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The place with four suns in the sky. . . four different colors. . . and in touch. . .  Well, first of all, most of the stars In the sky are not lone stars like the Sun but are binary or multiple star systems.  A fair fraction of binary stars are called "contact binaries", in which the gravitational attraction of the more master star pulls matter out of the less master star— it flows from the donor to the receiver.  Now, there are many cases where two binaries orbit each other. Two stars are revolving around a common center of mass. Another two stars are revolving around their center of mass, and the two centers of mass revolve around each other.

Now, as far as color goes, the Sun is a yellow dwarf.  A highly evolved star, like the Sun will be in another five billion years or so, is called a red giant.  A red giant usually winds up as a white dwarf.  And a very hot star but still in middle age like the Sun is called a blue dwarf.

A world with a million moons...   is Saturn. The Rings of Saturn are composed of snowballs which are certainly less than a meter across, perhaps ten centimeters across. There are millions of such snowballs making up the rings of Saturn.

A sun the size of the Earth and made of diamond . . .   Many white dwarfs fit that description. Where hydrogen has been substantially lost they are crystals, stars which are crystals, and they're cold and cooling still more.  So, for example, Sirius has a white dwarf companion.  It was the first one discovered, but there are enormous numbers of such white dwarfs, many of which are made largely of carbon in crystal form. Therefore diamond is the correct description.

An atomic nucleus a mile across that rotates thirty times a second ...   is a neutron star, which is the end product of the evolution of a star more massive that the Sun.  It becomes, not a white dwarf, but a neutron star.  That is, it's composed entirely of nucleons— the elementary particles which make up thenucleus of atoms.  Therefore they are atomic nuclei. And a mile across is how dense the thing shrinks to before the nuclear forces between particles pull the thing up against subsequent gravitational collapse.  And they're rotating thirty times a second because of the conservation of angular momenturn.  A star like the Sun spins once a month. When it contracts down to a mile across it's spinning something like thirty times a second.  A specific example— the one that rotates thirty times a second— is the pulsar in the Crab Nebula, which is a neutron star.
OK, Tiny grains between the stars with the size and atomic composition of bacteria . . .  Well, there's some absolutely tremendous number of them.  If you take a look at a typical dark nebula, like say the Horsehead Nebula, the dark stuff is the kind of grains I'm talking about.

Hm, Stars leaving the Milky Way . . .   Gas clouds falling into the Milky Way . . .  Well, again it's quite common. We are a star which is in the plane, one of the spiral arms, of the Milky Way.  But there are, for example, stars of a sort called "M dwarfs" which are oscillating out of the plane of the Milky Way— they spend most of their time out of it.

OK, Turbulent plasmas writhing with X- and gamma-rays and mighty stellar explosions. . .   Again, the Crab Nebula.  Not the star in the center of it, but the nebula itself, is a good example of this.

Places outside our universe ... Is a black hole. The nearest
object which is thought by many astronomers to be a black
hole is Cygnus X 1. I like to think of a black hole as a place
where the gravity is so great that the fabric of space has be
come puckered    isolated from the rest of space so that
light can't get out of it.

"Does that do it?" he asked.

"Perfectly,"said we.   "Another question:  What have you read that electrified you lately?" Sagan pondered, "Welt I'll tell you, the detective fiction of John D. MacDonald I find very interesting in terms of perception of character. Just for kicks it's far above Agatha Christie. . .   Urn, I've just reread The Odyssey.  It's a good book.  A lot of it takes place in Ithaca." "Which translation?" "Samuel Butler."