Sky and Telescope - June 2017 - 19

An Outwardly Ordinary Star
One of the most bizarre aspects of what is now called "Boyajian's star" or "Tabby's star" is just how average it looks. A
spectrum of the star reveals an outwardly normal, middleaged F-class star, almost half again as massive as the Sun. The
spectrum also shows that the star doesn't wobble in the way
we'd expect if a massive companion, such as a black hole or
another star, were in orbit around it.
While there are a handful of other (non-Kepler) stars
where the brightness dips irregularly similar to Tabby's star,
these objects are all young stars, surrounded by dusty disks,
which occasionally pass along Earth's line of sight. Archival
and follow-up observations of Tabby's star at infrared and
submillimeter wavelengths, on the other hand, demonstrates
that it has no such massive disk.
In January 2016, it became clear that Tabby's star was even
weirder than we thought. Bradley Schaefer (Louisiana State
University) published evidence that the star had been fading over the past century. Analyzing a series of photographic
plates at the Harvard College Observatory, Schaefer determined that between 1890 and 1989, the apparent brightness
of the star had decreased by approximately 15%.
This result was quickly called into question: Independent
researcher Michael Hippke and colleagues claimed the longterm fade wasn't real. Instead, they argued, a shift in photographic plate technology and data collection techniques in the
1960s had created artiﬁcial changes in the star's light curve.

Yet at least some long-term dimming appears to be real.
One of us (Montet) analyzed calibration data from the Kepler
mission, revealing that the star had dimmed by a whopping
3% over a period of four years, with a third of the fade occurring over just six months. (Automatic data analysis removes
long-term changes in order to improve the efﬁciency of the
search for transiting planets, but Kepler's calibration frames
reveal the trend.)
This four-year brightness change happened at a faster rate
than the change observed in Harvard's photographic plates.
But the Kepler observations proved the irregularity of Tabby's
star, so we can't rule out the possibility that we'd observe
different dimming rates over a century compared to a single
four-year stretch. Furthermore, while the Kepler data don't
directly conﬁrm Schaefer's result, it does seem unlikely that
errors made while measuring photographic plates would happen to predict a trend in Kepler data.
So any explanation of the star's behavior must explain
not only the short, extreme dips recorded by Kepler, but also
the slower dimming observed over the same four-year time
period, as well as potentially a long-term fade over the course
of the 20th century.
Let's investigate a few possibilities by following a single photon on its 1,300-year-long journey from Tabby's star to Kepler's
detector, all the while considering what could happen along
the way. We'll weigh each scenario by how well it explains the
data, as well as how plausible it is (see box on page 20).

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

12.25

Century's Fade
Data from the Harvard photographic
plate collection shows a possible drop
in Tabby's star's brightness between
1890 and 1989. The century-long
change is documented here in five-year
intervals. The line shows the trend that
best matches the data. The brightness
of 12 nearby F stars, which served to
check the trend, stayed level over the
same time period.

Magnitude (B band)

B. SCH A EFER / ASTR OPHYSICAL JOUR NAL, 2016 M AY 6

Day of Kepler mission

12.30
12.35
12.40
12.45
12.50
1890

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Year
s k y a n d t e l e s c o p e .c o m

* JUNE 2017

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Sky and Telescope - June 2017

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