New York Times Second Book of Science Questions and Answers 225 New, Intriguing, and Just Plain Bizarre Inquiries into Everyday Scientific Mysteries

CHAPTER 1 Reaching for the Stars Birth and Death Q. Are stars all burning out, or are new ones forming? A. Stars are being born as well as dying, but the rate varies greatly from galaxy to galaxy. Stars form from huge clouds of dust and gas. If a cloud begins to contract because of its own gravity, its interior heats up as gravitational energy is converted to heat energy, reaching millions of degrees, and nuclear reactions begin that change one element into another, releasing energy. The pressure tends to expand the cloud back out, but eventually equilibrium is reached. That is essentially what a star isa mass of gas at equilibrium between inward pressure from gravity and outward pressure from nuclear reactions. A star has a finite lifetime because it is burning fuel. For 90 percent of its life, it burns hydrogen into helium. When the hydrogen is used up, the pressure decreases, but gravity never disappears, so the star contracts until the temperature climbs again, this time reaching hundreds of millions of degrees, while reactions convert helium to carbon and oxygen. The star can then remain stable for a briefer time. Eventually the star dies, when the reactions no longer produce energy, but only consume it. In a Spin Q. Is the universe rotating? A. Most astronomers would say no. There is no known mechanism that would give the universe so much angular momentum, or spin, at its beginning, and few mechanisms for adding spin later. To know for sure if the universe rotates, scientists would need to know the velocities of millions of galaxies, over all regions of the sky and out to very great distances. Analyses of these velocities would be necessary to see if they indicated a common center about which galaxies were rotating, and a sense of direction on average. Since it may be a while before the velocities of millions of galaxies are known, astronomers are trying to answer a simpler question: Are there regions of space toward which large numbers of galaxies are moving as a group? Death by Black Hole Q. What would kill you if you fell into a black hole? A. You might not die right away, but you would eventually be pulled apart by the force of gravity. As you fell in and even afterward, you might not lose consciousness, but the pull of gravity on your feet would be stronger than on your head, and you would be stretched, then torn apart. The difference in force is called the tidal force and is like that in the ocean, except in more extreme form. The force would be less if it was a big enough black hole; in a small black hole it might kill you before you disappeared beneath the event horizon, the edge of the hole. But even in a larger black hole, the tidal force always gets you in the end. Once you fall in, you can't avoid falling toward the center, and the force would kill you before you reached the center. How long it would take depends on how big the black hole is. If it was big enough so that the tidal force didn't kill you before you fell in, you might have an hour or several hours before being torn apart. In a small one, such as one that forms when a star collapses, you wouldn't have much time, perhaps a thousandth of a second. That would happen with a run-of-the-mill black hole, like those that might be found in the Milky Way Galaxy. Averting Your Eyes Q. When I look at a constellation, I can see the fainter stars better out of the corner of my eye. Why? A. Because the eye has two kinds of receptors, cones for fine resolution and color and rods for dim light, and the rods tend to be located around the periphery, for viewing the edges of the field of vision. Cones are extremely good at high definition and for precise positioning of pinpoints of light. Rods don't give nearly as fine resolution and don't distinguish colors, but are much more sensitive.Ray, C. Claiborne is the author of 'New York Times Second Book of Science Questions and Answers 225 New, Intriguing, and Just Plain Bizarre Inquiries into Everyday Scientific Mysteries', published 2003 under ISBN 9780385722582 and ISBN 0385722583.