My first Magazine Sky & Telescope - 03.2019 | Page 21

2.0
Photoelectric
observations
2.1
2.2
2.3
2.4
Hevelius
William
Herschel
2.5
2.6
A star remains in the Cepheid stage for thousands to hun-
dreds of thousands of years, pulsating on time scales ranging
from 1 to 70 days. But a star doesn’t continue to pulse at
the same rate throughout its sojourn in this part of the H-R
diagram. As a star nears the end of its time in the instability
strip, it becomes fluffier and larger, and its period lengthens.
Polaris right now brightens and fades every four days, but
that rate is lengthening by 4½ seconds each year, a much
more rapid change than expected and one without clear
explanation.
From when astronomers first identified Polaris as a
Cepheid about 100 years ago to the 1990s, the difference
between the star’s brightest and faintest glow, or its light
amplitude, decreased from 5% to 1%. (So no, it’s not a change
you can see with the naked eye.) Due to a calculation error,
one team even predicted in 1993 that the pulsations were
about to die out and that Polaris was going to become stable.
Observers have seen one other Cepheid do that: V19, one of
the stars Edwin Hubble used in the 1920s to calculate the
distance to the galaxy M33.
But Polaris didn’t stop pulsing. The plummet stalled. In
2008, three independent groups confirmed that the ampli-
tude was increasing again and had been doing so since
1995, leading one team (in an obvious quip of the earlier
prediction) to title their paper “Welcome Back, Polaris the
Cepheid.” The rebound now appears to be complete: Based on
eight years’ worth of methodical observations by Rick Wasa-
tonic (Villanova) using an 8-inch and an 11-inch Schmidt-
Cassegrain in his backyard observatory, Polaris’s light ampli-
tude is now back up to 5%, Guinan says. 2.7
Polaris has not finished giving up its secrets. Engle, Guinan,
Evans, and their colleagues recently discovered that — much to
astronomers’ surprise — the star emits X-rays. They have since
found a few other Cepheids doing the same thing. Even the
original member of this stellar group, Delta Cephei, not only
shines in X-rays but in a pattern that varies periodically. Most
Cepheids are too far away for the current generation of space-
based X-ray telescopes to pick them up, so the team doesn’t
know if this behavior is the norm or the exception, Engle says.
Other, non-pulsing supergiants have also proven to be
X-ray emitters, bewildering observers. Astronomers didn’t
Estimated Visual Magnitude
of Polaris over Time
(AD 1660 to present)
1.9
Nearly Gone, Then Back Again
And Then There Were X-rays
1.8
magnitudes listed for stars both in Ursa Minor and elsewhere
in the sky, he discovered that al-S.ūfı̄’s values were a closer
match for today’s than Ptolemy’s were.
Except, that is, for Polaris. Once Guinan and his col-
leagues had calibrated the ancient data to our current system,
they discovered that both manuscripts record Polaris as some
2 to 4 times fainter than it is today.
“This is an entirely unexpected behavior for a Cepheid
variable,” the team wrote in 2014. They still don’t have an
answer.
1700
Pickering
John
Herschel
Visual
observations
1800
1900
2000
Year of observation
1.5
2.0
Estimated Visual Magnitude
of Polaris over Time
(AD 0 to present)
Magnitude
2.5
3.0
Al-S u ˉ fı ˉ
•
3.5
Ptolemy/Hipparchus
4.0
4.5
0
500
1000
1500
2000
Year of observation
p ON THE RISE Comparison of observations taken over the last
1,800 years or so indicates that Polaris has unexpectedly brightened by
a factor of two to four.
expect these stars to emit such fi erce radiation, because the
stars shouldn’t have the necessary superheated plasma near
their surfaces. This plasma is likely heated by tangled mag-
netic fi eld lines, which release energy as they rearrange them-
selves. But giants and supergiants shouldn’t be able to support
the necessary strong magnetic fi elds — at least not ones that
can poke out of the stars’ bloated surfaces where we can see
the X-ray-emitting plasma they produce, Engle explains.
“There’s something crazy going on,” he says.
So our familiar beacon of stability in the sky has proved
itself fi ckle. Polaris is not constant, but it is fascinating. Its
quirks may tell us much about what the oddball members of
the Cepheid class can do, and perhaps it will help us under-
stand aging stars in general. In that way, it might reorient
stellar astronomers just as effectively as it has the last millen-
nium’s travelers.
¢ Science Editor CAMILLE M. CARLISLE admits that some-
times she second-guesses herself when looking for Polaris in
the night sky.
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