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.

Science Illustrated - - CONTENTS -

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).

GBT

Apart from the net­work, the Green Bank te­le­scope is lis­ten­ing for ra­dio sig­nals from aliens.

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