Welcome to part three of my trilogy on stars’ birth, life, and death. In last week’s Skywatch column about the life of stars, I told you how stars cook up energy in their cores with nuclear fusion. Hydrogen atoms fuse to form heavier helium atoms because of intense pressure resulting in astronomically high temperatures. In the process, a tiny bit of hydrogen is converted into energy, which then forces its way to the outer levels of a star.
Hydrogen is the fuel of a star, and smaller and less massive stars like our sun sip their hydrogen supply slowly and live a long time. Our sun has been around for about 5 billion years and should have enough hydrogen fuel left in the tank to keep it going for another 5 billion years. More massive stars are not around for nearly as long, and they’re literally what you could call hydrogen gas guzzlers. The big guys of the stellar world may only last a few billion years. Sooner or later, all stars begin to run out of hydrogen in their cores, and stellar death gets underway. Smaller stars like our sun certainly die a violent but prolonged death, but the really massive stars go out with a huge bang!
Death of Smaller Stars
Low-mass stars like our sun get really fat as they die and gradually flicker out. In the case of our sun, it will run out of hydrogen in about 5 billion years, with helium building up as the hydrogen dwindles. When there is no longer enough hydrogen in the core, there’s no longer a balance between internal nuclear fusion energy and the never-ending external gravitational pressure. The core begins to contract, causing the temperature in the core to climb even higher. That causes the helium atoms to fuse into carbon and oxygen atoms. Some heat escapes beyond the core, reaching the cooler, outer hydrogen layers toward the edge of the sun. In time, the temperature rises high enough in these layers to fire up hydrogen nuclear fusion. All of this added energy causes the sun to bloat out into a red giant star. The reason the sun will turn red is that outer layers will expand and cool off.
When this happens to our sun, our home star will swallow up the planets Mercury and Venus and touch Earth. At that point, we’ll be toast! Even though the sun will have a cooler surface temperature of about 3,000 to 4,000 degrees F, it will be right on top of us! Also, toward the end of our sun’s red giant phase, excess energy “burps” in the outer layers will cause large clouds of gas to blow off and form large rings and shells of gas around what’s left of our star. Astronomers call these planetary nebulae. Even though they’re called planetary nebulae, they have nothing to do with planets. They got that name because back in the 1700s and 1800s telescopes weren’t quite up to the standards of the Hubble Telescope, and through those archaic scopes, these burping stars resembled giant planets.
Planetary nebulae don’t last long, though. Eventually, stars like our sun will totally run out of all fusion fuel and shrink into white dwarfs. With no more nuclear reactions inside the star to hold it up, gravity collapses the corpse of the once-proud star. In our sun’s case, it’s believed that whatever is left of the sun’s original mass will be squished into a ball about the size of Earth. When that happens, our sun will be considered a white dwarf or a “retired star.”
Death of Bigger Stars
Remember the old saying, “the bigger they are, the harder they fall”? Well, that’s certainly the case with behemoth stars. Massive stars, at least eight times more massive than our sun, die a spectacular death! Like smaller stars, they also bloat out into red giants; only they become super, super, big red giants! Helium atoms inside the stellar core fuse into heavier carbon and oxygen, and eventually, an iron core forms. That’s the end of the line, though, and a chain of reactions causes all nuclear you-know-what to break loose, and then you have an explosion beyond your wildest dreams.
The once supergiant red star explodes into a supernova. The supernova flings out material at beyond incredible speeds of 10,000 miles a second. In the process, heavier materials like gold, silver, and many other heavy elements are “cooked up” and become the building blocks of future stars and planets. Out of death comes new celestial life. This is recycling on a cosmic scale! The Crab Nebula in the constellation Taurus the Bull is the remnants of a supernova explosion that our distant ancestors saw back in 1054. Even with a small or moderate telescope, you can see the Crab Nebula near one of the horns in Taurus.
What’s left of an exploded star after a supernova can be one of two bizarre objects, depending on the mass of the remaining core. It may become a rapidly rotating neutron star, only about 10 to 15 miles in diameter, and so dense that one tablespoon would weigh billions of Earth tons.
Or, the core left behind may collapse very rapidly, possibly in a matter of hours, to an object so small and dense that not even light can escape it. When this happens, you have a black hole. What goes into a black hole stays in a black hole. Nothing escapes!
Black holes can literally suck material off of existing stars. When that happens, X-rays are produced as stellar material spirals to its doom in the black hole. These X-rays are signals that astronomers can detect. One of the first suspected black holes found using X-rays is in the bright constellation Cygnus the Swan, a constellation we see in the summer. Don’t worry, it’s over 46 thousand trillion miles away. You won’t get sucked in.
Celestial Happening this week: On Tuesday, Wednesday, and Thursday morning, the last quarter moon will be passing by the bright planets Jupiter and Mars in the pre- and early morning twilight southeastern sky. Jupiter is by far the brighter of the two planets. Mars will have a distinct orange-red glow to it.
Mike Lynch is an amateur astronomer and retired broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is the author of “Stars: a Month by Month Tour of the Constellations,” published by Adventure Publications and available at bookstores and adventurepublications.net. Mike is available for private star parties. You can contact him at firstname.lastname@example.org.