ALGOL

SHORT PERIOD ECLIPSING BINARY

Comparison Stars

45 epsilon Persei

Reference: Yale Bright Star Catalog 4th Revised Edition
HR 1220
HD 24760
Type B0.5V+A2V

(2000)
RA 03hr 57' 51.2"
Dec +40d 00' 37"

V= 2.89
B=2.71
U= 1.72
(B - V)= -0.18
(U - B)= -0.99
(R - I)= -0.18
J= 3.36
H= ?

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13 alpha Aries (Hamel)

HR 617
HD 12929
Type K2IHabCa-1

(2000)
RA 02hr 07' 10.3"
DEC +23d 27' 45"

V= 2.00
B=3.15
U= 4.27
(B - V)= +1.15
(U - B)= +1.12
(R - I)= +0.62
J= 0.09
H= -0.52

 

Program Star

beta Persei
(Algol)

Reference: Yale Bright Star Catalog 4th Revised Edition
HR 936
HD 19356
Type B8V

(2000)
RA 03hr 08' 10.1"
DEC +40deg 57' 21"

Period= 2.867315 days
Variation V= 2.1 to 3.4

V= 2.13
B=2.08
U= 1.71
(B - V)= -0.05
(U - B)= -0.37
(R - I)= -0.03
J= 2.16
H=?

Minima of Algol

Hopkins Phoenix Observatory Light Curves

Demon Star Photometry
by
Jeff Hopkins
Hopkins Phoenix Observatory
Phoenix, Arizona
December 2004

The night sky has fascinated man since the first time it was looked upon. When man began to study the sky closer, he found that things were not as steady as once thought. Besides the phases of the moon, wandering of the planets, occasional solar eclipses, comets and meteor showers, he saw some stars as unchanging while others seemed to vary in brightness. Most of these variable stars require careful observation to see them change. One star visible from the northern hemisphere stands out from all the others. The bright star beta Persei otherwise known as ALGOL has amazed and terrified people for hundreds if not thousands of years. Every 2.867 days just like clockwork, this 2.1 visual magnitude star winks to 3.4 magnitude. This wink can be seen with an unaided eye even in fairly bright sky locations.

This seemingly unnatural action of beta Persei was noted and the star received the name ALGOL which is Arabic for "Demon Star." All kinds of superstitions have been associated with it. Some astrologers have deemed it the most dangerous star in the sky.

While our star the Sun is unique in that there is only one star, many if not most stars have multiple stars in the star system. Because of the vast distances to even the closest stars, only one star can be resolved for most of these systems. Stars in a system revolve around a common center of gravity. When a star system has an orbital plane in line or at least partially in line with the Earth, the star system can be seen to change brightness as one star passes in front of the other. These star systems are known as eclipsing binary stars.

ALGOL is one of the brightest eclipsing binary star in our sky. While some report that ALGOL is the brightest eclipsing binary that we can see, not too far below ALGOL, the eclipsing binary beta Aurigae with a period of 3.960 day is brighter at visual magnitude 1.9. The ALGOL system is actually a three-star system, but only two of the stars play a role in what we see. The star system is located 92 light-years away. The primary star is a B8 while the secondary star is a less massive and dimmer K2 subgiant. The third star is an f1 star that orbits the primary and secondary stars every 1.86 years, however the orbital plane does not allow an eclipse to be seen.

Companion
The companion to ALGOL is a dying giant star. Yet it is the LESS massive of the two. The only explanation is that the dim companion has lost mass. The two stars are so close together, separated by less than a tenth the distance between the Earth and the Sun, and the giant has grown so large, that the bright star produces tides in the large one. Matter then flows in the reverse direction, from the large one to the small bright one, the effect directly observed.

ALGOL varies in V magnitude from 2.1 at maximum to 3.4 at primary minimum, with a period of 2.867315 days; this period, however, is slowly lengthening. The primary eclipse occurs when the fainter K2IV star passes in front of the brighter B8V star, and lasts for some 10 hours in total. Because the eclipse is partial, the minimum is not flat, but rounded. There is also a shallow secondary eclipse when the B8V star passes in front of the K2IV star. It can only be detected photoelectrically. The primary eclipse, however, can easily be detected with the unaided eye, and the magnitude and the time of minimum can be measured.

Because of the short period, producing a light curve from the photometric data can be a problem. The star system must be observed over multiple cycles. This is because part of the cycle occurs during daylight and twilight hours. If you plan to do differential photometry of ALGOL, use epsilon Persei as a comparison star as it's similar in color and magnitude.

Gamma Persei
In September 1990, another bright eclipsing binary was discovered in the same constellation. The star is 3rd magnitude gamma Persei. The eclipses of gamma Persei occur approximately every 14.67 years. The next eclipse is expected in April of 2005. However, because the star will be in superior conjunction with the Sun (on the opposite side) during the eclipse, the eclipse will not be visible from Earth.

Gamma Persei consists of a cool, giant G8 primary star in orbit with a hot, main-sequence A2 secondary. It is a composite-spectrum binary (also called a spectroscopic binary): spectroscopic analysis shows the presence of features from two different stars. The composite nature of the spectrum was recognized in 1897 by Antonia Maury at Harvard. Gamma Persei was resolved into its two components for the first time in 1973, and it was extensively analyzed in 1987. At this time it was predicted that the A2 star would pass behind the G8 star in the fall of 1990. The eclipse occurred on the evening of September 12th, and was recorded at several observatories. The secondary star "set" more or less vertically behind the giant star's limb, so the eclipse was central, or behind the middle of the G8 star, and lasted for an entire week. The eclipse was 0.3 magnitude deep visually, so it was detectable-though certainly not conspicuous-with the unaided eye. Gamma Persei will not eclipse again for unaided-eye observers until November 2019.

V Magnitude Phase
20 December 2004

B Magnitude Phase
20 December 2004

U Magnitude Phase
20 December 2004

A project is planned at the Hopkins Phoenix Observatory for early 2006 to obtain J and H baand infrared data on Algol using a 12" LX200 GPS telescope and Optec SSP-4 photometer.