188 ASTRONOMYANDTHEBIBLE
actualdifference in thestarsthemselves whenplaced
atequaldistances fromus. Menhada readyexpla-
nation forthis. Itshowed,theysaid,a differencein
size. Assumingthis,thediameter ofSirius maybe
held to exceedthat ofour sun intheproportion of
aboutfourteentoone,ortohaveadiameterofnearly
twelvemillion miles,andavolume twothousandsix
hundredandeighty-eighttimesasgreatasthesun's.
Butisthissecondexplanation,joinedwiththefirst,
abletoaccountforthedifferenceinstarluster?—No
;thereisstillanotherfactor. Astronomershavefound
thatthereisonemorecharacteristicofthestarswhich
theymustnotoverlookiftheywouldbewiththefacts.
"Thedifferencesofbrightnessaredue,first,todif-
ferenceof distance;second, to differenceof dimen-
sions,oroflight-givingarea;third,todifferenceinthe
brilliance ofthelight-givingsurface,dependingupon
differenceoftemperatureandconstitution....
"AsBesselputsit,thereisnoreasonwhytheremay
notbe'asmanydarkstarsasbrightones.'... The
companion of Sirius, though only giving about one
twelve-thousandthpartasmuchlightas Siriusitself,
isatleastonetenth partasheavy;so that,massfor
mass,itcannot be oneone-thousandth part as lumi-
nous."
InthecaseofSiriusanditscompanion,wehavetwo
starsequallydistant,ornearly so, fromus. Butby
investigationregardingtheperturbationsintheirorbits
throughtheir mutual attraction,which is directlyas
themass,ithasbeenascertainedthat Siriusis about
tentimestheheavier;andcarefulmeasureofthelight