StarDate

Globular star clusters are like skyscrapers in the suburbs. They’re densely populated structures surrounded by a lot of open space. Most of them are well outside the Milky Way’s disk. They’re tight balls of hundreds of thousands of stars, so they’re easy to spot.

An example is Messier 13, in the constellation Hercules. It climbs into view in the northeast by 9 or 10 p.m. Under dark skies, it looks like a faint smudge of light. Binoculars enhance the view.

M13 is about 25,000 light-years away. It contains several hundred thousand stars, packed into a ball about 150 light-years across. Most of its stars are right in the middle. They’re packed more than a hundred times tighter than the stars in our own galactic neighborhood. As a result, many more bright stars would speckle the skies of any planets in that region.

Most of those stars would look faint, and yellow or orange. That’s because M13’s original big, bright, blue stars have long since expired. The survivors are less massive than the Sun, and a good bit older. Such stars look redder and fainter than the Sun. A few stars have passed beyond the prime of life, so they’ve puffed up and gotten extremely bright. They’ll soon lose their outer layers, though, leaving only their hot but dead cores, known as white dwarfs.

A few stars in M13 have merged. That’s made them hotter, brighter, and bluer — the stand-out residents of a galactic skyscraper.
 

Script by Damond Benningfield

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Most stars are born in bunches — great swarms that form from the collapse of a single cloud of gas and dust. The stars tend to stick together for a while in clusters. Over time, though, the gravity of the galaxy’s other stars and gas clouds pulls most clusters apart, so the stars go their own ways.

Even then, many of the stars can share a common motion through the galaxy — they move at the same speed, and in the same direction. Such associations are known as moving groups. And the title star of one such group climbs into view in the northeast in late evening at this time of year.

Zeta Herculis is at the top right corner of the Keystone, a pattern of four stars at the center of Hercules. It’s about 35 light-years from Earth. It actually consists of two stars. One of them is bigger, brighter, and heavier than the Sun. The other is less impressive than the Sun.

Decades ago, astronomers discovered that the system shares its motion through space with several other stars, which are spread across several constellations. That suggested that the stars were born together, so they were called the Zeta Herculis Moving Group.

Over the years, though, observations have been unable to confirm whether the stars are all the same age. So the stars of the moving group could be related — or they could just happen to share the same motion through the galaxy.

We’ll talk about a cluster in Hercules that has stuck together tomorrow.
 

Script by Damond Benningfield

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A body in the outer solar system has a ring, and it’s a bit vexing. It’s so far from its host world that it shouldn’t exist. Instead, theories of ring behavior say its material should have stuck together to form a moon.

The host is Quaoar — a dwarf planet. It’s about 700 miles across — a third the size of our own moon. And it’s more than 40 times farther from the Sun than Earth is — in the deep freeze of the Kuiper Belt.

Over several years, astronomers used telescopes on the ground and in space to watch as Quaoar passed in front of four different stars. The way a star’s light fades reveals details about Quaoar, such as whether it has an atmosphere.

Instruments recorded flickers in the starlight shortly before and after each star passed behind the dwarf planet. That indicated the presence of something that’s not completely solid — a ring. It ranges from a few miles to a couple of hundred miles across.

The ring is about 2500 miles from Quaoar. Inside about half that distance, Quaoar’s gravity would keep the ring material from sticking together. At the ring’s current distance, though, a moon should form pretty easily.

The ring might be kept stirred up by Quaoar’s one known moon. Or in the extreme cold, the icy particles that make up the ring might be so hard that they bounce off each other. Whatever the reason, Quaoar has an inconvenient ring — one that hasn’t behaved the way it was expected to.
 

Script by Damond Benningfield

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Many of the big moons of Jupiter and Saturn appear to have oceans of liquid water below their icy surfaces. And a recent study says that the big moons of Uranus may be “juicy” as well.

The study was led by Julie Castillo-Rogez of NASA’s Jet Propulsion Laboratory. Her team looked at the moons Miranda, Ariel, Umbriel, Titania, and Oberon. Titania is the largest, although it’s less than half the size of our own moon.

The team simulated what’s going on inside the moons. The researchers considered each moon’s size, distance from Uranus, likely composition, and other factors. They also considered an alignment of the moons millions of years ago that could have heated their centers.

The simulations showed that all but Miranda — the smallest of the five — could have oceans. They could range from a few miles deep, to as much as 30 miles for Titania. They could be kept liquid by the internal heat of the moons, or by ammonia or other compounds that lower the freezing point.

A proposed mission to Uranus would scan the moons. Uranus’s magnetic field might cause the oceans to create their own magnetic signatures. The craft’s instruments should be able to detect those signatures — confirming a few more “juicy” moons in the outer solar system.

Uranus is low in the west in the evening twilight. It’s to the upper left of the planet Mercury, which looks like a bright star. Through binoculars, Uranus looks like a faint green star.
 

Script by Damond Benningfield

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Saturn has more than 80 known or suspected moons, including several that are pretty big. And a recent study says it might have had another big one in the distant past. Named Chrysalis, it might have broken apart a hundred million years ago, forming Saturn’s rings.

Planetary scientist Jack Wisdom at MIT was intrigued by Saturn’s tilt on its axis. It’s a lot more severe than it should be. That means something must have knocked the planet over. The gravity of the giant planet Neptune could have done the job, but only under special conditions — perhaps provided by Saturn’s largest moon, Titan.

Titan is moving away from Saturn at about four inches per year. Simulations by Wisdom and colleagues showed that, as Titan moved outward, it changed a “wobble” in Saturn’s rotation. That brought Saturn and Neptune into sync, tilting Saturn over.

Chrysalis was orbiting between Titan and another moon. As Titan moved outward, the gravity of Titan and Chrysalis increased Saturn’s tilt even more. It also pushed Chrysalis into a more and more lopsided orbit. That eventually brought it so close to Saturn that it was ripped apart by the planet’s gravity. The debris spread out to form the rings.

That scenario matches many estimates of the age of the rings — evidence that Saturn’s rings could be debris from a vanished moon.

Saturn appears near our own moon at dawn tomorrow. It’s to the left of the Moon, and looks like a fairly bright star.
 

Script by Damond Benningfield

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The space around Earth is getting crowded — thousands of satellites orbit our planet. The space around Jupiter is getting crowded, too — with natural satellites. A recent spate of discoveries has brought the number of known moons to 95 — more than any other planet in the solar system. And many more probably are just waiting to be discovered.

The new moons are small — no more than about five miles in diameter. And most of them are a long way from Jupiter — they take at least 11 months to make a single orbit.

The discoveries were made by a group led by Scott Sheppard of the Carnegie Institution for Science. Sheppard’s been involved in the discoveries of 70 of Jupiter’s moons. It takes careful observations to find the moons because they’re small, faint, and far away from Jupiter.

Three of the newly found moons orbit in the same direction as Jupiter’s rotation on its axis. They probably are chips of larger bodies that were blasted apart by big impacts.

The other new moons orbit in the opposite direction. They may have been asteroids that were captured by Jupiter when they wandered close to the solar system’s largest planet.

The new discoveries vaulted Jupiter past Saturn, which has 83 known moons, although its numbers are likely to swell, too. And a recent study said that Saturn might have had another big moon that was pulverized by Saturn’s gravity, forming the planet’s rings. More about that tomorrow.
 

Script by Damond Benningfield

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When the JUICE mission begins its trek to Jupiter later this month, it’ll be launched by one of the most powerful rockets on the planet, Ariane 5. And the probe carries more than three tons of propellants. Even so, it’ll need a lot of help to reach Jupiter, and to achieve its scientific goals.

JUICE is scheduled to enter orbit around Jupiter in 2031. But it can’t get there on its own. So as it loops around the Sun, it’ll stage some close encounters — three with Earth, and one each with the Moon and Venus. JUICE will “steal” a tiny bit of energy during each pass. That will give it a gravitational “kick,” which will boost its speed and sculpt its path.

The mission’s main goal is to orbit Ganymede, the solar system’s biggest moon. It appears to have a deep ocean of liquid water far below its surface. The ocean could be a habitat for life. And even if it’s not, studying Ganymede will tell us more about how Jupiter’s big moons formed, how they’re put together, and how they’re changing.

But JUICE will need more help to enter that orbit. It will stage more than two dozen flybys of Ganymede and Jupiter’s other big icy moons, Callisto and Europa — important targets on their own. JUICE will use each moon’s gravity to refine its path — like getting a free rocket burn with each pass. That will ensure that it reaches Ganymede at just the right time, angle, and speed — when it can use its own power to settle into orbit.
 

Script by Damond Benningfield

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[AUDIO: Ganymede radio]

That’s the “voice” of Ganymede, the largest moon of the planet Jupiter — radio waves recorded by a spacecraft in 1996. The waves tell us that the moon produces its own magnetic field — the only moon in the solar system that does so. And that helps us learn about Ganymede’s interior — including the likelihood that it has a hidden ocean.

Ganymede is the main target for JUICE, a European spacecraft scheduled to launch this month. It’ll enter orbit around Jupiter in 2031. It’ll make a couple of dozen close passes by Ganymede and Jupiter’s other big icy moons, Europa and Callisto. Then it’ll settle into orbit around Ganymede — the largest moon in the entire solar system.

Ganymede’s ocean appears to be about 60 miles deep, and it’s buried almost a hundred miles below the surface. It contains more water than all of Earth’s oceans combined.

Motions of the layers inside Ganymede generate the magnetic field, which was also “heard” by the Juno spacecraft a couple of years ago. Sloshing saltwater in the ocean may alter the field, causing a “wobble” in the moon’s auroras.

The ocean could have all of the basic ingredients for life: water, along with energy and minerals from volcanic “geysers” on the ocean floor. So it’s possible that Ganymede could host life. JUICE won’t tell us whether that’s the case. But it should provide new details about a giant ocean that’s hidden from view.

More tomorrow.
 

Script by Damond Benningfield

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The planet Jupiter is hiding. It lines up especially close to the Sun, so it’s hidden in the glare. It won’t return to view for a few weeks. But European scientists and engineers know exactly where it is. In fact, they’re getting ready to send a spacecraft toward it this month. It’ll arrive in 2031 to study Jupiter’s three largest moons.

JUICE — short for Jupiter Icy Moons Explorer — is the most ambitious mission to Jupiter so far. While it orbits Jupiter, it’ll make a total of 26 passes by the moons Ganymede, Callisto, and Europa. Then it’ll settle into orbit around Ganymede — the first spacecraft to orbit any moon other than our own.

Scientists are interested in the moons because they’re thought to have oceans of liquid water below their icy crusts. And each of the oceans is thought to hold several times more water than all of Earth’s oceans combined. That means the moons are possible habitats for life.

Europa is considered the best candidate. There could be fountains of hot, mineral-laden water jetting from its interior into the ocean. That would provide the main ingredients for life: water, the right chemistry, and energy.

Because of the shape of its orbit, JUICE will swing past Europa only twice. But its instruments will look for thin spots in the ice and jets of water squirting through the ice. They’ll also peek into the top layers of ice — another first for a “juicy” mission.

More tomorrow.
 

Script by Damond Benningfield

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Antares is a big spotlight. Not only is the star a brilliant beacon on its own, but it also lights up the stuff around it. In particular, it’s illuminating the Rho Ophiuchi complex — clouds and ribbons of gas and dust that are giving birth to more stars.

The complex is about 460 light-years away. Few stellar nurseries are closer. It’s named for one of the bright stars it’s created.

The complex consists of several clouds, plus some streamers that are dozens of light-years long. They’re sculpted by winds and radiation from hot, bright stars.

Some of the clouds and streamers reflect light from Antares and other big, bright stars. Others are energized by the stars, so they produce their own light. And still others remain dark — they’re dense and cold.

Those regions are where most of the new stars are being born. Astronomers have cataloged thousands of stars and potential stars there. Some have not yet flared to life. Others are no more than a million years old. The most massive will live short, brilliant lives, then blast themselves to bits — the same fate that awaits Antares in the next million years or so.

Antares stages a spectacular encounter with the Moon tonight. They climb into good view by about 1 a.m. Antares snuggles within a degree or so of the Moon. The Rho Ophiuchi complex spans about a dozen times the size of the Moon. It’s so faint that it’s best seen in photographs — lit up in part by bright Antares.
 

Script by Damond Benningfield

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