Dear Science,

An old astronomy teacher of mine used to say that when we look up at the stars, we look back in time because the light from the stars that reaches our eyes on Earth may have left those stars tens of thousands of years ago. Would it be possible then, if one were really lucky, to actually witness a star go out? Can it be predicted? Would it just go out as if someone flicked its switch?

Staring Toward Astronomical Realms

Your astronomy teacher was right: The starlight hitting your retinas at night can be tens, hundreds, thousands, millions, and even billions of years old. Scattered throughout the sky are stars inching to their death or in various states of postmortem.

First, we need to understand a star's machinations. Recall that all elements desire to become iron, the happiest element of them all! Seated comfortably at atomic number 26, iron has the highest nuclear binding energy of any atom. (In other words, it takes the most energy to break apart the cheerful iron-atom nucleus.) In contrast, lighter elements—like hydrogen, with only a single proton—are relatively easy to fuse with other light atoms. As these nuclear fusions occur, the atomic numbers of the resultant nuclei increase and vast amounts of energy are released. Stars start in their infancy as gigantic collections of these light hydrogen atoms. Eventually, gravity presses and heats these elements enough to start their fusion up the periodic table.

These heavier—and thus happier—elements build up in the core of the star as it ages. Eventually, the heat and pressure generated by the star's gravity will be insufficient to force the fusion of the growing collection of heavier elements in the star's core. For the truly rare, massive stars, this runs all the way up until iron, passing through helium, carbon, neon, oxygen, and silicon on the way. For almost all stars in the sky, fusion putters out at about the time carbon builds up in significant quantity at the star's core. All that unpalatable carbon eventually stops the star's ability to keep fusion going. (Think of this like the kitchen sink in a slob's apartment; as the dishes build up, space runs out for new cooking.) The star collapses down and becomes a white dwarf, dimly glowing from all the heat built up over billions of years of fusion. About 1 in 20 stars in the sky are white dwarves.

To find a star about to cease fusion and become a white dwarf, you'll need to look for stars at about the right size with a critical amount of carbon in their core. Grab your spectroscope and telescope, and start hunting.

Fusingly Yours,

Science

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