Sky & Telescope - April 2021 - 59

W Comae (yet another variable star
designation), is a BLO that exhibited
a peak magnitude of 11.5 in 1916 and
has one of the largest brightness ranges,
varying by up to 6 magnitudes. In
neatly encapsulated descriptions, we
learn that some sources vary over the
course of days, while others change over
months, years, or even decades - usually with no discernible periodicity.

Describing the Variability

3C 273: ESA / HUBBLE / N ASA; FINDER: POSS-II /
STSCI / CA LTECH / PA LO M A R OBSERVATORY

S QUASARS, BLACK HOLES, AND JETS The
word quasar is derived from quasi-stellar radio
source - indeed, they were first identified as
strong radio emitters. These objects appeared
pointlike on photographic plates, i.e., they
looked like stars (hence their moniker). We now
know quasars and their kin are galaxies at vast
astronomical distances that are powered by
supermassive black holes in their cores. Jets
of relativistic matter, such as the one captured
by the Hubble Space Telescope in this image of
3C 273, are often observed in quasars.

Swiss astronomer Fritz Zwicky
discovered the ﬁrst variable galaxy,
IV Zw 29 (later classiﬁed a Seyfert galaxy,
an object akin to a quasar but with a
more easily detectable host galaxy), on
photographic plates in 1965. In reexamining his own Palomar Observatory
plates dating back some 30 years, Zwicky
revealed that the galaxy's magnitude
varied irregularly during that timespan.
In another series of studies, researchers documented the galaxy's changing
brightness from 1958 to 1993, showing
that it ﬂuctuated between 17th and 18th
magnitude, punctuated by an occasional
rise to magnitude 16.7 followed by a
gradual decline. Zwicky's observations
marked the beginning of what would
become a new ﬁeld unto itself: variable
galaxies, which include BLOs, quasars,
and Seyferts.
Amateur astronomers will ﬁnd
detailed descriptions of speciﬁc variable galaxies in German astronomer
and author Wolfgang Steinicke's article
" Extragalactic Objects Discovered as
Variable Stars " (https://is.gd/steinicke_
agn). In it, he lists pertinent data and
characteristics of 21 variable galaxies, including ﬁnder charts and some
interesting historical facts. For example,

The exact origin of the variability is
still a much-studied subject, and many
theories abound. Various scenarios
invoke an accretion disk - matter captured from the host galaxy and swirling
around its central supermassive black
hole - or even the jets themselves.
Astronomers use light curves to decipher the variability of celestial objects:
By plotting the magnitude of the object
against time, patterns may emerge. The
nature of the variability depends on its
underlying causes. For example, Lola
Eachus and William Liller (both at the
Center for Astrophysics at Harvard College Observatory at the time) published
a study in 1975 on the long-term light
curve of 3C 279, a quasar in Virgo. They
plotted the object's magnitude (derived
from the blue-sensitive refractor plates)
from 1929 to 1952 and found that the
source brightened from around magnitude 16-17 to around 12-14 at regular
intervals of seven years (see the plot on
page 58). Some researchers propose that
this behavior arises from alternating
accretion-disk-dominated or jet-dominated emission from the quasar.

INSTRUMENTATION
The following setup was used to
obtain the images of three of the
targets presented here (Mrk 876,
Mrk 507, 3C 351):
System description: Starizona
Hyperion Telescope, aperture 12.5
inches (318 mm), FL 2520 mm
Exposure: 60 seconds, unguided
Camera: Apogee (Andor) U8300,
unbinned, -20°C, pixel size 5.4u
Filter: Luminance filter to limit
exposure to visible wavelengths
Mount: Software Bisque
Paramount ME

charts that indicate ﬁeld stars and their
magnitudes, which you can use to estimate the magnitudes of your target. See
also the article " Measuring the Stars " by
Richard Berry on page 60 of the December 2020 issue of Sky & Telescope - you
can use the same technique described
therein for galaxies.

Our Sample
3C 273 The ﬁrst quasar to be identiﬁed
as such (in 1963) is a good place to start.
Found in Virgo, it's about 11/3° almost
due west of the 10.6-magnitude galaxy
NGC 4536. I (Lamperti) ﬁrst observed
the quasar in 1991 with a 13-inch
reﬂector at 130× and saw it as a faint
3C 273

N

Want to Observe Variable
Galaxies?
The American Association of Variable
Star Observers (aavso.org) welcomes
your data on a number of objects,
including variable galaxies. You can ﬁnd
a handy table that lists variable galaxies (among other high-energy targets)
that should be within reach of amateurs
with larger telescopes at https://is.gd/
aavso_vargal. Speciﬁcally, the AAVSO
is looking for estimates of magnitudes,
along with the exact dates and times of
the observations. They provide ﬁnder

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http://www.aavso.org https://www.is.gd/steinicke_agn https://www.is.gd/aavso_vargal https://www.is.gd/steinicke_agn https://www.is.gd/aavso_vargal http://www.skyandtelescope.org
Sky & Telescope - April 2021

Table of Contents for the Digital Edition of Sky & Telescope - April 2021
