StarDate

A tiny arrow flies high across the sky on summer nights. Sagitta is quite high in the east as night falls, and arcs high overhead later on. Under dark skies, you can just make out the arrow, not far to the upper left of Altair, the bright star at the southern point of the Summer Triangle.

The constellation is home to an old star cluster that’s led a rough life.

M71 is a tightly packed group of stars known as a globular cluster. Such clusters are ancient, so they have low abundances of iron and other heavy elements. That’s because the clusters formed before the Milky Way’s earliest stars had much of a chance to enrich the galaxy with these elements.

But M71 is unusually metal-rich for a globular. And it’s in the Milky Way’s disk, not outside the disk, where most globulars reside.

That may explain an odd characteristic: M71 has fewer stars than most other clusters. The combined light of its stars is only about 15,000 times the brightness of the Sun. The most powerful globulars, on the other hand, radiate hundreds of thousands of times as much light as the Sun — a reflection of their much larger populations.

Because M71 orbits inside the Milky Way’s disk, it repeatedly hits gas clouds and spiral arms that tear away its stars. In fact, the cluster may have lost more than 90 percent of its original stars.

Even so, through binoculars or a telescope, M71 is a pretty sight: A star cluster that’s still shining in a galaxy that’s trying to tear it apart.

Script by Ken Croswell, Copyright 2017

A beautiful star cluster comes into view on summer evenings. Messier 11 is more than 6,000 light-years away, in the constellation Scutum, the shield. Long ago, an observer noted that the cluster’s brightest stars make the shape of the letter V, saying they resembled a flight of wild ducks. So M11 is also known as the Wild Duck cluster.

It consists of hundreds of young stars that were all born together, from the same cloud of gas and dust. And astronomers recently measured just how young the Wild Duck is.

Among its many residents are the two stars of an eclipsing binary. The stars orbit each other, and periodically pass in front of each other as seen from Earth. As they orbit, their gravitational pull on each other reveals how massive they are. And as they pass in front of each other, the duration of the eclipses reveals how big they are.

The brighter star is about three-and-a-half times as massive as the Sun, but almost five-and-a-half times larger. That means it’s used up the hydrogen fuel in its core and started to puff up. By comparing these numbers with models of how stars evolve, a team of astronomers concluded that the Wild Duck was born about 220 million years ago.

Here on Earth, the only witnesses to the cluster’s birth were dinosaurs and other creatures living during the Triassic Period. The dinosaurs have vanished, but the Wild Duck endures — a beautiful sight through a telescope on a summer night.

Script by Ken Croswell, Copyright 2017

If you look south shortly after sunset tonight, you’ll see eight moderately bright stars arranged in the shape of a teapot. That’s the constellation Sagittarius. It’s also where the center of our galaxy is — 27,000 light-years away. And this year marks the hundredth anniversary of the discovery that we’re far away from the Milky Way’s heart.

Early maps put us near the center, because dust in space obscures distant stars. As a result, we see only a small fraction of the Milky Way. And because of all the dust, we seem to be at the center of the little that we can see.

In the twentieth century, though, astronomer Harlow Shapley recognized the truth: that the galaxy is much bigger than had been thought, and that we’re off to one side.

Rather than mapping individual stars, Shapley looked at bright star clusters known as globulars. Most of these clusters are outside the Milky Way’s disk of stars, where the thickest dust resides, so dust doesn’t block our view of the clusters. And the clusters are quite bright, so we can see them even at large distances.

Shapley measured the distances to dozens of globular clusters. Some were so far away that, at first, he thought they were outside the Milky Way. But in 1917, he recognized that they not only belong to our galaxy, but they point the way to its center. That indicated that we don’t live in the Milky Way’s busy heart. Instead, we live in the suburbs of our galactic metropolis.

Script by Ken Croswell, Copyright 2017

If you look long enough and hard enough at Jupiter, there’s a good chance you’ll find another moon of the giant planet. Earlier this year, for example, astronomers announced the discovery of two moons, bringing Jupiter’s total to 69. The new discoveries aren’t much to get excited about on their own, though — each is a chip of rock only a mile or so across.

Last year, Scott Sheppard was scanning the outer solar system for a possible Planet Nine when Jupiter crossed his field of view. Using a large telescope in Chile, he discovered a possible new moon orbiting the solar system’s largest planet. The find was confirmed earlier this year when he teamed up with other astronomers, who were using an even bigger telescope.

The second moon was first seen in March of this year. A scan through photos of the region snapped last year also showed the moon, allowing the astronomers to confirm it and plot its orbit.

Both moons are many millions of miles from Jupiter, so it takes a year and a half for one of them to complete an orbit, and two years for the other. And both orbit backwards. That suggests that they were asteroids that passed close to the planet, where they were captured by its powerful gravity — adding to its big family of moons.

And Jupiter teams up with our own moon this evening. The planet looks like a brilliant star close to the lower right of the crescent Moon. The true star Spica stands below the Moon.

Script by Damond Benningfield

Fresh from its performance during Monday’s total eclipse, the Moon stages another beautiful encounter early this evening. It lines up with the planet Jupiter and the star Spica, the leading light of the constellation Virgo. Jupiter is by far the brighter of the two, with Spica close to its lower left. They’re quite low in the western sky at nightfall, and drop from view not long after.

During the eclipse, the Moon’s role was as a blackout curtain — it blocked the light from the Sun’s disk, allowing its wispy corona to shine through.

Tonight, though, the Moon itself takes center stage. It’s a thin crescent, with sunlight illuminating roughly one-eighth of the hemisphere that faces our way. As twilight begins to fade out, though, the rest of the lunar disk will become visible, too — illuminated by earthshine — sunlight reflected off of Earth.

And there’s a lot of it. As seen from the Moon, Earth would be in its gibbous phase this evening, with almost all of it in daylight. It would shine many times brighter than a full Moon, so the landscape would be plenty bright for an “earthlit” stroll.

Over the next few evenings, the Moon will stand higher in the sky at sunset, and it’ll be a fatter and fatter crescent. And the earthshine will begin to fade, because less of Earth’s daylight side will face the Moon. So the lunar night will get a little darker, under the light of a waning Earth.

More about the Moon and Jupiter tomorrow.

Script by Damond Benningfield

NGC 6946 is both beautiful and busy. It’s a spiral galaxy that we see face-on, providing a glorious view of its delicate spiral arms. And over the last century, astronomers have recorded 10 supernova explosions in the galaxy — the most recent just three months ago. That’s earned NGC 6946 a nickname: the Fireworks galaxy.

The number of firecrackers could have been even higher. But instead of exploding, one of the galaxy’s massive stars simply winked out of existence. That means it might have collapsed to form a black hole.

For a long time, theory has said that any star born with more than about eight times the mass of the Sun should end its life as a supernova. The titanic blast rips the star’s outer layers to bits, while the core collapses to form a neutron star or a black hole.

In recent years, though, astronomers have started to suspect that the most-massive stars skip right past the explosion part. They may start to explode, but as the core collapses to form a heavy black hole, its gravity grabs the exploding material and pulls it in.

An example of that may be the star in NGC 6946. Earlier this year, a team of astronomers reported that, in 2009, the star began getting brighter. But by 2015, it had vanished. There was no explosion, no debris — nothing at all. The team concluded that the star collapsed to form a black hole — depriving the Fireworks galaxy of another bright display.

Script by Damond Benningfield

Our home galaxy, the Milky Way, is a giant spiral that contains hundreds of billions of stars. And every now and then, one of its stars explodes as a supernova. The last time astronomers saw that happen was more than 400 years ago, but that may be because we can see only a small portion of our vast galactic home. If we were outside the Milky Way, astronomers estimate, we’d see about two supernova explosions per century.

We’ve seen supernova explosions in many galaxies beyond our own. But one prolific galaxy puts all the others to shame. It’s a beautiful spiral in the constellation Cepheus known as NGC 6946. During the past hundred years, astronomers have seen a record 10 supernovae in this one galaxy.

The galaxy owes those fireworks to its impressive star-spawning ways. Its gas and dust are giving birth to large numbers of new stars. The most massive of these stars explode a few million years after their birth, producing the fireworks we see.

As galaxies go, NGC 6946 is fairly close — about 20 million light-years from Earth. That means these stars actually died 20 million years ago, but it’s taken that long for the light of their explosions to reach us. Astronomers first saw a supernova in this galaxy a hundred years ago, back in 1917. The most recent popped off just a few months ago.

The supernova count might have gone even higher, but instead of exploding, one massive star collapsed to form a black hole. More about that tomorrow.

Script by Ken Croswell, Copyright 2017

Many skywatchers have been planning for this day for years. The Moon will pass directly between Earth and Sun today, creating a total solar eclipse from Oregon to South Carolina. Day will turn to night, and stars and planets will pop into view. And tens of millions of Americans are expected to see the spectacle in person.

A solar eclipse is the result of a complex ballet involving Earth, the Moon, and the Sun.

The Moon pirouettes against the background of stars and planets every 27 and a half days. But because the Sun is also moving against that background, it takes a couple of days longer for the Moon to catch up to it.

While the Moon passes the Sun roughly once a month, most months it doesn’t cause an eclipse. Instead, it usually glides a little above or below the Sun, because its orbit is tilted relative to the Sun’s path across the sky. A solar eclipse occurs only when the new Moon crosses that path. And even then, most eclipses are only partial: the Moon covers only part of the Sun’s disk, which is what most Americans will see today.

Total eclipses occur an average of about 18 months apart, although the gap can range from a few months to more than two years. The next total eclipse will take place in 2019, visible from parts of South America.

And there’ll be another eclipse over the U.S. on April 8th of 2024, from Texas to Maine. So if you miss out on today’s show, start planning for the next one — another disappearing act for the Sun.

Script by Damond Benningfield

For millions of skywatchers in the United States tomorrow, day will briefly turn to night as the Moon passes in front of the Sun, eclipsing its light. The Sun’s outer atmosphere, the corona, will shine silvery-white around the dark Moon. And the stars and planets will shine through the dark as well.

Regulus, the heart of the lion, will perch closest to the Moon. It’s usually lost from view at this time of year as the Sun crosses Leo’s borders. The planet Mercury will shine brightly a little farther away, with reddish Mars on the other side of the Sun.

Brilliant Venus and Jupiter will be farther from the Sun, with Sirius, the brightest star in the night sky, low in the southwest.

You may be tempted to shoot pictures and videos throughout the show. But a total solar eclipse is a once-in-a-lifetime experience, so most veterans say it’s best to simply drink in the spectacle. And it’s one that involves most of your senses — the air will get cooler, and you’ll hear the sounds of birds and other animals preparing for night.

There are a few safety tips to keep in mind. It’s completely safe to look at the Sun when it’s fully eclipsed, but not at other times; it’s so bright that it can damage your eyes. And highway patrols around the country remind drivers not to stop on busy highways. Instead, find a park or some other safe spot to enjoy the sights and sounds of this amazing lightshow.

We’ll have more about this skywatching spectacle tomorrow.

Script by Damond Benningfield

More information about the eclipse:

The Great American Eclipse is coming up on Monday. The Moon will briefly cover the Sun, turning day to night across a narrow slice of the United States.

The timing of the eclipse is known down to the second, and has been for decades. And today, astronomers can predict eclipses far into the future. But making such predictions isn’t easy. It requires a detailed knowledge of the Moon’s orbit around Earth, Earth’s orbit around the Sun, and even the shapes of Earth and the Moon.

Scientists had been trying to predict eclipses for a long time. Just when they first succeeded is a bit unclear, though.

There’s no doubt that people have been predicting lunar eclipses for thousands of years. But lunar eclipses are easier to predict. Earth’s shadow is roughly a hundred times wider than the Moon’s, so you don’t need as high a level of precision to get it right.

There are stories that the Chinese were predicting solar eclipses more than 4,000 years ago, but no confirmation. An eclipse in 585 BC that stopped a war supposedly was predicted by Thales, a Greek scientist. Many modern-day scientists doubt that, however.

The first confirmed prediction was made by Edmond Halley, using the laws of gravity devised by Isaac Newton. Halley forecast that an eclipse would cross England on May 3rd, 1715. And he was right. So the eclipse of 1715 is known as Halley’s Eclipse — honoring the prediction of an astronomical spectacle.

More about eclipses tomorrow.

Script by Damond Benningfield

More information about the eclipse: