The Turning of the Stellar Seasons
by Adam Hadhazy
Research Highlights from Kavli Astrophysics Institutes
The Author
With fall in full swing in many seasonal places in the Northern Hemisphere, the fierce verdancy of summer foliage has given way to crimsons and ochres. A changing of hues similarly happens for many kinds of stars as they, too, advance into the autumnal stages of stellar life. Our sun is a prime example. Classified as a G star, it presently appears white in the visible spectrum that our eyes see. The sun has blazed forth in this way for more than four billion years, as seemingly potent and steady as those green leaves soaking up that very sunlight for the last few months. But in another five billion years or so, the sun will likewise start Octoberizing. As is the way for all stars with about half the sun's mass, up through about five solar masses, the sun will evolve into a so-called red giant. The steady eons-long supply of fresh hydrogen to fuse into helium will have finally run low in the sun's core, and the aging star's outer layers will balloon outward, cooling and reddening. The sun's color-changing will not end there. Another billion years or so later, the sun will have worked through all its available nuclear fuel, shed its outer layers entirely into space, and at last come to its winter. Appropriately enough, this along-in-years remnant of the sun will be a gleaming white in appearance, a "white dwarf" in astronomer parlance, and evocative of a snow-locked barrenness. Scientists can predict these eventualities with extremely high confidence, thanks to our observations of countless other stars going through these stages and our deep theoretical understanding of the hows and whys. The scientific process thus enables us to know why the innumerable leaves above our heads change as they do, as well as also—though much farther above our heads—the innumerable stars.
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