StarDate

When early skywatchers looked into the night sky, they saw the stately progression of the stars. The stars march across the sky together, always returning to view at the same time each year. That led to ideas about the nature of the stars: lamps hung from a dark dome, for example, or holes in the dome letting in the light of heaven beyond.

There was a problem, though. A few especially bright lights moved at a different pace from the rest — and even from each other. They sometimes reversed direction, or even stood still. They got noticeably brighter and fainter, too. The Greeks called these oddballs “planets” — a word that means “wanderers.”

Figuring out what was happening with the planets helped us understand the workings of the entire cosmos. The planets are close by, and they move around the Sun, just as Earth does. In fact, they’re siblings of Earth. The other lights — the stars — are much, much farther. They form a distant background — a canvas for the “wandering” planets.

And one of the planets is at its best for the entire year this week. Jupiter is at opposition — it lines up opposite the Sun. It rises at sunset and is in view all night. And it’s closest for the year, too, so it shines brightest. It looks like a major star low in the east-southeast at nightfall — brighter than any true star. It’s on the border between Capricornus and Aquarius — the backdrop for a brilliant planet.

More tomorrow.
 

Script by Damond Benningfield

Support McDonald Observatory

If you bring your starship out of warp drive close to an unexplored star, be careful. There’s a slim chance it could be an anti-star — a star made of antimatter.

Antimatter is the oppositely charged counterpart of normal matter. When matter and antimatter meet, they destroy each other in a burst of energy. Other than the charge, though, they’re just alike. So from afar, it’s almost impossible to tell a normal star from an anti-star.

In fact, there probably shouldn’t be any antimatter stars at all. Matter and antimatter were made in the Big Bang. Today, all we see for sure is normal matter. The antimatter appears to have been cancelled out in the early universe.

But a few years ago, an experiment in space caught a few possible bits of antimatter. So astronomers went looking for a source.

They looked for unusual sources of gamma rays — the energy produced when matter and antimatter collide. And they found 14 candidate stars.

Based on that number, the researchers estimated there could be up to one antimatter star for every 400,000 normal stars in the galaxy’s disk. That could yield a total of a quarter of a million to one million anti-stars in the disk. And there could be even more anti-stars in the galaxy’s halo, outside the disk.

The candidate objects may well turn out to be something besides antimatter stars. Just in case, though, it may be a good idea to keep your shields up when you visit an unexplored star.
 

Script by Damond Benningfield

Support McDonald Observatory

If you ever see a straight lightning bolt, take a picture. It could be one of the most important discoveries in the history of physics: confirmation of dark matter.

Dark matter appears to account for 85 percent of all matter in the universe. It exerts a gravitational pull on the “normal” matter around it. It produces no energy of its own, though, so no one knows what it is.

The leading idea says it consists of subatomic particles. Experiments have been set up to catch these particles, to no avail. Dark matter has remained stubbornly dark.

Some scientists have proposed that dark matter particles are much bigger and heavier than most models suggest — weighing a few grams or even ounces. There would be far fewer of these giant particles, which means fewer encounters with Earth — and almost no way to catch them.

Earlier this year, though, researchers proposed a way to “see” these jumbo particles: lightning. As such a particle zipped through Earth’s atmosphere, it would ram into atoms and molecules, giving them an electric charge. If it plowed such a pathway during a thunderstorm, it would be heavy enough and move fast enough to zip straight through the air, clearing a straight path for lightning.

So far, researchers have found no record of any straight lightning — perhaps because no one has looked for it. So the next time you’re in a thunderstorm, keep your camera handy — for the possible mark of dark matter.
 

Script by Damond Benningfield

Support McDonald Observatory

The highest spot on the Moon isn’t a dramatic mountaintop. It barely qualifies as a hill — it’s more like a knob. It doesn’t loom above the surrounding landscape, and its slopes are gentle. Yet it’s almost seven miles high — more than a mile higher than the tallest spot on Earth, Mount Everest.

Since there aren’t any oceans on the Moon, scientists don’t use sea level to measure elevation. Instead, spacecraft have used lasers and cameras to produce 3-D maps of the surface. Combining all those observations provides an average elevation. Individual features are then plotted using that starting point.

The tallest mountain on the moon is Mons Huygens, which towers more than three miles above the surrounding plains. But its peak isn’t the tallest spot on the Moon. That honor goes to a feature called Selenean Summit. It’s near the rim of a crater on the farside — the side we never see from Earth. It’s 35,387 feet high. It probably formed as debris from a giant impact more than four billion years ago piled up.

The lowest spot on the Moon is the floor of a nameless crater that’s inside a bigger crater. It’s more than five miles below average. So that gives the Moon a range in elevation of more than 12 miles — almost identical to the range on Earth.

The Moon is low in the south as night falls. And it has a bright companion — Antares, the heart of the scorpion, which is close to the lower right of the Moon.
 

Script by Damond Benningfield

Support McDonald Observatory

The two brightest objects in the night sky other than the Moon bracket the early evening sky like a pair of bookends. Venus is the brighter of the two. It’s the magnificent “evening star,” low in the west as night falls. At the same time, Jupiter is on the opposite side of the sky — in the east-southeast.

Venus looks so bright for several reasons. It’s covered by bright clouds that reflect most of the sunlight that hits them. It’s close to the Sun, so there’s a lot of sunlight for the clouds to reflect back into space. And it’s close to Earth, so a lot of that reflected light shines our way.

Jupiter, too, is blanketed by bright clouds. But it’s much farther from both Sun and Earth. As a result, it gets much less sunlight than Venus does, and it reflects less toward us.

But Jupiter is the biggest planet in the solar system — about a dozen times wider than Venus. So while a given area on Jupiter reflects much less sunlight our way than the same-sized patch on Venus, there’s just so much of Jupiter that it shines brilliantly. Right now, in fact, it’s glowing at its brightest for the entire year — the second-brightest planet in the night sky.

There’s a short window for seeing Venus and Jupiter as bookends. That’s because Venus drops from view about an hour and a half after sunset. But Jupiter is lining up opposite the Sun, so it remains in view all night.

Tomorrow: high spots and low spots on the Moon.
 

Script by Damond Benningfield

Support McDonald Observatory

World War II ended 76 years ago. We can’t go back in time to see it happen. But we can see something from that same time: the light from the southern claw of the scorpion.

The claw is represented by the star Zubenelgenubi. It’s just a whisker below the Moon this evening — so close that it may be a bit difficult to see through the Moon’s glare. The star is in the constellation Libra, the balance scales. But its name comes from a time when it represented the southern claw of Scorpius, which is next door.

The system consists of at least four stars, which are split into two pairs. One pair is about 75 light-years from Earth, while the other is about a light-year farther.

A light-year is the distance light travels in one year — almost six million million miles. If a star is 76 light-years away, that means its light has taken 76 years to reach us. Put another way, the light we see from it tonight actually left the star 76 years ago — which is the case with one half of the southern claw of the scorpion. So we don’t know what’s going on with the star right now — and won’t until around the year 2097.

Of course, the reverse is true as well. If any of the stars of Zubenelgenubi have planets, and if there’s anyone on those planets who’s looking toward the Sun, they’ll see it as it looked 76 years ago — at the time a global conflict was ending on the Sun’s third planet.
 

Script by Damond Benningfield

Support McDonald Observatory

The two most prominent spiral galaxies in the night sky are in good view tonight — especially after the Moon sets in late evening. The Milky Way Galaxy arcs high across the sky, from teapot-shaped Sagittarius low in the south, to the Northern Cross high overhead, to W-shaped Cassiopeia in the northeast. And M31, the Andromeda Galaxy, is to the lower right of Cassiopeia. It looks like a tiny cloud. You need clear, dark skies to see Andromeda or the Milky Way.

A spiral galaxy consists of a wide, flat disk with a big bulge of stars in the middle. Its most beautiful features, though, are its spiral arms — bright ribbons that wrap around the galaxy’s heart.

The arms are outlined by young stars that are especially hot and bright. There are just as many stars between the arms, but they’re generally older and fainter.

Astronomers are still pondering what causes spiral arms. The leading theory says that a wave travels around the galaxy like an ocean wave rippling across the water.

There are several variations on how that creates the visible arms. One says the wave piles up clouds of gas and dust. The clouds are squeezed together, causing them to collapse and give birth to new stars.

Our solar system is in the Orion-Cygnus Arm of the Milky Way. It’s not a complete arm — it doesn’t wrap all the way around the galaxy. But it does contain some of the brightest stars in the night sky — stars that help outline our own spiral arm.
 

Script by Damond Benningfield

Support McDonald Observatory

Earth has dodged a cosmic bullet — at least for now. Tracking with a radio telescope early this year showed there’s no chance the asteroid Apophis will hit us any time in the next century. Until those observations, there was a tiny chance it might slam into Earth in 2068.

Apophis is about 1100 feet across — big enough to cause major damage if it hit. When it was discovered, in 2004, early calculations showed it might do just that — in 2029, ’36, or ’68. The first two possibilities were ruled out quickly, but the third persisted until this year.

Although it won’t hit us, Apophis will get close. In 2029, it’ll pass less than 20,000 miles away. And some teams of scientists want to take advantage of that approach. They’re drafting plans to send small satellites to intercept the asteroid.

Apophis Pathfinder would be launched a few years before the encounter. Two small probes would tag along with Apophis well before it gets here. They’d help define its orbit, measure its mass, map its surface, and more.

Another mission, Reconnaissance of Apophis, would launch a little later. A single spacecraft would look for changes as the asteroid flies past Earth. Watching those changes could help us better understand how asteroids behave as they orbit the Sun. That’s especially important for Apophis, which still could be a threat — but not over the next century.
 

Script by Damond Benningfield

Support McDonald Observatory

Comet Swift-Tuttle was last seen a quarter of a century ago, racing toward the outer solar system. We won’t see it again for a century. Yet it’s left us some little reminders of its presence: grains of ice and rock that create the Perseid meteor shower, which should be at its best the next couple of nights.

Swift-Tuttle has been called one of the most dangerous objects in the solar system. It orbits the Sun once every 133 years. Its orbit carries it slightly inside Earth’s orbit, so there’s a chance that it could hit our planet. And it’s a giant among comets — about 16 miles in diameter. If it hit Earth, it would be far deadlier than the asteroid that killed off the dinosaurs.

We have accurate predictions of its motions for the next few millennia. We know, for example, that its next close passage will take place on August 5th, 2126, at 10:50 a.m. Central Daylight Time — give or take a minute. It’ll miss Earth by about 14 million miles.

But a comet can be pushed around by other planets, or by “jets” of gas from its surface. That makes it hard to project an orbit very far into the future. So we don’t yet know when Swift-Tuttle might someday take dead aim at Planet Earth.

For now, it’s providing some smaller pyrotechnics — a meteor shower. The Perseids could produce a few dozen meteors per hour at their peak, before dawn tomorrow.

We’ll talk about another danger from space tomorrow.
 

Script by Damond Benningfield

Support McDonald Observatory

Scientists define the length of a day as 24 hours. But a day can vary by about a thousandth of a second — a millisecond — the result of storms, earthquakes, or other big events. The shortest day ever recorded came last July — one and a half milliseconds shorter than average.

A day on the planet Venus can vary by about 20 minutes.

Venus is roughly the same size and mass as Earth. But it spins in the opposite direction. And a recent study found that the average length of its day is a skosh more than 243 Earth days.

Measuring the length of a Venus day isn’t easy. The planet’s surface is hidden by clouds. That rules out using mountains or other features to time Venus’s rotation. Instead, scientists use radio telescopes as radar guns. They shoot beams of radio waves toward the planet, then measure the “echo.” A shift in the reflected waves reveals details about the planet’s rotation.

The recent study successfully “shot” Venus with radio waves more than 20 times over 15 years. Each observation revealed a slightly different rotation speed — a different length of day.

The changes probably are caused by the motions of Venus’s atmosphere, which is much heavier than Earth’s atmosphere. As it moves across the surface, it trades momentum with the solid planet — changing the length of a day.

Venus is the blazing “evening star” right now. Tonight, it’s close to the crescent Moon as twilight fades away.
 

Script by Damond Benningfield

Support McDonald Observatory