M71, a pretty little cluster of stars in the obscure constellation Sagitta, has given astronomers fits over the years.

To find it yourself, first look about halfway up the sky around midnight for the bright star Altair in the constellation Aquila, the Eagle. Up and to the left is a narrow grouping of four stars that form Sagitta, the Arrow. The leftmost star, Gamma, represents the point of the arrow. In binoculars or a small telescope, look halfway between Gamma and the next star to the right.

In binos, you’ll see a small hazy patch if the sky is clear and moonless. In a telescope six inches or more in diameter, you’ll see a fairly tight grouping of a couple of dozen stars with a gorgeous background layer of fuzzy light. The fuzz consists of stars that are too faint for your telescope to resolve into individual points of light.

M71 is the most mysterious object in the sky, even more mysterious than the 237 other “most mysterious” objects I’ve written about here. Really.

And why, you ask? Clusters of stars in our Milky Way galaxy come in two flavors, and M71 isn’t either one of them.

Open clusters are located in the main disk of our Milky Way galaxy. A typical cluster is composed of stars that were born from the same cloud of hydrogen gas and are therefore close together. Since the stars in a cluster were born at different times, astronomers see stars in the clusters at various ages – from young, hot, blue stars to old, cool, red ones.

Stars in open clusters also contain a wide variety of chemical elements. They are mostly composed of hydrogen and helium, the main constituents of most stars. But they also contain heavy elements like calcium, carbon and iron. Those elements can only be made at the end of a star’s life when it is dying. As some stars die, they send those heavy elements spewing into the hydrogen cloud, seeding the cloud with stuff that eventually forms into more stars. Thus, the “second-generation stars” in open clusters are fairly rich in heavy elements.

The second kind of star grouping, called a globular cluster, is generally located outside the main disk of the Milky Way. Globulars hover above and below the disk like bees slowly buzzing around a hive. A globular is shaped like an amazingly symmetrical ball with the stars very densely packed at the center and increasingly spread out toward the outside.

The stars in a given globular were all born at about the same time. The process used up most of the hydrogen gas cloud that gives birth to the stars.

Globular-cluster stars have reached old age but few have actually died. Thus, they are composed mostly of hydrogen and helium, with only tiny quantities of the heavy elements contained in second-generation stars.

The weird thing about M71 is that its stars contain the heavy elements that are the markers of an open cluster. On the other hand, M71 has few of the younger stars that are the benchmark of an open cluster.

M71 looks like a globular cluster in a telescope. It’s at the right distance of about 13,000 light years away, and it’s about the right size at 30 light years wide. (A light year is equal to six trillion miles). Like a typical globular, it has a plethora of old stars. However, it also has heavy elements, which are notable for their absence in other globulars.

So what is M71? Until the 1970s, astronomers classified it as a dense, old open cluster because of its abundance of heavy metals and its lack of central compression. Now, they think of it as a loosely packed, young globular cluster. As strange as it sounds, nine or 10 billion years old is “young” for a globular cluster.

Its relative youth explains the lack of heavy elements. Its lack of a central concentration means that the cloud out of which it formed must have been relatively small and relatively light on star-forming gases to start with. Globular-forming regions are usually characterized by bigger clouds and denser stuff.

It’s an odd bird, but them again, it’s a big universe. We can’t expect everything in a cosmos as vast and complex as ours to fit neatly into the frailty of our all-too-human classification systems.


Tom Burns


Tom Burns is director of the Perkins Observatory in Delaware.