Australian Sky & Telescope - 04.2019

Covered with spots

Thestarspothypothesismayseem

strangetomodernminds.Howcould

alargeandapparentlypermanent

spot cause the brightness to drop for

sevenhoursduringaperiodofalmost

threedays?Yetatthetimetherewas

considerable support for this idea. No

lessafigurethanIsaacNewtonhad

written, in Book III ofPrincipia(1687):

Astothosefixedstarsthatappear

and disappear by turns, and increase

slowlyandbydegrees,andscarcelyeven

exceed the stars of the third magnitude,

they seem to be of another kind, which

revolve about their axes, and, having a

lightandadarkside,shewthosetwo

different sides by turns.

Modern observers will recognise

theparticularpatternofbrightness

variation shown by Algol — nearly

constant brightness except for a

sharpdecreaseforafewhours—as

characteristicofadetachedeclipsing

binary system. While astronomers of

Goodricke’s time realised that binary

starsweretheoreticallypossibleand

quitelikely,nosystemshadyetbeen

showntobetruebinaries.Sunspots,

however, were a known phenomenon,

and established astronomers favoured

thestarspotexplanation.

In August 1783 the Royal Society

awarded the Copley Medal, its

highest honour, to John Goodricke

“forhisdiscoveryofthePeriodofthe

VariationofLightintheStarAlgol.”

Todaythisworkandlaterdiscoveries

arerecognisedasacollaborative

effortbetweenGoodrickeandPigott.

Thetwoobserverswereespecially

productive in the autumn of

1784,whentheydiscoveredthree

newvariablestars.InSeptember,

Goodricke noted that Beta Lyrae,

nowrecognisedasacontactbinary,

appeared to vary in brightness. On

thesamenight,Pigottsawachange

inthestarknowntodayasEta

Aquilae, later classified a Cepheid

variable. Goodricke noted the

variabilityofthenamesakeCepheid,

Delta Cephei, in mid-October. In

Pigott’s paper on the variability of

Eta Aquilae, he addressed possible

causes for the variation:

Hitherto the opinion of astronomers

concerning the changes of Algol’s light

seem to be very unsettled... though

various are the hypotheses to account

for it; such, as supposing the star of

some other than a spherical form,

or a large body revolving round it,

or with several dark spots or small

bright ones on its surface, also giving

an inclinations to its axis etc.; though

most of these conjectures with regard to

Algol be attended with difficulties, some

of them combined do, I think account

for the variation of Eta [Aquilae].

John Goodricke visited London in

1785, where he met the Astronomer

Royal, Nevil Maskelyne. In July of that

year, Maskelyne wrote Goodricke,

asking, “Did you ever doubt whether

some hemispheres of our Sun

(exclusive of the variable spots)

may not be brighter than others?”

Although the phrasing is odd, it is

clear that Maskelyne was attempting

to persuade his young colleague to

accept the starspot hypothesis.

One more observational detail

nudged Goodricke toward the spot

hypothesis and away from eclipses.

In his paper describing the variation

of Delta Cephei, Goodricke wrote

that “Even Algol does not seem to be

always obscured in the same degree,

being perceived to be sometimes

a little brighter than Rho Persei,

and sometimes less than it.” Rho,

a nearby star in Medusa’s head, is

a convenient comparison star for

a naked-eye observer as its average

visual magnitude is 3.4, the same as

Algol’s in eclipse. But here Goodricke

apparently fell prey to an error well

known to modern photometrists:

Rho is a semiregular variable

whose brightness ranges from

3.3 to 4.0! The eclipse hypothesis

depended upon Algol’s brightness

staying constant between eclipses

0

5.0

4.5

4.0

3.5

3.0

510

Days

Visual magnitude

15 20

0

5.0

4.5

4.0

3.5

3.0

510

Days

Visual magnitude

SA NEW VARIABLEGoodricke

detected Beta Lyrae’s variability in

September1784.ThelightcycleofBeta

isneverconstantbecausethestarisa

contact, or semi-detached, binary where

material transfers at a rapid pace from

thegaseousenvelopeofamassivestar

ontoalowermassstar.Beta’sapparent

visual magnitude peaks around 3.2. The

lightcurveshowsminimaatdifferent

depths,withthedimmestaslowas4.4.

SDISCOVERING DELTAGoodricke

noted the variability of Delta Cephei in

October1784.Delta’sbrightnesscycles

from3.5to4.4andbackupagainover

thecourseof5.4days.

0

3.5

2.0

2.5

3.5

123456

Days

Visual magnitude

STHE GORGON’S EYE Algol (Beta

Persei) dips from magnitude 2.1 to 3.4

and back every 2.7 days.

Algol

Delta Cephei

Beta Lyrae

24 AUSTRALIAN SKY & TELESCOPE April 2019

INCONSTANT STAR