Stellar mystery’s simple solution

THERE are many mysteries in science, and astronomy is no exception.

One of the biggest puzzles is that we don’t even know what most of the universe is made of, although there are plenty of others.

But one of them may have been solved. A star called Tabby’s star exhibits unusual changes in brightness, and we now think that these changes are caused by the star’s light being occasionally blocked by a ring of dusty material surrounding the star.

Stars called variable stars can appear to vary in brightness for several different reasons. One, called Algol, was known as the “Winking Demon” because of its regular changes in brightness, which we later discovered arose because it is an eclipsing binary star: a pair of stars orbiting each other and blocking each other’s light.

Another type is a star called a Cepheid variable, a type of star that pulsates on a regular basis. Astronomers have used these stars to great advantage in determining the distance scale of the universe, because it was discovered just over a century ago that their period of brightness change is related to their true average brightness.

Tabby’s star, however, has been a mystery. It was named after Tabetha Boyajian, of the Louisiana State University in the US. It is also sometimes named after her surname, being called Boyajian’s star.

Tabby’s star dims, but in a quite unpredictable way. The star does not “fit” into any of our classifications of variable stars, and it has been quite a problem to explain.

Whenever this sort of thing happens, there are all sorts of suggestions as to what may be causing such an unexpected phenomenon.

It has even been suggested that Tabby’s star may be surrounded by a huge structure built by aliens in order to harvest energy from the star, and that the dimming is caused by the structure occasionally blocking the star’s light as seen from Earth.

The idea that highly advanced alien civilisations may build such a structure has been mentioned before, and such a concept is actually quite interesting.

However, it is rather farfetched, and it is much better to look for simpler and more likely explanations.

This reminds me of the loss of contact with the Mars Observer spacecraft just before it arrived at the Red Planet in 1993, following which there were suggestions that it had been captured by Martians. The explanation is most likely to have been that the spacecraft exploded due to the leakage of fuel through a faulty valve.

So, back to Tabby’s star. The deduction was made by astronomers using the results from two space telescopes and an observatory in Belgium. Their finding that the star’s light is being partly blocked by dusty material was made because of observations of the star’s light in two different wavelengths: infra-red light and ultraviolet light.

They found that the infrared light — the longerwavelength light — was dimmed much less that the shorter wavelengths of ultraviolet light.

This was very much a telltale observation, because it indicated that the absorption was being caused by tiny particles.

You have probably made the same type of observation yourself, perhaps without realising it, by observing the moon when it is low in the sky.

It looks to have a distinctly orange-yellow colour at such a time, because its light is passing through more of the Earth’s atmosphere, scattering the shorter (bluer) wavelengths but allowing the longer ones through.

It was highly detailed work, but sometimes the simpler explanations are the most likely, which certainly seems to have been the case here.