StarDate

One of the loveliest events in the night sky is a close alignment of the Moon and the planet Venus. And just such an alignment takes place the next couple of days before and during dawn. Venus, the brilliant “morning star,” stands to the left of the Moon at first light tomorrow, and about the same distance to the upper right of the Moon on Wednesday.

A close alignment between two or more astronomical objects is called a conjunction. The Moon has several of them every month, because it makes a full loop against the background of stars and planets.

And it’s always the same roster of companions, because the Moon follows a narrow path across the sky. In fact, it stays close to the Sun’s path, known as the ecliptic. Several bright stars lie near the ecliptic, and so do the bright planets. So the Moon swings past each of these objects every month, staging beautiful conjunctions.

The Moon also has conjunctions with the Sun. Most months, these conjunctions are invisible. The Moon’s path is tilted just a bit with respect to the ecliptic, so the Moon usually swings just above or below the Sun, where it’s lost in the glare.

In August, though, the Moon will be crossing the ecliptic just as it passes the Sun. That will create the most beautiful of all astronomical encounters: a total solar eclipse. Its path will cross the United States from Oregon to South Carolina — a beautiful conjunction between the Moon and Sun.

More about the Sun tomorrow.

Script by Damond Benningfield

Summer arrives on Tuesday night, with the summer solstice. Tuesday and Wednesday will be the longest days of the year here in the United States — the greatest intervals between sunrise and sunset. But the light of day doesn’t begin or end at those moments. Sunlight filters into the sky long before the Sun breaks the horizon in the morning, and long after it disappears in the evening.

These periods are known as twilight. But there are several versions of twilight, all of which depend on how far below the horizon the Sun is.

What most of us think of as twilight is known as civil twilight. In the evening, it begins at sunset and ends when the Sun has dropped six degrees below the horizon — about the width of three fingers held at arm’s length. There’s still light in the sky at the end of civil twilight, but not enough for everyday activities, such as driving.

When the Sun is between six and 12 degrees below the horizon, it’s nautical twilight. When sailors navigated by the stars, they could begin to see stars during this period, but they could also still see the horizon, so it was a good time to take readings.

Finally, when the Sun is between 12 and 18 degrees below the horizon, it’s astronomical twilight. It’s dark enough for astronomers to observe brighter stars and planets, but not dark enough to see faint galaxies and nebulae.

When astronomical twilight ends, the sky’s just about as dark as it’s going to get — the darkness of another night.

Script by Damond Benningfield

In the modern world, there’s a rule, a regulation, or a definition for just about everything. And that includes such everyday events as sunrise and sunset.

There are several ways to think of these daily bookends. They can be the points at which the Sun is bisected by the horizon — half in view, half not. Or they can be the points when the Sun has dropped below the horizon. Or, in the modern definition, they can be the points when the Sun drops from view.

You might think those last two would be the same, but they’re not. Earth’s atmosphere acts as a lens, bending the Sun’s rays. So when you see the Sun standing just atop the horizon, it’s actually below the horizon, but the atmosphere has projected an image of the Sun into view.

So the official timekeeper for the United States, the Naval Observatory, defines sunrise and sunset as the moments when the center of the Sun is physically 50 minutes of arc below the horizon — a bit less than one degree. That accounts for the size of the Sun itself, and the “bending” properties of the atmosphere.

The atmosphere can bend the Sun’s rays at different angles at different times, though. So the predicted times of sunrise and sunset can be off by a minute or so. And when the Sun rises and sets at a low angle to the horizon, they can be off by several minutes.

And even when the Sun does drop from sight, the atmosphere scatters its light back into view, producing twilight. More about that tomorrow.

Script by Damond Benningfield

For most of a star’s long life, things happen slowly. A star like the Sun, for example, steadily converts hydrogen to helium in its core for about 10 billion years. At the end of that run, the pace picks up. And in the star’s final, colorful act, major changes can happen over just a few centuries.

One star that may be going through that last act is in Hydra, the water snake, which slithers quite low across the southwest at nightfall.

V Hydra appears to consist of two stars. The main star is the same mass as the Sun. It’s at the very end of its life, though, so it’s puffed up to more than 400 times the Sun’s diameter. The star pulses in and out like a beating heart, and it’s ejecting a lot of gas into space.

The second star orbits the bigger one every eight and a half years on a stretched-out path. When it gets close to the bigger star, it grabs some of the expelled gas, forming a disk. When enough gas builds up, some of it may be flung back into space as a “cannonball” — a blob that’s twice as heavy as the planet Mars.

So today, astronomers are seeing big changes in V Hydra every few years. And the changes could come even faster in the not-too-distant future. The bigger star may soon expel all of its outer layers of gas into space, forming a colorful nebula. The companion may sculpt it into an exotic shape — perhaps an hourglass, or even a butterfly. It’s a process that could play out very soon — a fast-paced change for a Sun-like star.

Script by Damond Benningfield

When a Sun-like star dies, it leaves a brilliant corpse — a glowing shell of gas that can look like a ring, a box, or even an hourglass. The colorful object doesn’t last forever, though — it slowly fades from sight.

Such a beautiful corpse is known as a planetary nebula — not because it has anything to do with planets, but because it can look like a planet when seen through a telescope. And one of the brightest and best known is in good view at this time of year.

The Ring Nebula is not far to the lower right of the brilliant star Vega, which is about halfway up the eastern sky at nightfall.

Long-exposure images show an oval that’s bright red and yellow on the outside, and pale blue and green in the center.

The nebula began forming several thousand years ago, as its star stopped producing nuclear reactions in its core. The core began to shrink and get much hotter. A couple of thousand years ago as seen from Earth, radiation from the core began pushing away the star’s outer layers. As they expanded into space, energy from the superhot core caused them to glow — forming the Ring Nebula.

The nebula is probably shaped like a barrel or a thick doughnut, and we’re looking straight through the hole in the middle. Other planetary nebulae may have companion stars that sculpt them into more exotic shapes, from hourglasses to butterflies — creating beautiful final acts among the stars.

We’ll talk about the possible birth of a planetary nebula tomorrow.

Script by Damond Benningfield

The surface of the largest moon of Saturn looks a lot like Earth, with mountains, riverbeds, lakes, and dunes. But while the dunes on Titan look like those on Earth, they’re really quite different. Their particles of “sand” may form in a different way, and the dunes align in the opposite direction from the prevailing winds.

Titan looks so Earth-like because it has a cold, thick atmosphere. Winds, along with rains of liquid methane and ethane, sculpt the surface — including the dunes. In fact, the grains that make up the dunes may consist of particles of organic compounds that stick together in Titan’s atmosphere and fall to the surface like sleet.

The dunes are found along Titan’s equator. They’re up to 30 stories high and hundreds of miles long.

Most of them point to the east. But climate models suggest that Titan’s prevailing winds are from the west, so the dunes ought to aim in that direction.

A recent study says they may align against the wind because the grains that make up the dunes have an electric charge, so they clump together. Titan’s everyday winds are too weak to move the clumps. At the equinoxes, though, jet streams that move eastward may give birth to big storms. Winds from those storms could be strong enough to sculpt the dunes on this intriguing moon.

Look for Saturn low in the southeast and south this evening. It looks like a bright golden star. Titan is visible through good binoculars or a telescope.

Script by Damond Benningfield

Lightning can make an AM radio station crackle. It can also produce radio static on other planets. In fact, the Cassini spacecraft recorded this “crackling” from storms on Saturn, the second largest planet in the solar system.

Lightning on Saturn can be up to 10,000 times stronger than the typical flash here on Earth. But it’s also much less common: thousands of flashes per year, compared to millions on Earth.

Lightning on Saturn forms in the same way as lightning on Earth: in big thunderstorms. Warm water vapor rises high into the atmosphere, creating static electricity. When that static discharges, it produces lightning.

Most of the lightning on Saturn has been concentrated in one region of the southern hemisphere, and a single monster storm in the northern hemisphere. The southern region is known as “storm alley.” One storm in that region lasted for nine months, although it didn’t produce lightning all the time.

The northern storm eventually wrapped around the entire planet. It produced the highest rate of lightning flashes yet recorded — about 10 per second: the most brilliant display yet seen on the giant planet.

We can’t see the lightning from here on Earth, but we can see Saturn. It’s in the southeast at nightfall, and looks like a bright golden star. It arcs across the southern sky during the night, and sets around sunrise. More about Saturn tomorrow.

Script by Damond Benningfield

It’s rare for second place to outshine first. Most of the accolades go to the winner of the NBA Finals or the gold medalist at the Olympics, not the runner-up. Yet among the planets of the solar system, second place is a lot more memorable than first.

That’s because the second-largest planet is Saturn, the most easily identifiable world other than our own. Its bright rings are both beautiful and memorable — the most distinctive feature of any planet or moon.

The rings are made of small bits of ice mixed with chunks of rock and a sprinkling of dust. Scientists aren’t positive just when they formed — whether it was billions of years ago, or much more recently.

But scientists are sure that the rings are dynamic. As small moons orbit within the rings, for example, their gravity pushes the ring material, creating ripples that look like the wake of a passing boat. Small moons are sometimes pulverized by collisions, adding material to the rings. At the same time, ring particles can bunch up to form new moons. And one of the outer rings gets fresh ice from geysers on one of Saturn’s bigger moons.

That helps make Saturn’s rings intriguing as well as beautiful — enhancing the appeal of the solar system’s runner-up.

And Saturn is putting on its best showing of the year this week. It’s low in the southeast as night falls, and looks like a bright gold star. It arcs across the south later on, and remains in view all night.

More about Saturn tomorrow.

Script by Damond Benningfield

Our home galaxy, the Milky Way, is about 13 billion years old. We know the age because some of its original stars are still around. Many of them congregate in giant ball-shaped clusters. And one of those clusters is in good view at this time of year.

Messier 5 is in the constellation Serpens, the serpent. As night falls, it’s well to the lower left of Arcturus, a bright yellow-orange star that’s high in the south. Through binoculars, the cluster looks like a fuzzy star. Small telescopes begin to reveal some of its individual stars.

In all, M5 has at least a hundred thousand stars, and perhaps as many as half a million. Most of them are packed into a spherical region that’s about 165 light-years across. For that reason, M5 is known as a globular cluster.

The cluster is about 13 billion years old. Astronomers determined that age by taking a census of its stars. More-massive stars burn out faster than less-massive ones. So finding the heaviest remaining stars in a cluster reveals its age. In this case, the oldest remaining stars are a bit less massive than the Sun, so they’re a bit fainter and oranger.

M5 orbits the center of the galaxy at more than a million miles per hour. That suggests that it comes from the galaxy’s halo — a spherical region that extends far outside the Milky Way’s bright disk. Right now, M5’s orbit has brought it near the disk. Over time, though, it will again loop into the outer reaches of the halo, leaving the disk behind.

Script by Damond Benningfield

As you listen to this program, you may be sitting still at your desk, or perhaps whisking down the highway at 70 miles an hour in your car. But those feelings of motion — or lack of it — are misleading. We’re all zipping through space at a couple of million miles per hour — the combined result of several different motions.

The most obvious of those motions is Earth’s rotation on its axis. It takes a bit less than 24 hours to complete one full turn. So anyone at the equator is moving at more than a thousand miles per hour. Those of us in the United States are moving a bit slower than that, since Earth’s circumference at higher latitudes is smaller than at the equator.

The next motion is Earth’s year-long orbit around the Sun, which we complete at an average speed of about 67,000 miles per hour.

These motions are small, though, compared to what comes next. The entire solar system is orbiting the center of the Milky Way galaxy, which is about 27,000 light-years away, at half a million miles per hour. Even at that speed, though, it takes about 230 million years to complete a single turn around the galaxy.

Finally, the entire Milky Way is moving toward a giant cluster of galaxies in the constellation Virgo, as well as a supercluster of galaxies beyond it. The gravitational pull of these great clusters accelerates us to about 1.3 million miles per hour — a high-speed jaunt across the universe.

Tomorrow: some of the Milky Way’s most ancient stars.

Script by Damond Benningfield