The Case for a Creator - Lee Strobel [90]
“Also, we’re very far from the nucleus of the galaxy, which is also a dangerous place. We now know that there’s a massive black hole at the center of our galaxy. In fact, the Hubble space telescope has found that nearly every large nearby galaxy has a giant black hole at its nucleus. And believe me—these are dangerous things!
“Most black holes, at any given time, are inactive. But whenever anything gets near or falls into one, it gets torn up by the strong tidal forces. Lots of high energy is released—gamma rays, X-rays, particle radiation—and anything in the inner region of the galaxy would be subjected to high radiation levels. That’s very dangerous for life forms. The center of the galaxy is also dangerous because there are more supernovae exploding in that region.
“One more thing: the composition of a spiral galaxy changes as you go out from the center. The abundance of heavy elements is greater towards the center, because that’s where star formation has been more vigorous over the history of the galaxy. So it has been able to cook the hydrogen and helium into heavy elements more quickly, whereas in the outer disk of the galaxy, star formation has been going on more slowly over the years and so the abundance of heavy elements isn’t quite as high. Consequently, the outer regions of the disk are less likely to have Earth-type planets.
“Now, put all of this together—the inner region of the galaxy is much more dangerous from radiation and other threats; the outer part of the galaxy isn’t going to be able to form Earth-like planets because the heavy elements are not abundant enough; and I haven’t even mentioned how the thin disk of our galaxy helps our sun stay in its desirable circular orbit. A very eccentric orbit could cause it to cross spiral arms and visit the dangerous inner regions of the galaxy, but being circular it remains in the safe zone.
“All of this,” he said, his voice sounding a bit triumphant, “works together to create a narrow safe zone where life-sustaining planets are possible.”
SCANNING THE STARS FOR LIFE
Suddenly, the Earth was sounding pretty special, nestled as it is in a sliver of space that gives it safe haven from the otherwise menacing conditions of the Milky Way. But what about other types of galaxies? Might they also provide threat-free neighborhoods for life-populated planets?
“What about elliptical galaxies?” I asked Gonzalez. “Do they have the potential to harbor life?”
“Elliptical galaxies look amorphous and are sort of egg-shaped, with stars having very random orbits, like bees swarming a beehive,” he explained. “The problem for life in these galaxies is that the stars visit every region, which means they’ll occasionally visit the dangerous, dense inner regions, where a black hole may be active. In any event, you’re less likely to find Earth-like planets in elliptical galaxies because most of them lack the heavy elements needed to form them.”
This was an important point, because I knew that most galaxies fall into the elliptical category.
“Most elliptical galaxies are less massive and luminous than our galaxy,” Gonzalez continued. “Our galaxy is on the top one or two percent of the most massive and luminous. The bigger the galaxy, the more heavy elements it can have, because its stronger gravity can attract more hydrogen and helium and cycle them to build heavy elements. In the low-mass galaxies, which make up the vast majority, you can have whole galaxies without a single Earth-like planet. They just don’t have enough of the heavy elements to construct Earths. Just like a globular cluster—you can have a whole globular cluster with hundreds of thousands of stars, and yet there won’t be a single Earth.
“If you look at the deepest pictures ever taken by the Hubble Space Telescope, they show literally thousands of galaxies when the universe was really young.