Sky & Telescope - March 2019 - 49

PUBLIC DO M AIN

7h 50m

7h 45m

7h 40m

-23°

M93

-24°

j
-25°

PUPPIS

Star magnitudes

A more imaginative take on the cluster had to wait for the pen of Admiral
William Henry Smyth. In the second
volume of A Cycle of Celestial Objects,
now known by amateur astronomers as
"The Bedford Catalogue" after Smyth's
town of residence in the United Kingdom, he directed readers to imagine two
rays, one shooting from Castor through
Procyon, another from Orion's sword
through Sirius. The cluster, "a neat
group . . . of a star-ﬁsh shape" shines at
the intersection of the two rays.
It's an oddly shaped starﬁsh, though.
Through 10×50 binoculars, M93 looks
more like a fat squid or a jellyﬁsh
ﬂoating across the sky, its body angled
northeast-southwest. More magniﬁcation strengthens the illusion. Tentacles ﬁll out with fainter stars, and a
8.2-magnitude orange-yellow star, HD
62679, caps the body.
Smyth mocked a fellow astronomer
who mistook M93 for a comet, but once
you've imagined the cluster as a sea
creature with two arms, it's not difﬁcult
to see how this could happen.
The stars comprising M93 formed
around the same time from the same
molecular cloud, so they're the same
composition, age (approximately 400
million years old), and distance from
Earth (about 3,600 light-years). As a
star cluster ages, more and more of its
higher-mass stars move off the main
sequence, evolving into red giants.
With an aged cluster, we'd expect a
Hertzsprung-Russell (H-R) diagram
to show fewer stars at the top of the
main sequence, with a large group of
red giants clumped at the top right (see
page 18). This holds true for M93 for
the most part, as the majority of the
cluster's 16 known red giants sit where
they're expected to on the diagram. But
three cluster stars, #26, #38, and #42,
aren't in their "proper" place; rather,
they've moved toward the Hertzsprung
gap, the less populated area of the diagram between the main-sequence and
red giant branches and above the subgi-

3
4
5
6
7
8
9

Ru 32
Haffner 16

-26°

t Follow William Henry Smyth's directions to
find the general location of M93: ". . . a line
from Orion's sword-cluster, led through Sirius,
strikes upon it 16° beyond, where it will be
intersected by a ray from Castor through Procyon." M93 is 1½° northwest of Xi Puppis.

Ru 36

g
-27°

ant branch. Spectral data obtained with
the Fibre-fed Extended Range Optical
Spectrograph at the European Southern Observatory's 2.2-meter telescope
at La Silla reveal these three as yellow
stragglers, red giants whose color may
be affected by the presence of a bluer
main-sequence companion. Instead of

hanging out with their buddies on the
right side of the H-R diagram, these
binary systems appear more centrally
located because the primary appears
more yellow in color thanks to "contamination" by its secondary. A fourth
red giant, cluster star #25, is also a suspected binary, but since its color hasn't
shifted toward the yellow, astronomers
think its companion is probably a very
a faint main-sequence star. Successful
detection of straggler stars can help
astronomers check their predictive
simulations on binary stellar evolution
in clusters.
While professional astronomers study
these straggler stars spectroscopically,
amateur visual observers can follow in
the footsteps of Messier, the Herschels,
and Smyth. A 5- or 6-inch reﬂector at
low magniﬁcations will easily provide
enough separation in the cluster stars
for you to detect #25, #26, #38, and
#42. Whether you see a starﬁsh or a
squid, M93 is a worthy object to include
on your springtime observing list.

u Charles Messier included M93 in the fourth
edition of his "Catalogue of Nebulae and Star
Clusters," published in 1781.
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Sky & Telescope - March 2019

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