StarDate

Ganymede, the largest moon of Jupiter, has a lot going for it. It’s the largest moon in the entire solar system — bigger than the planet Mercury. It probably has an ocean of liquid water far below its icy surface. And it’s the only moon that generates its own magnetic field.

Ganymede is about half again the diameter of our own moon. But its composition is quite different. It has an iron core, surrounded by a thick layer of rock, topped by a crust made of ice. The crust is hundreds of miles thick.

Ganymede’s ocean may lie between layers of that crust. And it may hold more liquid water than all of Earth\'s oceans combined.

The ocean probably is “salty” — it contains a lot of dissolved minerals. As Ganymede spins on its axis, the motions of the salty water contribute to the big moon’s magnetic field. The field is strong enough to produce auroras — like the curtains of color known as the northern lights here on Earth.

The magnetic field interacts with Jupiter’s field, which is much stronger. That interaction may contribute to Jupiter’s auroras as well — one more feature that makes Ganymede one of the more interesting worlds in the solar system.

Jupiter is just moving into view in the morning sky now. Tomorrow, look for it to the lower left of the crescent Moon in early twilight. It’ll climb higher over the coming weeks, making it even easier to see. And with binoculars, you’ll also be able to see its four biggest moons — including Ganymede.

Script by Damond Benningfield

Rigel is not just one of the brightest stars visible in the night sky. The supergiant is also one of the brightest stars in the entire galaxy — more than a hundred thousand times brighter than the Sun.

One indication of Rigel’s brilliance is a cloud of gas and dust about 40 light-years from it. Without Rigel’s light, the cloud would be completely dark. But the cloud reflects light from Rigel, making it visible. The cloud’s outline looks like the profile of a laughing witch. So it’s known as the Witch Head Nebula.

The nebula is about 900 light-years away, and spans about 70 light-years. It consists of the remains of a dead star. But it also appears to be giving birth to new stars.

Considering the nebula’s location, that’s not surprising. It’s on the edge of a region that’s given birth to many young, hot, bright stars — including Rigel. Winds and radiation from these stars are eroding the nebula, sculpting its “witch head” shape. But they’re also squeezing the nebula. That causes pockets of gas and dust to collapse and give birth to new stars. As those stars take shape, their own radiation and winds will further erode their birthplace — and the Witch Head Nebula will disappear.

Rigel is in the southeast as night falls, to the right of Orion’s Belt. The Witch Head Nebula is to the upper right of Rigel. It’s barely visible even through a telescope. It takes long-exposure images to show its outline — illuminated by a supergiant star.

Script by Damond Benningfield

The crescent Moon has a couple of colorful companions the next couple of mornings: the planet Mars and the star Antares. Both have a distinctly orange hue. Mars’s color is caused by reddish dust on its surface. Antares is colored by the temperature of the gas that makes up its surface.

There’s a bit of orange on the Moon as well — the residue of volcanic eruptions. The crew of the final Apollo mission found a patch of orange soil, which contained many orange glass beads. Scientists have seen similar beads mixed in with the soil from other locations. A few of the beads even contain water — suggesting there’s a lot of water mixed in with the lunar dirt.

Such color is a rarity on the Moon, though. Most of the surface is dark grey. It consists of plains of volcanic rock and jumbled highlands, all topped by the sticky, powdery dirt.

Neither pictures nor descriptions by astronauts depict much of any other color. Perhaps the most colorful bits of the Moon, in fact, are those that came from Earth. The Apollo landers were coated in gold foil. And astronauts set up American flags at all six of their landing sites. But exposure to unfiltered sunlight could have washed away much of that color — leaving the Moon as colorless as when astronauts arrived there.

Look for the Moon and its companions before sunrise. Mars is below the Moon tomorrow, with brighter Antares farther to the lower right. Both will be to the upper right of the Moon on Tuesday.

Script by Damond Benningfield

A pair of dogs trots across the night skies of winter: Sirius, the Dog Star, and Procyon, the little dog star. The names indicate that they’re the brightest stars of the constellations Canis Major, the big dog, and Canis Minor, the little dog.

The name “Sirius” means “glowing” or “scorching.” The first of those meanings refers to Sirius’s brilliance — it’s the brightest star in all the night sky. And the second refers to the time of year it made its first appearance in the morning sky when it was named — at the start of the hottest part of summer. In fact, that’s why we call that part of the year the Dog Days.

Procyon means “before the dog.” From most of the northern hemisphere, it rises a little earlier than Sirius does, so it precedes the Dog Star into the sky.

Sirius and Procyon are remarkably similar. They’re about the same distance from Earth, and each consists of two stars. The main star in each system is bigger and heavier than the Sun, and a good bit brighter. And the other star in each system is a white dwarf — the hot but tiny corpse of a once-bright star.

Sirius and Procyon are low in the east and southeast as the sky gets fully dark. Because they’re so bright, though, they’re quite easy to spot. Brilliant Sirius is directly below Orion’s Belt — a short line of three bright stars. Procyon is off to the upper left of Sirius — the little dog leading the big dog across the night sky.

Script by Damond Benningfield

Planets are everywhere. Astronomers have discovered more than 4,000 confirmed planets in star systems other than our own. And the list is likely to grow by thousands more in the years ahead.

One system where we’re not likely to find planets is Spica, the brightest star of Virgo. It’s well to the right of the Moon at first light tomorrow.

Spica is a binary — two stars that are bound to each other by their gravity. The surfaces of the two stars are only a few million miles apart. With such a small gap between them, no planet could orbit either star individually.

Astronomers have discovered planets in other binaries that orbit both stars in the system. But that’s less likely to be the case for Spica.

The surfaces of both of Spica’s stars are tens of thousands of degrees hotter than the Sun. They would vaporize any solid bits of ice or rock anywhere near the stars. And strong “winds” from the stars would blow these planet-making materials far away. So if any planets have formed at Spica, they would have to be many times farther out than Earth is from the Sun. Such worlds would be extremely hard to find with current techniques.

And even if Spica does have planets, they probably wouldn’t last for long. The heavier of Spica’s two stars probably will explode as a supernova. Planets aren’t strong enough to withstand the radiation and shockwave from such a blast. So if planets orbit Spica, they’ll live short lives — and die violent deaths.

Script by Damond Benningfield

In popular culture, a black hole is always a destroyer. It pulls apart stars and planets and gobbles up the pieces. And that certainly does happen. In fact, astronomers watched a black hole destroy a star as big as the Sun early last year.

Yet it might be possible to put black holes to work for less-destructive purposes. They might be used as massive power generators. They could produce enough energy to power an entire civilization.

A spinning black hole drags the space around it like water spinning around a drain. This creates a powerful magnetic field. And that’s where an advanced civilization might extract energy.

In one scenario, a large ring would be built around the black hole, aligned with its poles. Superconducting coils around the black hole’s equator would convert the magnetic energy to electrical current that would flow to the polar ring.

In another scenario, a spaceship would fly into the swirl of spacetime around the black hole and split apart. Half of the ship would fall into the black hole. But the other half would be thrown back into space. The escaping half would carry out much more energy than the whole spacecraft carried in.

“Stealing” this rotational energy would cause the black hole to slow down just a bit. Present-day spacecraft use a similar technique. They steal energy from a planet, getting a “kick” toward more-distant targets.

So black holes could be put to work — powering an entire civilization.

Script by Damond Benningfield

A year ago, astronomers watched the death of a star. For three months, the star blazed vividly as it was pulled apart by a black hole. At its peak, the dying star shined tens of billions of times brighter than the Sun.

The dying star was discovered on January 29th. It was seen by a network of telescopes that looks for exploding stars. Another network of telescopes quickly turned toward the event. So did two space telescopes.

By coincidence, another space telescope was already looking in that direction. TESS is a planet-hunting telescope that stares at the same patch of sky for weeks at a time. In addition to planets, it sees exploding stars and other objects that change in brightness. It first saw the dying star flare up on January 21st, although astronomers didn’t know about its observations for a few weeks.

The combination of views provided the most detailed view yet of a star being disrupted by a black hole. As the black hole pulled the star apart, a giant ribbon of hot gas was hurled into space, where it quickly cooled off. The rest of the gas eventually plunged into the black hole — never to be seen again.

The incident happened about 375 million light-years away. The black hole was about six million times the mass of the Sun — half-again the size of the black hole at the heart of the Milky Way. And the disrupted star was roughly the same mass as the Sun — an unlucky star consumed by a black hole.

Script by Damond Benningfield

A black hole can be one of the darkest objects in the universe and one of the brightest. In fact, some black holes can outshine the entire galaxy of stars that surrounds them.

A black hole itself is truly black. Stuff can fall in, but nothing can come back out — including light. So from here on Earth, it’s almost impossible to spot a black hole that’s on its own.

Fortunately for science, many black holes have companions — “normal” stars or clouds of gas and dust. The black hole pulls in material from such a companion, heats it up, and makes it shine brilliantly. Much of what we know about black holes comes from studying this process.

As material flows toward the black hole, it doesn’t fall straight in. Instead, it spirals around the black hole, forming a wide, thin accretion disk. This flow is turbulent, so particles ram together. This creates friction, which generates heat. The hot material radiates energy into space.

The friction also causes the particles to spiral closer to the black hole, where they move even faster. That generates more friction and more energy, so the disk gets hotter and brighter.

As much as 40 percent of the material in a disk can be converted to energy — compared to less than one percent for the process that powers the stars. So if a big disk surrounds a giant black hole, it shines as a quasar — one of the brightest objects in the universe — brighter than an entire galaxy of stars.

Script by Damond Benningfield

Robert Goddard is known as the “father” of American rocketry. Yet his contributions to the field are less than they might have been. Goddard did most of his work in secret — he didn’t collaborate outside his own team, and he didn’t publish many results.

In part, that secrecy was the result of an editorial that appeared in the New York Times 100 years ago today.

Goddard had just published a paper in which he outlined his ideas for sending rockets into space. To confirm that a rocket was actually working, he suggested using it to slam a small amount of explosives into the new Moon and watching through telescopes.

The Times objected. "That Professor Goddard ... does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react — to say that would be absurd. ... He only seems to lack the knowledge ladled out daily in high schools."

After that, Goddard avoided publicity. He continued his work, though, and launched the world’s first liquid-fueled rocket, in 1926. When that brought criticism from his neighbors, he moved his work to New Mexico.

There, he produced many patents for ideas that were incorporated into future rockets and spacecraft.

And on July 17th, 1969 — the day after Apollo 11 headed for the Moon — the Times published a retraction. "It is now definitely established that a rocket can function in a vacuum as well as in an atmosphere," the paper wrote. "The Times regrets the error."

Script by Damond Benningfield

The surface of the Moon is dotted with a score of lakes, even more seas, plus assorted bays and marshes. But there’s only one ocean: the Ocean of Storms. It’s part of a large basin that’s one of the biggest features on the entire Moon.

The “watery” features are actually vast plains of volcanic rock. To long-ago skywatchers, they resembled bodies of water. So they called them seas, lakes, or other watery appellations.

The Ocean of Storms is in the northwestern quadrant of the lunar hemisphere that faces Earth. It’s the largest of the dark patches visible to the unaided eye.

The volcanic plain is part of a larger feature — a shallow depression known as a basin. It covers about 10 percent of the lunar surface — roughly half the land area of the United States. At most, though, it’s only two or three miles deep.

Geologists don’t agree on what created the basin. It could have been an impact by a giant asteroid into the infant Moon. Or it could have been created by heating inside the Moon itself. Whatever the mechanism, though, it left a giant feature on the lunar surface: its only “ocean.”

Look for the Moon climbing into good view in late evening. The Ocean of Storms is near the lower left edge of the Moon at that hour, but wheels to the top of the lunar disk later on. And the Moon has a bright companion: Regulus, the brightest star of the lion. It’s below the Moon as they climb into view, and closer to the Moon at first light tomorrow.

Script by Damond Benningfield