My first Magazine Sky & Telescope - 02.2019 | Page 41

Modern observers will recognize
the particular pattern of brightness
variation shown by Algol — nearly
constant brightness except for a
sharp decrease for a few hours — as
characteristic of a detached eclipsing
binary system. While astronomers of
Goodricke’s time realized that binary
stars were theoretically possible and
quite likely, no systems had yet been
shown to be true binaries. Sunspots,
however, were a known phenom-
enon, and established astronomers
favored the starspot explanation.
In August 1783 the Royal Society
awarded the Copley Medal, its high-
est honor, to John Goodricke “for his
discovery of the Period of the Varia-
tion of Light in the Star Algol.” Today
this work and later discoveries are
recognized as a collaborative effort
between Goodricke and Pigott.
The two observers were especially
productive in the autumn of 1784,
when they discovered three new vari-
able stars. In September, Goodricke
noted that Beta Lyrae, now recog-
nized as a contact binary, appeared
to vary in brightness. On the same
night, Pigott saw a change in the
star known today as Eta Aquilae,
later classified a Cepheid variable.
Goodricke noted the variability of
the namesake Cepheid, Delta Cephei,
in mid-October. In Pigott’s paper
on the variability of Eta Aquilae,
he addressed possible causes for the
variation:
Hitherto the opinion of astronomers
concerning the changes of Algol’s light
seem to be very unsettled . . . though
3.0
3.5
4.0
4.5
5.0
0
5
15
10
20
Days
p A NEW VARIABLE Goodricke detected
Beta Lyrae’s variability in September
1784. The light cycle of Beta is never
constant because the star is a contact,
or semi-detached, binary where mate-
rial transfers at a rapid pace from the
gaseous envelope of a massive star onto
a lower mass star. Beta’s apparent visual
magnitude peaks around 3.2. The light
curve shows minima at different depths,
with the dimmest as low as 4.4.
Delta Cephei
3.0
As to those fixed stars that appear
and disappear by turns, and increase
slowly and by degrees, and scarcely even
exceed the stars of the third magnitude,
they seem to be of another kind, which
revolve about their axes, and, having a
light and a dark side, shew those two
different sides by turns.
Beta Lyrae
3.5
4.0
4.5
5.0
0
5
10
Days
p DISCOVERING DELTA Goodricke
noted the variability of Delta Cephei in
October 1784. Delta’s brightness cycles
from 3.5 to 4.4 and back up again over
the course of 5.4 days.
Algol
2.0
this idea. No less a figure than Isaac
Newton had written, in Book III of
Principia (1687):
various are the hypotheses to account
for it; such, as supposing the star of
some other than a spherical form, or a
large body revolving round it, or with
several dark spots or small bright ones
on its surface, also giving an inclina-
tions to its axis etc.; though most of
these conjectures with regard to Algol be
attended with difficulties, some of them
combined do, I think account for the
variation of Eta [Aquilae].
John Goodricke visited London
in 1785, where he met the Astrono-
mer Royal, Nevil Maskelyne. In
July of that year, Maskelyne wrote
Goodricke, asking, “Did you ever
doubt whether some hemispheres
of our Sun (exclusive of the variable
spots) may not be brighter than oth-
ers?” Although the phrasing is odd, it
is clear that Maskelyne was attempt-
ing to persuade his young colleague
to accept the starspot hypothesis.
One more observational detail
nudged Goodricke toward the spot
hypothesis and away from eclipses.
In his paper describing the variation
of Delta Cephei, Goodricke wrote
that “Even Algol does not seem to be
always obscured in the same degree,
being perceived to be sometimes
a little brighter than Rho Persei,
and sometimes less than it.” Rho,
a nearby star in Medusa’s head, is
a convenient comparison star for
a naked-eye observer as its average
visual magnitude is 3.4, the same as
Algol’s in eclipse. But here Goodricke
apparently fell prey to an error well
known to modern photometrists:
2.5
29
3.5
18
3.5
30
0
1
2
3
4
5
PERSEUS
6
Days
pu THE GORGON’S EYE Algol (Beta
Persei) dips from magnitude 2.1 to 3.4
and back every 2.7 days. Use this chart
to estimate its brightness with respect to
the comparison stars of magnitude 2.1
(Gamma Andromedae) and 3.4 (Alpha Tri-
anguli). Rho Persei is also a variable star.
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Algol
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l
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