it is possible that Marvin was directing the ship in the direction of whichever planet the restaurant is located (Magrathea, Frogstar World B, or otherwise).

http://en.wikipedia.org/wiki/Places_in_ ... the_Galaxy

[War Arrow]

So what happens when you reach the edge of the universe and turn your back on it?
What do you see? And what happens if you keep on going?
I know the official answer, but I have my doubts.

You don't see much apparently. Despite all the galaxies out there, most of the light is shifted so far below the visible spectrum that it's just black. I think.

Oh sorry. You said the universe. I misread you there.
I don't think you can get to the edge of the universe for similar reasons that you can't get to the edge of the earth. We are all puny ants scurrying around in our lowly three dimensions etc. etc.

[Digit]

When I were a little lad!
Eistein was arguing that the universe was 'closed', that light from the distant edge of the universe passed so many magnetic fields that it was bent so badly that in theory you should be able, with a powerful enogh 'scope, to see the back of your head. Therefore all travell would be effectively random and you could never reach the edge of the universe.
His idea, not mine.
Now the experts tell me that there is nothing beyond the edge, no time, no space, no existance, that the universe exists where it exists, that it creates its own existance by existing.
How ever! The universe is supposed to be expanding, logic therefore says that there must be something for it to expand into!
Bit like falling off the edge of a flat as far as I can see.

[Beagle]

Here's one for you FM.
http://johnhawks.net/weblog/topics/scie ... _2008.html

For this, they will need new ways to keep track of data about giant sets of objects, with varied observation conditions. Call it "astroinformatics?"

Going to be very interesting.