StarDate

Many astronomical discoveries have come in stages – a series of “aha” moments where we learn more about the nature of an object.

A good example is Messier 13, the Great Hercules Cluster. Under especially dark skies, it’s just visible to the unaided eye, so people have known about it forever. It looks like a faint, hazy star. But during the 1700s, the cluster was “discovered” several times.

The first discovery was made by Edmond Halley. Using a small telescope, he came across it in 1714. He described it as “a little patch.” Charles Messier saw it a half-century later. He described it as “round, beautiful, and brilliant.” But, he wrote, “I am sure it doesn’t contain any star.” He made it the 13th object in his catalog.

In 1779, though, William Herschel contradicted Messier. M13 “is a most beautiful cluster of stars,” he wrote.

Many other discoveries have followed. They’ve told us that M13 contains hundreds of thousands of stars packed into a tight ball. And the cluster is ancient – 12 billion years old or older.

Messier 13 is 25,000 light-years away. In early evening, look in the east-northeast for the Keystone of Hercules – a lopsided “square” of stars. M13 is between the two stars at the top of that pattern, a bit closer to the one on the left – a giant cluster that’s still producing amazing discoveries.

Script by Damond Benningfield

Most of the stars in the Milky Way orbit the center of the galaxy in the same direction as all the other stars around them, and at about the same speed.

But a few follow their own paths. An example is a star at the tip of the Guitar Nebula. The nebula is a bubble of gas with an outline that resembles a guitar.

It’s in Cepheus, which is low in the north at nightfall. The king’s brightest stars form an outline that resembles a child’s drawing of a house. Don’t look for the nebula, though – it’s so faint that it wasn’t discovered until 1992.

The guitar was sculpted by a pulsar – the crushed corpse of a mighty star. It spins once every two-thirds of a second, emitting a beam of energy that sweeps past Earth on each turn.

The pulsar was born when the star exploded as a supernova. The explosion must have been off-center, so it gave the dead core a powerful kick. The pulsar is plowing through clouds of gas and dust at almost two million miles per hour. It leaves an expanding wake behind it, like a ship traveling across the ocean.

That wake is what we see as the Guitar.

But there’s more to the nebula than meets the eye. X-ray telescopes in space reveal a long, high-speed “jet.” It’s firing away from the tip of the nebula at a right angle to the nebula itself. The jet most likely is powered by the pulsar’s magnetic field, which funnels charged particles away from the pulsar – an interesting note from a celestial guitar.

Script by Damond Benningfield

The Milky Way Galaxy is home to a few hundred billion stars. And on average, it gives birth to a couple of Sun’s-worth of stars every year. But a much smaller galaxy about 12 million light-years away puts the Milky Way to shame. It is spawning about 10 times as many stars per year.

Like the Milky Way, Messier 82 is a thin disk, with spiral arms wrapping around a dense core. It’s less than half the size of the Milky Way.

M82 is a starburst galaxy. It had a close encounter with another galaxy within the past hundred million years or so. That caused huge clouds of gas and dust to collapse, triggering the starbirth.

The new stars are concentrated in the center of the galaxy, where astronomers have cataloged more than a hundred super star clusters. Each one contains hundreds of thousands of stars. Many of the stars are especially hot and massive, which makes the clusters especially bright.

A strong “wind” of hot gas races away from that region. It squeezes the surrounding clouds, giving birth to more stars. But within another hundred million years, all the gas and dust will have been used up. Then, M82 will settle down to the same quiet life as the Milky Way.

M82 is in Ursa Major. As night falls, it dangles below the upside-down bowl of the Big Dipper. It’s an easy target for small telescopes. We see it edge-on, so it looks like a small, bright slash.

Script by Damond Benningfield

The brightest star of the Southern Cross is like a whole episode of “Dancing With the Stars.” It consists of perhaps six or more stars. They’re all twirling through their own ballroom, linked by the strong hands of gravity.

Alpha Crucis is 320 light-years away. To the eye alone, it looks like a single point of light – the 13th-brightest star in the night sky. But binoculars or a telescope show two stars. Both of them are at least a dozen times as massive as the Sun, and thousands of times brighter. They’re so far apart that it takes about 1300 years for them to complete a single orbit around each other – a slow turn across the dance floor.

But one of those stars is actually two stars on its own. They’re so close together that not even the biggest telescopes can see them individually – the second star reveals its presence only to special instruments. But it’s also bigger, heavier, and brighter than the Sun. The two stars are dancing to a faster tempo – one turn around each other every 76 days.

Those three stars might have three more companions. They’re a long way from the first trio, and they’re not as impressive. But they appear to share a common motion through the galaxy with the brighter trio. That means the two groups could be gravitationally bound to one another – dancing a waltz that would require a hundred thousand years to complete one turn across the floor.

Script by Damond Benningfield

The farther north you live, the less of the universe you can see. Earth itself blocks the view of a large swath of the southern celestial hemisphere. That’s the half of the sky that’s south of the celestial equator – the projection of Earth’s equator. So those of us in the United States miss out on at least part of the southern sky.

One of the treasures we miss is Crux, the southern cross. It’s the smallest of the 88 constellations. But it’s also one of the prettiest and most prominent.

Four of its stars are fairly bright, and they do form a shape that looks like a cross. If you include one more star in the pattern – the faintest of the five – the pattern looks more like a kite. It points the way to the south celestial pole.

Not surprisingly, that pattern has played a big role in the skylore of many southern-hemisphere cultures. Several saw the cross as the footprint of a big bird. Others saw it as a stingray, the anchor of a giant canoe, or some other prominent object or animal.

Today, Crux is featured on the flags of Australia, New Zealand, and Brazil. It’s also on the flag of the European Southern Observatory – which has a great view of the southern cross.

From the United States, Crux is barely visible from the Florida Keys, far-southern Texas, and Hawaii. At this time of year, it’s quite low above the southern horizon in early evening – pointing the way to the celestial pole.

More about Crux tomorrow.

Script by Damond Benningfield