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described it as “a cluster of scattered
stars, pretty close and nearly of a size,
the densest part of it about 15′ diam.,
but the rest very extensive”.
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. 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
subgiant branch. Spectral data obtained
with the Fibre-fed Extended RangeOptical Spectrograph at the European
Southern Observatory’s 2.2-metre
telescope at La Silla reveal these three as
yellow stragglers, red giants whose colour
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 thanks to ‘contamination’
by its secondary. A fourth red giant,
cluster star #25, is also a suspected
binary, but since its colour 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 12.5- or 15-cm 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 autumn observing list.M93Ru 32Ru 36Haffner 16gkj
PUPPIS7 h 50 m 7 h 45 m 7 h 40 m3 4 5 6 7 8 9Star magnitudes–24°–23°–25°–26°–27°XCharles Messier included M93 in the fourth
edition of his ‘Catalogue of Nebulae and Star
Clusters,’ published in 1781.