h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This was very interesting and as you said , written so that the layman could understand it . And men do not come any more lay then me I can tell you .

I'm glad you liked it.

Very nice basic primer. Adequate definition is always an essential of quality popular science.

I enjoyed the piece but it didn't actually say what happens at the end of the life of a white dwarf. I realise that, as you say it stops shining, but what is actually left? A ball of solid matter or a ball of inert gas? Or does it just go quietly into the night?
Any ideas, I'd love to know.

I didn't say because I don't think the astronomers know for sure, but I think it would be a sphere of inert ultra-dense matter, which we could call a black dwarf.

So we'd better put on the headlights when swanning around the galaxy.
Many thanks

Huge though such a black dwarf may be, the chances of hitting it in space are small, as space is big.

I was just thinking, before I read your post, that if we ever send a spaceship out to the stars to set up a new colony, it should be called the "Swanfleet".

Maybe there's one near Alpha Cygnet!

With some estimates, cooling times for white dwarfs exceed the probable life expectancy of the Universe.

I thought the universe was going to live forever.

I guess it depends on what one means by the term 'universe'. Currently its restricted to the local C-theta sector and under the current accelerating model of cosmological expansion that is expected eventually to decay. On the other hand, there's a growing concensus that the local C-theta sector must be embedded in a larger plenum.

Sorry, you're using terms I don't know.

I'm still stuck in Alpha quadrant!

On the C-theta sector, best illustrated by means of taking a couple of distant galaxies half way to Hubble's limit (or the Wilson-Penzias threshold) in opposite directions. Having selected one's pair of antipodal galaxies, the next question is how much of what we can see from our position half way between them would be visible from the point of view of an observer in either of the two antipodal galaxies.

The answer is that anything beyond its antipodal galaxy, which would from the point of view of an observer in either be located at Hubble's limit, would be lost to view, even though its visible from our central position.

Each of those antipodal galaxies is located at the limit of their C-theta sector from the point of view of an observor in either.

The C-theta sector is simply the limit at which electromagnetic observations can be made.

G-theta sectors are the regions over which cosmological objects are gravitationally bound and are considerably smaller than C-theta sectors. The limit of the G-theta sector is the limit at which the principle of cosmological expansion begins to govern relative motion.

Another illustration of the C-theta sector can be made by means of taking a galaxy located at Hubble's limit and asking how much of the universe we can see would visible by an observer located in that galaxy. The region of overlap is only 120 degrees wide and there are perspective effects. From our point of view with respect to the region of overlap, we're located at the apex of a conical sector which appears to be bounded by a semi-spherical section at Hubble's limit. From the point of view of a observer there the reverse would obtain, they would seem to be located at the apex of that cone and our position would appear to be at the center of a semi-spherical section at Hubble's limit. Their own position would mark the center of their local C-theta sector and ours would be at its limit.

on post 14, paragraph 4, I should have made that "...the region over which electromagnetic observations can be made." rather than "...the limit at which..."

with reference to white dwarfs (or should that be dwarves). what are they actually converting? I assume they have used up all their hydrogen and helium etc'(not forgetting that any assumtion is the mother of all *%@ups)and once you get to lead I thought that was it unless you are a super nova?

I believe they're just still very hot and it takes them a long time to cool down. I don't think they're actively involved in fusion any more.

Oh, and it should be dwarfs. It's only the dwarves in fantasy novels that are dwarves. This plural was invented by Tolkien. You'll notice that it is Snow White and the Seven Dwarfs.