New grasp of galaxies: varied - and violent
Some 75 years ago, Edwin Hubble trained his telescopes at a faint point of light in the constellation of Andromeda and revolutionized astronomy. Where others had only seen a star or cloud of cosmic dust, he saw a whole new system of stars, for the first time proving that galaxies exist outside our Milky Way.
Today, at the turn of a new century, the orbiting telescope that carries Hubble's name is leading another astronomical renaissance. And this time, the research is changing fundamental ideas about some of the most massive objects of creation.
Individually, none of the new discoveries is as groundbreaking as Hubble's seminal find. Taken together, however, they represent a significant advancement of our understanding of every aspect of galaxies - from what they are to how they are formed. Moreover, while the data are in some cases raising as many questions as they answer, they are also showing scientists that galaxies are far more varied and violent than ever imagined.
"In some ways, the changes have been pretty dramatic," says John Gallagher, an astronomer at the University of Wisconsin in Madison.
During the past 15 months:
* Scientists have found a black hole at the center of the Milky Way. Once thought to be merely galactic exotica, black holes have now been located in the center of more than three dozen galaxies, leading researchers to suggest they may play some integral role.
* The location of a gas cloud at the edge of the universe - big enough to create 100 billion suns - has forced astronomers to rethink theories on galaxy formation.
* Pictures from Hubble have offered new clues about how galaxies evolve.
* Infrared data have shown that some galaxies were born much earlier than previously believed possible.
Across the board, scientists' perceptions of galaxies today are vastly different than they were even a few decades ago. Indeed, some researchers are even going so far as to challenge the very idea of what a galaxy is.
A clump of dark matter
Last week, a team from the University of Cambridge in England suggested that 99 percent of the universe's galaxies may not even be visible. The idea is that galaxies aren't collections of stars, but rather clumps of dark matter - a substance that has never been proved to exist, but that scientists say makes up 90 percent of the mass of the universe. Without the explanation of dark matter, for instance, the gravitational actions of the cosmos don't make sense.
Huge amounts of dark matter are apparently at work in typical galaxies, but smaller amounts may be collected throughout the universe with no stars in them. Says Cambridge astronomer Neil Trentham: "We're biased toward seeing big things with a lot of light."
Back in Hubble's day - and for some time after that - galaxies were seen as "island universes" of stars and dust, created in the distant past as a single, monolithic unit and then left to spin serenely through space for the rest of time.
Images from the Hubble Space Telescope refute that notion in a number of ways. They show how large and small spiral galaxies are evolving differently - the cores of many large spiral galaxies have become stable and sterile, while smaller ones continue to suck in material from the perimeter, creating cataclysmic reactions and new star births.
Space is a dangerous place
More than that, they show that space is a dangerous place. Trails of gas streaming from the edges of galaxies indicate that star systems sometimes graze each other in cosmic near-misses. A picture released yesterday shows that a galaxy in the constellation Taurus is actually siphoning material from a neighbor through an intergalactic "pipeline"
Other galaxies have two nuclei - evidence that larger galaxies can cannibalize smaller neighbors. The Milky Way, in fact, is currently consuming two smaller galaxies called the Large and Small Magellanic Clouds.
It's all a natural part of the cosmos's life cycle, says Dr. Gallagher. In a Darwinian twist, the most robust galaxies tend to be those that have eaten some of their peers, he says. Those that have remained isolated are often wispy and weak.
In part because of these observations, scientists are coming to believe that this growth by accumulation is the way that many galaxies are formed.
The theory has been backed up by infrared images of the farthest reaches of the universe, billions of light years away - and therefore billions of years in the past. Astronomers have unexpectedly found hundreds of tiny galaxies there. This may suggest that such small galaxies formed soon after the Big Bang, and later coalesced to form the larger galaxies we see today.
Yet another paper released last week reports that scientists found, 12 billion light years away, a huge reservoir of cold gas big enough to have perhaps created an entire galaxy. "We're probing right to the edge of the dark ages, right up to the time when stars first began to form," says Chris Carilli, an astronomer at the Very Large Array radio telescope in New Mexico, who worked on the paper.
What role black holes play in all this remains perhaps the most intriguing mystery. Scientists have so far been unable to prove a causal link between one or the other, but the observations have been tantalizing.
In some systems, we've seen both stars and black holes forming, says Dr. Carilli. "At some point, you have to say it might be a fundamental part of the [formation] process."
(c) Copyright 2001. The Christian Science Publishing Society