Chapter 2

For the first 400 million years the universe was hot and filled with neutral atoms. Light moved freely, not bouncing off anything, so the universe looked dark.

For the next 600 million years, the universe was cool enough so that gravity could pull atoms together to form dwarf galaxies. Galaxies formed the way our solar system formed – gas in the center condensed into a black hole (a very compact collection of matter with such strong gravity that even light can’t escape, so it looks black). The rest of the gas around this black hole condensed into small clumps dense enough to start a nuclear reaction (where hydrogen and helium combined into heavier elements and released a lot of energy) becoming stars.

These galaxies emitted ultraviolet light that knocked the electrons off atoms, re-ionizing and re-energizing them. After 1 billion years, the ionized gas was so energetic that only massive galaxies, over 100 times bigger than dwarf galaxies, with enough gravity to capture these high speed ions, could form.

Then the galaxies themselves started clumping together to form galaxy clusters or Groups. For instance our galaxy (the Milky Way) is one of the biggest of a Local Group of 30 galaxies (Virgo). These clusters then formed super clusters, which in turn formed filaments, the highest known level of organization in our universe. About 90% of the universe is empty space in between filaments that look like soap bubbles.

Within 2 billion years, after converting all of their hydrogen into heavier elements, like carbon, oxygen, and iron some stars exploded in a spectacular supernova. The ejected heavier elements in turn attracted other gas gravitationally to form a new kind of star, one with more than just hydrogen and helium and one that allowed planets to be formed. This is how our sun and planets were formed 4.6 billion years ago.

As galaxies formed the remaining gas was more thinly spread. The maximum rate of star formation was 7 billion years ago.

Notable Events

Source www.sciencemag.org Volume 313 Page 931 18 Aug 2006

Time Gya Event

13.7 Universe started

13.4 First stars formed from Hydrogen and Helium

13.3 First Galaxies formed

12.7 Universe re-ionized by energetic photons

12 Clusters of galaxies formed

10 Milky Way Galaxy formed

9 Spiral Arm formed where Sun eventually formed

6.6 Some Galaxies bumped into each other and merged

5.0