H   H
What fills the Universe
Formation of the Galaxies

The Big Bang sent out matter in all directions.
Matter coalesced from the energy in the hot young universe.
First, strings (maybe) or quarks, then protons and neutrons.
Next hydrogen, helium and lithium atoms were all formed.
As the universe expanded and cooled, the seeds of galaxy formation
were sown in the very early universe due to
small density fluctuations in the primordial matter
as atoms gravitated toward each other to form Superclusters.

And from these gigantic clusters, galaxies were formed.
Left in the wake of the expansion of the universe, these islands of gas and dust created by gravitation coalesced into billions of stars.

During the past 10-15 billion years, rotation has flattened the gas in our galaxy into a thin disk.

Stars, gas, dust, brown dwarfs and black holes all isolated in space and held together by gravity.
The Milky Way was discovered to be a Galaxy by Edwin Hubble in 1930. Before Hubble's realization, what did people believe that huge cloud in the night sky to be?
                Thanks to the pioneering work of Henrietta Leavitt, who had developed a technique using variable stars as a measure of distance,
Henrietta Leavitt spent a great deal of time searching the photographic plates for these variable stars in the small and the large Magellanic clouds. Leavitt discovered around a thousand variable stars in these clouds. Among these, she found 25 Cepheid variables in the small Magellanic cloud and noted that the period of these variable stars were correlated with the peak brightness. The brighter the star was, the longer it took to vary its brightness. In other words, the period was proportional to the brightness. Leavitt argued that since all the stars in the small Magellanic cloud were situated in a small part of the sky, it was reasonable to assume that they were more or less at the same distance from us.
Her discovery led to Hubble's Law

One of many 'Computers'
A Galaxy Merger
  Hubble Heritage Project
         There are hundreds of billions of galaxies. . .
                                             . . .each with hundreds of billions of stars!

  How do we 'see' the Stars, Galaxies, Quasars and Black Holes?
Electromagnetic Spectrum

courtesy of UCB

The Milky Way in many Wavelengths

        Hubble Deep Field

    Quasars and Black Holes