50 AUSTRALIAN SKY & TELESCOPE February | March 2019
OBSERVING by S. N. Johnson-RoehrO
n these summer nights, Puppis
wheels high above the southern
horizon. Divorced from the
larger constellation Argo Navis by
thepenofNicolas-LouisdeLacaille
duringhisvoyageof1750–1754,Puppis
representstheship’sstern,floatingaft
ofCanisMajor.Theconstellationholds
some intriguing stellar sights, one of
them being the variable starRS Puppis.
RS Puppis is easy to find and remains
relatively bright throughout its period
ofvariability.Alineextendedfrom
Delta (δ)CMathroughthetipofthe
Big Dog’s tail, Eta (η)CMa,across
Puppis leads to the star’s general
location. Begin your star-hop from
Zeta Pup, moving 3° north-northeast
fromthestartotheopenclusterNGC
2546.Fromthisscatteringofstars
continueabout2°north-northeastto
findadelicatestellararcthatcurves
northward to RS Pup.
RSPup’svariabilitywasdetectedin
1897byMs.Reitsma,anobservatory
assistantattheUniversityof
Groningen, during her examination of
Cape Photographic Durchmusterung
(CPD)platesfrom1888and1890.The
star’s variability was confirmed visually
in1897–1898byAlexanderW.Roberts,
observing from Lovedale Observatory,
SouthAfrica,andR.T.A.Innes,
observingfromtheRoyalObservatory,
Cape of Good Hope. From this, David
Gill,HisMajesty’sAstronomeratthe
Cape of Good Hope, concluded that
thestar’sbrightnessvariedfrom6.8
to7.9.Hisnumbersareverycloseto
today’sestimations:Overaperiodof
41.3days,RSPuppisdimsgraduallyto
magnitude7.67beforerapidlyrisingto
magnitude 6.52.
TheshapeofRSPup’slightcurve
revealsthatit’saCepheidvariable;the
relationship between its luminosity
(intrinsic brightness) and period ofvariability is directly proportional.
The longer a Cepheid’s period, the
moreluminousitis.RSPupturnsout
to have one of the longest periods of
any Cepheid in our galaxy, so it’s also
oneofthemostluminousCepheids
discovered.
Although Cepheids are famous
fortheirutilityinmeasuringgalactic
distances (using the period-luminosity
relation, astronomers can determine
a Cepheid’s absolute magnitude, and
from that, along with its apparent
magnitude, calculate the distance),
there remains some uncertainty about
thedistancetoRSPup.In1961,Bengt
Westerlund (Uppsala Southern Station)
discovered that the star is embedded
in a reflection nebula, parts of which
exhibitavariationinbrightness
relatedtothestar’spulsationperiod.
In 2008, astronomers using the
European Southern Observatory’s New
Technolog y Telescope geomet rically
analysed these ‘light echoes’ in the–34°–36°–38°–40°
8 h 40 m 8 h 30 m 8 h 20 m 8 h 10 m 8 h 00 mc_`PUPPISPYXISAPASATRSGOqw2546Star magnitudes435
6
7
8SThe variable star RS Puppis lies 5¾º north-northeast of the 2.3-magnitude star Zeta Puppis.nebula and determined the distance
to be close to 6,500 light-years.
Geometric analyses based on Hubble
Space Telescope data shaved about 300
light-years off that number in 2014. A
geometric parallax measurement based
on the Gaia Data Release 2 dropped the
distance to about 5,600 light-years.
It’s fun to think about RS Pup
‘getting closer,’ but its distance doesn’t
make any practical difference for
backyard observing. But because the
star’s period is long, it’s useful to watch
it over the course of a couple months.
This is an easier task for those who live
in the Southern Hemisphere, but it’s
not out of the question for observers in
the north. In February, the star transits
close to midnight.
If observing variable stars is just
your thing, consider participating
in an observing campaign led by the
American Association of Variable Star
Observers (AAVSO); visit aavso.org/
observers for more information.Echoes from a variable star
RS Puppis continues to delight astronomers more than 120 years after its true nature was discovered.