My first Magazine Sky & Telescope - 02.2019 | Page 51

O
Cepheid in our galaxy, so it’s also one of
the most luminous Cepheids discovered.
Although Cepheids are famous
for their utility in measuring galactic
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
the distance to RS Pup. In 1961, Bengt
Westerlund (Uppsala Southern Sta-
tion) discovered that RS Pup is embed-
ded in a refl ection nebula, parts of
which exhibit a variation in brightness
related to the star’s pulsation period. In
2008, astronomers using the European
Southern Observatory’s New Technol-
ogy Telescope geometrically analyzed
these “light echoes” in the nebula and
determined the distance to RS Pup to
be close to 6,500 light-years. Geometric
analyses based on Hubble Space Tele-
scope data shaved about 300 light-years
off that number in 2014. A geometric
parallax measurement based on the
Gaia Data Release 2 dropped the dis-
tance to about 5,600 light-years.
It’s fun to think about RS Pup “get-
ting closer,” but its distance doesn’t
make any practical difference for back-
yard 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 January, RS Pup is a late riser,
transiting around 2:00 a.m. local stan-
dard time on New Year’s morning. In
mid-January, it shines about 15° above
the southern horizon as it culminates
an hour after midnight. Observing
conditions improve as February begins,
with the star gaining another degree of
altitude before it transits near mid-
night. At mid-month, RS Pup’s altitude
has dropped 1°, but culmination occurs
near 11 p.m. By the end of February, RS
Pup transits at a convenient 10 p.m.,
but you’ll probably be scraping the edges
of your observing zone as it stands only
12° at its highest.
If observing variable stars is just
your thing, consider participating in an
observing campaign led by the Ameri-
can Association of Variable Star Observ-
ers (AAVSO); visit aavso.org/observers
for more information.
FURTHER READING: To watch a time-
lapse of the light echoes around RS
Puppis as captured by the Hubble Space
Telescope during a 5-week observation
period, visit spacetelescope.org/videos/
heic1323a.
w
_
–34°
RS
`
magnitudes
n these midwinter nights, the
constellation Puppis wheels slowly
across the southern horizon. Divorced
from the larger constellation Argo Navis
by the pen of Nicolas-Louis de Lacaille
during his voyage of 1750–1754, Puppis
represents the ship’s stern, fl oating aft
of Canis Major. Puppis remains some-
what unfamiliar to many northern
skywatchers: Our scopes seem to stop
at Canis Major, possibly because the
brightest stars in Puppis — Zeta (ζ),
Rho (ρ), and Pi (π) — are no match
for Sirius. But Puppis does hold some
intriguing stellar sights, namely, the
variable star RS Puppis.
RS Puppis is easy to fi nd and remains
relatively bright throughout its period of
variability. A line extended from Delta
(δ) CMa through the tip of the 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 from the star to
the open cluster NGC 2546. From this
scattering of stars continue about 2°
north-northeast to fi nd a delicate stellar
arc that curves northward to RS Pup.
RS Pup’s variability was detected in
1897 by Ms. Reitsma, an observatory
assistant at the University of Gronin-
gen, during her examination of Cape
Photographic Durchmusterung (CPD)
plates from 1888 and 1890. The star’s
variability was confi rmed visually in
1897–1898 by Alexander W. Roberts,
observing from Lovedale Observatory,
South Africa, and R. T. A. Innes, observ-
ing from the Royal Observatory, Cape
of Good Hope. From this, David Gill,
His Majesty’s Astronomer at the Cape
of Good Hope, concluded that the star’s
brightness varied from 6.8 to 7.9. His
numbers are very close to today’s esti-
mations: Over a period of 41.3 days, RS
Puppis dims gradually to magnitude 7.67
before rapidly rising to magnitude 6.52.
The shape of RS Pup’s light curve
reveals that it’s a Cepheid variable;
the relationship between its luminos-
ity (intrinsic brightness) and period of
variability is directly proportional. The
longer a Cepheid’s period, the more
luminous it is. RS Pup turns out to
have one of the longest periods of any
PYXIS
–36°
q
3
4
5
6
7
8
AT
2546
–38°
AS
PUPPIS
c
–40°
AP
GO
8 h 40 m
8 h 30 m
8 h 20 m
8 h 10 m
8 h 00 m
p The variable star RS Puppis lies 5¾º north-northeast of the 2.3-magnitude star Zeta Puppis.
sk yandtele scope.com • FE B RUA RY 2 019
49