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Hello, ours is about a hundred thousand light years across.

It shaped like a fried egg and we travel inside it at approximately 220 kilometers per second.

The nearest one to us is much smaller and is nicknamed the Sagittarius dwarf.

But the one down the road called Aldrometer is just as large as ours and in 10 billion years we'll probably crash into it.

I'm talking about galaxies, the vast islands in space

of staggering beauty and even more staggering dimension. But galaxies aren't

simply there to adorn the universe, they house much of its visible matter and maintain the stars in a constant cycle of creation and destruction.

But why do the galaxies exist? How have they evolved?

And what lies at the center of a galaxy to make the stars dance around it

at such colossal speeds.

With me to discuss galaxies at John Gribbin, visiting fellow in astronomy at the University

of Sussex, Carolyn Crawford, Royal Society University

Research Fellow at the Institute of Astronomy, and Robert Kennecutt, Plume and

Professor of Astronomy and Experimental Philosophy at the University of Cambridge.

John Grubman, let's start with a

simple definition. What's a galaxy and what do galaxies consist of? You've done it

already. It's this flattened disk of stars. When we look up at the sky

...