2019-07-01_Australian_Sky_&_Telescope

58 AUSTRALIAN SKY & TELESCOPE July 2019

factorssuchasradius,ageandmass.Withoutsuchbasic

elementsnaileddown,guessingwhena supergiantlike

Betelgeusewillgosupernovais a shotinthedark.

Togaugethedistanceofa star,astronomersoftenrely

onparallax. AsEarthorbitstheSun,theangleofourviewof

a nearbystarchangesslightly.Thestarappearstoshiftits

positionbya fractionofanarcsecondagainstthebackdropof

moredistantstars.Astronomerscanusethat,alongwiththe

baselineofEarth’sorbit,tomeasurea star’sdistance.

Parallaxworksonlyforrelativelynearbyobjects.While

AlphaOriis closebyastronomicalstandards,gettinga precise

parallaxforit is challenging,becausethatmeasurementis

smallcomparedtothesupergiant’sangulardiameter.In

otherwords,Betelgeuseis justtoobigtomakethismethod

useful.Also,thestarmightnotevenhavea symmetrical

shape,andthebrightspotsonitsphotospherecomeandgoin

variouspositions,makingit hardtolocateitscentre.

Othercommontacticsforassessingdistancealsooffer

nohelp.Astronomersoftenusea companionstartoaid

inestimatingdistance,aswasdonewithPolaris.But

AlphaOrihasnoknowncompanions.Thestaris alsotoo

brightforGaia,theEuropeanSpaceAgencysatellitethatis

successfullyfixingthedistancetoa billionstarsandother

celestialobjects.

In2017,GrahamHarper(UniversityofColorado)and

colleaguespublishedoneofthemostthoroughlycalculated

distanceestimates.Theteamcombineddatafromfour

differentinstruments:theHipparcossatellite,theEuropean

SouthernObservatory’sVeryLargeTelescope,andtwoarrays

ofradiotelescopes,e-MERLINandALMA.Theirstudyput

thedistanceat about 222 parsecs,or 717 light-years.Yet

Harperandhiscoauthorsadmittheycouldbeoffby20%.

“Itis frustratingthatitsparallaxis currentlysopoorly

constrained,whichseverelylimitswhatcanbegleanedfrom

alltheresearcheffortintothissystem,”theylamentinthe

2017 paper.

Radius

Diameterscaleswithdistancelinearly,soourgriponAlpha

Ori’ssizehasthatsame20%uncertainty.Plus,Betelgeuse

hasshellsofgasanddustthatitsstrongstellarwind

continuouslyblowsout.Thesemakeit difficulttodefinethe

star’svisiblesurface.

ButthatBetelgeuseis mind-bendinglyhugeis notin

question.Estimatesofradiusovertheyearshaveranged

between3 and5 astronomicalunitsormore.(Onea.u.is

themeanEarth-Sunaveragedistance,orabout 150 million

kilometres.)A recentestimateputtheradius— withthe

expectedlargeerrorfactor— at 955timesthatoftheSun.

(Thisis slightlylargerthanthe 2016 estimateweuseat right.)

Thatgivesusa starthatmightbe1.3billionkilometersacross.

Brightnessandmass

That20%uncertaintyindistancedoubleswithhowmuch

lightthestarputsout.“That’sa 40%uncertaintyinthe

luminosity,andthatgetstobebigenoughyoucandrivea

modeltruckthroughit,”saystheoreticianCraigWheeler

(UniversityofTexas,Austin).

0.015 ̋

Saturn

Jupiter

Mars

Earth

Venus

Mercury

TSUPERGIGANTIC Taken by ALMA in 2015, this picture (left) of hot gas in the star’s lower chromosphere is the highest-resolution image of

Betelgeuse. The overlay of planet orbits (right) shows how far the star would reach into our Solar System if it stood in place of the Sun.

ALMA (ESO / NAOJ / NRAO) / E. O’GORMAN / P. KERVELLA (2)

RED STAR