All_About_Space_-_Issue_94_2020

“To me, it makes far more sense that this

system is some previously unknown

natural phenomenon” HughOsborn

How to find Tabby’s Star

See the 'megastructure star' yourself – if

you have dark skies and a good telescope

consider, but this looked like something youwould

expect an alien civilisation to build.”

As well as exciting the media, the hint ofan

alien megastructure galvanised the Search for

Extraterrestrial Intelligence (SETI) community.

Follow-up searches were instigated to look forany

other technosignatures that might be emanating

from Tabby’s Star. The searches were unsuccessful–

but that had been the realistic expectation allalong.

As Doug Vakoch of the SETI Institute explainedat

the time: “Our assumption is that there’s goingtobe

a natural explanation for this, we just haven’tgotten

clever enough to find it.”

By the end of 2017 that ‘natural explanation’

for the mysterious behaviour of Tabby’s Star

was becoming clearer. The dimming was found

to be less pronounced at the infrared end of

the spectrum than at the ultraviolet end. That’s

exactly what astronomers would expect to seeif

the dimming was caused by a cloud of tinydust

particles – but not from an alien megastructure.As

Boyajian explained: “If a solid, opaque objectlike

a megastructure was passing in front of thestar,it

would block out light equally at all colours.Thisis

contrary to what we observe.”

In view of this, the scientific consensus isnow

veering towards the idea that the peculiar light

curve,withitsdeep,randomlyoccurringdips,is

producedbyanunevenringofdustcirclingthe

star.Thatstillleavesa mystery,though,becausethe

onlystarsnormallyobservedwithlargeamountsof

dustintheirvicinityareveryyoungones.Tabby’s

Star,ontheotherhand,isa maturemainsequence

star,somewhatlargerandhotterthantheSun.

Astronomerswouldhaveexpectedthatanydust

thatoncecircleditwouldhavecondensedinto

planetslongago– justashappenedhereinthe

SolarSystem.Onepossibility– thoughnomore

thana hypothesisatthisstage– isthattwoofthose

planetscollidedwitheachotherintherecentpast,

shatteringthemselvesintoa cloudofdustydebris

that’sresponsibleforthedimmingswesee.

EvenifTabby’sStarturnsouttobesomething

otherthananalienmegastructure,theargument

thatsayssuchthingsmightexist– andthatwe

coulddetectthemthroughanomalouslightcurves

• remainsvalid.Asastronomerscontinuetoscour
  throughKeplerdata– andyetmoredataisgathered
  byitssuccessor,theTransitingExoplanetSurvey
  Satellite(TESS)– anothermegastructurecandidate
  mightcropupatanymoment.InJune2019,that’s
  exactlywhathappened.
  Thisonehasn’tgota nice,friendlynickname
  yet,butitsofficialdesignation– HD139139,with

Wheretopointyourtelescope

Tabby’sStarisabouttwo-thirdsoftheway

alongfromDenebtoDeltaCygni.It’squite

close to a small star cluster, NGC 6866.

What to look for

Tabby’s Star is faint at magnitude

11.7, but should be visible with a good

amateur telescope if you’re lucky

enough to have dark skies.

DistancefromtheSolarSystem

At1,470lightyears,Tabby’sStarisn’tone

ofournearestneighbours– butit’sclosein

comparisontothesizeofthegalaxy,which

isaround100,000lightyearsacross.

Landmark: Deneb

This is the brightest

star in Cygnus, at the

tail end of the Swan. To

its east is Delta Cygni,

marking one wing tip.

Physical

characteristics

Tabby’s Star is a main

sequence star like the

Sun, but F-type rather

than G-type – making it

about 40 per cent more

massive and a thousand

degrees hotter.

DeNeB

CyGNuS

OmICrON

CyGNI

KIC 8462852

NGC 6866 VeGA

© NASA

Constellation:

Cygnus the Swan

One of the easiest

constellations to

spot – the prominent

‘northern cross’, which

is high in the sky

throughout the short

summer nights

The weirdest star