StarDate

The insides of the planet Uranus are weird. Some models of the planet suggest it could have “diamondbergs” floating in an ocean of metallic liquid carbon.

Uranus is the seventh of the Sun’s major planets. And it may be the solar system’s weirdest planet, period. It’s the third-largest planet, but only the fourth most massive.

That’s one indication that most of Uranus consists of hot “ices” — water, methane, and ammonia. They’re heated by the great pressure in the mantle — the layer below the planet’s atmosphere.

There are several ideas about what’s happening in that layer. The most popular says that high temperatures in the mantle cause the methane to separate into its individual molecules. The carbon forms diamonds. They rain through the mantle, settling in an ocean around the core. The carbon is squeezed so tightly that it forms a metal. Electric currents in the ocean then generate the planet’s magnetic field.

It’s hard to know what’s happening, though, because only one spacecraft has visited Uranus — four decades ago. There are plans for a new mission, but it won’t get there for a long time — providing a better look deep into this weird planet.

Uranus stands close to Venus, the “evening star,” the next couple of nights. Uranus is easily visible through binoculars. It looks like a faint green star. It’s to the upper left of Venus this evening. It’ll be about the same distance to the lower left of Venus tomorrow night.
 

Script by Damond Benningfield

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Phobos, the larger moon of Mars, could be one of the solar system’s most exciting thrill rides. But if you’re not careful, it could become a little too thrilling — one big push and you’d float off into space.

Phobos is a lumpy boulder that’s only about 15 miles in diameter. Its main feature is a giant impact crater. The impact almost blasted the moon to bits.

Phobos orbits just 3700 miles above the Martian surface. At that altitude, it moves so quickly across the Martian sky that it rises in the west and sets in the east, with moonrises about 11 hours apart.

Seen from Phobos, Mars would span about a quarter of the sky, and it would slide past at about 5,000 miles per hour. So in a single orbit around Mars — daylight permitting — you’d see everything from the giant volcanoes of Tharsis Ridge to Valles Marineris, a complex of canyons that dwarf the Grand Canyon here on Earth. But because of Phobos’s low altitude, you wouldn’t see the Red Planet’s polar ice caps — they’d be hidden around the curve of the planet itself.

You wouldn’t want to get too excited by the view, though. The gravity of Phobos is so weak that a single big jump would launch you into space — leaving the Martian moon behind.

Mars is in our own sky right now, well up in the west-southwest at nightfall. It looks like a bright orange star. Tonight, it’s well below the first-quarter Moon, and leads the Moon down the western sky.
 

Script by Damond Benningfield

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The star El Nath is best known for its position in the sky. It forms the tip of one of the horns of the bull, so it’s nicknamed “the butting one.” And it stands directly opposite the center of the Milky Way Galaxy. But the star itself is interesting in its own right.

What we see as El Nath is a giant. It’s about five times the mass of the Sun, and more than four times the Sun’s diameter. And it shines about 700 times brighter than the Sun.

The word “giant” doesn’t apply only to the star’s size, though. It also describes its phase of life. El Nath is only about a hundred million years old — a few percent the age of the Sun. Because of its extra weight, though, it aged much, much faster. It’s already passed through the prime of life, and is into the next phase. As that phase continues, the star will get even bigger and brighter.

El Nath is also intriguing because there are hints that it has a companion. In one study, astronomers split the star’s light into its individual wavelengths. That produced patterns that are like the star’s “barcode.” But there were hints of a second barcode. If it’s real, then El Nath has a companion star that’s too close and faint to see directly. But so far, no one has been able to confirm a companion for the butting one.

El Nath stands close to the lower right of the Moon this evening. And bright orange Mars is a little farther to the upper left of the Moon. More about Mars tomorrow.
 

Script by Damond Benningfield

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20 years ago tonight, a little before midnight, a cosmic missile streaked above the Midwest. It startled witnesses across five states. It then exploded with the force of perhaps 500 tons of TNT, raining shrapnel on Park Forest, Illinois, near Chicago. Debris hit houses, cars, and a fire station. A several-pound fragment punched a hole in the roof of one of the houses, bounced around the top floor, and settled not far from the head of a sleeping teenager.

The missile was a small asteroid — a space rock. It slammed into Earth’s atmosphere at about 45,000 miles per hour. Its outer layers vaporized, forming a brilliant meteor. The pressure of its trip through the atmosphere then caused it to explode, raining fragments across the landscape — the most heavily populated region ever to be pelted by a major “rain” of meteorites.

Several dozen pounds of those chips have been recovered. Studies of them have shown that the original space rock was a fragment of a much larger asteroid that broke up about 470 million years ago. The original body was in the asteroid belt, between the orbits of Mars and Jupiter. It was made of rock, not metal.

There’s a lot of disagreement about details of the fragment that hit Earth. Estimates of its original size range from about four to 10 feet, and a mass of a couple of tons to about 15 tons — a big missile that exploded 20 years ago tonight.
 

Script by Damond Benningfield

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The Big Dipper moves high overhead this evening, with the bowl turned upside down. As you take in the view, link the two stars at the outer edge of the bowl. If you follow that line down toward the horizon, the first moderately bright star you come to is Polaris. Earth’s north pole aims directly at the star, so it’s also known as the North Star or the Pole Star — it marks true north in the sky.

Another north pole stands at about the same height at that hour. It’s in Coma Berenices, a faint constellation that’s well above Arcturus, a bright yellow-orange star that’s low in the east.

That spot marks the north galactic pole — the projection of the north pole of the Milky Way Galaxy.

The galaxy is shaped like a disk. It’s about a hundred thousand light-years across, and about ten thousand light-years thick. If you draw a line through the center of the galaxy, at a right angle to the disk, and project it above the disk, it aims toward Coma Berenices.

When we look in that direction, we’re looking through a thin section of the Milky Way, so there aren’t many really bright stars in that part of the sky. Instead, we’re looking into intergalactic space. And in fact, a telescope reveals many other galaxies in that direction. More than a thousand of them form a giant cluster, known as the Coma Cluster — a beautiful collection of galaxies that stands due north.

Tomorrow: A shower of space rocks pelts Illinois.
 

Script by Damond Benningfield

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The glowing band of the Milky Way is always tough to find. It’s so faint that you need to get far away from city lights to see it. But this time of year is especially challenging, because the Milky Way doesn’t soar high across the evening sky as it does in summer and winter. Instead, it brackets the sky — it’s low in the west after nightfall, and in the east before dawn.

It’s all a matter of perspective. During summer, Earth faces the center of the Milky Way Galaxy around midnight, so the galaxy’s crowded inner regions climb high overhead. And during winter, we face opposite the galactic center, so the galaxy’s faint outer precincts top the sky.

About now, though, we’re looking at a right angle to the galaxy’s disk around midnight — away from our galactic home and toward intergalactic space. You can see the Milky Way, but not at its best.

In early evening, it arcs across the west. Its path is traced by Sirius, the brightest star in the night sky, which is in the south-southwest; bright orange Betelgeuse, well to the upper right; and W-shaped Cassiopeia, low in the northwest.

And as Earth turns during the night, a different region of the Milky Way rotates into view before dawn. It arcs high across the east, from Cassiopeia — now in the northeast — to the Northern Cross and over to teapot-shaped Sagittarius in the south — a section of the Milky Way that will highlight the evening skies of summer.
 

Script by Damond Benningfield

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The Moon creeps up on the brightest point of light in the night sky tonight: Venus, the “evening star.” Venus stands above the Moon as they come into view. The gap between them will shrink as they set, a couple of hours later.

The Moon was “new” on Tuesday, as it crossed between Earth and the Sun, starting a new month-long cycle of phases. At sunset tonight as seen from the United States, it’s about two and a half days into that cycle — it’s two and a half days “old.” It’ll grow 27 days older before the cycle ends at April’s new Moon.

This evening, the Sun will light up only about seven percent of the side of the Moon that faces our way. It’s daylight in that slim crescent, and nighttime everywhere else.

A lunar night lasts about two Earth weeks. But on the Earth-facing side, the nights are quite bright. They’re lit up by earthshine — sunlight reflecting off of Earth. It’s just like moonshine here on Earth, but much brighter. Earth is much bigger than the Moon, and about three times more reflective. So a full Earth is dozens of times brighter than a full Moon. That’s a view to look forward to for the astronauts who are expected to travel to the Moon later in this decade and beyond.

Right now, Earth is more than 90 percent illuminated, casting its bright blue glow across the lunar landscape. From here on Earth, that glow is easily visible on the Moon’s nightside — the ghostly light of Planet Earth.
 

Script by Damond Benningfield

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If you’re an astronomer who studies faint, wispy objects like spiral galaxies, then this is your time to shine. The Moon was new yesterday, so it doesn’t fill the sky with its glow. With the pesky moonlight out of the way, those faint galaxies will shine at their best.

One example is M63, a spiral galaxy in the constellation Canes Venatici. It’s in the eastern sky in early evening, and soars high overhead later on. It’s also known as the Sunflower Galaxy for its beautiful shape and color. It’s bright, so it’s visible through just about any telescope. But the moonless night makes it easier to see some of the details.

In some ways, M63 resembles our own galaxy, the Milky Way. It’s about the same size and mass. And it appears to have a central black hole, although if it does it’s a lot bigger than the Milky Way’s.

Also like the Milky Way, the outer edge of the galaxy’s disk is bent, and probably for the same reason: the gravitational pull of companion galaxies. In the case of M63, the companions appear to pull out long, faint ribbons of stars that show up in long-exposure photographs.

And there’s one more similarity: Both galaxies are surrounded by haloes of dark matter. It far outweighs the bright matter, so it exerts a strong pull on the galaxy’s stars and gas clouds. But the dark matter produces no energy of its own, so there’s no way to see it — no matter how dark the skies are.
 

Script by Damond Benningfield

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The planet Jupiter is disappearing from the evening sky. And tomorrow, it’ll briefly disappear from the daytime sky — but only along a narrow path across the southern hemisphere.

Jupiter is quite low in the west as night falls, so you need a clear horizon to spot it. It looks like a bright star shining through the twilight. It’s far below much brighter Venus, the “evening star.” Jupiter sets about the time the sky gets fully dark.

Jupiter is vanishing from the evening sky because of our changing perspective. It’s about to pass “behind” the Sun, so it’ll be hidden in the Sun’s glare. It’ll return to view — in the morning sky — in a couple of months.

For tomorrow’s daytime disappearance, Jupiter will be hidden by the Moon — an act called an occultation. It happens because both Jupiter and the Moon stay close to the ecliptic — the Sun’s path across the sky. They both stray from it a little bit, so most months the Moon passes right by Jupiter. When the alignment is just right, though, it passes in front of the giant planet, hiding it from view.

This occultation takes place during the daylight, so it’s visible only through a telescope — and only along a path through north and central South America and the southern Pacific Ocean.

Tomorrow evening, though, the Moon and Jupiter will stage a nice encounter. They’ll stand side by side in early evening — the crescent Moon and the “disappearing” planet Jupiter.
 

Script by Damond Benningfield

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If it’s been a nasty winter where you are, then you have our sympathy. On the bright side, though, it’s almost over — spring arrives in the northern hemisphere today.

That doesn’t mean the weather will change overnight. The shift of the seasons is a long, slow process, not something that happens with the flip of a switch. It’s governed by the steady progression of the Sun across the sky, which is the result of the changing angle between the Sun and Earth’s poles.

At the start of spring — the vernal equinox — the Sun crosses the equator from south to north. Neither pole dips toward the Sun, so the entire planet will see roughly equal amounts of daylight and darkness.

Over the next three months, though, as Earth continues in its orbit around the Sun, the north pole will gradually dip toward the Sun. That will bring longer, warmer days to the northern hemisphere, and shorter, cooler ones to the southern hemisphere. It’ll also bring non-stop sunshine to the north pole — a “day” that will stretch almost six months from sunrise to sunset.

One other item of note is that the Sun rises due east and sets due west on the equinox. That’s not the case at other times of the year. As the Sun moves northward across the sky, its rising and setting points move northward, too. So the equinox is a good time to get your bearings right here on Earth — all part of the constant cycle of the seasons.

Another cycle in the sky tomorrow.
 

Script by Damond Benningfield

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