Sky and Telescope - June 2017 - 20

Tabby's Star

º To determine whether a given scenario
is "likely," astronomers consider two
metrics: How well does the explanation
match our observations? And how plausible is the scenario based on what we
already know? Both of these questions
are important!
If someone were to tell me that they
flipped a quarter six times and it

came up heads each time,
the likelihood that this coin
has two heads is 60 times
higher than that the coin is
perfectly fair. However, I've
seen lots of quarters, but I've
never seen one with two heads.
I also know that someone is more likely
to tell me about their strange coin-flipping experience than their normal one.

Inside the Star
Let's start tracking our photon at the moment it's created:
via a hydrogen fusion reaction in the star's core. One popular
suggestion to explain Tabby's star's long-term dimming is the
sputtering out of hydrogen fusion in its core. We expect at least
one of the many stars that Kepler observed to exist at this particular stage of evolution, so it's a plausible scenario. However,
it doesn't ﬁt the data well at all. It takes more than 100,000
years for a photon to escape the star's core. So even if fusion
suddenly stopped completely, causing the star to contract, the
change required to dim the star's brightness by 1% would span
hundreds of thousands of years. Fusion's end couldn't produce
4% dimming over four years, much less 15% dips over a day.
After rising towards the star's surface our photon will
escape the star - but not quite in a random way. Concentrations in magnetic ﬁeld lines may cause dark patches to

1.020
1.015

Relative brightness

1.010
1.005
1.000
0.995
0.990

¢ BENJAMIN MONTET

appear brieﬂy on a star's surface. On the Sun such spots are
small. From minimum to maximum sunspot activity, the
Sun's brightness changes by less than 0.1%. But on other
stars, spots can be bigger and their effects more dramatic. The
brightnesses of some stars in the Kepler ﬁeld have varied by as
much as 5% as spots rotate in and out of our view.
So could Tabby's star be exhibiting an extreme case of
starspots? Probably not. Because of the way they form,
starspots similar to the ones observed on the Sun require that
the star have a convective zone near its surface, where boiling
motions carry out energy produced within the star. Less massive stars have thicker convective zones, while more massive
stars have shallower ones. With 1.4 times the mass of the Sun,
Tabby's star should have a small convection zone or maybe
even none at all. We'd be surprised to see starspots as big as the
Sun's, much less ones big enough to cause 20% variability.
Even if giant starspots were plausible, the dips we observe
don't look like the spots observed on other stars. Starspots
tend to evolve fairly slowly, taking a few stellar rotations to
grow and then slowly fade away, so we see the same spots
over and over. Yet on Tabby's star each dip appears only
once. What's even more concerning is that the single dips
last much longer than the star's rotation period, when they
should be rotating in and out of view. Starspots simply
couldn't create the signals we see.

0.985

Around the Star

0.980

Once our photon has left Tabby's star and begins traversing
space, it will ﬁrst encounter any material in orbit around the
star. It's possible that the cause of the dips isn't something
intrinsic to the star itself but some material that occasionally
passes between the star and us.
We can narrow down the orbit of this presumed material in two ways. First, objects close to the star will become
hot and emit infrared radiation. However, observations from
the Spitzer Space Telescope and others haven't detected any
unusual infrared radiation other than the amount expected
from Tabby's star itself. So we know that the material, if it's
there, can't be too close to the star.
Second, based on the durations of the dips, the material
would need to take several days to pass in front of the star.

0.975
0.970
0.965

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Day of Kepler mission
p THE FOUR-YEAR DIM While Kepler photographed every star in its
field every 30 minutes, it only recorded the brightness from a few pixels
near each star. (Data is expensive to store and transmit, so the less data
saved on board the spacecraft, the better.) This strategy induced longterm trends that overwhelmed any slow astrophysical signals. But about
once a month, the Kepler spacecraft downlinked raw calibration frames
of the entire field. Investigating these frames, Montet and coauthor
Joshua Simon found that Tabby's star had faded for the entire primary
Kepler mission, with a more precipitous drop in the final few months.

Therefore,
my prior knowledge
T
tells me that it's significantly more
likely that the quarter does indeed
have a tails side, even though
our
o two-headed model provides a
much better fit to the data. Astronomers make the same calculations every
time we analyze possible explanations
for our observations.

J U N E 2 0 1 7 * SK Y & TELESCOPE

CH A RT: LE A H TISCIONE / S&T / B. M ON TE T & J. SIM ON; COIN: BIGSTOCK PHOTOS.CO M / V ECTORK AT

How Likely Is It?

http://www.BIGSTOCKPHOTOS.COM

Sky and Telescope - June 2017

Table of Contents for the Digital Edition of Sky and Telescope - June 2017