Once a visitor sees M13, call their attention to the Milky Way and explain that the arc it sweeps out approximates the plane of our galaxy. Once they grasp that concept, point to the scope and say, “Notice how it is pointing with respect to the Milky Way plane?” Then I ask, “Does M13 lie above the plane of our galaxy?” They usually see the geometric relationship; suggest that on their way home they ponder whether M13 lies “above” or “below” our galaxy.
Globular clusters actually abound around galaxies. Of the 110 mostly northern hemisphere Messier objects, 29 are globular clusters in our galaxy. While M13 is the most observed, I have included a sampling of other showpiece clusters that can occupy much rewarding eyepiece time. If you want to start astrophotography, an image of M13 can be obtained with ISO 400 film in about 3 minutes. Other clusters require somewhat longer exposures.
Globular Versus Galactic Clusters
A globular cluster aptly represented by M13 consists of hundreds of thousands of stars so that each cubic light year contains one or two stars. If a cluster is only 20 light years in radius that means it’s an enormously massive object.
In contrast, the galactic cluster represented by M29 is a grouping of stars—a “knot” of stars in the stellar backdrop. Indeed, the stars may constitute a “local system” and be traveling together, but they are not the tightly bound massive objects that globular clusters represent. Read the paragraph for M2 and consider its enormous size; you can approximate its mass by simply using the mass of our Sun as typical.
One major difference between globular and galactic clusters is easily seen in the telescope. Compare a galactic cluster, M29, to a globular cluster, M13, in the typical eight-inch Schmidt Cassegrain Telescope (SCT). Start observing each at low power, say about eight per inch of aperture; then increase magnification in increments up to about 25 times per inch of aperture. At low power (eight times per inch), M29 is a spectacular knot of stars and, in contrast, M13 is a hazy patch which is undefined. At high power (25 times per inch), M29 has lost its cluster identity while M13 has gained identity as a spectacular globe of tightly packed stars.
Clusters Are Old
Clusters usually are considered “old” as stars go since they formed early in our galaxy’s history. So, they are generally “metal poor.” Hence, cluster stars don’t exhibit the extensive periodic table of elements characteristic of our Sun and its planets. Gravitational lensing used to search for planets in clusters—where there are lots of stars and the chances of lensing events is probable—indicates that planets are scarce if not absent. Since they formed early, long before there was abundant dust from which a “sun nebula” could form, the absence of planets could be typical of clusters.
However, in 1993, astronomers observed that new globular clusters form in dust-rich colliding galaxies. Consequently, in some clusters that belong to galaxies that collided long ago and far away, stars similar to our Sun might abound. Do they have planets? If so, imagine how the nighttime sky would appear to any inhabitants. Millions of years in the future, when our galaxy and the Andromeda galaxy (M31) collide, clusters that form will be different than those that now exist.
A Sampling of Globular Clusters
This is by no means a complete listing; however, it provides an amateur, or even casual observer with a nice sampling that illustrates how each globular is different.
M2:
Globular Cluster in AquariusM2 is about 50,000 light years away and is easily resolved in an eight-inch SCT. Consequently, its 12 arc minutes translates to 175 light years in diameter. Assume there’s one star in every cubic light year, and since the volume of a sphere is 4pr3, that calculates to nearly 3 million stars! Suppose there are two stars in every LY3! Now you know why a globular is a massive object.
M3:
Globular Cluster in Canes Venatici
M15:
Globular Cluster in Pegasus
M56:
Globular Cluster in Lyra
M71:
Globular Cluster in Sagitta
M92:
Globular Cluster in Hercules
M5: Globular Cluster in Canes
Venatici
M30:
Globular Cluster in Capricorn