Sky and Telescope - September 2018 - 32

Mystery Object

lished, compiling everything then known on these objects.
Several entire pages were devoted to stars such as Mira and
Algol, but only a few sentences mentioned BL Lac.
It seemed like this star would fade to obscurity; the case
went cold for decades.

Case Reopened
In the 1960s the burgeoning ﬁeld of radio astronomy opened
up a whole new "eye" on the sky, reaching the point where
all-sky surveys could be carried out to some
degree of positional accuracy. Radio astronomers began revealing a slew of interesting new
objects, such as quasars - objects ﬁrst thought
to be radio stars but in 1963 discovered to be
at cosmological distances (see S&T: Sept. 2013,
p. 24). Then in 1968 astronomers discovered
a strange point-like source. Its radio spectrum
was relatively ﬂat, with emission just as bright
at low frequencies as at high ones. Moreover,
the radio waves were polarized - when the
vibrations of an electromagnetic wave are
restricted, even partially, in one direction, so that there's less
random orientation - indicating the presence of magnetic
ﬁelds. Curiously, the source sometimes varied in brightness in
a span as short as a few days.
Shortly thereafter, optical astronomers found that this radio
source coincided positionally with BL Lac. Because no star had
q HISTORIC HARVARD COLLEGE OBSERVATORY This is the observatory as it appeared in 1899, well after the advent of the use of photography in astronomy. The largest dome on the right still houses the 15-inch
"Great Refractor," which is primarily used today for public observing
nights (cfa.harvard.edu/hco/grref.html).

been discovered before with such peculiar radio properties,
astronomers conducted a comprehensive visible-light campaign
on this object. They conﬁrmed historical observations that
indicated the light curve varied by up to several magnitudes
with no obvious pattern. Even stranger things were discovered,
however. The visible light was also polarized, and the direction
of the polarization varied, sometimes from day to day.
But the most amazing thing was the spectrum - astronomers were amazed and aghast to ﬁnd that it was completely
featureless. Examining the spectrum was
exactly like looking at a sheet of blank paper. It
conveyed no information, preventing astronomers from learning about the true nature of
BL Lac. The only thing they did know was that
the spectrum was unlike anything they had
observed before. There were some peculiar
white dwarfs with spectra superﬁcially similar
to BL Lac's. However, photometric analysis
ruled out the white dwarf scenario. But something else came up: To their surprise, astronomers noted that BL Lac fell close to quasars on
a color-color diagram, where the differences in brightness in
several wavebands are plotted for comparison. This suggested
that perhaps BL Lac was extragalactic. If it were truly at the
distance of quasars, that would make it one of the brightest
objects in the universe.
There was one problem with this line of inquiry. Without any emission or absorption lines in the spectrum, it was
impossible to determine BL Lac's redshift and hence measure
its distance. And, at the time, no other method for establishing cosmological distances was available.
One curious footnote to this ﬂurry of observations was
written when famed astronomer Halton Arp observed BL Lac
using the 200-inch Mount Palomar reﬂector. He noted that
BL Lac was not stellar in appearance and speculated that it
might be a peculiar planetary nebula.
Curiouser and curiouser!

Astronomers knew that ﬁnding more candidates would
help them in their investigations. They scoured variable star
catalogs and identiﬁed many possibilities, but when they
crosschecked against lists of radio sources none proved to be
similar to BL Lac.
So they tried it the other way around. Astronomers
searched through databases of radio sources and selected
a sample with ﬂat spectra and highly variable polarization,
then compared their positions with those of known variable stars. This exercise proved quite fruitful. Soon they had
identiﬁed about three dozen possible candidates.
In the meantime, astronomers chomped at the bit to come
up with an explanation for BL Lac. Three scenarios prevailed:
* If BL Lac were extragalactic, then one possibility was that
it was a blueshifted quasar. Back when quasars were ﬁrst
identiﬁed, not everyone agreed they were at the great dis32

S E P TE M B E R 2 018 * SK Y & TELESCOPE

H A RVA RD COLLEG E OBSERVATORY

Rounding up the Unusual Suspects

http://cfa.harvard.edu/hco/grref.html

Sky and Telescope - September 2018

Table of Contents for the Digital Edition of Sky and Telescope - September 2018