Astronomers: ‘We were here; This is who we were’


By Tom Burns - Stargazing



Last week, I told the story of my first view of the globular cluster M13 in a large telescope that I had built myself.

I was looking at uncountable stars collapsed into a single telescope field. M13 is an explosion of starlight so dense that it is difficult to resolve the stars into individual points of light except in a large telescope.

A verbal description of the view hardly does it justice, but I will try anyway. It looks like a glittering pile of diamond dust with a dense central core of stars that tapers off as your eye moves toward its outer edge. It looks like someone has shot a paintball gun filled with stars at the velvety dark wall of night.

This time of year, about half-a-dozen globular clusters are worth looking at in a telescope. That’s out of the total of more than 150 Milky Way globular clusters known to astronomers.

Globular clusters are clusters of stars that huddle around the dense galactic core but outside the disk of galactic disk — out in the galactic suburbs, you might say. M13 is perhaps 100 light-years, or 600 trillion miles, in diameter.

“Globulars” like M13 are composed of ancient stars very close together. At about 22,200 light-years away, M13 is about as far away as an astronomical object can get and still be in our Milky Way in that direction.

M13 must have been easily visible to the ancients without optical aid. Even with today’s light pollution, it is visible to the naked eye from dark, rural sites.

However, if ancient astronomical scholars wrote about M13, I’ve never run across it.

The first mention of M13 had to wait for the invention of the telescope. Unfortunately, the first telescopists didn’t concern themselves much with the stars or anything outside our solar system. They were primarily interested in the planets and comets orbiting the sun.

The first mention of M13 came in 1714, some 80 years after Galileo first pointed his telescope at Jupiter and Venus.

But Edmund Halley, of Halley’s Comet fame, was looking for comets, those hazy patches of light that occasionally zoomed past Earth. He spied M13 but saw it only as a round, milky patch of white that did not move against the starry background. Not a comet. Time to move on.

Fifty years later, in 1765, Charles Messier saw it the same way. Like Halley, he marked its presence as non-comet #13 in his catalog of non-comets that has come to be called the Messier catalog. And M13 finally had a nicknumber that it carries to this very day.

By 1789, telescope technology had improved enough that the great British astronomer Sir William Herschel could observe M13 as more than just a fuzzy patch. He saw a spherical ball dense with stars. To describe such objects, he coined the term “globular cluster.”

Again, interest in those star balls languished. Fast-forward to 1918. American Astronomer Harold Shapley determined their great distances and positions outside the main disk of the Milky Way.

His discoveries were startling. At the time, most astronomers assumed that the sun and Earth were at the center of the Milky Way. By studying the exact positions of globular clusters, he determined that the center of our galaxy was somewhere in the direction of the constellation Sagittarius. We humans were out in the galactic periphery — no place in particular.

You have to wonder, as astronomers did, why the flattened disk of our Milky Way has such little mini-galaxies hovering around it.

Galaxies like the Milky Way started as giant amorphous clouds of mostly hydrogen gas. Gravity causes the cloud to collapse slowly into a spinning disk.

But some patches of gas are left behind in the collapse. They form into balls of stars called globular clusters consisting of hundreds of thousands of stars loitering around the primary galactic disk.

As much as I like M13, my favorite globular is a ball of diamond dust called M5 in the rather inconspicuous constellation Serpens, the Serpent.

M5 is one of the largest globular clusters at about 165 light-years wide, making M13 look like a shrimp by comparison. You’ll need a large telescope to see it in all its glory. Its stars are somewhat fainter than M13’s stars because of their greater distance at 26,000 light-years from Earth.

But what a view, especially in a large telescope! According to some estimates, it contains roughly half a million stars, double that of M13.

In a medium-sized amateur telescope, M5 explodes into countless tiny points of light, thick with stars at its core and thinning out as you look outward from the center. Like M13, less than 1/2 of a light-year separates the stars in its crowded center. M5 thus rivals even the dense central hub of our galaxy in its abundance of stars.

If some lucky aliens live on a planet at the edge of a globular cluster facing the pinwheel of the Milky Way, half their sky would be filled with the gigantic arcs of the spiral arms of our galaxy. The other half would be thick with the abiding light of tens of thousands of stars.

The stars of M5 and M13 are among the oldest in the Milky Way. Stars form from giant clouds of gas and dust called nebulae. Perhaps 13 billion years ago, the stars of M5 and the other globulars were among the first to form.

In the main disk of the Milky Way, where we reside, plenty of nebulae were left over from the initial formation of stars. Those nebulae continue to fuel the birth of stars today, and they will continue to do so for billions of years into the future. Not so with the stars of globular clusters.

The oldest of them formed near the beginning of the universe. Consequently, most of them have reached stellar decrepitude.

Such first-generation stars are composed of mostly hydrogen and helium. As stars die, they form all the heavier elements above hydrogen and helium on the periodic chart.

When some stars reach their explosive death throes, they seed any leftover gas clouds with their heavier elements. Those elements show up in a second generation of stars and the planets that form around them.

For second-generation stars to form, leftover gas clouds have to be present, but globulars like M13 and M5 are thick with stars. They used up virtually all their gas clouds on the first go around.

The stars in globular clusters generally lack the heavier elements that signal the eventual second round of star formation. As the old stars die, no new stars have been born — or will be born — to take their places.

They will begin to wink out in a relatively short time, and M5 and M13 will fade from view. Better check it out now. In a billion years or so, it will be too late.

You can also expect that those old stars didn’t form the tiny, dense planets like those in our solar system. They lack the heavier elements that tend to coalesce into planets like Earth.

Thus, no (or only a few) verdant earths have formed on which life can develop. Still, if any life exists on a stray planet orbiting around a star in M13, that life must be ancient indeed since their stars and planets formed 13 billion years ago.

In perhaps a stray case or two, civilizations most likely would have long ago been born, evolved, and died during the long lives of the stars of M13.

Still, that didn’t stop a few earthling astronomers from making a grand, symbolic gesture. On 16 November 1974, they broadcast a three-minute radio signal at the core of M13. The signal’s 1,679 bits of data contained crude graphic representations of a standing human figure, a DNA molecule, and Puerto Rico’s Arecibo radio telescope, which astronomers used to broadcast the signal.

Is there intelligent life somewhere in M13 to see and interpret the signal? Probably not. Would anyone be looking in our particular direction for those three minutes? Almost certainly not.

Could the signal make it past the globular cluster’s periphery? Could it penetrate that wall of stars that isolate the core from the outside universe? No way.

Even if some intelligent entity sees the signal, the message will take 22,200 years to get there. Another 22,200 years would pass before their return signal reaches us.

Will humans still exist on our planet in 44,400 years to hear the reply? Given the current state of human affairs, the possibility seems highly unlikely.

But they did it anyway. Against all odds, they threw into the vast cosmic ocean a message in a bottle. On behalf of all humanity, it said, “We were here. This is who we were.”

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By Tom Burns

Stargazing

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

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