2019-07-01_Australian_Sky_&_Telescope

Accuratelydeterminingluminosityis knottyevenif

astronomersareconfidentaboutthedistance.“It’sreally

hardtofigureoutexactlyhowluminousa staris,”saysEmily

Levesque(UniversityofWashington).Beyondknowing

howfarawayit is,researchersneeda solidfixona star’s

temperature.WehavethatforBetelgeuse,whichis knownto

berelativelycool,witha surfacetemperatureofabout3500K

(3226°C)versustheSun’s5778K(5504°C).Buttheyalso

needa griponhowthetemperatureaffectsthelighttheyget

inbluerversusredderwavelengths(hotterstarslookbluer).

Plustheyneedtounderstandhowmuchdustencirclesthe

star,becausedustcanblockandreddensomeofthelight.

Intrinsicbrightnessoffersinsightintostellarmass,which

itselfhelpsreveala star’sevolutionaryhistoryandexpected

lifetime.Asit stands,Betelgeuse’sabsolutemagnitudeis

thoughttobeabout–6.Thatimpliesa minimummassof

at least5 or 10 timesthatoftheSun,andcurrentestimates

putthestar’smassat roughly 10 to 20 solarmasses.Having

luminosityandmassonlyballparkedfurtherlimitsour

chancesofguessingwhenBetelgeusewillripitselfapart.

Rotation

Anothercomplicatingfactorconcernsthespeedof

AlphaOri’srotation.In1995,AndreaDupree(Harvard-

SmithsonianCenterforAstrophysics,orCfA)andRonald

Gilliland(thenSpaceTelescopeScienceInstitute)used

theHubbleSpaceTelescope’sFaintObjectCameratotake

anultravioletpictureofBetelgeuse.It wasthefirstdirect

imageofa stellarsurfaceotherthantheSun’s.Inthatstudy

anda lateronewithHanUitenbroek(thenCfA),thepair

determinedthatBetelgeuse’srotationalvelocitywasabout

15 km/s.That’sextraordinarilyhigh,evenclosetobreak-up

speed,Wheelersays.YetrecentobservationswithALMA,the

AtacamaLargeMillimeter/submillimeterArray,appearto

confirmthatrotationalspeed.

Wheelertriedtomatchthatvelocityinmodelshecreated

witha groupofhisTexasundergraduates.Butheand

hisstudentscouldcomputea velocityonly^1 / 50 ofwhat

Dupree’steamhadfound.Theonlywaytheycould

getto 15 km/sintheirmodelswasby‘inventing’a

companionstarthatBetelgeuseswallowedearlier

initslifetime.If thecompanionwasaboutone

solarmass,beinggobbledwouldprovidejustthe

rightamountofangularmomentum,theteam

discovered,tospinupthestarto 15 km/s.

ButwhetherBetelgeuseatea staris

speculative,andthosegianteddiesregularly

wellingtothesurfacefurthermuddythe

picture.Spectrarevealthosesupergranules,

asastronomerscallsuchmassiveconvection

cells,havetheirownvelocityassociated

withthem.“Youcanseea blueshiftinthe

upwellingstuffanda redshiftinthedown-

wellingstuff,”Wheelersays,indicating

http://www.skyandtelescope.com.au 59

Betelgeuse

D= 887

Polaris

D= 45

Proxima

Centauri

D=0.15

Alpha

CentauriB

D=0.86

Alpha

CentauriA

D=1.2

Sun

D= 1

TERRI DUBÉ /

S&T

q

uOUR STELLAR CAST This diagram

shows the sizes of several famous

stars, each of them shown to scale.

Betelgeuse’s size is represented by the

large orange curve. Listed diameters

(‘D’) are approximate and given as

multiples of the Sun’s diameter. We’ve

included the Sun for reference.

“Most likely

it’s in helium

core-burning

and we still

have, I’m

sorry to say,

100,000 years

to go.”