Galactic Family Tree Sheds Light on Cosmos
Pathfinder charms us with new views of Mars. The Galileo spacecraft hints at life-sustaining waters on Jupiter's moon Europa. But have you noticed the scene at the other end of the universe?
For the first time, astronomers can survey conditions deep enough in space - and far enough back in time - to see the first stars turn on and the first galaxies arise. At a time of extraordinary space exploration, this is one of the most scientifically rewarding fields of discovery.
The light that reveals this scene left its sources when the universe was less then 20 percent of its present age. Reviewing this exploration in the journal Nature earlier this summer, Roberto Abraham of Britain's Royal Greenwich Observatory in Cambridge noted that the new galaxy surveys "are now seeing almost all of the galactic history of the universe."
This is the kind of basic knowledge scientists need to understand how the universe got to be the way it is. They need it to understand the history of our own Milky Way galaxy. They need it, ultimately, to understand how our solar system with its life-sustaining planet Earth arose within that galaxy. One of the primary reasons the United States and Europe invested in the Hubble Space Telescope was to gather that primordial cosmic knowledge. Their investment now is beginning to pay its expected dividend.
Scientists working with the Space Telescope Science Institute in Baltimore revealed a remarkable scene at an American Astronomical Society meeting in Austin, Texas, a year and a half ago. It covered a tiny patch of northern sky near the Big Dipper where astronomers had thought there was little to see. The sharp-eyed Hubble revealed it to be rich in distant galaxies. Subsequent analyses by many scientists have located what appear to be young galaxies that are just getting their act together. They are sucking in hydrogen-helium gas from the surrounding primordial medium. Some appear to be merging with each other.
Taken together, these analyses of Hubble images supplemented by data from other astronomical satellites and ground-based instruments sketch a story of cosmic evolution in which star formation began somewhat slowly. It then rose to a peak when the universe was about half its present roughly 13-billion-year age. It has slacked off since then.
At least, that's how it looks now. Dr. Abraham warns that the story may change as the observations and analyses continue. One big uncertainty is just how much star formation there was in the beginning. If dust in young galaxies hid some of the action, star formation may have gotten going more vigorously than scientists now realize. The infrared instruments astronauts installed in the Hubble last winter and new infrared observing satellites could settle the question. Infrared light would cut through a dust veil and reveal any hidden star formation.
Not all the action at the distant epoch was in the young galaxies. The European Space Agency (ESA) in Paris earlier this month reported a new view of what was happening in the hydrogen-helium medium at the time of early galaxy formation. This gas was formed in the first few minutes of the universe's birth in the so-called big bang primordial explosion of energy and matter. Radiation from newly formed stars - especially from the most energetic galaxies - began to strip electrons from the gas atoms, leaving behind electrically charged atoms called ions.
ESA reports that research over the past eight years now has let astronomers observe the crucial epoch when some, but not all, of the primordial helium was ionized. This has involved research by astronomers in Germany at the University of Hamburg using the Hubble telescope and ground-based data.
When earth scientists try to reconstruct the evolution of our planet they have to work backward from the fossils and geological evidence they have today. Astronomers are more fortunate. They can actually look back in time and see what was happening when the universe first evolved its galactic structure.