Suburbs of our galaxy


By Tom Burns - Stargazing



The Milky Way, our galaxy, is one of trillions of tiny islands of stars sprinkled throughout the vast cosmic ocean of space.

When we look at images of galaxies, we see only their densest parts. Seen from the top, they are often shaped like flat spirals — children’s pinwheels of uncountable stars.

Seen from the side, galaxies look much like lenses bulging at the center and tapering to points at the edges. Most of a galaxy’s stars are spread throughout the lens-like structure, the galaxy proper.

However, appearances can be deceiving. Strictly speaking, galaxies are shaped more like balls than lenses.

Above and below the lens, filling in the sphere, is a sparse sprinkling of relatively dense star balls called globular clusters. In artist renderings of the Milky Way, they look at bit like tiny bees swarming around the galactic hive.

If we think of the Milky Way as its main spiral, globular clusters like M13 form a kind of halo around the cluster. Now add every stray speck of dust and gas dragged along by the Milky Way’s gravity as it moves through space. You get a rarefied, more-or-less spherical halo, perhaps a million light-years wide, surrounding the pinwheel.

In other words, globulars are much like the small satellite cities that surround major metropolises. Every Columbus must have its New Albanies, its Picuas and Pataskalas.

Astronomers have identified about 150 of such fellow travelers gravitationally bound to the Milky Way.

The best quality of late springtime in the early evening is that the dense disk of the Milky Way proper is out of the way. We can thus see above and below the disk to the realm of the globulars. The best of them is visible right now in binoculars or a telescope. Look for M13 pinned to the chest of the mighty constellation Hercules.

First, you’ll have to find Hercules. Look low in the east around 11 p.m., and you’ll see a fairly faint, four-star box, wider on the left than on the right, called the Keystone. Locate the top two stars of the Keystone.

If your eyes are good and you live under a deep, dark rural sky, you’ll see a small, fuzzy star at the top of the Keystone. If you live under a more urban sky, use binoculars. Sweep about one-third of the way from the left star toward the right. The little, fuzzy lint ball is the globular cluster M13.

That’s about what it looked like to Edmund Halley (you know, the comet guy) when he discovered it in 1714. Halley’s telescope was so small that all he saw was a little fuzz ball.

Later that century, British astronomer and large-telescope builder Sir William Herschel used one of his behemoths to estimate the number of stars in M13 at 14,000. People thought it was a misprint. Surely, he had meant to write 4,000.

Over a century later, Harold Shapely counted 30,000 stars. As the telescopes got bigger, the estimates got larger.

The bottom line: You are looking at about 300,000 stars — although you can’t possibly see them all. A small telescope will begin to resolve the outer stars of the ball. A larger one like the Schottland Reflector at Perkins will dazzle your eyeballs with countless points of light resolved to the core of the cluster.

“300,000 stars” sounds like a lot, but it isn’t. It would take one million M13’s to equal the star count in our Milky Way galaxy.

Globulars tend to be denser with stars at their center. As we move toward the edge, the stars are less tightly packed together. The reason a small telescope can’t resolve the stars at the core of a globular is that they are too close to each other. From our vantage, you can’t see the stars on the side of the ball facing away from us. The stars look like they are on top of one other.

That feature is, of course, an illusion caused by our distance from M13. The cluster is perhaps 22,000 light years distant — out at the fringes of our galaxy. One light year is equal to about six trillion miles. That’s a heck of a lot of miles, Earthlings.

M13 is perhaps 35 light-years wide, small by comparison to the Milky Way galaxy, which is 100,000 light years wide from one end of the pinwheel to the other. It is even small by comparison to the larger globular clusters, which can be over 150 light years wide.

That’s huge by earthly standards, however. Our planet is about 8.3 light minutes away from the sun, making M13 over two million times wider than the Earth/sun distance.

As a result, M13 is not all that crowded, even with 300,000 stars to work with. On average, each cubic light year of space is occupied by only one star.

When we look at it on that level, the gulf between stars seems enormous. Even in globular clusters, among the most closely packed places in the universe, the stars are still like dust.

But it isn’t when we compare the stars of M13 to the stars in our section of the Milky Way’s disk. Our Earth and sun are out in the fringes of the Milky Way where stars are relatively sparse. The closest star to our sun is over four light years away, and that’s relatively close. Out in the galactic suburbs, where we live, the stars average about seven light years apart.

M13 is a glorious sight in a large telescope, and most amateur astronomers agree that the “Great Globular in Hercules,” as it is often called is the finest of the globulars.

However, 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, which makes M13 look like a shrimp. You’ll need a large telescope to see it in all its glory. Its stars are somewhat fainter than M13’s stars because of its 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, 200,000 more than M13.

In binoculars, M5 will, as usual, look like a tiny fuzzy patch. Look for it just above the star marked as “5 Serpens” on most star maps. If you’re observing from the dark skies of rural Ohio on a moonless night, you might glimpse it without any optical aid at all.

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, in its dense core, only about a light year separates the stars on average. 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 the 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 and the other half would be thick with the unremitting light of 500,000 stars.

The stars of M5 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 gas and dust were left over from the initial formation of stars to fuel the birth of stars until this very day and for billions of years into the future.

As the stars of M5 formed, they used up all the available gas and dust in the region. The stars of M5 have become old and no new stars have risen up to take their places. In a relatively short time, they will begin to wink out, and M5 will fade from view. Better check it out now. In a billion years or so, it will be too late.

In a very large telescope, the view of M5 is a jaw-dropping experience. Up at Perkins, we got as many “oh, wows” from the sight of M5 and its globular sibling M13 than we did the famous rings of Saturn.

Sadly, the light of the globulars is so much fainter than the light of the much closer planets. The growth of outside lighting near Columbus and Delaware will inevitably wash out their dim but magnificent glow. Better check them out now. In a few years, it might be too late.

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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.