# Need help with some Astronomy facts about universe

I’m making a wallpaper for my computer, and I need some help with numbers. I’ve poked around a bit but thought that perhaps someone with Astronomical knowledge might be able to give me the answers. Here is what I have so far:

The background is a view of the Milky way from above it’s axis as a barred spiral galaxy. Sol is indicated on the picture (You can get this pic from the NASA image site btw) Underneath Sol I have added the comment “You are here”, I’ve added a double ended arrow from Sol to the center of the galaxy and the comment “approx. 28,000 Light years”, I have another double ended arrow crossing the diameter of the galaxy that says “Approx. 100,000 light years” At the bottom I have written “The Milky way is one of an estimated 100 Billion Galaxies.” and “The estimated number of Galaxies has only increased over history”

I want to add some things on the side, first I want to say “The Milky way contains approximately 200 billion stars” “If intelligent life occurs on at least one out of every 100 billion stars we are not alone in the Galaxy”

Next I want to say “The universe contains approximately (S) stars (I don’t know the value of S) that is (X) times as many stars as our Galaxy.” and " If intelligent life occurs on at least one out of every (S/2) stars, we are not alone.

Lastly I want to say "The estimate of 100 Billion Galaxies and (S) stars includes only the observable universe. Because of the speed of light we can only see part of the universe, it is highly likely that there are more Galaxies and Stars than we can see.

Can anyone fact-check what I have so far, and tell me the value of S?

Just in case anyone is interested, the title at the top of this wallpaper is “Your place in the universe”

AllFree

Your numbers are too absolute. There are more than 100 billion galaxies. There are approximately 200-400 billion stars in the milky way, etc.

Given the uncertaintly of the counts you can probably safely assume the Milky way to be average and multiply the number of galaxies times the number of stars in the milky way and get a horribly inaccurate number, but as accurate as any.

We may not be alone in our galaxy, but given what we know about physics and space travel (not a whole lot, really) Life needs to occur a whole lot more often than 1 in 100 billion stars if we ever hope to encounter it elsewhere in the galaxy. It might be there, but we’d never interact. But it looks more and more likely every day that life is a lot more common than that.

This is the Drake Equation

2.31? Are we the .31?

You absolutely can’t safely assume that the Milky Way is average, since it’s larger than the vast majority of other galaxies. The Magellanic Clouds are probably closer to average.

Here is the archive for the Astronomy Picture of the day. I think you should look at as many of these picutes as you have time for, before settling on your wallpaper.

http://antwrp.gsfc.nasa.gov/apod/archivepix.html

>Because of the speed of light we can only see part of the universe, it is highly likely that there are more Galaxies and Stars than we can see.

This line is a bit weird. We can reliably assume that there are certainly more than we can see. We only see the Sun as it was 8 minutes ago, but none of us seriously consider that it may have vanished in the meantime. We accept that it is more than “highly likely” the Sun is still there. Likewise for the stars and galaxies further away, except that no doubt some have vanished since their light started toward us (and a similar number have been born since light from their neighborhood would have started toward us). So if your “highly likely” refers to this issue, it is probably not mentioning it as a liklihood. It is a different problem that due to Einsteinian relativity there isn’t one completely applicable notion of what we mean by “now” when we ask how many galaxies there are out there now (and this is after everybody has taken the speed of light into account). Yet a third issue is what cosmology is currently saying about the Universe, which (if I understand the state of the art correctly) is that the Universe is infinite in size. Perhaps it is better to say that places we can see now on the edge of the “observable universe”, which we see by the light they emitted near the dawn of time, are in their nature no different than here, and have a similar history, including being able to see out a similar distance in all directions to their version of the edge of the observable universe. If this understanding is correct as far as it goes, then there are an infinite number of intelligent civilizations, and there are even an infinite number of them in which there is a SDMB having this conversation, and an infinite number in which this conversation looks the same except my posting spelled some word differently, and so forth. The distances between these, of course, might be large enough to guarantee they will never know of one another, but it doesn’t change the conclusion. Max Tegmark has written some interesting papers on the topic.