Sky and Telescope - November 2017 - 45

Under the Stars by Fred Schaaf

A Thrilling Trio
These three variable stars will hold
old
d
your attention for nights on end.

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racking variable stars requires
meticulous observation habits,
with careful attention paid to a single
characteristic of a star, its brightness.
Many amateur astronomers lean toward
observing larger, richer, more detailed, or
more colorful deep-sky objects, presuming that monitoring variable stars
requires more work or more prolonged
effort than they want to put out.
So people may be surprised to learn
that three of the greatest amateur
astronomers of the 20th century, all
widely famed for their interest in more
visually rich classes of astronomical
objects, all widely treasured for their
skilled and beautiful astronomical
writing, were devoted, masterful, and
passionate observers of variable stars.
Three men and three stars. These
three great amateur astronomers are,
in chronological order: William Tyler
Olcott; Leslie Peltier; and Walter Scott
Houston. This is a great time of year to
ponder the topic of variable stars and
what may have inspired the interest of
these men in them. Our current sky
offers the best evening visibility for
the three most important, prototypical
variable stars - all within a few hours
of right ascension and within a few
constellations of one another. The three
variable stars are, west to east: Delta (δ)
Cephei, the prototype of the Cepheid
variable class; Omicron (ο) Ceti (Mira),
the prototype of long-period or Mira
variables; and Beta (β) Persei (Algol),
the prototype of eclipsing binaries.
Let's take a closer look at these these
three variable stars.
Delta Cephei, Mira, and Algol.
I wrote about Delta Cephei in last
October's installment of this column.
This pulsating star drops in brightness
from magnitude 3.5 to 4.4 and back

to maximum in 5 days, 8 hours, and
48 minutes. It forms a small triangle
with excellent comparison stars for its
maximum and minimum, and it's also
a gorgeous and easy orange and blue
double star.
Delta heads a class of variable star
known as "the Cepheids," which offered
us a key to establishing distances in the
universe. The variability of Delta Cephei
was discovered in 1784 by astronomer
John Goodricke. Goodricke lost his
hearing after falling ill at age 5, but he
received a solid education in mathematics and natural philosopy nonethless. He
died of pneumonia at the age of 21, but
not before proposing the correct explanation for the variability of Algol.
Mira is a long-period variable in the
neck of Cetus, the Whale, that goes
from one maximum to the next in
about 332 days. Mira typically ranges
from about 9th or 10th magnitude to
4th or 3rd magnitude, but it occasionally peaks at 2nd magnitude - and once
even rivaled 1st-magnitude Aldebaran.
Mira is approximately 500 times the
diameter of the Sun and so cool (for a
star) that it has water vapor in its outer

atmosphere. The star's variability was
ﬁrst noticed by German pastor David
Fabricius back on August 13, 1596, but
he seems to have believed it was a nova.
Algol marks the severed head of the
monster Medusa in the grasp of Perseus. Algol dims from magnitude 2.1 to
3.4 over a period of 2 days, 20 hours, 48
minutes, and 56 seconds, with both the
dimming and brightening taking about
5 hours each. Though Goodricke was
the ﬁrst to ﬁgure out Algol dims due to
eclipses by its cooler companion star,
the variability of Algol seems ﬁrst to
have been recorded by Italian astronomer Geminiano Montanari in 1667.
Next time: the 20th-century three.
I've only just gestured at the accomplishments of Goodricke, Fabricius,
and Montanari this month, but in next
month's column, where we take further
looks at awe, total solar eclipses, and
more, we'll also discuss the "20th-century three" mentioned above and their
love of variable stars.
¢ Contributing Editor FRED SCHAAF
was long ago struck by the wonder of
variable stars.
s k y a n d t e l e s c o p e . c o m * N O V E M B E R 2 0 17

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Sky and Telescope - November 2017

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