Feb 15

Skyward for March 2021. David H. Levy

Skyward for March 2021.
David H. Levy

Stars are people too.

In last month’s Skyward, I included that four-word phrase, but the first time I used it was actually in an article about the life of the star Betelgeuse, for Astronomy magazine. When I met Richard Berry, the editor at the time, he began by reciting those words: “Stars are people too.” He added that he accepted the article for publication in his magazine after he read those words. (It turns out that wasn’t my only unusual experience with that magazine. A few years later David Eicher, the current editor, and I witnessed a construction crew blowing up a freeway overpass near the magazine’s headquarters in Milwaukee.)

As I explained last month, stars live out their lives much as do. They are born in gaseous stellar nurseries, or diffuse nebulae. In our sky two of the most famous nebulae appear are in summer, the Lagoon Nebula in Sagittarius, and in winter, the Orion Nebula. The little stars within the nebula vary in brightness, usually by a few tenths of a magnitude, but they can change quite quickly. There are a few others in the Hyades star cluster in Taurus, the bull. I saw one star there change rapidly over a period of a few minutes. These stars mimic the behavior and misbehavior of human youth.

Also like us, stars settle down as they grow older. Our Sun is an example of a star in middle age. It has shone steadily for almost five billion years and will continue this way for another several billion. Except for a cycle of eleven years during which the numbers of sunspots, which are storms on the face of the Sun, rise and fall, the Sun behaves constantly and predictably. There are vague hints of a 12,000-year cycle dating back to biblical times but I have not found any evidence for this.

As our Sun enters old age it will begin to act erratically again. Its hydrogen supply will be almost exhausted. It will begin to fuse its helium. At some point during its red giant phase, it will suffer a helium flash. This event might feature only a few minutes of strong helium fusion, but during which the Sun briefly will emit an enormous amount of energy equivalent to that of our whole galaxy. As it continues its red giant phase it might vary in brightness by several magnitudes over many months. Mira, a star in Cetus the whale, is such a star. A Mira star’s core begins to contract under the force of its own gravity and whatever hydrogen is left will ignite into a shell around the core. Mira, like other red giants, was once a Sun-like star that has used up its supply of hydrogen. Once the helium is exhausted, its core will be left with heavier elements like oxygen and carbon. The outer layers of these old stars will explode as novae every few hundreds or thousands of years. Eventually, with their outer layers gone, the core will become a white dwarf star.

If a star is much more massive than our Sun, it would end its life far more dramatically—as a supernova. Such an event is really catastrophic. There are two kinds. In the first, the smaller member of a two-star system will keep on attracting material from its larger companion. But instead of repeated nova explosions, the small star will get more and more massive. When that star’s core reaches a certain limit, in less than a second, the star finally will collapse on itself and will blow itself apart.

The other kind involves a very massive star, say three or four times the mass of the Sun. Just like in the smaller star, its supply of hydrogen will be gone. With little helium left the still contracting core is left with carbon and oxygen. When the core reaches a certain temperature, the remaining carbon will ignite all at once tear the star apart.

If the star is very massive, say nine or ten times the mass of the Sun, its very hot core allows the carbon to ignite and burn as before, but gradually, not all at once. Heavier elements like phosphorus and sulfur will be formed in shorter and shorter intervals, until silicon is generated. After just one day, the silicon will fuse into iron. Iron cannot fuse to anything heavier. Instead, in less than a second the core will crash in on itself. In the resulting explosion, the star’s outer layers will be blown away. The brightness rise is so dramatic that the single supernova will outshine its entire galaxy. What is left is either a very dense neutron star, where a cubic inch of matter would weigh as ton or more here on Earth, or in the most massive stars, a black hole from which even light cannot escape.

Although stars do not have consciousness like we do, they lead extraordinary lives that are well worth our appreciation and study. Don’t forget: Stars are people too.

David Levy

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