Albireo: a binary star at the southern end of Cygnus the Swan. Although Albireo appears as a single star when observed with the naked eye, its two stars are resolvable when seen through even smaller telescopes. Astronomers who separate these stars in their telescopes refer to the practice as "splitting Albireo."
If the orbital plane of a binary star system is aligned with Earth, each component star will periodically move in front of the other from our perspective. During one of these "transits," one component will be blocked from our view and so the brightness of the star, itself, will appear to diminish. The diminishment will be greater when the secondary star (the dimmer) moves in front of the primary star.
The archetype of eclipsing binaries is Algol, the second brightest star in the constellation Perseus. In this system, the two component stars are separated by 0.05 AU (astronomical units), about 4.7 million miles. The revolutionary period equals 2.86 days. [We are ignoring Algol C, a third component of the Algol system that is not a part of the eclipsing binary system.]
The primary eclipse occurs when Algol B, the secondary star, passes in front of the brighter Algol A. A secondary eclipse occurs when Algol A moves in front of Algol B. The diagram below shows the light curve of the Algol system during an entire orbit.
____________________________________________________
Reminder: apparent magnitude measures a celestial body's apparent brightness. The dimmer the body, the higher the magnitude number.
____________________________________________________
At the far left we see the green curve at magnitude 2.2: the apparent magnitude of Algol when we see its two main stars together. We soon observe the curve "dip" as the secondary star transits the primary over a ten hour period. During that passage, Algol's brightness first decreases. It reaches a minimum when Algol B blocks Algol A to its fullest extent. As Algol B begins to move away from Algol A, the star's brightness increases until the stars are once again both wholly visible. A secondary eclipse -when A blocks B- decreases the brightness only slightly.
Our study of variable stars requires examination of light curves
Astronomical almanacs will often include listings of "Algol at minimum" times. At those times, Algol's brightness is at a minimum. The following are the next few dates for these minima. (We did notice include the July 1, 2020 at 7:04 a.m. minimum as that time will precede today's posting.)
- 4 July 2020 3:53 a.m.
- 7 July 2020 12:41 a.m.
- 9 July 2020 9:30 p.m.
- 12 July 2020 6:19 p.m.
One can observe Algol in the pre-dawn eastern sky presently. Algol, otherwise known as Beta Persei, is marked above. Only the July 4th minimum will be observable. We listed the others just for "fun."
_______________________________________
Remember: the Bayer Nomenclature System assigns Greek letters to stars within a constellation in accordance to their relative brightness. The brightest is marked alpha, the second brightest beta, the third gamma, et cetera. (Of course, this system has not proven to be the most accurate as its formulation (early 17th century) predated the development of photometers and other devices that can accurately measure stellar brightness.) Beta Persei is the second brightest star in Perseus
____________________________________
Astronomers have identified many Algol variables in our sky. They are generally not as well known because (a) the magnitude range is small (b) the stars are not visible without telescopes or (c) the variability period is quite long.
For example:
Mintaka: Orion's western belt star (Delta Orionis)
Mintaka is an 'Algol variable.' A 'minimum of Mintaka' occurs every 5.7 days. However, the magnitude range is small: 2.14 at maximum ; 2.26 at minimum. Hardly noticeable. We won't bother listing Mintaka at minimum times for July because Orion isn't currently visible. The constellation will return to the pre-dawn eastern sky by mid August.
-ROTATING VARIABLES-
The second type of extrinsic variable is the "rotating variable." As the star rotates, its brightness changes. Quite often, large darker regions along the star's photosphere pass in and out of our sight range. When the dark spot is in view, the star's brightness is at a minimum. When the star's brighter regions are aligned with our sightline, the star is at a maximum brightness.
Ellipsoidal binaries provide another interesting example of rotating variables.
These binary stars are close enough to induce tidal distortions in each other. One can think of these shapes -roughly- like footballs. As the stars revolve around each other while rotating on their axes, we will alternately see the shorter side of the "football" and then the longer side. When the shorter side comes into view, the star is dimmer than it appears when we see the longer side.
These variables are considered extrinsic because the brightness variation results from their rotations and not due to any internal process.
Tomorrow, we'll begin our examination of intrinsic variables which constitute a much larger subset of variable stars.
______________________________________
Skywatching Tip!
See the full moon on July 4
The last quarter moon on July 12
Moon is new on July 20th.
_______________________________________
To subscribe or unsubscribe from the Daily Astronomer: