1. Jul 6, 2008

### Dumbmum

Hello,

I have registered here in the hope that someone can answer a couple of dumb questions! My 6 year old son is very interested in planets etc and i cannot answer some of his questions. If anyone is inclined to help me out I'd be so grateful.

1) How do we know that there are millions of galaxies when we cannot see or travel that far?
2) How can we estimate how many stars exsist in unexplored galaxies?
3) Suppose the big bang started with a single particle, where did that particle come from?
4) Something cannot just appear out of nothing, right? So where did the gases and space dust come from in the first place?

See just an old dummy here who needs a friendly scientist to help her out!

Thank you

2. Jul 6, 2008

### mathman

1) Current telescopes (especially Hubble) can see lots of galaxies. There have been complete sky surveys.
2) Using the Hubble relationship (red shift to estimate distance) we can estimate the absolute brightness of galaxies, which can give an estimate of the number of stars.
3) Big unknown!!!!
4) All stuff came out of the big bang.

3. Jul 6, 2008

### Dumbmum

Thank you! So can I be cheeky and ask another question?
If the universe is constantly expanding what is it expanding into? I cannot understand how , if nothing is beyond, how something can occupy a space that is not there.

Can you recommend a book that is somewhere between a childs book and something not-too-complicated that a non-physicisty type of person can understand?

Many thanks

4. Jul 6, 2008

### B. Elliott

Stephen Hawking's books, 'The Universe in a Nutshell' and 'A Brief History of Time' are both very to easy to read and very illustrated. They should be exactly what you're looking for.

On your question about what space is expanding into, it's not really expanding into anything. The Universe its self is expanding. The simplest way to describe is is the classic balloon filled with air analogy, but keep in mind this is a two dimensional model of a three dimensional (actually multidimensional) universe.

Imagine a very tiny balloon which is deflated (when the Universe was a singularity... a single point). Now, imagine that you started to pump up that balloon with air. Galaxies would begin to form on the surface of the balloon as dots... like markings on a balloon from a magic marker. Nothing is 'inside' of the balloon since that would be a three dimensional model. Anyway, as you begin to pump up the balloon more and more, the galaxies which were at one time very close together, are now starting to slowly move away from one another. The positions of the galaxies (in this analogy) stay in place, but the 'space' (surface) which the galaxies exist on is expanding and growing larger and larger, which gives them the appearance of drifting away from each other.... when it's really the space its self which is expanding!

Here is a site with a few good pictures if I didn't explain it that well...
http://www.astro.ucla.edu/~wright/balloon0.html

And here's a Java applet...
http://www.astro.ucla.edu/~wright/Balloon2.html

5. Jul 7, 2008