October 24, 2019

AstroEvent: Catching the Variations of Beta Lyrae.

For the most part, the heavens beyond the solar system are unchanging throughout a normal human life span. Sure, stars evolve and move about the center of our galaxy, but a student of the universe will scarcely notice this over decades. One spectacular exception, however, is the realm of variable stars. One of the most famous and easy to observe is the “demon star” known as Algol in the constellation Perseus. This week, we’d like to introduce you to its summer counterpart, rising in the constellation Lyra in the north east after dusk for mid-northern latitude observers.

A Beta Lyrae Finder chart. (Created by the Author using Starry Night & Paint).

Beta Lyrae is the second brightest star in the constellation Lyra after Vega, the fifth brightest star in the sky. Also known as Al Sheliak which is Arabic for “the Tortoise,” (The ancient name of the constellation Lyra, you can sort of see how they got there with its parallelogram shape!) this star has been known to be variable in nature since John Goodricke noticed its changes in 1784. In fact, Beta Lyrae is an eclipsing variable of a most peculiar nature, and one of the most intensely studied stars in the sky. Burnham’s Celestial Handbook describes Beta Lyrae as “one of the most frustrating stars in the sky to an astrophysicist.” An eclipsing variable occurs when two stars pass behind and in front of each other as seen from our earthly vantage point. In the case of Beta Lyrae, the orbit is demonstrated by this mind blowing video constructed by data from the CHARA interferometer.

Beta Lyrae varies by not quite 1 magnitude (or about 2.5 times) in brightness, dipping from magnitude +3.4 to 4.3 every 12.9 days as the companion star vanishes behind the primary. Nearby Gamma Lyrae serves as a good comparison star, nearly equaling Beta Lyrae at normal brightness. Unlike Algol, these dips in brightness are gradual; the star is about half as bright as Gamma Lyrae at mid-eclipse and has a smaller dip to magnitude 3.8 midway between the larger eclipses.

Just what is Beta Lyrae? That’s what has frustrated astronomers for generations. Current models suggest a pair of B-type stars. When one star reached red giant stage later in its life, it expanded beyond its Roche limit and the embrace began. Currently, a 2.8 solar mass star is shedding material onto a 12.8 mass star; the star is shrouded in a dense disk of material and is also shedding extra mass being ejected from its poles. The pair is currently 11 million miles apart from center to center (Mercury orbits our Sun at an average distance of 35 million miles) and the orbital period of the pair is increasing by about 19 seconds a year. Incidentally, it’s this shroud of in falling material that has frustrated astrophysicists in identifying the companion star; Beta Lyrae is thought to be surrounded by rings of luminous material and has been the subject of several mind-blowing portraits. And like Epsilon Aurigae, its existence has led to much bizarre speculation over the years!

Clearly, a system like Beta Lyrae can’t last long; its exchanging mass in the order of one Sol mass star every 50,000 years and shedding large amounts of material in the process. Beta Lyrae is one of the few systems where we can see this process as it occurs, gives us a snapshot look at a unique stage of stellar evolution. The system is about 950 light years distant; with an absolute magnitude of -3.9, this system would be 7 times brighter than Sirius if you moved it closer to the solar neighborhood around 32.6 light years distant…

There are two schools of thought that I’ll throw out there for watching the variability of Beta Lyrae; one is to note the brightness against such nearby stars as Gamma Lyrae and note the nights that it appears to dip in brightness. If you want to know ahead of time when that will be worldwide, I’ll refer you to this excellent projected forecast. HOWEVER, for the slightly more daring, I’m going to suggest that you watch Beta Lyrae for a couple weeks first WITHOUT looking at the predictions, & THEN compare your “best guesses” as to when the mutual eclipses occurred. Do they jibe? Hey, you’ve done science and verified a true astronomical phenomenon! And as is the case with Algol and Epsilon Aurigae, you can observe the changes in Beta Lyrae with the naked eye, no equipment required!

Bonus Sci-Fi Bizarre trivia fact: Remember the old Star Trek cartoon series? Spock, Sulu, & Uhura visit Beta Lyrae in the episode entitled “The Slaver Weapon” written by science fiction author Larry Niven!

Comments

  1. Beni says:

    This game (memories) brought me to this article:
    https://www.youtube.com/watch?v=AgIJMN6Mtso

    (“The Tail of Beta Lyrae” for Atari 600XL/800XL)

    I played this game my whole childhood long. I never made it over the 3rd section….for years of play. That is how a game has to be. And here I read that this system is called “the Tortoise” which remembers me of “Discworld” from Terry Pratchet. Maybe it really is a tortoise? =)

    Regards,
    beni

Trackbacks

  1. [...] for an ‘occasional Diadem’ to indeed occur. Most famous eclipsing variables, such as Algol or Beta Lyrae have periods measured over the span of days or hours. Incidentally, these also make great [...]

  2. [...] The first pair in the system are contact binaries of 0.9 and 0.3 solar masses respectively in a tight embrace revolving about each other in just under six hours. Contact binaries consist of distorted stars whose photospheres are actually touching. A famous example is the eclipsing contact binary Beta Lyrae. [...]

  3. [...] The initial span in a complement are hit binaries of 0.9 and 0.3 solar masses respectively in a parsimonious welcome revolving about any other in usually underneath 6 hours. Contact binaries include of twisted stars whose photospheres are indeed touching. A famous instance is a eclipsing hit binary Beta Lyrae. [...]

  4. [...] stars whose photospheres are actually touching. A famous example is the eclipsing contact binary Beta Lyrae. [...]

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