UpDate - Vol. 11, No. 9, Page 4
October 31, 1991
Algol, the Demon Star, shines with variable brightness

     High over head in November is a star that has been called the
Demon Star. It is not exceptionally bright nor does its position
aid in forming a conspicuous constellation. Its call to fame is the
way its brightness varies with time. This star, Algol, designated
Beta in the constellation of Perseus, was the first short-period
variable star to be discovered.
     No telescope is needed to observe this variation in the light
from Algol. All that is required is a willingness to make repeated
checks on the brightness of Algol in comparison with the stars
around it.
     At age 18, John Goodricke Jr., who enjoyed star gazing with
his friend Edward Pigott, diligently observed the fluctuations of
Algol and discovered that the variation occurred regularly with a
repetition period of only 68.8 hours.
     That was in the year 1782. Some other variable stars were then
known, but they were all of long periods (hundreds of days) or were
irregular.
     Today, with highly precise instruments to measure stellar
brightnesses, we know of many variable stars. Indeed, it may be
argued that ALL stars are variable when viewed with sufficiently
sensitive detection instruments tuned to the right spectral
wavelengths. This branch of astronomy is called photometry.
(Photometry is one of the major research activities at Mt. Cuba
Observatory, known to many northern Delaware residents because of
its many public education programs.)
     It was Goodricke's careful and repeated observations of Algol
that enabled him to visualize the way the light changed regularly
with time and to suggest a reason for the variation.
     Most remarkable is that the explanation that this young man
proposed in 1783 turned out to be exactly correct and continues to
this day to be the accepted model for this star.
     Goodricke hypothesized that Algol is not a single star but a
binary--two stars so close together in space that they are in orbit
about one another. Algol's companion is cooler and, thus, darker
than Algol. As they circle one another, the darker star passes in
front of Algol, partially eclipsing it at the regular intervals of
the orbital period.
     Such a system is called an eclipsing binary. Several thousand
are now known, but Algol remains the best to be seen with the naked
eye.
     We call this a model because no one has ever seen Algol up
close. Through the largest telescopes, it remains a single point of
light. All that we know concerning it comes from painstaking
analysis of that speck of light and the way it changes with time.
     The raw materials for our model are the data accumulated from
astronomical observations. The tools to construct the model are not
chisels or knives but mathematics and reason. What takes form is a
picture for the mind, not the eye. Yet the resulting model can be
just as stimulating and beautiful to the mind's eye as the palpable
product of an artist's creativity.
     Goodricke, who had the disadvantage of being unable to hear or
speak, clearly had the qualities of a great astronomer. The world
was deprived of his scientific abilities and genius by his untimely
death at the age of 21. In his short career, he made several
important discoveries in astronomy.
     To find Algol in November, look to the northeast at about 9
p.m. Capella is the brightest star in this part of the sky at this
time.
     Later in the night, some other bright stars will be rising,
but they will always be lower in the sky than Capella. Confirm that
the star is Capella by finding the nearby triangle of faint stars
known as the Kids (Capella, you recall, is the goat star). Now
trace all of the constellation Auriga and use it to orient the map.
     Although the relative positions of the stars, one to another,
will not change appreciably throughout a human lifetime (the map
will remain valid, though you live a thousand years), the
orientation of the map with respect to which direction is "up"
changes continually.
     Included on the map are some better-known references that may
help you in orienting the map and finding the less familiar
constellation of Perseus.
     These references are Polaris (the North Star), the Pleiades
star cluster (Seven Sisters) and the W-shaped constellation of
Cassiopeia. Note how the string of stars forming a J in Perseus,
with the bright star Algenib midway in the J, is positioned among
these references.
     You are now prepared to trace down the string of fainter stars
in the other half of Perseus. They curve away in the opposite
direction to roughly form a backward J. Here is where we find
Algol, the brightest star in that line and, normally, the second
brightest star in all of Perseus.
     That it is not always second brightest is the attribute that
has brought Algol to our attention. Fifteen percent of the time
Algol is being eclipsed, at least in part, by its darker companion,
and the total light that we see is dimmed.
     The eclipses begin rather suddenly with the light decreasing
steadily for five hours until a minimum is reached where Algol is
no longer the second brightest star in Perseus. Compare it to stars
on the J side of Perseus or to the Kids in Auriga.
     Upon reaching minimum, Algol immediately reverses and begins
the five-hour return to its former prominence where, once again, it
will be comparable in brightness to Almach in Andromeda and to
Polaris.
     The times when Algol is at its extreme minimum may be
predicted quite exactly since they recur at the regular intervals
of the orbital period with which Algol and its companion circle one
another.
     This period is two days, 20 hours and 49 minutes, enabling us
to make predictions of when to observe future minima. Thus we know
that minima will occur on Nov. 8 at 8:06 a.m., on Nov. 11 at 4:54
a.m., on Nov. 14 at 1:43 a.m., etc.
     The minimum on Nov. 16 is at a more convenient time, 10:32
p.m., but the encroaching bright moon will make it more difficult
to find the stars. It's not worth trying to see these
constellations when the moon is bright, so avoid the full-moon
nights around Nov. 18-24.
     The last minimum in November will be on the 28th at 9:48 a.m.,
when daylight will make most of the 10-hour decline and return
unobservable. However, by successively adding two days, 20 hours,
49 minutes, you can extend these predictions to more favorable
future dates: 6:37 a.m. on Dec. 1, 3:26 a.m. on Dec. 4, and so on.
- Richard B. Herr,
Associate Professor of
Physics and Astronomy