StarDate

If a cosmic giant sat on a big, gassy planet, it would look a lot like Saturn, the second-largest planet in the solar system. It’s 10 percent wider through its equator than through the poles. But Saturn flattened itself – a result of its low density and fast rotation.

Saturn consists of a series of layers. Its core is a dense ball of metal and rock. Around that is a layer of hydrogen that’s squeezed so tightly that it forms a metal. Around that is a layer of liquid hydrogen – the lightest and simplest chemical element. And the planet is topped by an atmosphere that contains methane, ammonia, water, and other compounds.

Despite its great size, Saturn spins once every 10.7 hours. That pushes material outward, making the planet fatter through the equator. The combination of its composition and rotation makes Saturn especially light – it’s less dense than water.

Saturn doesn’t have a solid surface. But scientists have defined a “surface” as the depth in its atmosphere where the pressure equals the surface pressure on Earth. At that level, Saturn’s gravity is only a bit stronger than Earth’s gravity. So if you were floating at that altitude, you’d feel like you’d added a few pounds. And because of Saturn’s flattened shape, you’d feel heavier at the poles than the equator.

Look for Saturn near the Moon tonight. It looks like a bright star to the right of the Moon in early evening, and farther below the Moon at dawn.

Script by Damond Benningfield

Building the planets of the solar system was like building a city – it didn’t happen all at once. Instead, it probably took a hundred million years or more to complete the construction project.

The first to be completed were Jupiter and Saturn, the Sun’s largest planets. They came together in the prime real estate for planet building – the region with the most raw materials. Closer to the Sun, it was so hot that ices were vaporized and blown away. Farther from the Sun, the material thinned out. But at the distance of Jupiter and Saturn, the balance was just right.

The two giants took shape in a hurry. Small grains of ice and rock stuck together to make pebbles, then baseball-sized chunks, then boulders, and so on. That quickly built massive cores, which then swept up huge amounts of leftover hydrogen and helium gas. So within just a few million years, Jupiter and Saturn have grown to monstrous proportions.

Uranus and Neptune took shape a little later – within tens of millions of years. Earth and the other rocky inner planets took a bit longer – at least a hundred million years. So the biggest planets of the solar system are also the oldest – dating to shortly after the birth of the Sun.

Saturn stands close to the Moon the next couple of nights. The planet looks like a bright star. It’s to the lower left of the Moon as darkness falls tonight, and about the same distance to the right of the Moon tomorrow night.

Script by Damond Benningfield

The 41st episode of a celestial series plays out tomorrow: a total lunar eclipse. It’ll be visible around much of the world – but not the Americas.

Every eclipse belongs to a series, called a Saros. The eclipses in a Saros are separated by 18 years plus 11 and a third days. If we could watch all the eclipses in the cycle play out, we’d see the Moon pass through Earth’s shadow from top to bottom or bottom to top. So the Moon barely dips its toe in the shadow at the beginning and end of the sequence. But it’s fully immersed during the middle of the cycle, creating total eclipses. And because of that extra third of a day in the cycle, each eclipse occurs a third of the way around the world from the previous one.

This eclipse is part of Saros 128. The cycle began in 1304 and will end in 2566 – 71 eclipses in all.

Most of Asia and Australia will see this entire eclipse, from beginning to end. And most of the rest of the world will see at least part of it. Totality – when the Moon is completely immersed in the shadow – will last for an hour and 22 minutes. But the eclipse occurs during the middle of the day for those of us in the United States, so we won’t see any of it.

What we will see the next couple of nights, though, is a beautiful full Moon – the Fruit Moon or Green Corn Moon – completely free of Earth’s dark shadow.

Script by Damond Benningfield

The Moon will briefly cover up the tail of the sea-goat tonight – Deneb Algedi, the brightest star of Capricornus. The sequence will be visible across much of the United States.

This vanishing act is an occultation – a type of eclipse in which one object completely covers another. But eclipses are nothing new for Deneb Algedi. Not only does it periodically get covered up by the Moon, but it stages its own eclipses – two of them every day.

What we see as Deneb Algedi is a binary – two stars in a tight orbit around each other. The main star in the system is about twice as big and heavy as the Sun, and much brighter. Its companion is a little smaller and fainter than the Sun.

We’re looking at the system edge-on, so the stars pass in front of each other – creating eclipses. When the fainter star crosses in front of the brighter one, the system’s overall brightness drops by about 20 percent – enough for a skilled skywatcher to notice. But when the brighter star eclipses the fainter one, the dip is much smaller, so it’s detectable mainly with instruments.

The stars orbit each other once a day. That means we see two eclipses per day – just 12 hours apart.

Deneb Algedi isn’t especially bright, so it’s hard to see through the bright moonlight. But binoculars will help you pick it out. From much of the western U.S., the Moon will just miss the eclipse-happy tail of the sea-goat.

Script by Damond Benningfield

The planet Jupiter will slide past one of the brighter stars of Gemini the next few mornings. At their closest, they’ll be separated by just a fraction of a degree.

The star is Wasat – from an Arabic phrase that means “the middle.” But the middle of what has been lost over the centuries.

The star also is known as Delta Geminorum – its Bayer designation. The system was devised in the early 17th century by German astronomer Johann Bayer. He named all of the stars in the constellations that were visible from the northern hemisphere. Each star was given a Greek letter followed by the constellation name. If he ran out of letters, he switched to the Latin alphabet.

In most constellations, Bayer named the stars in the order of their brightness. The brightest was alpha, the next-brightest was beta, and so on. Sometimes, he ranked the stars on their location or some other system. And he named the stars based on how they looked to the naked eye, so the rankings were completely subjective. So even though delta is the fourth letter in the Greek alphabet, Delta Geminorum is only the eighth-brightest star in Gemini.

Jupiter and Wasat are well up in the east at dawn. Jupiter looks like a brilliant star, far to the upper right of even-brighter Venus. Wasat will stand below Jupiter tomorrow. Jupiter will drop past it over the following couple of days, so they’ll be at their closest on Saturday and Sunday.

Script by Damond Benningfield

Water is the key ingredient for life on Earth. And as far as we know, it’s a key ingredient for life everywhere else in the universe as well. That shouldn’t be a problem, though, because there’s plenty of water to go around.

Water is common in part because it’s made of two of the three most common elements in the universe – hydrogen and oxygen. They come together in the cold of deep space to make grains of ice. Some of those grains are found in the clouds of gas and dust that give birth to new stars and planets. Others form inside those clouds.

In recent years, astronomers have found evidence of water in other star systems, and even in other galaxies. They’ve found grains of ice in the disks of material around newborn stars. They’ve seen giant belts of comets, which contain a lot of ice. They’ve discovered water vapor in the atmospheres of a few planets. And they’ve even found evidence that some planets could be covered in oceans of liquid water.

One example is TOI 1452 b, which orbits a star that’s much smaller and fainter than the Sun. The planet itself is bigger and heavier than Earth. Given its details and its distance from the star, scientists say it could have a deep global ocean – a possible home for life.

TOI 1452 is about a hundred light-years away, in Draco. The dragon twists high across the north at nightfall. But the star is much too faint to see without a telescope.

Script by Damond Benningfield

Earth is the only body in the solar system with liquid water on its surface. But it’s not the only one where you can find water. In fact, water is everywhere – from comets and asteroids to the giant planets.

Comets and asteroids are chunks of rock, metal, and ices – including water ice. Comets have more ice, but most asteroids probably have large amounts as well. Such bodies might have supplied much of the water on Earth when they collided with our planet billions of years ago.

Water ice is common throughout the solar system. It’s been seen at the poles of the Moon and the planet Mercury – the Sun’s closest planet. It forms large polar caps on Mars. And it coats many of the moons of the giant outer planets.

Water also has been detected in the clouds of Jupiter and Saturn. And it may be a major component of the outer layers of Uranus and Neptune, the Sun’s most remote planets.

To find liquid water, you have to go deep. There may be global oceans of water far below the surfaces of some of the big moons of Jupiter and Saturn, and perhaps some moons of Uranus and Neptune as well. Some of those oceans could hold more water than all of Earth’s oceans combined.

On Earth, water is a key ingredient for life. So some of the moons of the outer planets are considered good places to look for life – swimming in oceans of liquid water.

We’ll talk about water beyond the solar system tomorrow.

Script by Damond Benningfield

Water is all about extremes. The atoms that make up water molecules were forged in some of the hottest environments in the universe. But most of the molecules formed in the cold of deep space.

A water molecule consists of two hydrogen atoms plus one oxygen atom – H-2-O. A hydrogen atom contains one electron and one proton. The electrons formed in the first fraction of a second after the Big Bang, when the universe was extremely hot and dense. The protons formed a few minutes later.

By about 380,000 years, the universe had expanded and cooled enough for the electrons and protons to stick together to form atoms. And today, hydrogen accounts for more than 90 percent of all the atoms in the universe.

Hydrogen and helium, the other major element forged in the Big Bang, soon came together to make stars. And a star’s core is hot enough to “fuse” lighter elements to create heavier ones.

The first steps in that process create carbon, nitrogen, and especially oxygen – the third-most abundant element in the universe. When stars die, they expel some of those elements into space. And in the cold away from the stars, hydrogen and oxygen can stick together to make molecules of water. Some of the water’s incorporated into planets – including our own.

So the next time you take a cool drink of water, think of the hot-and-cold origins of this important compound.

We’ll have more about water tomorrow.

Script by Damond Benningfield

To have a strong heart, you naturally need strong arteries. And that’s not a problem for Antares, the heart of the scorpion. It’s flanked by two fairly bright stars that historically have shared a name: Alniyat – an Arabic name that means “the arteries.”

The stars probably are siblings of Antares. They all formed from the same giant complex of gas and dust, within the past 10 million years or so.

Alniyat I is also known as Sigma Scorpii. It’s a system of four stars. Two of them form a tight pair, with a third close by. The fourth star is farther out.

Both stars in the tight grouping are much like Antares. They’re many times the mass of the Sun, so they’ll probably end their lives with titanic explosions. Antares is a little farther along its lifecycle, so it’s closer to that showy demise.

Alniyat II is Tau Scorpii. It’s a single star. It, too, is destined to explode as a supernova, but not for several million years – a little later than Antares and the main star of Sigma. On the astronomical clock, though, that’s close – just a few ticks away.

Antares and its arteries are close to the right of the Moon at nightfall this evening. Sigma is close to the right or upper right of Antares. Tau is about the same distance to the lower left of Antares. The arteries aren’t as bright as the scorpion’s heart, though, so you might need binoculars to see them through the glare.

Script by Damond Benningfield

A spacecraft that’s on it way to Jupiter is “pinballing” around the solar system, getting an extra “kick” as it zips close to the planets. It’ll get the next kick tomorrow, from Venus.

The spacecraft is JUICE – Jupiter Icy Moons Explorer. It’s scheduled to arrive at Jupiter in 2031. But it needs help to get there. And it gets that help from the gravity of Venus, Earth, and the Moon.

During each encounter, the craft “steals” a bit of gravitational energy. That speeds it up and sculpts its path around the Sun. The encounters drastically reduce the amount of fuel JUICE must carry, cutting its size and weight and reducing its cost. JUICE flew past Earth and the Moon a year ago. It’ll get additional boosts from Earth in 2026 and ’29.

JUICE will scan Venus as it flies past. That will give scientists some extra information about the planet. And it’ll give engineers a chance to check out the craft’s instruments.

When JUICE arrives at Jupiter, it’ll orbit the planet for almost three years. After that, it’ll begin orbiting the planet’s largest moon, Ganymede. Its observations of Ganymede and Jupiter’s other icy moons will reveal details about their possible buried oceans, which could be habitats for microscopic life.

Venus and Jupiter are in the dawn sky now. Venus is the brilliant “morning star,” with slightly fainter Jupiter to its upper right – two destinations for a “pinballing” explorer.

Script by Damond Benningfield