StarDate

The Cassini spacecraft has been orbiting Saturn for almost 13 years. During that time, it’s made many visits to the giant planet’s moons. It’s paid special attention to the biggest moon, Titan, passing less than a thousand miles from it dozens of times. And on Saturday, it’ll make one final pass by the big moon — its final close encounter with any of Saturn’s moons.

Titan is veiled by smog — an orange haze that makes it impossible to see the surface. But Cassini has viewed the moon at wavelengths that are invisible to the eye. Those wavelengths pass through the haze, allowing Cassini to study the surface in detail.

Among other things, those observations have revealed sand dunes that can be hundreds of miles long, and lakes and seas that are filled with liquid hydrocarbons. The largest is bigger than Lake Superior. Occasional rains may fill rivers, which then flow into the lakes and seas.

Cassini has also given us hints of even more liquid far below Titan’s surface — an ocean of water mixed with ammonia. It could be many miles thick, and contain far more water than in all of Earth’s oceans.

Cassini will fly about 600 miles above Titan on Saturday, giving scientists a farewell look at the intriguing moon. As a bonus, Titan’s gravity will reshape Cassini’s orbit, placing it on a course that’ll carry the craft between Saturn and the inner edge of its rings — beginning the final phase of its lifetime of discovery. More about that tomorrow.

Script by Damond Benningfield

For sheer grandeur, no astronomical object beats a spiral galaxy. And one of the grandest of them all climbs high across the sky on spring nights: Messier 100. We see it face-on, providing an amazing view of its delicate spiral arms.

The galaxy is one of the leading members of the Virgo Cluster — a couple of thousand galaxies that travel through the universe together.

M100 is bigger and more massive than our home galaxy, the Milky Way. But like the Milky Way, it’s a wide, flat disk. Waves ripple through the disk, squeezing clouds of gas and dust and causing them to give birth to new stars. Many of the newborn stars are especially hot and bright, so they outline the waves, forming the spiral arms.

M100 has another thing in common with the Milky Way: It has a couple of smaller companion galaxies. The gravity of these galaxies is tugging on M100. That may be responsible for a slight offset in one side of the spiral, making at least one of the spiral arms look a bit deformed.

A couple of decades ago, Hubble Space Telescope discovered some stars in the galaxy that are good mile markers. That allowed astronomers to make a good measurement of M100’s distance: about 56 million light-years.

And a small telescope reveals M100 shining from that great distance in the corner of the constellation Coma Berenices, near Virgo. A slightly bigger telescope begins to reveal its great beauty — a grand spiral galaxy whirling through the cosmos.

Script by Damond Benningfield

Galaxies don’t like to be alone. They congregate in clusters — collections of a few dozen to a few thousand of them, all bound by their mutual gravitational pull. Our own galaxy, the Milky Way, belongs to a small cluster known as the Local Group.

The nearest big cluster is about 50 million light-years away. It’s in the constellation Virgo, so it’s known as the Virgo Cluster. It contains a couple of thousand galaxies.

Most of them are puffballs — dwarf galaxies that are only a fraction the size and mass of the Milky Way. But a few are giants — they’re the size of the Milky Way or bigger, and some may contain dozens of times as many stars as the Milky Way.

The Virgo Cluster is at the center of a much larger collection of galaxies: the Virgo Supercluster. It includes several clusters that move through space as a unit, tied together by gravity. Membership includes the Local Group, which is on the supercluster’s outskirts. We’re becoming more tightly integrated into the supercluster, though, as the Local Group is pulled toward the Virgo Cluster.

And it’s easy to spot the cluster’s location right now. It’s high in the east and southeast at nightfall. It’s roughly between the planet Jupiter, which is the brightest object in the sky at that hour, and the tail of Leo, the lion. Binoculars reveal a few of the galaxies as hazy smudges of light, while a small telescope brings several galaxies into sharper view.

More about the Virgo Cluster tomorrow.

Script by Damond Benningfield

There are lots of ways to think of galaxies. We might look at them as cosmic cities, each with a densely populated “downtown” and more thinly settled suburbs. More poetically, we might think of them as “island universes” — collections of stars, gas, and all the other stuff that makes up the universe, all in one big, bright, beautiful package.

However you look at them, galaxies are the biggest individual objects in the universe. They contain anywhere from a few million stars to trillions. And they span anywhere from a few thousand light-years to a million or more.

These giant agglomerations are dynamic — they grow and evolve. Their clouds of gas and dust collapse to give birth to stars, for example. That can happen steadily, like the drip of a leaky faucet — just what’s happening now in our home galaxy, the Milky Way. Or it can happen in a hurry, like the rush of a fire hose — usually when two galaxies interact with each other — passing close to each other, or even merging.

In fact, big galaxies like the Milky Way probably got so big through mergers. In the early universe, most galaxies were pretty small. They were packed together so tightly, though, that their gravity pulled them together. Today, big galaxies continue to grow by gobbling up smaller ones; the Milky Way is ingesting several right now.

Gravity also holds together big groups of galaxies, known as clusters. And we’ll talk about one of the most impressive clusters tomorrow.

Script by Damond Benningfield

Most stars consist almost entirely of hydrogen and helium, the lightest and simplest chemical elements. But there’s a smattering of other ingredients as well — everything from oxygen to platinum. These elements can be tough to detect, though. For one thing, there’s not much of them — they add up to only a fraction of one percent of all of a star’s atoms. And for another, they’re usually concentrated deep inside the star, where they’re hidden from astronomical instruments.

But one class of stars likes to show off some of those elements. The stars in this class all have about the same surface temperature — they’re much hotter than the Sun.

They’re known as mercury-manganese stars, because they can show concentrations of these and similar elements that are thousands of times greater than in other stars.

They don’t really have more of these elements, though. Instead, more of the elements have been dredged up from deep inside. Radiation from the star’s core may push these elements to the surface, where astronomers can see them. On the other hand, gravity pulls other types of elements toward the core, so the stars show unusually low levels of them.

One good example of a mercury-manganese star is known as Gienah. It’s the brightest star of Corvus, the crow. The constellation is in the southeast as night falls, and due south around midnight. Its brightest stars form the outline of a small sail. Gienah is at the top right corner of the sail.

Script by Damond Benningfield

Saturn follows the Moon across the sky late tonight. The planet looks like a bright star, and perches close to the lower left of the Moon at first light tomorrow.

The Moon is in its “gibbous” phase now. That means that sunlight illuminates more than half of the hemisphere that faces our way.

The portion that’s in view offers a good look at the contrast between the two main landforms on the Moon: the dark “seas” and the lighter highlands.

As the name implies, the highlands are generally at a higher elevation than the seas. They’re the fractured remains of the Moon’s original crust. They consist mainly of a mineral known as anorthosite. It’s not very heavy, so when the Moon formed, it floated to the top. It contains a lot of calcium, which is why it has a lighter color.

Over the eons, impacts by space rocks blasted the crust, churning it up — like pounding the tiles on your kitchen floor with a hammer. That created a jumbled landscape of hills, mountains, canyons, and other dramatic features.

Some of the biggest impacts blasted holes all the way through the crust, or created deep cracks in the crust. That allowed molten rock to bubble up from below, forming volcanic plains. This rock, known as basalt, contains a higher concentration of dark materials, so the plains look dark. They reminded long-ago skywatchers of bodies of water, so they were called maria, the Latin word for “seas” — dark seas of rock on the surface of the Moon.

Script by Damond Benningfield

Weather forecasts tell us the odds that it’ll rain tomorrow. Betting lines tell us the odds that one team will beat another. And the Rio Scale tells us the odds that a possible signal from ET is either the real thing or just wishful thinking.

Scientists developed the Rio Scale almost two decades ago, after reports of the discovery of intelligent signals from another star turned out to be a hoax. The scientists wanted an easy way to rate the odds that a possible discovery was the real deal — not only for themselves, but for the media and public as well.

So during a conference in Rio de Janeiro, Jill Tarter and Ivan Almar devised a rating system. It evaluates two main factors. One is the importance of a possible discovery. A signal from a star a few light-years away — one we could communicate with over a human lifetime — rates higher than a signal from a star across the galaxy.

The other factor is the credibility of any reported discovery. A signal that’s been detected several times by reputable institutions would get a higher rating than a signal heard by a guy with a backyard radio dish.

When those two factors are combined, they produce a number from zero to 10. Right now, the only discovery that rates as high as a 3 or 4 is Tabby’s Star — an odd system in which many objects routinely pass in front of the star, blocking some of its light. That doesn’t mean ET is there, only that it’s worth doing some more looking — just in case.

Script by Damond Benningfield

Venus is just beginning its long reign as the “morning star.” It’s quite low in the east as dawn begins to color the sky. It’s so bright, though, that if you have a clear horizon, it’s easy to pick out. It’ll remain in good view in the morning sky until after Thanksgiving.

Despite its beauty, Venus is a hellish world. Surface temperatures are hot enough to melt lead, and the atmosphere is dense and toxic. There’s nothing else like it in the solar system. But Venus-like worlds may be common in other star systems. In fact, a recent study found a system that could have two Venuses.

Wolf 1061 is just 14 light-years away — closer than all but a few dozen other stars. Yet it’s only one percent as bright as the Sun — so faint that you need a telescope to see it.

Three planets are known to orbit the star. All three probably are rocky worlds a bit bigger and heavier than Venus and Earth. One of them is so close to the star that it’s almost certainly like Venus. Another follows an orbit that’s so stretched out that the planet is quite hot for part of its year, and quite cold for the rest.

A third planet appears to spend about two-thirds of its year inside the habitable zone — the distance from the star where temperatures are just right for liquid water. But it spends the rest of the time closer than the habitable zone, where it’s hot. That may trigger conditions like those on Venus — keeping all three worlds of Wolf 1061 too uncomfortable for life.

Script by Damond Benningfield

If you love gemstones, scientists have discovered a couple of worlds that might be just right for you. One of them probably has a layer of garnet below its crust. And the other may have clouds composed of the mineral that makes up rubies and sapphires.

Kepler 407b is a dense, rocky world like Earth. It’s about the same size as Earth, but up to three times heavier. It orbits a star that’s almost identical to the Sun.

Astronomers looked at the chemical composition of the star. They then worked with geophysicists to estimate the composition of the planet. Their work suggested that Kepler 407b could have a layer that’s made mainly of garnet.

The other planet is HAT-P-7b. It’s bigger and heavier than Jupiter, the giant of our own solar system. But it’s only a few million miles from its star. At that range, the planet is not only extremely hot, but it’s locked so that the same hemisphere always faces the star.

Like Jupiter, the planet is probably a big ball of gas, with clouds atop its atmosphere. Although no one has seen the clouds, they have detected bright spots moving across the planet — cloud banks moving from the nightside, where they form, to the hotter dayside.

Those clouds are so hot that they probably are made of solid particles, including the mineral that makes up rubies and sapphires. As the clouds move into daylight, those particles “rain” back into the giant planet’s atmosphere — a shower of gems on a distant world.

Script by Damond Benningfield

The Moon is just past full this evening. Sunlight illuminates all but a sliver of the lunar hemisphere that faces our way, so the Moon is bright and beautiful. And it has a couple of bright, beautiful companions. The star Spica stands to its upper right, with the brilliant planet Jupiter above Spica.

If you look at the Moon, you’ll see the same features that are always in view — dark volcanic plains and lighter-colored highland regions. They’re the same features you’ll see tomorrow night, and a month from now, and a hundred years from now.

Since the same side of the Moon always faces our way, you might think that the Moon doesn’t turn on its axis. But that’s not the case. In fact, if it didn’t turn we’d see the entire Moon — the familiar near side and the hidden far side. Instead, it’s the rate at which it turns that keeps it aiming our way.

Over the eons, Earth’s gravity created tides in the lunar surface. That slowed the Moon’s rotation. Eventually, it slowed enough that the same side always faces our way. So the Moon turns on its axis at the same rate at which it orbits Earth. By the time it’s moved a quarter of the way around us, it’s also completed a quarter turn on its axis, and so on.

The Moon, of course, creates tides in Earth’s oceans, which are slowing its rotation. So if Earth and Moon are around long enough, then just as the same hemisphere of the Moon always faces Earth, the same hemisphere of Earth will always face the Moon.

Script by Damond Benningfield