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

Send your science questions to
dearscience@thestranger.com

Jonathan Golob is an actual doctor.

5 replies on “Dear Science”

  4. No, what needs to be said is that starts don’t go out over two minutes. They fade gradually. There’s no way to observe this one night. I presume it would take many many years, besides, as he points out, lots of stars in the sky ARE white dwarfs so all we’d see is a decrement not a switch off. More interestingly, why not discuss supernovae, which are much more dramatic both in terms of visibility and timing.

  5. Well, if we were to assume that stars “blink out” when they die — that is, their photon emissions instantaneously drop to zero — then what is the probability that, while gazing into the sky, STAR could witness such an event?

    Assuming in STAR’s area there are 1,000 visible stars in the sky, and his central visual field takes in 30 percent of the sky.

    That would mean he could see, on average, 300 stars at any one moment.

    This seems a little high to me, though — is his attention span good enough to notice if any one of those 300 stars disappears? Let’s say 100 of them are in his very central field, so that he would be sure to say “holy fuck!” if one suddenly winked out.

    Now, I guess, we’ve got to assume that the region of sky STAR is staring at contains 100 stars whose ages are evenly distributed. (This may not a very good assumption, considering that stars may be born in clusters, but, averaging everything out seems to be the simplest thing to do …)

    If the average lifespan of a star is 10 billion years, then, if STAR stared at the night sky for a year, the likelihood that he would witness the death of a star would be:

    100 / 10,000,000,000 = 1 in 100 million.

    However, it’s not possible to gaze into a night sky for a year; sooner or later, daylight will come.

    Let’s say he stares for an hour, and that there are 8,766 hours/year.

    Then, the probability would be cut by a ratio of 1/8,766, and his chances of witnessing a star death would be:

    1 in 877 billion.

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