Spring evenings are usually reserved for galaxy hunting. With the winter Milky Way setting in the west and the summer Milky Way just coming into view in the east, the domain of the open clusters is poorly placed for their observation. Yet there is a handful of interesting clusters located well away from the band of the Milky Way that exhibits a wide range of properties and distances.
A glance at almost any star atlas will reveal that the majority of open star clusters are found in and near the band of the Milky Way. Indeed, more than 90 percent of those known lie no farther than 10 degrees from the galactic equator. There are two reasons for this.
Open clusters form within the spiral arm system, where the molecular gas clouds are mostly confined to a thin disc. These clouds tend to revolve about the galaxy in nearly circular orbits and with very little vertical motion. Initially, stars and clusters born in these clouds have a similar orbital motion and hence distribution. The scale height of the open clusters as a system — where the density distribution decreases by a factor of 2.72 — is about 180 light-years. This tells us that few clusters are farther than 500 light-years or so from the midplane.
Contributing to this is the fact that most open clusters are not particularly robust. The gravitational potential of the galaxy tidally strips away stars, and encounters with massive molecular clouds do their part in this process. The average lifetime of an open cluster is not much longer than 300 million years, with perhaps only 20 percent surviving beyond 500 million years (some two circuits about the galaxy). Most of the noncluster field stars must have once been members of clusters or associations.
The longest-surviving clusters are those which contain many stars, the total mass and sheer numbers delaying dissolution. If the cluster has a component of vertical motion that carries it well above and below the gaseous disc, there are fewer opportunities for the massive clouds to do their work in helping to tidally strip away stars. And if the cluster is far from the galactic centre, the tidal force induced by the galaxy is weaker.
The clusters that can survive for many orbits about the galaxy tend to have their orbits ever more perturbed by the massive molecular clouds, becoming more eccentric and having greater vertical motion. And so we find that the older the cluster, the farther from the disc it is. The oldest open clusters could well have formed initially from clouds in decidedly out-of-plane orbits. About one-third the age of the ancient globular clusters, these objects could be regarded as a kind of transition of sorts between the two extremes. In any case, our small sample here supports the trend of increasing height above the midplane with age.
The charts shown here plot one cluster that has already been disrupted, another that is not far from dissolution, three robust clusters and a very young association that hosts a cluster being freshly minted inside a dense cloud. We will examine each in this order, which is also the order of both increasing age and distance from the disc midplane (with the exception of the young association). The labels show the name; distance (d) from Earth in light-years; distance (h) above the galaxy’s midplane in light-years; and age in millions of years (Myr) or billions of years (Gyr). Also shown are the North Galactic Pole (NGP), the galactic equator and galactic latitude +20 degrees.
The nearest of all clusters is the Ursa Major stream, so termed because it has recently become fully disrupted. The core group is the five middle Dipper stars and twice as many fainter members. The “halo”’ has dozens of stars scattered over the entire celestial sphere, with us passing through it.
The third nearest cluster (the Hyades being second) is the Coma Berenices star cluster, Melotte 111. It lies almost exactly “overhead” at the North Galactic Pole. Its lower-mass stars appear to have been completely stripped away, and those remaining do not have the combined mass to keep the group together for very much longer.
The seventh nearest cluster is M44, the Beehive. With about 1,000 members, it has some time yet before it disappears from the scene. It has already completed a bit more than one orbit. Interestingly, its similar age and space motion suggest a relationship with the Hyades. Sufficient time has passed for former B-type stars to have evolved to white dwarfs.
We now make a big jump in distance to M67, almost in the line of sight with M44. From the determined mass, the member count could approach 3,000, with a significant number of white dwarfs. As it happens, the cluster is almost at its farthest distance from the galactic plane. The farthest and faintest cluster in our sample is NGC188. Under a dark sky, a 10×50 binocular will reveal a faint smudge, but a sizable telescope is required to detect individual stars out of the estimated total of more than 2,000.
Our last system is very different, in that it’s an unbound association and of considerable youth. Embedded within the associated very opaque rho Ophiuchi molecular cloud is a hidden star cluster in the making. The Scorpius OB2 Association is part of the tilted Gould’s Belt system, located not far from where the belt rises highest above the midplane. Of the known dozens of such youthful star-forming regions, only three others have a similarly large angular distance from the galactic equator. But for Sco OB2, a large part of the reason for this is that it’s the nearest to us. Here is a good example of the effect of perspective; if it were located at a more typical, farther distance, it would lie closer to the galactic equator.
The spring sky is not considered open cluster territory. Partly by accident and partly due to their great distance from the galaxy’s disc, a handful of open clusters are on display, well removed from the band of the Milky Way. They range in distance from 75 light-years to more than 5,000 light-years, in resolvability from naked-eye to those requiring a large telescope and in age from a few million years to nearly as old as our solar system. They afford a fine opportunity for us to appreciate the effects of distance and age on stellar systems.
Deep-sky expert Glenn LeDrew of Ottawa has been a regular contributor to SkyNews for more than a decade.
