M71: Mysterious star cluster in Sagitta

By Tom Burns - Stargazing

Last week, we discussed the minor constellation called Sagitta, the Arrow.

Partly because of Sagitta’s small surface area, the constellation contains few astronomical objects of interest to amateur or professional astronomers. However, one object has given astronomers fits over the centuries.

Let’s give a warm, quizzical welcome to Messier 71 (M71, for short).

First, let’s have a look at it. Go out and find Sagitta.

The leftmost star, Gamma, represents the point of the Arrow. Look halfway between Gamma and the next star to the right in binoculars or a small telescope.

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 relatively tight grouping of a couple of dozen stars with a gorgeous background layer of faint light. The fuzzy glow consists of stars 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.

Swiss astronomer Philippe Loys de Chéseaux discovered M71 in 1745 as he swept the sky for comets.

Five years later, Charles Messier, another comet hunter, included it in his famous catalog of objects that looked vaguely comet-like but were not, in fact, comets. In other words, for Messier and other astronomers like him, M71 was a discard. It was not always to be so.

And why is M71 so mystifying, you ask? As interest in Messier’s non-comets increased, astronomers couldn’t figure out what it was or what to call it.

They knew at least that it was some sort of star cluster. But clusters of stars in our Milky Way galaxy come in two flavors, and M71 didn’t seem to fit in either one of them.

Open clusters are located in the primary disk of our Milky Way galaxy. A typical open cluster is composed of stars 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 primarily 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 as it dies. As some stars die, they send those heavy elements spewing into the hydrogen cloud, seeding the cloud with stuff that eventually forms into new stars. Thus, the “second-generation stars” in open clusters are rich in heavy elements.

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

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

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

M71 looks a bit like a globular cluster, albeit a tiny one. It has a diameter of about 27 light-years. By comparison, the Great Globular Cluster in Hercules (M13) has a diameter of 168 light years.

M71 has a mass of about 17,000 suns and produces around 19,000 times more energy than the sun. Astronomers call that energy output luminosity.

M13 has a mass 600,000 times that of the sun. M13’s luminosity is over 300,000 times that of the sun.

In total, M71 may have as many as 20,000 stars. M13 has as many as 500,000.

In short, M71 is large for an open cluster but small compared to a typical globular cluster.

M71 looks a bit like a globular cluster in a telescope. It’s at the correct distance of about 20,000 light years away. (A light year is equal to six trillion miles.)

However, it lacks the central concentration of stars characteristic of globular clusters. But like a typical globular, it has a plethora of old stars.

The weirdest thing about M71 is that its stars contain the heavy elements that mark an open cluster.

Still, heavy elements are notable for their absence in globulars. So it must be an open cluster.

But those heavier elements are relatively scarce compared to the second-generation stars in open clusters. M71 has only 17 percent of the metals of the sun, a nearby second-generation star very dear to our hearts.

Also, M71 has few of an open cluster’s benchmark younger stars. So it must be a globular cluster.

But M71 is notable for the relative absence of RR Lyrae variable stars. RR Lyrae variables are very old stars that once were the mass of our sun or slightly below the sun’s mass.

Astronomers attribute their varying brightness to their old age. Their pulsations are their death throes. Soon they will use up their available fuel and collapse into white-dwarf stars.

Since globular clusters contain mostly older stars, the absence of RR Lyrae variables suggests that M71 must be an open cluster.

There things stood until 1943. Astronomers concluded that M71 was an open cluster, albeit a densely packed one.

However, that same year, James Cuffey of Kirkwood Observatory in Bloomington, Indiana, concluded that M71 was more like a loosely populated globular cluster.

In that regard, M71 is the odd cluster out, but it is not alone. M71 resembles M68, a globular cluster in the constellation Hydra.

However, Cuffey continued to study the confounded thing. In 1959, he concluded that M71 had characteristics more akin to an open cluster.

By the 1970s, detailed studies of M71 led astronomers to lean again toward the globular-cluster identification. They argued that the key to the problem was M71’s age.

A study in 2008 by a team led by Brazilian astronomer Alan Alves-Brito has, for the moment at least, settled the argument.

They discovered that M71 had stars with sun-sized masses that had evolved past the red-giant phase into the last stages before they collapsed into white dwarfs.

That phase, called the horizontal branch, is characteristic of globular clusters. However, M71’s horizontal branch was relatively short compared to most globular clusters.

Most globular clusters formed 11-13 billion years ago in the halo of the Milky Way. In fact, they probably formed earlier than the main disk of the Milky Way.

The M71 confusion resulted from its relative youth of only 9-10 billion years. M71 was a late bloomer.

Most open clusters are far younger than that. Their very loose gravitational connection causes them eventually to drift apart. M71 has held together for billions of years.

At nine or ten billion years old, the stars of M71 would have had sufficient time to develop their sparse collection of heavier elements.

However, its relative youth for a globular cluster explains the dearth of RR Lyrae variable stars. Most of its sun-like stars have not had enough time to age into the last stages of their lives.

Thus, after a long struggle, astronomers have finally achieved a consensus concerning M71. It is a low-mass globular cluster akin to M68 in Hydra. It lacks a concentrated central mass of stars, which is unusual but not unique among globulars.

If you end up at a star party or public observing session before the summer ends, ask the telescope operator to point their telescope at M71 and revel in the three-century-long struggle to answer a seemingly uncomplicated question — what to call the blasted thing.


By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.