Australian Sky & Telescope - 02.2019 - 03.2019

20 AUSTRALIAN SKY & TELESCOPE February | March 2019

browndwarfs.Thus,whileatleasthalfofthecensusstarsare

in multiple systems, most systems are not multiples.

Thelistofourstellarneighboursillustratesperfectlythe

initial mass function(IMF),whichdescribesthefrequency

withwhichstarsofdifferentmassesform.TheIMFpredicts

thatthemostmassivestarsaremuchrarerthanlower-mass

stars.Itthereforecomesasnosurprisetofindthatmostof

our neighbours are fairly modest stars, with our Sun actually

standingoutasoneofthemoreimpressive.

Within10parsecs(pc)therearenoO-orB-type stars,

whicharethehottest,brightestandmostmassivestarsonthe

main sequence. The next class down is theA-typestars,with

surfacetemperaturesbetween7600and11,500kelvin.There

arefourofthesenearby,allofwhichamateurastronomers

knowwell:Altair,Sirius,VegaandFomalhaut.

Next are theF-typestars,alittlecoolerthantheA-types

butalittlewarmerthantheSun,andtherearesevenof

these, including Procyon in Canis Minor. The Sun belongs

to theG-type stars and, again, there are more of these than

theF-types,includingourstar,AlphaCentauriA,TauCeti

and16others.CoolerthantheSunaretheK-typestars;these

outnumber all theA-,F-, andG-typestarsputtogether.They

includeAlphaCentauriB,bothmembersof61Cygni,Epsilon

Eridani and 39 others.

However,themostintriguingfindingoverthelast24years

ofRECONSistheubiquityofM-typestars,oftenreferredto

asMdwarfsorreddwarfs.Thesearethe

smallest,coolestandfainteststars,with

asurfacetemperaturelessthan4000K

andmassesfromhalfasolarmass

down to just 7.5% of the Sun’s mass.

When RECONS began, it was thought

thatMdwarfs might account for half

ofallthestarsinthegalaxy.However,

thanks in part to the work RECONS

hasdone,wenowknowthattheyare

evenmorecommon,makingupthree-

quarters of all stars.

“Threeoutoffourisanawfullot

ofstars,”saysHenry.“That’sabitofa

surprise.”

There are also 21 ‘dead’ stars, the

cooling stellar cores named white

dwarfs,withtheclosestbeingSiriusB,

8.6 light-years away.

Ashortageoffailedstars

Theotherkeyresultrelatestobrown

dwarfs. Smaller than red dwarfs but

larger than gas giant planets, brown

dwarfs are the awkward in-betweeners,

notquitemassiveenoughtogenerate

the required temperatures and pressures

to ignite the nuclear fusion of hydrogen

within their cores.

Sincediscoveringthefirstofthese

failed stars in the 1990s, astronomers

have suspected that brown dwarfs form

the same way that stars do, condensing

fromfragmentingcloudsofmolecular

gas. In that case, one might expect them

tofollowtheIMFtrendandbefound

even more frequently than red dwarfs.

Butthatbeliefwasnotdata-driven,

Henryexplains.“Intheearlydays

people would get very excited about

browndwarfsandsaytherearemore

browndwarfsthantherearestars,”he

says. “And I thought, ‘Based on what?’”

Henry’sskepticismhassincebeen

borneout.TheRECONSdatashowthat

thereare8timesmorestarswithin

10 pc than there are brown dwarfs.

Yetthereappearstobeadisparity

between the number of brown dwarfs

in the local neighbourhood and those

fartherafield.Astronomersarenow

routinely detecting them in young star

clusters,withtheSubstellarObjects

in Nearby Young Clusters (SONYC)

1:2 to 1:

SONYC star-forming

regions

1:

RECONS solar

neighbourhood

Ratio of brown dwarfs to stars

VEGA:

AKIRA FUJII; MULTIPLICITY AND RATIO INFOGRAPHICS: TERRI DUBÉ /

S&T

; NGC 1333: X-RAY: NASA / CXC / SAO / S. WOLK ET AL., OPTICAL: DSS & NOAO / AURA / NSF, INFRARED: NASA / JPL-CALTECH; SIRIUS:

AKIRA FUJII

VEGATheA-type

star Alpha Lyrae lies

25 light-years away.

73 %

21 %

4 %

1 %

1 %

66 systems with

14 systems with

3 systems with

2 systems with

232 systems with

THE NEAREST STARS