Sky & Telescope - March 2019 - 17

TERRI DUBÉ / S&T

Betelgeuse
D = 887
slice of the H-R diagram (see page 18). Stars in the instability
strip are variable, their light changing in predictable patterns.
Almost all of them have moved out of the main sequence,
the prime of life when stars fuse hydrogen in their cores, and
have entered their golden years. This latter period splits into
three sequential branches: red giants with inert helium cores
surrounded by a shell of fusing hydrogen; horizontal-branch
giants, when the helium core has reignited and is now transforming itself into carbon; and asymptotic giant branch (AGB)
stars, those that have a carbon core surrounded by nested
shells of fusing helium and hydrogen. The instability strip
cuts a path through these stages.
Perhaps the most famous denizens of the instability strip
are Cepheids. Cepheids are yellow and orange giants and
supergiants, with masses of 3 to 18 Suns and girths 35 to 80
times larger than our star. They used to be big, bluish B stars,
but such massive stars live short, frenetic lives, and at 50 to
200 million years they have already entered middle age.
Cepheids breathe in and out in a predictable pattern,
controlled by a thin layer of ionized helium in their interiors. When this layer compresses, its temperature rises, and
its helium atoms lose a second electron. The extra electrons
prevent radiation from below from traveling freely through
the layer. Heat that once passed through the helium is now
trapped beneath it. The temperature and pressure below
increase. As the pressure skyrockets, it shoves the layers
above it outwards, and the helium expands and becomes
more tenuous, permitting heat to pass through it again and
lowering the interior pressure. But once the pressure below
the layer drops, gravity takes over and the layer sinks and
compresses again. This cycle repeats many times, causing the
star to pulse.
Polaris is the closest Cepheid, lying some 445 light-years
away. For several years astronomers debated its precise distance - which is ironic, given that Cepheids derive most of
their fame from their role as rungs on the cosmic distance
ladder. In 1912 Henrietta Swan Leavitt discovered that the
brighter a Cepheid is intrinsically, the longer it takes to cycle
through a pulsation period. Because Leavitt was studying
stars in the Small Magellanic Cloud dwarf galaxy, all the stars
had roughly the same distance. Once astronomers calculated
how far away one Cepheid variable lay, they could deduce
the distance to any other Cepheid in the universe simply by
measuring how quickly the star cycles through its pattern and
how bright it appears. And because Cepheids radiate some
10,000 times more energy than the Sun does, they can be
seen to very great distances indeed.
Depending on their mass, stars may cross the instability
strip multiple times. A star of Polaris's mass will cross the
Cepheid region thrice. But due to the previous disagreement
about the North Star's distance, no one has been quite sure
where Polaris is in its evolution. Part of the problem is, Polaris
isn't your typical Cepheid. It has a more complicated jiggle
than the simple breath-in-and-out pattern that most Cepheids follow, so astronomers don't use it in the distance ladder.

It also may be brightening 50 to
100 times faster than expected,
explains Edward Guinan (Villanova University).
"It should not be doing this,"
Guinan says. Cepheids can
brighten with time, but Polaris
has shot up more than 10% in the
last century. "Doesn't make any
sense."
Guinan, Scott Engle (Villanova), and their colleagues discovered the change while gathering
data for another Polaris project.
They dug up measurements from
previous centuries and traced the
decline back to Johannes Hevelius
in the 1600s. To be thorough, they
decided to go even further back
and check the data in two ancient
star catalogs: Ptolemy's Almagest
from around the year 150, and
al-S.u
¯fı¯'s 964 Kita
¯b al-Kawa¯kib
al-Tha
¯bitah ("Book of the Fixed
Stars"). Many assume al-S.u
¯fı¯'s
book is a copy of Ptolemy's, but
when Guinan looked at the

u OUR STELLAR CAST In our three-part
series we'll discuss Polaris, the Alpha
Centauri system, and Betelgeuse. These
stars span a range of sizes, shown to
scale. Listed diameters ("D") are approximate and given as multiples of the Sun's
diameter. We've also included the Sun for
visual reference.

Sun
D=1

Alpha
Centauri A
D = 1.2

Alpha
Centauri B
D = 0.86

Proxima
Centauri
D = 0.15

Polaris
D = 45

Massive stars live fast
and die young, and Polaris
is already a puffy, aging
star that has devoured
the supply of hydrogen
fuel in its core.

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Sky & Telescope - March 2019

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