StarDate

Two of the constellations of summer bookend the waxing Moon tonight. Sagittarius is to the left of the Moon at nightfall. Some of its bright stars form the shape of a teapot, with the Moon near the tip of the spout. Scorpius curls below and to the right of the Moon. The stars that mark the scorpion’s stinger are directly below the Moon, with the scorpion’s bright heart, the star Antares, well to the lower right of the Moon.

Tonight is known as International Observe-the-Moon Night. It’s an event started by NASA to draw attention to the Moon and the agency’s efforts to explore it. Hundreds of groups across the country and around the world organize special events — Moon-watching parties, lectures, workshops, and much more. We have a link to some listings on our web site.

NASA has grand plans for sending people and robots back to the Moon. Most of those plans are years behind schedule. Some of the automated probes, for example, were originally planned for launch as early as 2020. Technical issues, COVID, supply-chain bottlenecks, and other problems have slowed them down. So as things stand now, the first of the robotic missions won’t launch until late this year at the earliest. The others have been delayed to next year or beyond.

When they do get there, they’ll concentrate on finding water and other resources for future human explorers — people who will observe the Moon from close range.
 

Script by Damond Benningfield

Support McDonald Observatory

The heart of the scorpion is doomed. Sometime in the next million years or so, the star Antares is likely to blow itself apart as a supernova. Only its tiny, dead core will remain.

That same fate awaits several other bright stars in Scorpius. A good example is Acrab — from an Arabic name that means “the scorpion.” It may have not one, but two future supernovas. The star is off to the right of the Moon as darkness falls. It’s at the top of a short line of three stars that represents the scorpion’s head.

Acrab is actually a stellar sextuplet. It consists of two tight pairs of stars, each of which has a distant companion. The two triplets are then bound to each other as well, giving Acrab six stars in all.

Two of those stars appear to be at least 10 times as massive as the Sun. That’s above the weight limit that determines which stars will explode as a supernova. So within a few million years, the cores of each of these stars probably will collapse, and their outer layers will blast into space. For a few weeks, each explosion will shine as brightly as billions of normal stars. After that, the stars will fade from sight.

For now, though, Acrab remains in good view. Look for it to the right of the Moon by about the width of your fist held at arm’s length. Much brighter Antares, the orange star that marks the scorpion’s heart, stands much closer to the Moon. They drop from view about 9:30 or 10.
 

Script by Damond Benningfield

Support McDonald Observatory

Earth faces many threats from the cosmos. Asteroids or comets could slam into our planet. We could be zapped by radiation from an exploding star. And our technology could be zapped by outbursts from the Sun.

One thing we probably don’t have to worry about is being sucked in by a black hole. The odds of such an encounter are
astronomically small.

Most of the known black holes are a few times more massive than the Sun. The gravity of such a black hole creates a strong “warp” in the space around it. That distorts the appearance of stars and galaxies that line up behind it. For any black hole within a few light-years, that distortion would be easy to see through a telescope. Since no such black holes have been seen, it’s safe to say that none threatens Earth.

In fact, the space between stars is, well, spacious. That makes it almost inconceivable that a black hole or any other stellar object could nail the solar system. But it is possible that something could pass within a couple of light-years. At that range, its gravity could stir up the Oort Cloud, a shell of trillions of big iceballs on the fringes of the solar system. A gentle nudge could push some of those iceballs toward the inner solar system, where they could slam into Earth.

So far, though, astronomers have found no black holes, stars, or other bodies that are even remotely close enough to trigger such a bombardment. There’s nothing to worry about. For now.
 

Script by Damond Benningfield

Support McDonald Observatory

“Spaghettification” sounds like prep work for a big pasta dinner. But for scientists, it’s something even tastier: the destruction of a star by a black hole. It reveals details about the star, the black hole, and the galaxy they inhabit.

It happens when a star passes too close to a black hole — especially a supermassive black hole at the heart of a galaxy. As the star gets close, the black hole’s gravity starts to pull it apart.

The destruction is known as a T-D-E — a tidal disruption event. The black hole’s gravity pulls more strongly on the side of the star that’s closest to it, so it rips the star apart in strands — like pieces of spaghetti.

The star doesn’t go quietly, though. It produces a huge outburst of energy. And it produces more energy later on. Some of the star’s hot gas forms a disk around the black hole. That produces X-rays. The disruption also creates a cloud of gas and dust, which absorbs the X-rays and emits infrared energy.

Powerful magnetic fields also direct some of the star’s gas back into space in high-speed “jets,” which produce radio waves.

Astronomers have discovered more than a hundred T-D-Es in the last couple of decades. They can use the details of each event to calculate the mass of both the dying star and the black hole. They also can learn about the environment around them — like figuring out which spices are in the sauce as a black hole consumes a “spaghetti” dinner.
 

Script by Damond Benningfield

Support McDonald Observatory

Astronomers have been debating the fate of some of the hottest, brightest, and most massive stars in the universe for years. Some studies had concluded that such a star would produce a brilliant supernova. Others said the entire star would collapse to form a black hole. But some recent discoveries suggest a hybrid scenario.

Wolf-Rayet stars can be dozens of times as massive as the Sun, and millions of times brighter. As such a star nears the end of its life, it blows material into space through powerful winds, forming a nebula around the star.

When the star reaches the end of its life, its core collapses — probably forming a black hole. Its outer layers then may blast into space, producing a brilliant supernova. On the other hand, there might not be an explosion — the entire star would collapse, winking out with no fireworks to mark its death.

But a couple of discoveries in the last few years suggest there’s something to both scenarios. Astronomers watched as two supernovas grew extremely bright, then vanished within weeks. The stars were surrounded by elements that likely were expelled by Wolf-Rayet stars. But the total amount of material in each explosion was quite small.

So the stars may have produced a bright outburst as their exploding outer layers rammed into the clouds of gas around them, then vanished as most of their material collapsed to form a massive black hole.

More about black holes tomorrow.
 

Script by Damond Benningfield

Support McDonald Observatory

Earth and Jupiter wouldn’t appear to have a lot in common. Earth is a ball of rock surrounded by a thin atmosphere. Jupiter is a ball of gas 11 times wider than Earth, with a thick, dense atmosphere.

One thing they do have in common is thunderstorms. On both planets, rising columns of water vapor condense to form clouds when they reach higher, colder altitudes. If there’s enough water vapor and enough energy, the clouds can reach towering proportions — the kind that produce thunderstorms. On Earth, they can be 10 miles high. But on Jupiter, they can be dozens of miles tall.

Several missions to Jupiter have recorded lightning in its clouds. Some have recorded radio waves produced by the lightning. Others have actually seen lightning. Some of the lightning bolts are thousands of times stronger than anything on Earth.

The Juno mission has also seen smaller flashes above the clouds. And so far, scientists don’t have a great explanation for them. But they continue to work on the problem as Juno takes more readings of Jupiter’s big thunderstorms.

Jupiter’s great size and its clouds make it especially bright. And right now, it’s shining at its brightest. It looks like an especially brilliant star, outshined only by the Moon and Venus. Jupiter is low in the east at nightfall, climbs across the south during the night, and is ready to vanish in the west at dawn.
 

Script by Damond Benningfield

Support McDonald Observatory

When Voyager 1 flew past Jupiter more than four decades ago, it recorded radio waves produced by the planet. Scientists use these and other radio studies to learn about the solar system’s largest planet — its auroras, the strength of its magnetic field, links to its big moons, and more.

The radio waves have been especially helpful in figuring out the length of Jupiter’s day. Because the planet is a big ball of gas, there are no solid features to tell us how fast Jupiter spins. To make it even tougher, cloud bands in Jupiter’s upper atmosphere move in opposite directions.

The radio waves are produced by the motions of different layers deep within the planet. The motions generate a powerful magnetic field. As Jupiter spins, the magnetic field gives particles around the planet a charge. That causes them to emit radio waves. So those waves tell us Jupiter’s internal rotation rate, which is the official definition of a Jovian day: nine hours, 55 minutes, and 30 seconds — the shortest day of any planet in the solar system.

Jupiter is putting in its best showing of the year. It’s opposite the Sun, so it’s in view all night. It’s also closest for the year, so it shines brightest. It looks like a brilliant star. It’s low in the east at nightfall, arcs across the south later on, and is low in the west at first light.

More tomorrow.
 

Script by Damond Benningfield

Support McDonald Observatory

Jupiter is the king of the planets. It’s the biggest of the Sun’s eight major planets — about 11 times the diameter of Earth. And it’s more massive than all the other planets and moons combined. And right now, it really looks the part. It’s lining up opposite the Sun, so it’s closest and brightest for the entire year, and it’s in view all night.

Jupiter’s great heft means it must have been in the “sweet spot” for planet formation. It was close enough to the Sun that there was a huge supply of raw materials for making planets. And it was far enough that the mixture included a lot of ices. Small grains of ice, rock, and metal stuck together to make larger and larger chunks. The chunks then stuck together to form planets.

As Jupiter grew, its increasingly powerful gravity pulled in more and more of these building blocks. In addition, there was a lot of leftover gas from the formation of the Sun. The more massive Jupiter got, the more of this hydrogen and helium gas it pulled in. So today, Jupiter consists mainly of those two elements, with a melty core of heavier materials in the middle — making it the king of the Sun’s planets.

Jupiter is low in the east as night falls, arcs high across the south during the night, and is low in the west at dawn. It looks like a brilliant star. In all the night sky, it’s outshined only by the Moon and by Venus, which is ending its rein as the “morning star.”
 

Script by Damond Benningfield

Support McDonald Observatory

The Hopewell culture flourished across the American Midwest for centuries. A network of nations linked by trade routes, its people farmed, settled in villages, and built massive earthworks — mounds of dirt sculpted into geometric shapes.

But about the year 400, the culture began to vanish. Archaeologists have blamed its demise on war, climate change, and other factors. Scientists at the University of Cincinnati recently added another possibility: an exploding comet.

They looked at 11 Hopewell sites in Ohio and two other states. And they found several odd features. One was a layer of charcoal — the result of an extensive fire. That layer contained high concentrations of elements that are rare on Earth but common in comets and asteroids. And the layer also held lots of tiny beads of iron and silicon, which also could have come from space.

The researchers also studied oral histories of descendants of the Hopewell. They found stories of a horned serpent that dropped rocks, a sky panther that could tear down forests, and others.

The team concluded that a comet exploded high in the sky between the years 252 and 383. It started a fire that burned 9,000 square miles of forest, sprinkled meteorites across the landscape, and probably destroyed villages.

The idea needs more study before it can be confirmed. But it suggests that the Hopewell could have been doomed — at least in part — by fire from the sky.
 

Script by Damond Benningfield

Support McDonald Observatory

Autumn arrives this evening as the Sun crosses the equator from north to south — the moment of the September equinox.

Over the last few decades, the most common date for the equinox here in the United States has been the 22nd. But it also can occur a day earlier or a day later.

That’s a result of the difference between the calendar year and the true year — the time it takes Earth to make one full turn around the Sun — a gap of 5 hours, 48 minutes, and 45 seconds.

On average, the equinox advances by that extra bit each year. But Leap Year basically pushes the equinox back to its starting point. Today, the equinox occurs on September 22nd for most of the Lower 48 states for three years in a row. It then jumps to the 23rd the year before Leap Year for the eastern part of the country.

This arrangement doesn’t completely balance the books, though. If uncorrected, it would cause the equinox to move about one day earlier every 128 years. But the calendar drops three Leap Years every four centuries. For the most part, these adjustments keep the September equinox on the 22nd or 23rd — for now.

As you might expect, though, it won’t stay that way. The equinox will begin to shift toward September 21st later in the century.

Tomorrow: a possible rain of fire across the American Midwest from an exploding comet.
 

Script by Damond Benningfield

Support McDonald Observatory