Non-intrinsic Variables

Eclipsing binary

An eclipsing binary is simply where one star orbits a larger companion that affects the apparent magnitude during the eclipse. Alpha Herculis is a cool red supergiant 382 light years away. This star has a magnitude of 3, and this brightness varies by almost one magnitude over a period of about 128 days. Its companion is itself a binary system orbiting Herculis, comprising a giant and another smaller sun-like star.

A faster eclipse is presented by Lambda Tauri, 370 light years distant. Magnitude 3.4. This brightness drops by almost half a magnitude every 3.95 days when its dwarf companion passes in front. It is only a partial eclipse. Interestingly the stars are incredibly close, just 15 million kilometres apart. This probably causes mass exchange and tidal distortions, and this affects their luminosity even when there is no eclipse taking place.

In some cases, where binaries appear as one star because they are too close to distinguish in a telescope, they can be detected by the change in the level of light emanating from them. The first such example to be discovered was Algol (Beta Persei), also known as the Demon Star. In 1783 John Goodricke (1764 – 1786) noted its light variation which dipped for a period of about 10 hours, and that this occurred every 2 days and 21 hours. He hypothesised that this was because a dark body was orbiting the star and passing in front of it. Today we know this dark body is in fact another star. Algol is now the prototype for the eclipsing binary.

Eta Carinae
The Carina Nebula (NGC 3372) is home to young, extremely massive stars, including the still enigmatic and violently variable Eta Carinae, a star system with well over 100 times the mass of the Sun. The two-lobed Homunculus Nebula appears to surround Eta Carinae just below and left of centre. Eta Carinae is likely on the verge of a supernova explosion. Image Credit: NASA, ESA, Hubble, ESO.

Beta Lyrae is also an eclipsing binary but with a difference. Also known as Sheliak, it is 880 light years away and has a magnitude of 3.5. The brightness of the system varies by about one magnitude every 12 days 22 hours and is easily visible to the naked eye. It was also discovered by John Goodricke, a year later in 1784.

In its own way it sets the prototype for an “eclipsing interacting binary star”. In this configuration the two stars are so close together that they are greatly distorted by their mutual attractions. They are just 35 million kilometres apart. Given their relative size their surfaces are a mere 11 million kilometres apart. They exchange material and share a common atmosphere. Material pouring out of the stars is forming a thick accretion disc. Both stars are giants, being 20 and 10 times the sun’s mass. As with most binaries the smaller star is more massive than the larger star.

Symbiotic variable

Symbiotic variables are binary stars that live on each other in the sense that they exchange material. Unlike most binaries there is no one way street. They are of late spectral types K and M. Besides absorption lines they have emission lines of ionised helium and oxygen.

Irregular variable

Last but not least is the irregular variable. This is the category astronomers use for variables that defy explanation, the “odd bin” of variable stars, if you like. They spin rapidly and some cast off shells of matter. Intense disturbance in the shells of matter are the cause of the irregularities in light emissions. External factors can play roles in the light intensity that a star emits. Stars shrouded in nebulosity are known as nebular variables.

Here are a few examples.

Epsilon Aurigae is a supergiant with eclipses that last for two years. What it is eclipsing cannot be seen, but whatever it is it must be far larger than supergiant Aurigae. Astronomers suggest it is being eclipsed by a massive dust cloud from a nearby star system. Maybe the dark cloud is orbiting a small binary system that takes it across the line of sight of Aurigae every two years.

R Coronae Borealis is a low-mass yellow supergiant, whose luminosity drops in magnitude from 5.9 to 14.4 at irregular intervals. That is eight levels of magnitude, which is theoretically impossible. It certainly cannot be explained merely by physical changes occurring in the star itself. Either it is being orbited by a massive dust cloud, or perhaps, when it ejects material caused by shells of carbon dust, it is obscuring some of the light.

As a prototype for its strange behaviour it is sometimes abbreviated R CrB.

Gamma Cassiopeiae is a variable star with unpredictable changes in magnitude, ranging from first to third. Astronomers reckon it is rotating at more than a million kilometres an hour (at the equator). It is not surprising this behaviour results in massive loss of material creating a disc with varying and unpredictable emissions. Cassiopeiae could also be donating material to an undisclosed companion star.

RW Aurigae variables present sudden maxima and minima luminosities. They can go through four of five maxima over a period of 40 days. These rapid changes can be succeeded by long periods of steady brightness. It is thought this happens when the star passes through a nebula-free region of space. There are quite a number of these characters in the Orion nebula. Their spectral type is dG, that is, dwarfs with temperatures of about 5 700°C.

An even more peculiar variable is Eta Carinae. Located some 7 500 light years distant it has an extensive history of random massive eruptions, sometimes becoming one of the brightest objects in the night sky. It is thought Eta Carinae may actually be four stars, but together are described as a “blue supergiant”. In the 17th and 18th centuries it was a magnitude 2 star, but later this started to fluctuate. There was a very sharp increase in brightness in 1838 until the star reached a magnitude of -0.8 by 1843. It was during this period, in 1841, when it underwent a giant outburst that produced two distinctive lobes of outflowing material, now referred to as the Homunculus Nebula. These lobes are moving away at about 2 million kilometres an hour. In 1867 its brightness suddenly dropped to 7.6 and has since fluctuated between 6 and 7. This is certainly a highly unstable region and Eta Carinae has not long to go before erupting into a supernova.

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.

Last updated: Tuesday, 2nd March 2021