The stunning Orion Nebula - The Royal Astronomical Society of ...

On Another Wavelength
M56 – A Globular Cluster in Lyra
by David Garner, Kitchener-Waterloo Centre
(jusloe1@wightman.ca )
Lyra is a favourite constellation to be
hunting around at this time of the year.
Not only does it have several variable stars,
including the well-known RR Lyr, it has: Vega (α Lyr), the
fifth-brightest star in the sky and one corner of the Summer
Triangle; M57, the Ring Nebula; and the “Double Double”
(ε Lyr)—a pair of bright double stars. Most of us have seen
these before, so something a little different might be in order:
try looking about 8 degrees south of α Lyr for a small globular
cluster known as M56 (see Figure 1).
Globular clusters are a spherical collection of stars bound
together by gravity (Figure 2), and are frequently found in the
halo of our galaxy orbiting the galactic core. In 1917, Harlow
Shapley studied RR Lyrae variable stars contained within
these clusters, as they are often used as standard candles to
measure galactic distances. He noticed that these clusters
centred on Sagittarius and outlined the shape and size of the
Milky Way.
Figure 1 — A map of the constellation Lyra.
Figure 2 — M56, the Globular Cluster in Lyra, courtesy of Ron Brecher,
Kitchener-Waterloo Centre. Ron acquired this image using a QSI583wsg
camera, Astrodon RGB Gen-2 filters, and an 8-inch f/8 RC on a MI-250
mount, from my SkyShed in Guelph, Ontario.
M56 (NGC 6779) is a ball of a few hundred thousand stars
that is approximately 85 light-years across (which corresponds
to a linear dimension of 8.8 arcmin) and 32,900 light-years
distant. Lacking a bright core, it is somewhat dim, with an
apparent brightness of magnitude 8.3. It was first discovered
by Charles Messier in 1779 and described as a “nebula
without stars, having little light.” Later, in 1784, it was resolved
into stars by Sir William Herschel. The NGC lists the stars
between 11th and 14th magnitude.
M56 was described by Harlow Shapley as an elongated cluster,
and he found only one variable star there. Today, approximately
a dozen variables have been identified in M56. In 2008, the
CURiuos Variables Experiment (CURVE) detected several
new variables in the cluster, one of which is an RR Lyrae star,
the third star of that type found in M56.
M56 does not have nearly as many variables as the globular
cluster M15 (as described in a previous article) and it is
somewhat difficult to see with a small telescope, but is
certainly worth a try. You can find it at RA 19 h 16 m 35 s and
Dec +30° 11′ 4″. V
Dave Garner teaches astronomy at Conestoga College in Kitchener,
Ontario, and is a Past President of the K-W Centre of the RASC.
He enjoys observing both deep-sky and Solar System objects, and
especially trying to understand their inner workings.
RASC members receiving this Journal
in electronic format are hereby
granted permission to make a single
paper copy for their personal use.
October / octobre 2011 JRASC | Promoting Astronomy in Canada
209