Sky and Telescope - September 2018 - 33

FINDING BL LAC Cuno Hoffmeister
would have perused a plate very much
like this one (taken by Harvard astronomers on the evening of November 1,
1934, in Oak Ridge, Massachusetts) when
he discovered the "variable star" that
turned out to be BL Lac (position approximated by the box).

Cassiopeia

Cepheus

Cygnus
Andromeda

H A RVA RD COLLEG E OBSERVATORY /
G L ASS PL ATE COLLECTION

Lacerta

tances their redshifts implied. Some astronomers searched
for blueshifted quasars to counter the implied extragalactic
distances of the redshifted ones. In fact, in 1967, Peter
Strittmatter and Geoffrey Burbidge of UCSD proposed a set
of properties to describe blueshifted quasars, and it turned
out that BL Lac exhibited many of these.
* BL Lac might be an unusual planetary nebula, as had been
suggested by Arp, amongst others.
* BL Lac was a black hole within our galaxy that was accreting matter. This theory, proposed in 1974 by Stuart Shapiro
and James Elliot of Cornell University, fit just about everything observed in BL Lac.
But ultimately BL Lac's nature depended on determining its
distance, and in the 1960s and into the 1970s there was no
reliable way of measuring this.
Then two astronomers came up with an interesting idea.

Investigating the Crime Scene
James Edward Gunn and John Beverley Oke (both at Caltech)
took on the challenge of determining BL Lac's distance. They
accepted that the spectrum was of no help due to the lack of
emission or absorption lines. Instead, they proposed that BL
Lac was the core of a galaxy. If this were the case, then perhaps they could devise some way of observing the galaxy itself.
The galaxy had to have been very faint, otherwise it would
have been detected by now. Halton Arp's earlier observations
had hinted at an extension, but obtaining a spectrum of that
faint outer region was challenging due to BL Lac's overwhelming brightness.
In the early 1970s, Gunn and Oke used the largest telescope at the time, the Mount Palomar 200-inch Hale telescope, in order to collect as much light as possible. They
fabricated a special mask that blocked the light from BL
Lac but allowed the immediate surrounding region to shine
sk yandtelescope.com * S E P TE M B E R 2 018

33

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

Contents
Sky and Telescope - September 2018 - Cover1
Sky and Telescope - September 2018 - Cover2
Sky and Telescope - September 2018 - 1
Sky and Telescope - September 2018 - Contents
Sky and Telescope - September 2018 - 3
Sky and Telescope - September 2018 - 4
Sky and Telescope - September 2018 - 5
Sky and Telescope - September 2018 - 6
Sky and Telescope - September 2018 - 7
Sky and Telescope - September 2018 - 8
Sky and Telescope - September 2018 - 9
Sky and Telescope - September 2018 - 10
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Sky and Telescope - September 2018 - 31
Sky and Telescope - September 2018 - 32
Sky and Telescope - September 2018 - 33
Sky and Telescope - September 2018 - 34
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Sky and Telescope - September 2018 - Cover3
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