StarDate

The stars that dust the night sky are all moving around the center of the galaxy at hundreds of thousands of miles per hour. Yet they’re so far away that their motion is imperceptible — across not just a human lifetime, but hundreds of lifetimes. In fact, even one of the fastest stars as seen from Earth will move just one degree over the next 5,000 years — less than the width of a finger held at arm’s length.

Altair is the brightest star of Aquila, the eagle. In fact, the name “Altair” means “the flying eagle.” The star is in the southeast at nightfall, at the lower right corner of the bright, widespread Summer Triangle.

Altair is only about 17 light-years away — closer than all but a handful of the stars visible to the unaided eye. That’s the main reason it’s moving in such a hurry. It’s like watching race cars on opposite sides of a track. Although the cars are all moving at about the same speed, those on the side closer to you cover a larger angle in a given time than those on the far side.

In the stars, though, that motion is too small to see with the eye alone. Instead, astronomers make precise measurements of a star’s position against the background of more-distant stars. Comparing Altair’s position over a period of years reveals its apparent speed across the sky.

Again, look for Altair high in the southeast at nightfall, climbing high across the south during the night, and almost due west as it sets, before dawn.

Script by Damond Benningfield

Support McDonald Observatory

Most of the stars that populate the galaxy are in the prime of life. It’s a phase that can last anywhere from a few million years to a trillion years or longer.

In astronomical jargon, that phase of life is known as the main sequence. And a prime example of a star that’s on the main sequence is Altair, the leading light of Aquila, the eagle. The star is high in the southeast at nightfall, at one corner of the bright Summer Triangle.

The main sequence is named for a relationship between a star’s brightness and its surface temperature or color. More than a century ago, two astronomers noticed that if you plotted those two characteristics on a graph, most stars lie along a narrow line — the main sequence. And it’s all because of what’s going on inside those stars.

Altair is bigger and brighter than the Sun, for example, and its surface is thousands of degrees hotter. Yet both are on the main sequence because they’re both “fusing” the hydrogen fuel in their cores to make helium. That keeps both of them shining steadily.

A star’s time on the main sequence depends on its mass. The Sun, for example, will spend about 10 billion years in that phase. But Altair is almost twice the Sun’s mass, so it “burns” through the hydrogen much more quickly. As a result, it’ll spend just a quarter as much time on the main sequence. After that, it’ll puff up and enter the next phase of life: a red giant.

We’ll have more about Altair tomorrow.

Script by Damond Benningfield

Support McDonald Observatory

The Sun illuminates about half of the lunar hemisphere that faces our way this evening. The angle of the sunlight makes it easy to see the patterns of light and dark on the surface.

One of the dark markings is the Sea of Serenity — an ancient volcanic basin that might have caused the Moon to “wander” a bit.

The Sea of Serenity formed billions of years ago. A giant asteroid slammed into the Moon, creating a crater that’s more than 400 miles wide. Over hundreds of millions of years, dense magma from deep below the surface oozed into the crater, forming a smooth, dark plain.

The volcanic rock was much heavier than the rocks around it. And a recent study suggests that caused the Moon to tilt over a bit. So its poles changed position — by about 300 miles.

In fact, the study says that several big events caused the lunar poles to wander. Each event created an area of dense rock at the surface, which shifted the Moon’s balance. So each event caused a shift in the poles.

The last of these events took place billions of years ago. So today, the Moon is stable — its poles haven’t moved in a long time. And that’s important for lunar exploration. Deep craters at the poles haven’t seen the Sun in billions of years. So vast deposits of ice are trapped there — possible resources for future explorers.

Look for the Moon in the southwestern quadrant of the sky this evening. The bright star Antares, the heart of the scorpion, lurks nearby.

Script by Damond Benningfield

Support McDonald Observatory

Scientific discoveries take time. They have to be confirmed by other scientists. That can come through new observations, a dive into old ones, or a combination of the two.

An example of the “combo” approach is the discovery of a planet orbiting Proxima Centauri, the closest star to our own solar system.

Astronomers had already confirmed one planet in the system, called Proxima Centauri b. It’s about the size of Earth, and it’s in the region where conditions are most comfortable for life.

Early this year, a team led by Mario Damasso in Italy reported the discovery of a possible second planet. Their work indicated that Proxima Centauri c is heavier than Earth. And it’s so far from the star that it probably would be an iceball.

A few months later, another team reported snapping possible pictures of the planet, which might have rings.

And in June, Fritz Benedict of McDonald Observatory added more details. He looked at observations that he and colleagues had made with Hubble Space Telescope in the 1990s. They were looking for planets in closer orbits, but didn’t find any. When he looked again, though, he found evidence of planet “c.”

When he combined the observations of all the teams, Benedict came up with a planetary mass of roughly seven times that of Earth, and an orbital period of about five and a half years.

Other teams continue to study the system to tease out details about this and other possible planets around our close neighbor.

Script by Damond Benningfield

Support McDonald Observatory

Many of the star pictures that adorn the night sky are big and bold. The curving body of Scorpius, the scorpion, strides low across the southwestern horizon on August evenings, for example, while Cygnus, the swan, climbs high across the top of the sky.

But many other constellations are much more difficult to see. They’re small or faint or both — little more than filler between the better-known star patterns.

Three of them stairstep up the east as night falls. One of them is small but fairly easy to find, while the other two require some work and imagination.

The easy-to-spot constellation is Delphinus, the dolphin. It’s about halfway up the eastern sky, to the lower left of the bright star Altair. It’s one of the smallest of the 88 constellations. But it stands out because its main stars really do form a pattern that resembles a dolphin. Its body is to the left, and its long tail to the right.

An even smaller constellation is directly below Delphinus — Equuleus, the little horse. Unlike the much bolder Pegasus, the flying horse, which is next door, Equuleus is drawn as only the horse’s head. All of its stars are quite faint, so you need especially dark skies to see it.

And Sagitta, the arrow, is above Delphinus. Its stars are faint as well. Under dark skies, though, you might just make out the shape of the arrow — four stars pointing roughly to the lower left — a tiny star picture in the broad summer sky.

Script by Damond Benningfield

Support McDonald Observatory

NASA’s working on plans to send astronauts to Mars. But Ray Bradbury has already taken us there. The author published “The Martian Chronicles” in 1950 — stories of the Martians’ futile resistance to human colonists — and how humans became the Martians.

Bradbury was born 100 years ago today, in Waukegan, Illinois. He read the works of H.G. Wells, Jules Verne, and others. And when his family moved to California, in 1934, he set up camp in the UCLA library. He couldn’t afford to go to college, so he educated himself at the library. He also joined a local science-fiction group that included some of the top writers in the field.

Bradbury published his first paid story in 1941. And over the next five decades, he published hundreds of works in many genres. His stories were among the most literary of all sci-fi and fantasy. The list included such famous works as “Fahrenheit 451,” about dictatorship and censorship, and “The Illustrated Man.”

Bradbury’s works were adapted for radio, and he wrote screenplays for movies and TV shows. He also headed a theater group.

Shortly after Bradbury died, in 2012, NASA named the landing site of the Curiosity Mars rover in his honor: Bradbury Landing — a spot on a world that Bradbury had “seen” long before any craft ever visited there.

Script by Damond Benningfield

Support McDonald Observatory

Millions of black holes may populate the Milky Way Galaxy. So far, though, only one of them has been found in a star system that’s visible to the unaided eye. HR 6819 is in the constellation Telescopium. Unfortunately, though, it’s visible mainly from the southern hemisphere.

The black hole is the closest to Earth yet seen. It’s about a thousand light-years away — a third of the distance to the previous record holder. It contains two stars that are much bigger, heavier, and brighter than the Sun. Years ago, astronomers took a close look at it with a telescope in Chile. But they filed the observations away without a careful analysis.

But they recently noticed that a similar system might have a black hole. So they took a better look at their data. And they found a hidden presence. One of the bright stars is orbiting an invisible object that’s at least four times the mass of the Sun. If the object was a star it would be easy to see. Since it’s not visible, it must be a black hole.

Unlike most black holes, this one is quiet. Most of the other known black holes — a few dozen in all — are stealing gas from companion stars. The gas forms a hot disk around the black hole that shines brightly. In this case, though, the black hole is too far from its companions to pull away any gas.

Despite its proximity, the black hole is nothing to worry about. It’s so far away that it’s no threat to Earth or the rest of our solar system.

Script by Damond Benningfield

Support McDonald Observatory

The neck of one of the Milky Way Galaxy’s spiral arms is a land of giants. Astronomers have discovered several monster star clusters there. And the clusters are dominated by some of the biggest and heaviest stars in the galaxy: red supergiants.

The clusters reside in the Scutum-Centaurus Arm — a lane of stars and gas clouds that curves around the galaxy. It spirals off the end of a “bar” of stars at the galaxy’s heart. The arm funnels gas toward the bar. The gas piles up, then splits into clumps that give birth to stars.

Those “clumps” include at least four red-supergiant star clusters — the only ones yet seen in the entire galaxy.

Red supergiant stars are much bigger and heavier than the Sun, and tens of thousands of times brighter. But they’re rare. In fact, these clusters and the space around them contain a large fraction of all the Milky Way’s red supergiants.

The clusters are quite young — no more than 20 million years old. Yet because the red supergiant stars are so massive, they’re already in the final stages of life, so they’re huge and brilliant. And soon, they’ll shine even brighter. Every red supergiant will explode as a supernova — depriving the clusters of their most impressive residents.

The clusters are in the constellation Scutum, which is in the south at nightfall, above teapot-shaped Sagittarius. Despite their brilliance, the clusters are veiled by clouds of dust, so they’re hidden from view.

Script by Damond Benningfield

Support McDonald Observatory

Everything about our home galaxy, the Milky Way, is humongous. The galaxy’s disk spans at least a hundred thousand light-years. It contains hundreds of billions of stars. And a giant bar of stars may inhabit the middle of the galaxy.

Figuring all of that out hasn’t been easy. It’s like the old adage about not being able to see the forest for the trees. Since we’re inside the galaxy’s disk, our view of the Milky Way’s overall design is limited. So the bar wasn’t discovered until a few decades ago. And even now, astronomers aren’t sure about its details.

Recent observations suggest it’s shaped a little like a baguette, with rounded, tapered ends. It’s anywhere from a few thousand to tens of thousands of light-years long. And one of its ends may point toward the solar system.

The bar may have formed from a giant rotating disk of stars in the center of the galaxy. Gravitational interactions between stars pushed many of the stars into elongated orbits — forming the bar.

Spiral arms loop off both ends of the Milky Way’s bar. They may funnel gas toward the bar. So the intersection of a bar and a spiral arm is a busy stellar nursery; more about that tomorrow.

The end of the Milky Way’s bar is in Sagittarius, which is low in the south at nightfall. Look for a “teapot” of moderately bright stars. The center of the galaxy is in the “steam” above the teapot created by the glow of millions of stars in the galaxy’s disk.

Script by Damond Benningfield

Support McDonald Observatory

Sagittarius is home to some monster stars. That’s because one of the Milky Way Galaxy’s spiral arms passes through its borders. The arms are where new stars are being born, and some of those stars are especially big and bright.

One example is VX Sagittarii. It’s classified as a red supergiant or hypergiant. It’s probably a billion miles in diameter or bigger. That’s more than a thousand times the size of the Sun. It’s so big that if it took the Sun’s place, it would engulf the five closest planets.

The star has grown so puffy because it’s used up the original hydrogen fuel in its core. Now, it’s “burning” other elements. That raises the temperature of the core, so radiation pushes outward on the surrounding gas.

The process has made the star’s outer layers unstable, so it puffs in and out like a beating heart. Over a couple of years, its diameter varies by tens of millions of miles.

The star also produces a strong “wind.” It carries away enough gas to make a star as heavy as the Sun in just a few thousand years.

And before long, it’ll blow a lot more gas into space. The star will explode as a supernova. That’ll blast away its outer layers, leaving behind only its crushed core.

Sagittarius is low in the south at nightfall. Look for a group of eight stars that forms the outline of a teapot. VX Sagittarii is well above the teapot. It’s more than 5,000 light-years away, and it’s behind clouds of dust, so despite its brilliance, you need a telescope to see it.

Script by Damond Benningfield

Support McDonald Observatory