StarDate

The lowest spot on Earth is the Challenger Deep — a part of the Mariana Trench that lies seven miles below the surface of the Pacific Ocean. But that’s barely a wading pool compared to a possible ocean on Titan, the largest moon of Saturn. It could be 150 miles deep and hold far more water than Earth’s oceans.

Titan is enveloped in a cold, thick atmosphere that’s topped with an organic “smog.” Lakes of liquid methane and ethane dot its surface. But there appears to be far more liquid beneath Titan’s crust, which is made largely of frozen water.

The Cassini spacecraft measured how much the surface of Titan flexes — the result of tides caused by the gravitational pull of Saturn. They found that the tides are more than 30 feet high — much higher than if Titan were made just of rock.

So, instead, it must be made of roughly half rock and half water. Some of the water makes up the frozen crust, but much of it must still be liquid — warmed by the tides and the decay of radioactive elements inside Titan.

The ocean, which may contain ammonia, may begin 30 miles down or farther, and could be 150 miles deep. If so, then it contains about four billion cubic miles of water — more than a dozen times more water than all of Earth’s oceans combined.

Look for Saturn rising well to the left of the Moon late this evening. The planet looks like a bright star. But you need a telescope to pick out watery little Titan.
 

Script by Damond Benningfield

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The star system U Scorpii ran a little late. It was expected to erupt in 2020. But it took some extra time — it didn’t pop off until last month.

U Scorpii is a binary system. One of its members is the corpse of a once normal star, known as a white dwarf. The other is in the prime of life. They’re so close together that they orbit each other once every 1.2 days.

At such close range, the white dwarf pulls gas off the surface of its companion. The gas forms a disk around the white dwarf. It spirals inward, piling up on the surface of the white dwarf.

But that can go on for only so long. In the case of U Scorpii, that’s about 10 years. At the end of that period, the new layer of gas is so hot that it triggers a nuclear explosion — the surface becomes a giant H-bomb — an eruption known as a nova. In just a few hours, that makes the system about 10,000 times brighter. It also blows away the disk around the white dwarf, although it begins to re-form within days.

U Scorpii’s first outburst was seen in 1863. Until June, the most recent was in January of 2010. Astronomers had expected it to erupt again as early as two years ago. But it came a little late — making the system flare thousands of times brighter than average.

U Scorpii is in Scorpius, which is low in the south at nightfall. The star is above Antares, the scorpion’s heart. It’s faded dramatically since its outburst, so it’s much too faint to see without a telescope.

Script by Damond Benningfield

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To the human eye, the brightest star in the scorpion is Antares, its orange heart. If our eyes were sensitive to X-rays, though, scorpion’s leading light would be Scorpius X-1. In fact, it would be the brightest star in all the night sky.

Scorpius X-1 was discovered 60 years ago — the first source of X-rays ever seen beyond our own solar system.

It’s actually two stars locked in orbit around each other. Yet the X-rays aren’t directly produced by either star. Instead, it’s the interaction between them that sets the system ablaze.

One of the stars is a neutron star — the ultra-dense corpse of a once mighty star. It’s almost one-and-a-half times the mass of the Sun, yet only as big as a small city. Its companion is a “normal” star, but it’s less than half the mass of the Sun.

The two stars are quite close together. The neutron star pulls gas off the surface of the companion. The gas forms a swirling disk around the neutron star. As the gas spirals inward, it’s heated to millions of degrees — creating X-rays. Eventually, the gas slams into the neutron star at a large fraction of the speed of light, creating even more fireworks.

Antares is almost due south at nightfall, about a third of the way up the sky. Scorpius X-1 is directly above it, by the width of your fist held at arm’s length. It’s too faint to see with the eye alone — unless you just happen to have X-ray vision.

More about the scorpion tomorrow.
 

Script by Damond Benningfield

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Summer is the season of the scorpion. The constellation claws low across the south on summer nights. And it’s pretty easy to pick out. Its stars really do outline what looks like a scorpion — a wide, flat head; a long body curving to the lower left; and a sharp “stinger” at the end. And that outline is visible even from fairly light-polluted suburbs.

Scorpius is one of the 12 classical signs of the zodiac. In the pseudo-science of astrology, the Sun is “in” that sign for about a month, from late October to late November.

That has no relationship to the Sun’s true position. The Sun crosses the actual constellation from roughly November 23rd to the 28th. Scorpius is more than just the stick-figure outline of the scorpion. Astronomers have divided the sky into 88 constellations, each with its own formal borders, like the borders of states or countries. So we know to the minute when the Sun will both enter and exit the boundaries of the scorpion.

Scorpius lies along the plane of the Milky Way. When we look that way, we’re looking into the densest, thickest portion of the galaxy. So the constellation is filled with many bright stars, star clusters, and clouds of gas and dust. Several of the clusters and clouds are faintly visible to the unaided eye. But they require nice, dark skies — letting the wonders of the scorpion shine through on a summer night.

We’ll talk about a few of those wonders tomorrow.
 

Script by Damond Benningfield

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Parker Solar Probe has passed as close as about five million miles to the surface of the Sun. That’s well inside the Sun’s outer atmosphere. And in a few years, it’ll get closer still — less than four million miles. A thick shield will keep its instruments cool and safe as they go where no spacecraft has gone before.

Its achievement suggests that probes in the far-distant future might be able to get even closer to an even bigger star: Antares, the bright orange heart of the scorpion.

Antares is a supergiant. It’s much brighter, heavier, and larger than the Sun. In fact, if it took the Sun’s place, it would extend beyond the asteroid belt, engulfing the four innermost planets — including Earth.

But it’s also several thousand degrees cooler than the Sun. And because it’s so big, its outer layers are thin and puffy — not much denser than the Sun’s outer atmosphere. So even with present-day technology, a craft like Parker Solar Probe should be able to dip well into the outer layer of Antares, grabbing samples of its material from inside the star.

Of course, that won’t happen anytime soon. We can’t even journey to the nearby stars, and Antares is more than 500 light-years away — an immense gulf between us and this immense star.

Antares is easy to find tonight because it huddles quite close to the Moon. They’re in the south at nightfall and set in the wee hours of the morning.

More about Scorpius tomorrow.
 

Script by Damond Benningfield

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A bright star in Cassiopeia is in a short but impressive stage of life: It’s beginning to puff up like a giant balloon.

Beta Cassiopeia is in the north-northeast at nightfall. It’s at the top right point of the letter “W” formed by some of the constellation’s brighter stars.

Beta Cass is probably less than half the age of the Sun. But it’s almost twice as massive as the Sun, so it burns through its nuclear fuel much faster. In fact, it’s probably already consumed the original hydrogen in its core, converting it to helium. Gravity is causing the core to shrink and get hotter, which will allow it to start burning the helium to make even heavier elements.

In reaction to the changes in the core, Beta Cass’s outer layers are starting to puff up. Today, the star is roughly four times wider than the Sun. That’s big, but not nearly as big as the star will be. Over millions of years, it’ll puff up to dozens of times the Sun’s diameter. It’ll then shrink and cool a bit as it goes through another series of changes, before puffing up to even bigger proportions.

At the end of that second cycle, Beta Cass will expel its outer layers. That will briefly surround the star with a colorful bubble of gas and dust. The bubble will dissipate, though, leaving only the star’s now-dead core: a stellar corpse that will slowly cool and fade away into the long cosmic night.
 

Script by Damond Benningfield
 

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The Moon slides toward one of the claws of the scorpion tonight as though it were being dragged in.

The claw is represented by the star Zubenelgenubi. The name means “the southern claw,” even though the star is well away from the scorpion. Instead, it’s part of the present-day constellation Libra, the balance scales. But before Libra was first drawn, the scorpion’s grasp extended far from its body.

No one knows exactly when the claws were lopped off. As early as 3,000 years ago, Babylonians called that region “the balance of heaven.” At the time, the Sun appeared against those stars at the fall equinox — a time when day and night are roughly equal — a time of “balance” in the heavens.

Libra doesn’t show up in the skylore of Greece or Rome until centuries later. And even then, some records continue to describe the region as the scorpion’s claws.

Libra is one of the 12 constellations of the zodiac — a word that means “circle of animals.” It’s the only constellation that doesn’t represent a living thing — not since it was severed from the body of the scorpion.

Zubenelgenubi stands close to the left of the Moon at nightfall. As the hours tick by, the Moon will slide closer to the star. The scorpion itself curls well to the lower left of the pair, down to just above the horizon. It’s easy to spot because its “heart” is marked by the bright orange star Antares.
 

Script by Damond Benningfield

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In the Cold War summer of 1947, flying-saucer mania gripped the United States. The phrase “flying saucer” entered the lexicon in June, when a pilot reported seeing several of them near Seattle. Over the following six weeks, thousands of sightings were reported. And 75 years ago this week, the Army announced it had recovered a saucer that crashed near Roswell, New Mexico.

It’s unlikely that alien spacecraft visited Earth in 1947 or any other year, though. Traveling between the stars isn’t like the movies — you can’t just whoosh from star to star.

The closest star to the Sun is Proxima Centauri, four and a quarter light-years away. The light that leaves Proxima Centauri tonight — traveling at 670 million miles per hour — won’t reach Earth until 2026. And the speed of light is the universal speed limit — nothing in the “normal” universe can go faster.

The laws of physics do allow some shortcuts to the stars. But most of them require huge amounts of energy, they need exotic forms of matter, or they face other obstacles. Engineers have tinkered with some ideas for interstellar ships, but the craft tend to be ponderous, not jaunty little saucers.

A team is working on a way to send an entire fleet of ships to Proxima Centauri. Lasers on Earth would zap “sails” on the ships, pushing them outward at up to 20 percent of the speed of light. There’s only one drawback: Each ship would be about the size of a postage stamp.
 

Script by Damond Benningfield

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On a stormy night 75 years ago, something crashed in the desert near Roswell, New Mexico. The military says it was a balloon carrying top-secret instruments for sniffing out atomic-bomb tests in the Soviet Union. But others say it was a flying saucer carrying visitors from another world — the most famous and persistent UFO story to date.

Saucer mania gripped the country that summer. Thousands of sightings were reported. And Roswell was a perfect place for a close encounter. Robert Goddard had tested his rockets just outside town during the 1930s. The first atomic bomb was detonated not a hundred miles away, the army was testing V-2 missiles at White Sands, and the local air base housed the world’s only atomic-bomb group.

The story of the Roswell Incident broke on July 8th, 1947. Roswell Army Air Field issued a press release saying it had recovered a “flying disk” from a ranch near town. The army soon backtracked, saying the disk was nothing more than a weather balloon.

The story died, but it didn’t rest in peace. Decades later, the base intelligence officer said he really had discovered a flying saucer. He said it was fashioned of a thin, tough metal covered with odd markings. Later versions of the story said investigators had recovered alien bodies, too.

It’s a great story, but scientists will tell you it’s only that — a story — and we’ll tell you why tomorrow — on Star Date.
 

Script by Damond Benningfield

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Objects in the night sky aren’t always what they seem. Take Messier 71, a star cluster in Sagitta, the arrow. Originally, it was classified as an open cluster. In recent decades, though, it’s been reclassified as a globular cluster.

Both types of clusters are big groups of stars that were born together, from the same cloud of gas and dust. But that’s where the similarities end.

Most globular clusters contain a hundred thousand stars or more. The stars clump together in a tight ball. The clusters move well beyond the galaxy’s wide, flat disk. And their stars are among the oldest in the entire Milky Way — typically 10 billion years or older.

Open clusters, on the other hand, generally contain a few hundred to a few thousand stars, they’re concentrated in the disk, and they tend to be young. The combined gravity of their stars isn’t enough to hold a cluster together, so most open clusters vanish within a couple of billion years.

M71 has many of the traits of an open cluster: It’s in the galaxy’s disk, its stars are widely spread, and they’re younger than those in most globulars. But recent work has shown that M71 has more stars than first thought, the stars are older than expected, and the cluster travels far outside the galaxy’s disk. So M71 is a globular cluster — a not-too-big, not-too-tight family of ancient stars.

M71 is due east at nightfall, a third of the way up the sky. Through binoculars, it looks like a small, hazy patch of light.
 

Script by Damond Benningfield

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