Sky & Telescope - May 2019 - 55

SS Vir

Virgo
η

δ
γ Vir
γ
Crater

V Hya

p Left: Though located in the constellation Hydra, the variable carbon star V Hydrae may be easier to find by star-hopping from Crater, the Cup. Find
it 3½º south-southwest of Alpha Crateris and about 5º west-northwest from Beta Crateris. Right: The semi-regular variable star SS Virginis is an easy
star-hop from Eta Virginis. SS Vir reached maximum in March and is now slowly dimming and reddening. The green light shining south of SS Vir in this
2009 image is the non-periodic Comet Lulin (C/2007 N3), discovered by Ye Quanzhi and Lin Chi-Sheng from Lulin Observatory.

V H Y DR A E: TILL CREDNER; SS VIR: JEREM Y PERE Z;
RY DR ACONIS: A K IR A FUJII; T LY R A E: A K IR A FUJII

Not quite as red but better placed in
the sky, SS Virginis gives us a longer
window of opportunity. This semi-regular variable has a color index of +4.2
and a period of 361 days. The rise from
minimum to maximum light takes
up 53% of the cycle. Individual cycles
have an amplitude of about 2.2 to 2.6
magnitudes, with some maxima reaching magnitude 6.0 and some minima
dipping as low as magnitude 9.6. SS Vir
reached maximum in mid-March, and
you'll be able to watch the star fade and
redden through the summer months for
as long as it graces your evening sky.
Conveniently located about halfway
between 8 Draconis and the red giant
9 Draconis, RY Draconis is a semiregular variable with a color index of
+3.3. A 2016 paper by John Percy and
Emily Deibert in The
Journal of the American
Association of Variable Star Observers
indicates a pulsation

period of approximately 277 days and a
long secondary period of 1136 days or
more. No matter when you see it in its
cycle, RY Dra is a cinch to spot because
its visual-magnitude range is 5.9 to 8.0.
The bracketing stars 8 and 9 Dra shine
at magnitudes 5.2 and 5.4, respectively.
If you miss V Hydrae, you'll have
another chance to spot a deeply red
star. T Lyrae also has a color index of
+5.5 and will be with us throughout the
summer and much of the fall. According to the International Variable Star
Index (aavso.org/vsx), this irregular
variable shines anywhere from a visual
magnitude of 7.5 to 9.2, so it's another
carbon star that's easy to observe anytime it's well placed in your sky.
Take the color indices of extremely
red stars with a grain of salt. They are
Draco
RY Dra
8

κ
7

Lyra
ε

u Left: RY Draconis lies
near the Dragon's tail,
between 8 and 9 Draconis.
The line of stars defined
by Kappa, 4 Dra, and 6
Dra serves as a convenient
starting point for a starhop. Right: The ruby-red
carbon star T Lyrae forms
a triangle with brilliant
white Vega and 4.4-magnitude Zeta Lyrae.

well beyond the accurately determined
colors of the standard stars that are the
basis for calibration. The far blue end of
the color scale is also problematic.
The blue of our celestial sapphires
isn't as easy to discern as the red of
carbon stars. The negative range of a
color index is quite small. In fact, for a
theoretical star of inﬁnite temperature,
the color index would only be −0.46,
not very far removed from a star with
a color index of +0.2, which appears
essentially white to the eye. Because of
this, we gather no deep blue sapphires,
just pale ones.
Main-sequence stars (dwarf stars,
designated by the letter V) reach their
visually bluest at spectral types near
O9V. Stars hotter than this release
their additional output in the invisible

α Vega

δ
Ursa
Major

T Lyr

s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 9

http://www.aavso.org/vsx http://www.skyandtelescope.com

Sky & Telescope - May 2019

Table of Contents for the Digital Edition of Sky & Telescope - May 2019