StarDate

With another spring only a memory now, one of the season’s leading star patterns is plunging from view in the evening sky.

Leo, the lion, is in the west as darkness falls. It’s dropping head first, like a real lion pouncing on its prey. Its brightest star, Regulus, is quite low in the sky, so any trees or buildings along the horizon will block it from view.

The lion’s head and mane spread out to the upper right of Regulus. His body is above Regulus, with his tail, the star Denebola, far to the upper left of Regulus.

Leo is associated with spring because it springs highest across the sky then, and is in view for most of the night. But every night, the stars rise and set about four minutes earlier than the night before. So as the weeks roll by, so do the stars. Those that are prominent during spring disappear during the longer nights of summer.

And on the opposite side of the sky, stars that were hidden during spring evenings rise to prominence. So while Leo and other spring constellations vanish, Scorpius and Sagittarius line up for their own prime-time appearances.

Leo isn’t quite done, though. The Sun is setting a little earlier each night, which gives Leo a bit of extra time before it vanishes from the evening sky. Look for it low in the west as the color of twilight drains away. Regulus will disappear by month’s end, with the rest of the lion following as we head into August.

Tomorrow: the eye of the bull in the morning sky.

Script by Damond Benningfield

Thunderstorms can roll across much of the country at this time of year. They can bring strong winds, hail, and flooding rains. That seasonal pattern is reflected in one of the names of July’s full Moon: the Thunder Moon.

The most commonly used full-Moon names are the Harvest Moon, in September or October, and the following Hunter’s Moon. But every full Moon has its own name. And they all reflect what’s happening in nature at that time of year.

Most of the names were created by the native tribes and nations of North America. The names were part of a system for tracking the turning of the seasons. June, for example, had the Rose or Strawberry Moon, and August has the Green Corn Moon. These names helped people know when to plant or harvest crops, or when to move to grounds with better hunting or better weather for the coming season.

Some cultures relied on more than just the Moon, though. They tracked the Sun’s progress along the horizon, for example, and followed the appearances of key stars. These astronomical cues made it easier to follow the seasons — year in and year out.

And the Moon will be full at 11:07 Central Daylight Time tonight, as it lines up opposite the Sun. The Sun will illuminate the entire lunar hemisphere that faces Earth, lighting up the sometimes stormy night skies of early summer.

Tomorrow: An encounter in the morning sky.

Script by Damond Benningfield

A bright star with a much brighter future scoots quite low across the sky on these early July evenings. To see it, though, you need to be south of about Dallas or Charleston.

Alpha Lupi is the leading light of the constellation Lupus, the wolf. The star is due south as darkness falls, and sets in the southwest a few hours later.

Alpha Lupi is classified as a blue giant. The “blue” part means its surface is many thousands of degrees hotter than the surface of the Sun. And the “giant” part means the star is many times bigger than the Sun. Add them up and the star shines about 20,000 times brighter than the Sun.

Alpha Lupi is likely to get a lot brighter over the next few million years, though — a result of its fast evolution. Even though it’s only a few percent the age of the Sun, it’s already at the end of its normal lifetime. It’s probably converted the original hydrogen fuel in its core to helium. Now, it’s burning the helium to make heavier elements. Radiation from the hot core is pushing outward on the surrounding layers of gas, causing the star to puff up.

As Alpha Lupi continues to evolve over the next few million years, it’s likely to become bigger and brighter.

Eventually, though, the core will no longer be able to produce nuclear reactions. At that point, the star is likely to explode as a supernova. For a few weeks or months, it’ll shine as brightly as billions of normal stars — bringing its bright but short life to a brilliant end.

Script by Damond Benningfield

The Moon has a beautiful companion tonight: the planet Saturn. It’s quite close to the lower right of the Moon at nightfall, and looks like a bright golden star. It stays close to the Moon all night long.

The Moon is a dead world. But in centuries past, many thought the Moon had water, air — and life. In the 17th century, for example, German astronomer Johannes Kepler described a world populated by snakelike creatures with wings.

Kepler is one of the pioneers of modern astronomy. He was the first to describe the orbits of the planets mathematically, proving that Earth is not the center of the solar system.

Kepler wrote about an imaginary journey to the Moon in a book called “The Dream,” which was published after his death. Many have described it as the first science-fiction story.

Kepler wrote that Moon creatures lived inside cave-like fortresses built to protect them from the Sun. These creatures were heavier than people, and looked like snakes. Some had wings, but others lived in the water. They rested during the long lunar day — which is equal to 14 Earth days — and were active during the night.

Late in his life, Kepler used a new device to study the lunar surface: the telescope. He thought the lunar craters must have been made by the Moon creatures.

Today, though, we know that the craters were carved by the impacts of mountain-sized boulders long ago. One crater is named for Johannes Kepler — a man who dreamed of a living Moon.

Script by Damond Benningfield

A stellar corpse in the constellation Scorpius isn’t a good neighbor. It’s blasting a small companion star with a particle beam. That causes the companion to flare up every couple of minutes.

AR Scorpii stands close to Antares, the scorpion’s bright orange heart, which is not far to the lower right of the Moon at nightfall. AR Scorpii is quite faint, though, so it takes a big telescope to spot it.

The system consists of two stars. One of them is a white dwarf — the hot, dense “corpse” of a once-normal star. The other is a red dwarf — a small, cool star that’s still in the prime of life. The two are separated by about half a million miles, which is only twice the distance between Earth and the Moon.

The white dwarf spins once every couple of hours — a fast rate for a white dwarf. It might have spun up by “stealing” gas from its companion. As the gas poured onto the white dwarf, it sped up the star’s rotation like a parent pushing a child on a merry-go-round.

That high-speed ride helps the white dwarf generate a powerful magnetic field, which accelerates charged particles near the white dwarf to almost the speed of light. As the white dwarf rotates, that creates twin beams, like those from a lighthouse.

The beams sweep across the companion star every couple of minutes. That zaps the surface of the star, making it much hotter for a few seconds. So AR Scorpii grows noticeably brighter every two minutes — as a dead star zaps its close companion.

Script by Damond Benningfield

Rovers are a common sight on Mars these days; two of them are trundling across the planet right now. But the first Mars rover arrived at the Red Planet 20 years ago today. In three months of operations it covered only about the length of a football field. But it provided new information on Martian history, and tested the technology that made it possible to build more-capable rovers later on.

Protected by a cocoon of airbags, Mars Pathfinder tumbled to a landing in Ares Vallis, a region where a mighty flood took place a couple of billion years ago. The petal-shaped lander soon opened up, then turned on its instruments. And it sent the Sojourner rover on its way.

Sojourner wasn’t much bigger than a microwave oven, but it was the first rover on any solar-system body other than Earth and the Moon. It carried cameras and an instrument for measuring the composition of rocks and soil. That experiment found evidence that the region had once been wet — not bone dry, as it is today.

The Pathfinder base also found evidence of water in the distant past. And it conducted extensive observations of the Martian weather. It measured temperatures from near freezing to more than a hundred degrees below zero, and found that the winds were fairly light, with occasional strong gusts.

The mission showed that it was possible to land and operate a rover on another planet — making it possible to build the much bigger rovers that are exploring Mars today.

Script by Damond Benningfield

We think of orbits as circles. But that’s not the case. The orbits of the planets and moons are all ellipses, which look like circles that have been squished a bit. So the distance between a planet or moon and the body it orbits changes. The Moon’s distance from Earth, for example, varies by about 30,000 miles.

And Earth’s distance from the Sun changes, too. In fact, today we’re farthest from the Sun for the entire year — a bit more than a million and half miles farther than the average distance.

Because of that extra gap, Earth receives about six percent less solar energy at this time of year than when we’re closest to the Sun, in early January. But Earth’s atmosphere and oceans do a good job of distributing the Sun’s heat around the globe, which keeps the northern and southern hemispheres relatively balanced.

There’s one other effect of Earth’s elliptical orbit. Our planet moves fastest when we’re close to the Sun, but slowest when we’re far from the Sun, as we are now. As a result, summer is the longest season here in the northern hemisphere — it lasts about five days longer than winter.

Of course, the situation is reversed in the southern hemisphere — winter is the longest season, while summer is the shortest.

After today, Earth will start moving closer to the Sun, at an average of more than 15,000 miles per day — carried inward by our planet’s elliptical orbit.

Tomorrow: a pathfinding rover arrives at the Red Planet.

Script by Damond Benningfield

In the 1960s, the United States launched a series of satellites to verify a treaty by watching the skies for nuclear bombs set off by the Soviet Union. The Vela satellites didn’t sniff out any bombs. But they did find some big explosions: titanic blasts by dying stars. And the first one was detected 50 years ago today.

The discovery remained a secret until 1973, when project scientists reported on 16 such explosions in a scientific journal. They called the blasts “gamma-ray bursts.” It took decades for astronomers to get a pretty good idea of what caused them. And even today, their story is incomplete.

We do know that all the bursts seen so far have occurred far outside our home galaxy. Yet when they appear, they far outshine entire galaxies of stars. Something that’s so bright yet so far away must be extremely powerful — so powerful that it took a long time to figure out the source.

The longest-lasting bursts probably represent the death throes of massive stars. At the end of such a star’s life, its core collapses to form a black hole or a neutron star. The star’s outer layers fall inward, with some of their gas forming a superhot disk around the collapsed core. Some of the material in the disk blasts its way through the poles of the dying star, forming powerful jets of gamma rays. We see the outburst only if one of the jets aims our way.

This model isn’t complete. But it’s the best idea so far to explain outbursts that were first seen 50 years ago today.

Script by Damond Benningfield

The same hemisphere of the Moon always faces Earth, locked in place by the gravitational interaction between the two bodies. The other hemisphere is known as the back side, the far side, or the dark side — not because it doesn’t get any sunlight, but because it’s hidden from view.

Spacecraft have shown us the dark side in detail, though. And it doesn’t look much like the view we’re used to. That side of the Moon has almost none of the dark, smooth volcanic plains found on the near side. Instead, it’s blotchy, with lots of impact craters and jumbled mountains. Apollo 8 astronaut William Anders — one of the first three men to see the far side with his own eyes — described the view from lunar orbit in 1968:

ANDERS: The back side looks like a sand pile that my kids have been playing in for a long time — it’s all beat up, no definition. Just a lot of bumps and holes.

The crust on the far side is miles thicker than on the near side. That may be because a smaller moon splatted into the far side billions of years ago. The impact wasn’t strong enough to blast out a crater. Instead, the smaller moon simply spread out across the landscape — creating a view unlike anything on the Moon’s familiar face.

And that face is in the southwest as night falls this evening, with two bright companions. The star Spica stands below the Moon, with the brilliant planet Jupiter a little farther to the right of the Moon.

Tomorrow: distant fireworks.

Script by Damond Benningfield

If the night sky was a TV show, tonight’s episode might be called Luna and the Giant. That’s because the Moon has a big companion tonight — the planet Jupiter. It looks like a brilliant star quite close to the left or lower left of the Moon.

Jupiter is the biggest planet in the solar system — about 11 times the diameter of Earth. As astronomers look around the galaxy, though, they find that Jupiter’s size is quite popular.

Hundreds of planets found in other star systems are about the same size as Jupiter, even though many of them are a good bit heavier.

Jupiter is also about the same size as the typical brown dwarf — an object that’s more massive than a planet, but not massive enough to shine as a star. And it’s close to the size of the smallest stars. At a minimum, these stars are about 80 times the mass of Jupiter. But the least massive are actually smaller than Jupiter.

All of these objects are about the same size because of gravity. If you add more material to an object the mass of Jupiter, it doesn’t get much bigger because gravity squeezes it more tightly.

Once a star reaches a certain critical mass, though, it starts to get bigger in a hurry. For one thing, there’s just too much stuff to pack into a smaller space. More important, though, is that the core of a star generates nuclear energy, which pushes outward on the gas around it. That causes the star to get bigger — pushing it out of the realm of “Jupiter-sized.”

Script by Damond Benningfield