ALL EYES ON A BIZARRE STAR
A flash. Pause. Two minor flashes. Long pause. Since amateur astronomers spotted a star’s mysterious light variations, cosmologists have tried to explain the odd behaviour. They are tackling the mystery with the help of a network of telescopes.
There’s a star, out there in space, that keeps changing its light output in a very weird way. And scientists want to know why.
Is the explanation a construction boom in an alien civilization? Did a star consume a planet? Or is the confusion caused by huge groups of comets? Astronomers are at a loss, when it comes to the KIC 8462852 star. Originally discovered in 1890, the star never really attracted much interest, before a team of American amateur astronomers took a closer look at data from NASA’s Kepler space telescope, which observed the star for four years. During that period of time, two major and one series of minor dimmings of the starlight have occurred at irregular intervals. This is very unusual and unlike anything astronomers ever observed.
So, the mysterious star will now be monitored closely. For 12 months, a global network of telescopes will keep a constant eye on the star, measuring the spectrum of light during the next major dimming. Nobody knows when it will come, but hopefully, new measurements can reveal what is blocking out the light of the star, answering the question once and for all. Although scientists have introduced one explanation after the other, none of them seem to tell the whole truth.
TELESCOPE MISSED DIMMING
The KIC 8462852 star is located in a dwarf galaxy 1,276 light years from Earth, and for many years, its life was an anonymous one. Not until the Planet Hunters amateur astronomers took a closer look at the large quantity of Kepler telescope data, scientists took an interest in it.
In 2009-2013, the space telescope studied 150,000 stars to find any orbiting planets by measuring their brightness. The subsequent analyses were made by computers programmed to look for changes in brightness of a few %, and so, they did not find the major dimmings. But when the Planet Hunters scrutinised the Kepler data, the team discovered that during the four year observation period, the star's brightness had been reduced twice, by 16 and 21 % respectively, and by 0.2-8 % 10 times for periods of up to several months.
The KIC 8462852 is an ordinary, Sun-like star, which normally shines stably with the same brightness for billions of years. Something large was bound to block out the light of the star, but the team did not know
what, so the members contacted their mentor, astronomer Tabetha Boyaijian from the US Louisiana State University US. At first, she thought that the dimmings were due to an error in the analyses of the Kepler data, but everything was perfect, and in late 2015, she and the Planet Hunters published the surprising observations of the star, which was nicknamed Tabby’s Star.
NO THEORIES FIT COMPLETELY
Astronomers were speechless, as they had never before seen a Sun-like star behave so strangely. Scientists throughout the world immediately set out to investigate it.
Astronomers soon ruled out the most obvious explanations. The huge dimmings cannot be due to planets, as even large gas giants would only dim the light by a few % at the most. Moreover, planets block out starlight at regular intervals, when, during their orbits, they pass between the star and Earth, but the changes in the brightness of Tabby’s Star are completely chaotic. The star might also be surrounded by huge dust clouds, which originate, when planets or asteroids collide, destroying each other. However, this is not likely. When the light from a star hits dust clouds, they are heated and emit infrared heat radiation, but several telescopes have already been unsuccessfully looking for increased infrared radiation.
Personally, Tabetha Boyajian first thought that the major and irregular starlight dimmings could be due to huge comet clusters. If the clusters were affected by the gravity of a neighbouring star, groups of comets could pass in front of the star at varying intervals, so their tails, which can grow millions of km long, would block out some of the starlight for days or weeks. However, the theory encounters the same problem as if the "shadow" had been produced by dust from planets and asteroids. The water vapour in the comet tails will be heated by the starlight, causing heat radiation, and scientists have not observed any signs of this.
However, the theory has not been totally forgotten. The shadow could be caused by a host of free-flying comets located in empty space between the star and Earth. Such comets have not yet been spotted , but they could exist, as comets emit so little light that they cannot be seen by a telescope, if they are located outside the Solar System.
OLD PHOTOS, NEW QUESTIONS
The mystery became even more tricky, when astronomy professor Bradley Schaefer scrutinised an archive with photographs of the star dating back to 1890. Much to his surprise, Schaefer discovered that the light from Tabby’s Star has become 14 % weaker over the past 100 years, which ought not be possible in the case of an ordinary star.
The accuracy of old observations is always doubtful, and consequently, astronomer Benjamin Montet from the California Institute of Technology in the US analysed the Kepler telescope’s recordings of the star once again. According to his analyses, the brightness is still reduced, and even faster than it used to. During the first three years of the observation period, the brightness was reduced by 0.3 % per year. During the next seven months, the brightness was reduced at a speed equivalent to 2.5 % per year. And during the last five months of the observations, the light was stable.
So, astronomers not only need to explain the major, sudden dimmings, but also the gradual dimming over the past 100 years.
CONSTRUCTION CASTS SHADOWS
A few theories, which can each explain everything, have already been introduced. According to some scientists, an unseen planet orbiting the star could be inhabited by a sophisticated civilization, which has built huge solar power plants around Tabby’s star over centuries. Over time, the scattered clusters of solar power plants have grown so large that they block out ever more of the star's light. The numerous minor/major dimmings are
due to the fact that
clusters of solar power plants of different sizes and paths pass between the star and Earth. The civilisation could even be so sophisticated that the flying solar power plants do not only convert the starlight, but also the heat of the solar panels into power. That could explain the low levels of thermal radiation when compared to similar stars. Astronomers admit that the theory sounds far-fetched, but a construction boom near the star cannot be ruled out. So, the theory is now being scrutinised by astronomers, who have pointed radio telescopes at the star to see, if any civilisation is communicating by radio signals, like we do on Earth.
Recently, another astronomer introduced a theory, which has attracted much attention. The theory involves that within the past 10,000 years ,Tabby’s Star swallowed a planet. This is rare, but a collision with an asteroid or a large comet could have sent the planet into the burning hell of the star. If so, the star’s brightness would increase, only to be gradually reduced to the normal level. The scenario could explain the dimming over the past century. If the planet were a gas giant, it might have been surrounded by large moons, which were not swallowed, but rather sent into orbit close to the star. If they melted, they would produce the large clouds of dust and gas, which could explain the sudden dimmings captured by Kepler.
TELESCOPES TO SOLVE MYSTERY
Astronomers want to record the spectrum of light during the next dimming. They'll use a global network of coordinated telescopes known as the Las Cumbres Observatory. A detailed spectrum could indicate what is blocking out the light of the star. A dust cloud would reduce the light of the blue and red wavelengths only. Rarer elements will absorb other colours. And if huge clusters of solar power plants are in the way, the light will be equally reduced at all wavelengths.
None of the theories have been proved yet, but hopefully, the telescopes will provide the new knowledge required to solve the mystery.
Astronomers do not know why the star's brightness varies so much, but it might be due to huge groups of comets. The Kepler telescope has observed the star for four years (2009-2013).
Apart from the network, the Green Bank telescope is listening for radio signals from aliens.