Collapsing stars create black holes

You wanted to know

A student in Gregg Thompson's sixth-grade social studies class at Woodland Primary School in Gurnee asked, "How does a black hole form?"

Black holes are a fairly recent discovery. The world-famous physicist Albert Einstein, whose 100-year-old theories are key to our understanding of how the universe works, was not able to confirm black holes existed.

Astronomers and scientists today have proof black holes exist; our own Milky Way galaxy features a few hundred million stellar black holes.

"We think black holes were once stars," said Steve Beck, astronomer and student at the University of Chicago. Once 10 times more massive than the Sun, a black hole is a fuel-depleted star that shrinks down to a fraction of its former self.

What makes it tricky to define black holes is you can't see them. They have no light whatsoever, so experts can only identify them because their gravitational pull is so strong it affects how surrounding stars behave.

"Stars are gigantic nuclear generators that are formed by hydrogen fusing into helium. As the star burns hydrogen fuel, it ages," Beck said. "When the fuels run out, the star starts burning other fuels, signaling that it is in its final stages of life."

The chain reaction started by the hydrogen burnout continues as the star fuses oxygen, carbon, nitrogen, neon, silicon and finally iron, Beck said.

"Iron does not fuse very well, as it takes a large amount of energy to undergo the process of nucleosynthesis," he said. "Iron then begins to build up at the star's core. When this happens, the outer layer of the star implodes and can cause the iron core to shrink extremely quickly.

"It's a matter of seconds before a core the size of the Earth becomes the size of a singular, one-dimensional particle."

This is called gravitational collapse, and happens when the internal mass of the iron core can no longer support the gravity of the star's outer shell. This final collapse is observed as a supernova explosion.

"What is left behind is a singularity with a gravitational force so great that photons, or particles of light, cannot escape from it," Beck said.

"Black holes are still an incredibly recent discovery," he continued. "We now assume that a black hole is a very normal celestial body, and they are fundamental in our understanding of the mechanics of the universe. There is a super-massive black hole at the center of every galaxy, and probably millions of smaller ones littered about, much like stars."

It's the supersized gravitational force concentrated in that tiny grain of a "singularity" that is the hallmark of the black hole. The force makes all surrounding space objects respond with uncharacteristic behavior, leaving clues that it does, in fact, exist.

There's one more step in the star-to-black hole conversion that has to do with time. Time actually slows down. This is called time dilation, Beck said.

"Time dilation occurs due to gravity. If you and I had super accurate watches set at the same time, and I approached a black hole, as the gravity became greater and matter and particles began to slow down, I would observe your watch as going faster than mine, while mine was normal.

"So, if I entered a black hole, time would move normally for me as I observed it. This is because time is affected by gravity, which is affected by mass. The extreme mass of the black hole creates so much gravity, even the fastest particle, the photon, can't escape its pull.

"When I exit the black hole and get back to you, I find that you've aged much more than I have. Since you were not under such gravitational pressure, time continued to pass normally to you, while I slowed down."

Black holes come in three sizes - stellar, intermediate and super massive. NASA uses its space traveling Hubble telescope as one tool to detect black holes and other stellar features, such as quasars. Astronomers would like to take pictures of a black hole, but since black holes emit no light, they have to wait until technology and science advance. Beck relates his experience with seeing the invisible black hole.

"I have seen the way black holes affect stars. The constellation Sirius has a star, Sirius B, having its orbit seriously affected by a black hole. So, in a way, I have seen a black hole in that I've viewed its affect on the celestial bodies around it," he said.

Check it out

The Warren-Newport Public Library District in Gurnee suggests these titles on black holes and celestial bodies:

• "A Black Hole is Not a Hole" by Carolyn DeCristofano

• "Seven Wonders of the Sun and Other Stars" by Rosanna Hansen

• "The Mysterious Universe" by Ellen Jackson

• "Black Holes and Other Bizarre Space Object" by David Jefferis

• "Black Holes, Pulsars and Quasars" by Isaac Asimov