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


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

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

A verbal description of the sight doesn’t do it justice, but I’ll try anyway. It looks like a shimmering diamond dust heap with a dense central core of stars that tapers off as your eye moves to its outer edge. Looks like someone shot a paintball gun filled with stars at the dark, velvety wall of the night.

At this time of year, half a dozen globular clusters are worth viewing with a telescope. This is out of the total of more than 150 globular clusters in the Milky Way known to astronomers.

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

The “globulars” like M13 are composed of old stars very close to each other. At about 22,200 light years, M13 is about as far as an astronomical object can go and still be in our Milky Way in that direction.

M13 was to be easily visible to the elders without optical aid. Even with today’s light pollution, it is visible to the naked eye from dark rural locations.

However, while ancient astronomical scholars have written about M13, I have never met it.

The first mention of M13 had to wait for the invention of the telescope. Unfortunately, the early telescopes didn’t care much about the stars or anything outside of our solar system. They were mainly interested in planets and comets orbiting the sun.

The first mention of M13 dates from 1714, about 80 years after Galileo first pointed his telescope at Jupiter and Venus.

But Edmund Halley, of Comet Halley’s fame, was looking for comets, those hazy specks of light that sometimes passed beyond Earth. He spied on M13 but only saw him as a round, milky spot of white that did not move against the starry background. Not a comet. It is time to move on.

Fifty years later, in 1765, Charles Messier saw it in the same way. Like Halley, he marked his presence as non-comet # 13 in his catalog of non-comets which became the Messier catalog. And M13 finally had a nickname he wears to this day.

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

Once again, interest in these star balls was languishing. Fast forward to 1918. American astronomer Harold Shapley determined their great distances and positions outside the main Milky Way disk.

His findings were surprising. 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 the globular clusters, he determined that the center of our galaxy was somewhere in the direction of the constellation Sagittarius. We humans were in the galactic periphery – no place in particular.

You must be wondering, as astronomers have done, why the flattened disk of our Milky Way is surrounded by such small mini-galaxies.

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

But a few gas plates are left in the collapse. They form starballs called globular clusters made up of hundreds of thousands of stars lurking around the primary galactic disk.

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

M5 is one of the largest globular clusters around 165 light years wide, which makes M13 look like a shrimp in comparison. You will need a large telescope to see it in all its glory. Its stars are somewhat fainter than the stars of M13 due to their greater distance 26,000 light years from Earth.

But what a view, especially in a large telescope! By some estimates, it contains around half a million stars, double the size of M13.

In a mid-sized amateur telescope, M5 explodes into countless tiny dots of light, thick with stars at its core and brightening when you look out from the center. Like M13, less than half a light-year separates the stars in its crowded center. M5 therefore competes even with the dense central core of our galaxy in its abundance of stars.

If a few lucky aliens lived on a planet on the edge of a globular cluster facing the Milky Way’s pinwheel, half of their sky would be filled with the gigantic arcs of our galaxy’s spiral arms. The other half would be covered with the permanent 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, stars in M5 and other globulars were among the first to form.

In the main Milky Way disk, where we reside, many nebulae were left behind during the initial star formation. These nebulae continue to fuel star birth today, and they will continue to do so for billions of years into the future. This is not the case for stars in globular clusters.

The oldest of them was formed near the beginning of the universe. Therefore, most of them reached stellar decrepitude.

These first generation stars are composed mainly of hydrogen and helium. When stars die, they form all of the heaviest elements above hydrogen and helium on the periodic table.

When certain stars reach their explosive agony, they seed the remaining gas clouds with their heaviest elements. These elements appear in a second generation of stars and the planets that form around them.

In order for second generation stars to form, remaining gas clouds must be present, but globulars like M13 and M5 are thick with stars. They used up virtually all of their gas clouds in the first round.

Stars in globular clusters generally lack the heaviest elements that signal the possible second round of star formation. As the old stars die, no new star is born – or will be born – to take their place.

They will start to go out in a relatively short time, and M5 and M13 will disappear from view. Better check it out now. In a billion years, it will be too late.

You can also expect these old stars to have not formed the tiny, dense planets like those in our solar system. They lack the heavier elements that tend to blend into planets like Earth.

Thus, no (or only a few) verdant lands have formed on which life can thrive. Yet if there is life on a wandering planet orbiting a star in M13, that life must be old since their stars and planets formed 13 billion years ago.

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

Still, that didn’t stop a few Earthly astronomers from making a big symbolic gesture. On November 16, 1974, they broadcast a three-minute radio signal to the heart of M13. The signal’s 1,679 data bits contained raw graphical 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 look in our particular direction during those three minutes? Almost certainly not.

Could the signal go beyond the periphery of the globular cluster? Could it penetrate this wall of stars which isolates the core from the outside universe? Certainly not.

Even if an intelligent entity sees the signal, it will take 22,200 years for the message to reach it. It will be another 22,200 years before their signal to return reaches us.

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

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

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


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