StarDate

Hydra, the water snake, is the largest of all the constellations — and by far the longest. It spans more than 100 degrees — almost a third of the way around the sky. So it takes a long time to climb into view. Tonight, for example, its head is in the east-southeast at nightfall. It’s well to the left of Sirius, the brightest star in the night sky. But the snake’s tail won’t climb into view until after midnight.

The head consists of five or six faint stars, depending on how it’s drawn. Most of them are so faint that they’re not visible from cities, or even the suburbs. Five of them share a name: Minazal. It comes from an Arabic phrase that means “belonging to the uninhabited spot” — an indication that they’re not much to look at.

The Minazals are numbered, based on their position in the head. The brightest is Minazal 1. It’s a binary — two stars locked in orbit around each other. The system is about 160 light-years away. Its main star is about three times the size and mass of the Sun, and dozens of times brighter. The companion is smaller and fainter.

The closest of the Minazals is number three, at 135 light-years. And the farthest is number 2, at almost 600 light-years. As seen from Earth, it’s the faintest member of the quintet. In reality, though, it’s the brightest — more than 2500 times brighter than the Sun — the most impressive star in the head of the water snake.

Script by Damond Benningfield

The Moon slides along one of the horns of Taurus tonight. At nightfall, the bull’s bright eye, the star Aldebaran, is to the lower right of the Moon. And the tip of the horn, El Nath, is closer to the left or lower left of the Moon. By the time they set, in the wee hours of the morning, El Nath will stand close above the Moon.

The Moon is in its “waxing gibbous” phase. “Gibbous” means the Sun is lighting up more than half of the lunar hemisphere that faces our way. “Waxing” means that more of that half is being lit up every night. The Moon will keep getting “fatter” until it’s full, a week from today.

“Gibbous” comes from a Latin word that means “hump.” It can be applied to anything that appears to have a hump on its back, such as a camel. A gibbous Moon appears to have a hump on top of its round half.

Planets can appear gibbous as well. Mercury and Venus are inside Earth’s orbit around the Sun, so they can follow the same cycle of phases as the Moon — from thin crescents to almost full. We don’t see them as new or full because, during those phases, they’re passing between Earth and the Sun, or behind the Sun.

Planets outside Earth’s orbit never appear in a crescent phase, but they can appear gibbous, depending on our viewing angle.

Stars, on the other hand, are always full. They generate their own light, so the viewing angle doesn’t matter — unlike the Moon, which will have a “hump” on its back for another week.

Script by Damond Benningfield

The Sun’s birthplace has long since fallen apart. The Sun almost certainly was born in a cluster, with anywhere from dozens to thousands of siblings. Over a few hundred million years, though, the cluster fell apart, with all of the stars going their own way.

We can see that process at work in the best-known star cluster of all: the Pleiades. The cluster is close to the Moon tonight. Its brightest stars form a tiny dipper. It may be a bit hard to pick out through the moonlight, but binoculars will help.

The cluster is about 445 light-years away. It’s about a hundred million years old. And it consists of more than a thousand stars.

But the cluster is already starting to break up. It’s held together by the gravity of its stars. But that “stickiness” is overpowered by the gravity of the rest of the galaxy. It pulls at the Pleiades, causing the stars to spread out. Stars at the edge of the cluster are pulled out into their own orbits. That reduces the cluster’s overall gravity, making it easier to strip away even more stars.

Most estimates say the Pleiades is likely to completely fall apart over the next 250 million years. By then, several of its brighter stars will have expired. Those stars are especially heavy, so they “burn” through their nuclear fuel in a hurry. By the time the Pleiades disintegrates, only their dead cores will remain — remnants of a once-mighty cluster.

Script by Damond Benningfield

The Sun is near the top of its class — class “G.” Such stars all have about the same temperature, so they look yellow. Members of the class that are in the prime of life, as the Sun is, are also close to the Sun’s mass, size, and brightness.

That doesn’t mean the stars are just alike, though. They’re different ages, they spin at different rates, and they have different levels of activity.

A close example is Kappa Ceti, in Cetus, the whale. The star is about 30 light-years away. It’s almost exactly the same size and mass as the Sun. But it’s only 85 percent of the Sun’s brightness, and it spins about three times faster. And those two facts are related.

Kappa Ceti is a few hundred million years old — less than 10 percent the age of the Sun. Younger stars rotate faster than older stars — a result of their formation. They spin fast as the cloud of dust and gas that gave them birth collapses. A rapidly spinning star generates a stronger magnetic field. That creates more and bigger “starspots” — dark storms on the surface. And that reduces the star’s overall brightness. In the case of Kappa Ceti, some of the spots are so big that they cause the star’s brightness to change as the spots rotate into and out of view.

Kappa Ceti is high in the south-southwest at nightfall. Under dark skies, it’s just visible to the unaided eye. It’s well to the left of the brilliant planet Jupiter, and to the lower left of the Moon.

Script by Damond Benningfield

The average depth of the oceans on Earth is a little more than two miles. And the greatest depth is about seven miles. But that’s little more than a big puddle compared to the ocean on Ganymede, the biggest moon in the solar system. Its ocean appears to be about 60 miles deep. And it covers the entire moon. That means that Ganymede’s ocean probably contains several times as much water as all of Earth’s oceans combined.

Ganymede is the largest moon of Jupiter, the largest planet. It’s almost 3300 miles in diameter — bigger than the planet Mercury. It’s more than twice as far from Jupiter as the Moon is from Earth. Its crust is about a hundred miles thick, and is made of frozen water. The ocean is below that cap.

Scientists are pondering whether the ocean could be a good habitat for life. There’s another layer of ice below it. But if the ocean comes in contact with rocks, then geysers of hot water with lots of minerals could spew into the water. That would provide the energy and chemistry for life.

Observations by the Juno spacecraft suggest there could be such contact. Juno found salts on the surface of Ganymede. It might also have seen organics — the chemical building blocks of life. So water could have “leaked” to the surface — providing intriguing evidence of conditions in Ganymede’s deep ocean.

Jupiter is close to our moon tonight. The planet looks like a brilliant star above the Moon.

Script by Damond Benningfield

Astronomers use many techniques to find planets in other star systems. One of those is looking where there doesn’t appear to be anything — wide gaps in disks of dust around young stars. Most of these stars are only a few million years old — billions of years younger than the Sun. They’re encircled by cold dust — raw materials for making planets.

The disks are easy targets for ALMA — an array of telescopes in Chile that studies a form of radio waves. It sees the disks in good detail. And most of the disks have wide gaps — zones with very little dust. Such regions could have been cleared out by the gravity of orbiting planets. So astronomers are trying to find those planets.

One system where they’ve been successful is A-b Aurigae. It’s more than 500 light-years away, in Auriga, the charioteer. The constellation is high overhead at nightfall, although the star is too faint to see without a telescope.

The star is bigger and heavier than the Sun. But it’s an infant — it’s not yet fully formed. It’s encircled by a giant disk. And observations have revealed a likely planet in a gap in the disk. The planet is bigger and heavier than Jupiter, the giant of our own solar system. It’s so far out that it takes hundreds of years to orbit the star. But that helped make it a little easier to spot. In fact, astronomers found it by taking its picture — a planet discovered lurking in a dark zone around an infant star.

Script by Damond Benningfield

The Vikings liked to see the world. For a couple of centuries, they plied northern oceans, exploring much of Europe, establishing colonies in Greenland, and even reaching North America. They did so with a good sense of the natural world, including the sky. But they might’ve had a little help. They might have pinpointed the Sun’s position through fog and light cloud cover using crystals — sunstones.

Sunstones are mentioned in the Norse Sagas — accounts written a couple of centuries after the peak of Viking exploration. No one has found such a stone in any Viking sites. But scientists have explored the possibility that ancient navigators could have used them. And some of the results suggest that it’s plausible.

Most studies have focused on a crystal known as calcite. Researchers paint a black dot on one face of the crystal, then aim the stone at the sky. The crystal splits a beam of sunlight into two beams. That creates two images of the dot. The observer turns the crystal until the dots are the same brightness. At that point, the angle of the crystal reveals the direction of the Sun — a big help when navigating through skies that obscure the Sun. Computer simulations back up that possibility.

None of the experiments prove that Vikings actually used sunstones — scientists need better evidence for that. Still, it’s at least possible that these master navigators used the stones to help them see the world.

Script by Damond Benningfield

We can never just look up in the sky, point to one of the dots of light there, and say, “Oh, yeah — that’s Neptune!” Although it’s the fourth-largest planet in the solar system, Neptune is so far that it’s never visible to the eye alone. Tonight, for example, it’s almost three billion miles away — 30 times the distance from Earth to the Sun. So it’s only about a tenth as bright as it would need to be to see it with your eyes alone.

But we can pinpoint Neptune’s location. That’s easy to do this evening because the planet stands above the crescent Moon. As seen from the East Coast, they’re about six degrees apart — the width of three fingers held at arm’s length. They’re even closer from western parts of the country.

From parts of the southern hemisphere, they’ll be closer still. The Moon will pass in front of the giant planet, briefly blocking it from view. Even then, though, it’ll happen during daylight, limiting the view.

The celestial cover-up isn’t visible from the United States for a few reasons. For one thing, the Moon slides across the sky in a hurry, so it covers the planet for only a short time. And the viewing angle isn’t quite right. From most of the globe, Neptune will pass a little above the Moon. If the Moon was bigger, it would block out the planet from a wider region of Earth. So, when the Moon covers a planet or star, you have to be in the right place at the right time to see it happen.

Script by Damond Benningfield

You’ve probably never seen two donkeys perched on the back of a crab. Unless, that is, you’ve spent much time looking at the stars. Two faint stars in Cancer have names that mean the northern and southern donkeys — Asellus Borealis and Asellus Australis. They’re on opposite sides of the crab’s back. They flank the constellation’s most famous member — a star cluster known as both the manger and the beehive.

The star names come from Greek mythology. The donkeys carried gods who were involved in a war between two sets of gods — those of Olympus, and their predecessors, the Titans.

The northern donkey is about 180 light-years away. It’s a little more than twice the size and mass of the Sun. But it’s billions of years younger than the Sun.

The southern donkey is a little closer — about 130 light-years. It appears to consist of more than one star. The main star is bigger and heavier than the Sun, and about half the Sun’s age. It’s nearing the end of its life, so it’s puffed up to become a giant — a stage in life that awaits the Sun in several billion years.

Cancer is in the east at nightfall. Pretty much everything in the constellation is faint, so you need a dark sky to see much. A moonless night like tonight is a good time to look. The Beehive star cluster looks like a hazy patch of light. The donkeys flank it — the northern one to the left, and the southern one to the right.

Script by Damond Benningfield

Most stars begin their lives in colorful style — inside giant, glowing clouds of gas and dust. One example is the Rosette Nebula, which looks like a delicate rose. It’s in Monoceros, the unicorn. The constellation is in the south-southeast at nightfall, to the lower left of much brighter Orion.

The Rosette spans more than a hundred light-years, and it’s about 10,000 times more massive than the Sun. It’s already given birth to hundreds of stars, and many more are still taking shape.

The biggest and brightest of these stars form a cluster at the center of the nebula. Energy from the stars zaps the remaining hydrogen gas around them, causing it to glow reddish pink.

The stars also produce powerful winds. The winds have cleared out a wide cavity around the stars, shutting down the process of starbirth.

But farther away, the winds actually help that process. They squeeze clumps of gas and dust, causing them to collapse. When a clump gets dense enough, its own gravity takes over, squeezing the material into a tight ball. The center of the ball gets extremely hot, igniting nuclear fusion — and giving birth to a new star.

The stars of the Rosette are only a few million years old. Eventually, the remaining gas and dust will either be incorporated into stars, or blown out into space. Over time, the stars most likely will spread out and go their own way — and the colorful Rosette will vanish.

Script by Damond Benningfield