StarDate

         Mercury is one of the hottest places in the solar system. Daytime temperatures at its equator can reach 800 degrees Fahrenheit -- hot enough to melt lead. But it also has some cold spots. In fact, they may be so cold that they hold huge deposits of ice -- billions of tons of it.

         Mercury is the closest planet to the Sun, so it makes sense that it would get hot. But the planet doesn’t have an atmosphere, so the heat isn’t distributed around the planet. At the poles, where the Sun is always quite low in the sky, temperatures can be hundreds of degrees below zero.

         It’s especially cold inside deep craters, where the Sun never shines at all. Radio telescopes on Earth, as well as instruments on a Mercury orbiter, found evidence of water ice inside some of those craters. They also found evidence of ice between the craters, buried under layers of dirt.

         It’s not certain where the ice came from. It might have been deposited when water-bearing asteroids or comets slammed into Mercury. Or it could have come from ice volcanoes on the planet itself. Regardless of its origin, though, there’s a lot of ice on this hot little planet.

         Mercury is in view shortly after sunset right now, quite low in the west-northwest. It looks like a bright star, but it’s so low in the twilight that it’s hard to spot. It’s a little easier to see this evening, though, because the planet is close to the right of the Moon.

         More about the Moon and planets tomorrow.

Script by Damond Benningfield

Otto Struve came from astronomical royalty. His grandfather and great-grandfather had been leading astronomers in Russia. And his father was director of Kharkov Observatory in Ukraine.

Young Otto embarked on a career in astronomy as well, taking classes in astronomy and math. His academic career was interrupted by World War I, when he joined the Russian Imperial Army.

Struve returned to college after the war, and earned his degree in 1919. But conditions in that part of the world remained unstable. After the Russian revolution, a civil war was under way. And 100 years ago this month, Struve joined it.

He enlisted in the White Russian Army, which opposed the Bolsheviks. Struve was a member of the cavalry. He was wounded in action in July, and he once had his horse shot out from under him.

By late 1920, the war was lost for Struve’s army. He was evacuated to Turkey, where he spent a year and a half eating at soup kitchens. He worked at any job he could find, including lumberjacking.

Through a network of connections, though, Struve was offered a job at Yerkes Observatory, which was operated by the University of Chicago. Struve arrived in late 1921.

By 1932, he was directing the observatory. And that year, Chicago signed a deal with the University of Texas to operate the new observatory it was planning. Struve led the observatory’s construction, and he became its director -- the first director of McDonald Observatory.

Script by Damond Benningfield

A relic from the early universe is cruising across the constellation Libra right now. It’s only about 200 light-years away, but it’s moving in a hurry -- about 800,000 miles an hour. That’s far faster than most of the stars around us. So it’ll move out of Libra in a few thousand years -- the blink of a cosmic eye.

The star is known as HD 140283. But it also has a nickname -- the Methuselah Star. That’s because measurements of its age say it’s at least 13 and a half billion years old, and perhaps a good bit older. Since the universe itself is only about 13.8 billion years old, that makes the star one of the oldest around.

Astronomers have pieced together several bits of evidence to arrive at the star’s age. The key bit is its composition. HD 140283 has very low levels of heavy elements -- anything other than hydrogen and helium, the simplest elements. Heavier elements were created in the hearts of earlier stars, then spewed into space when those stars died. The Sun has relatively large amounts of these elements, which tells us that it followed many generations of stars.

HD 140283 has less than half a percent as much iron as the Sun, with slightly higher levels of oxygen and a few other elements. Those abundances tell astronomers the star must have been born when the universe was in its infancy. That makes it one of the older stars around -- and the oldest in our region of the galaxy.

Script by Damond Benningfield

Scorpius is missing its claws. The celestial scorpion has a curving body, a wide head, a bright heart, and a prominent stinger. But if you look at depictions in modern atlases, it doesn’t have claws. Those stars were stripped away to form part of Libra, the balance scales.

         Yet the stars that once represented the claws have maintained their arachnoid names. Zubeneschamali comes from an Arabic name that means “the northern claw.” And Zubenelgenubi is “the southern claw.”

         Zubeneschamali is the brighter of the two. It’s a single star about 185 light-years from Earth. The star probably is about 80 million years old, although the number could be off by a good bit in either direction. Either way, it’s only a tiny fraction the age of the Sun.

         The star is quite impressive. It’s about three and a half times the mass of the Sun, and about 130 times the Sun’s brightness. Because of its surface temperature, it shines white or blue­-white. Over the centuries, though, many skywatchers have described it as green -- perhaps a simple trick of the eye.

         Zubenelgenubi is more complicated. It consists of two pairs of stars. The pairs are separated by more than 5,000 times the distance from Earth to the Sun. And a star that’s even farther away may be bound to the system as well -- giving the southern claw at least five stars.

         Look for these stars in the southeast as night falls, to the upper right of the scorpion’s head.

Script by Damond Benningfield

A constellation with a split personality is beginning its annual migration across the southern evening sky. It’s not especially bright, but it’s close to a more prominent constellation, which makes it easier to find.

Libra stands to the upper right of Scorpius as night falls, low in the southeast. Its classical outline shows a triangle of stars, with a couple of lines of stars dangling below.

Libra represents a set of balance scales held by the Greek goddess of justice, Astrea. She\'s represented by the nearby constellation Virgo.

Libra may have gained its reputation for balance because it represented balance in the heavens. Thousands of years ago, the Sun passed across the constellation at the time of the autumn equinox — a time when day and night are roughly equal lengths — a time of balance.

But Libra also has a second story — as the claws of Scorpius. It was known as the scorpion’s claws as far back as ancient Babylon. In fact, the proper names of several of Libra’s brighter stars represent that history. Zubeneschemali is an Arabic name that means “the northern claw.” Zubenelgenubi is “the southern claw.” And Zubenelakrab is simply “the scorpion’s claw.”

Zubenelgenubi is at the top of the constellation’s triangle of stars, so it represents the fulcrum of the scales. Zubeneschemali is to its upper left in early evening, with Zubenelakrab below it — the severed claws of the scorpion turned into the scales of justice.

Script by Damond Benningfield

Like bowlers trying to pick up a spare, physicists keep trying to knock down general relativity — Albert Einstein’s theory of gravity. But they keep throwing gutter balls. The theory has withstood every test.

Einstein developed the theory in the early 1900s. It passed its first big test 100 years ago yesterday, during a solar eclipse. Relativity predicted that the light of stars that appeared near the Sun would be deflected a bit by the curvature of space around the Sun. Astronomers observed that effect — turning Einstein into a major celebrity.

Since then, relativity has been confirmed in many ways. Astronomers found, for example, that gravity causes a shift in the wavelength of light leaving stars and other massive objects. They’ve discovered that galaxies and clusters of galaxies act as gravitational lenses, bending the light of galaxies behind them. And they’ve discovered the “jiggles” in spacetime known as gravitational waves — something predicted by Einstein’s equations.

And astronomers at McDonald Observatory and elsewhere have bounced laser beams off reflectors on the Moon. Those observations show that Earth and the Moon “fall” toward the Sun in the same way — gravity treats them equally, just as predicted.

While general relativity works well at large scales, it falls apart at the smallest scales. So scientists keep probing its limits — trying to unify the science of the big with the science of the small.

Script by Damond Benningfield

When the year 1919 began, Albert Einstein was a little-known German physicist. By year’s end, though, he was an international celebrity — probably the most famous scientist in the world. The difference? A total solar eclipse that took place 100 years ago today.

A few years earlier, Einstein had published his theory of gravity, known as general relativity. Among other things, it said that mass causes space-time to curve. That could be confirmed during a solar eclipse. As light from background stars passed close to the Sun, it would be deflected a tiny bit. That would make the stars appear to be out of their normal positions.

Frank Watson Dyson, Britain’s astronomer royal, proposed testing that idea during the solar eclipse of May 29th, 1919. So two expeditions headed out to watch it. Arthur Eddington, another top British astronomer and an admirer of Einstein’s work, led a group to an island off the coast of Africa. The other group went to Brazil.

Weather conditions weren’t the best. Clouds and rain blocked part of Eddington’s view. And the heat in Brazil warped the mirror of the Brazilian group’s main telescope. Even so, both groups got some observations. They then spent months analyzing their findings. They reported that their observations were a good match to Einstein’s theory. Einstein appeared on the front pages of newspapers around the world — making him an instant celebrity.

More about general relativity tomorrow.

Script by Damond Benningfield

One of the stars that marks the head of the scorpion is like a matryoshka doll — it has lots of hidden layers. To the eye alone, it looks like a single point of light. To a telescope, it looks like two stars. And to special astronomical instruments, it looks like at least six stars.

Beta Scorpii is at the left side of a row of stars that represents the head of Scorpius. It’s low in the southeast at nightfall, above Antares, the scorpion’s bright orange heart.

The entire system is about 400 light-years away. And it’s probably only a few million years old.

The brighter of the two stars visible through a telescope is in fact a binary on its own. And one of those stars also consists of two stars, for a total of three.

The stars of the close double are the most impressive of the entire system. One of them is about 10 times as massive as the Sun, and about 8,000 times brighter. The other is about 15 times the Sun’s mass and 30,000 times its brightness. At such great heft, both stars likely will explode as supernovae. The bigger star will go first, sometime in the next few million years.

The other point visible through a telescope also consists of three stars — one on its own, the other two in a tight pairing. They’re not as impressive as the members of the other tight pair. Even so, one of them may be near the mass required to become a supernova. So bright, busy Beta Scorpii could have an even brighter future.

Script by Damond Benningfield

A planet that never was is putting in its best appearance of the year this week. It lines up opposite the Sun, so it’s in the sky all night. It’s brightest for the year, too, although you still need binoculars to find it.

Ceres is the largest member of the asteroid belt, between the orbits of Mars and Jupiter. It\'s about 600 miles in diameter — roughly a quarter the size of the Moon.

Ceres is put together more like a planet than like the other objects in the asteroid belt. It consists of a series of layers — a solid core, surrounded by a mantle of ice, topped by a crust of ice and rock.

Scientists aren’t sure just where Ceres was born. Its composition suggests it might have formed beyond the realm of the planets, then migrated inward to its current location. But it’s also possible that it formed right where it is.

Either way, it probably was prevented from growing any bigger by nearby Jupiter. The gravity of the giant planet kept all the other material in the asteroid belt stirred up. That kept Ceres from pulling in smaller bodies and adding to its own heft. So Ceres remained fairly small — bigger than an asteroid, but too small to be a full-fledged planet.

Ceres is straddling the border between Scorpius and Ophiuchus right now. As darkness falls, it’s to the upper left of Antares, the bright orange star that marks the scorpion’s heart, by about the width of your fist held at arm\'s length — a tiny planet that never was.

Script by Damond Benningfield

Most of the stars in the Milky Way orbit the center of the galaxy in the same direction, and at about the same speed, as all the other stars around them.

But a few follow their own paths. An example is a star at the tip of the Guitar Nebula. The nebula is a bubble of gas with an outline that resembles a guitar. It’s in Cepheus, the king, which is low in the north at nightfall. The constellation’s brightest stars form a shape that resembles a child’s drawing of a house. Don’t look for the nebula, though — it’s so faint that it wasn’t discovered until 1992.

The guitar was sculpted by a pulsar — the crushed core of a once-mighty star. It spins once every two-thirds of a second, emitting a pulse of energy with each turn.

The pulsar was born when the star exploded as a supernova. The explosion must have been off-center, so it gave the dead core a powerful kick. The pulsar is plowing through clouds of gas and dust at more than two million miles an hour. It leaves an expanding wake behind it, like a ship traveling across the ocean. That wake is what we see as the Guitar Nebula.

But there’s more to the nebula than meets the eye. X-ray telescopes in space reveal a long, high-speed “jet” of material. It’s firing away from the tip of the nebula at a right angle to the nebula itself. The jet most likely is powered by the pulsar’s magnetic field, which funnels charged particles away from the pulsar — adding an interesting note to the celestial guitar.

Script by Damond Benningfield