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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

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

Sagittarius is packed with star clusters. That’s mainly because the center of the Milky Way Galaxy lies within its borders. So as we look in that direction, we’re looking into dense clouds of stars, plus the gas and dust for making more stars. One of the most prominent clusters is Messier 22. It’s about 10,000 light-years away. But it contains hundreds of thousands of stars, so it’s an easy target for binoculars. It’s in the south at nightfall, above the “lid” of the teapot formed by some of the brightest stars of Sagittarius. M22 is a globular cluster. All of its stars are packed into a sphere that’s about a hundred light-years across. And they’re among the oldest stars in the galaxy. So almost all of them are fairly small and faint. The cluster was born with some stars that were big, bright, and heavy. But such stars expire quickly. Some of them explode, leaving behind a neutron star or a black hole. Others skip the explosion and collapse directly to form a heavier black hole. And astronomers have found two black holes in M22’s core. Both of them are fairly heavy — between 10 and 20 times the mass of the Sun. And each of them is devouring a companion star. Most models say a globular cluster should have just one black hole at its core. The rest should be kicked out by encounters with other black holes. Since astronomers have found two black holes in M22, though, even more could be awaiting discovery. Script by Damond Benningfield  

Our home galaxy, the Milky Way, is home to hundreds of billions of stars — and the population is growing all the time. New stars are taking shape inside vast clouds of gas and dust. And several of these clouds are visible in the constellation Sagittarius, which scoots low across the south on summer nights. The brightest of these clouds is M8, the Lagoon Nebula. If you have especially dark skies, it’s visible to the unaided eye as a faint smudge of light. But telescopes reveal rich details, including a dark streak across the nebula — like a lagoon cutting into a tropical island. The bright part of the nebula is too hot to give birth to new stars. But dark veins and bubbles of colder gas are mixed in, and these are busy stellar nurseries. They’ve given birth to several dozen stars in the Lagoon. At around four million years old, they’re some of the youngest stars in the entire galaxy. Another nursery is just to the northwest of the Lagoon: M20, the Trifid Nebula. Through a telescope, it looks like it’s split into thirds by veins of cold gas and dust. Hot young stars near the center of the nebula pump out ultraviolet energy. That causes the surrounding gas to glow like a fluorescent bulb — the light of a stellar nursery. Sagittarius is low in the south as night falls. Its brightest stars form the shape of a teapot. M8 and M20 stand a little above the spout of the teapot. We’ll have more about Sagittarius tomorrow.  Script by Damond Benningfield

Our solar system is on a high-speed merry-go-round. The Sun orbits the center of the Milky Way Galaxy at half a million miles per hour. The center of the galaxy is in Sagittarius. The constellation rolls low across the southern sky on summer nights. Some of its bright stars form the outline of a teapot. The galactic center is above the “spout” of the teapot — immersed in the faint “steam” of the Milky Way. We’re roughly 27,000 light-years out from the center. At that distance, it takes the solar system about 230 million years to complete a single revolution. The solar system’s orbit isn’t a smooth path, though. If you could plot it out, you’d see a lot of little bumps and wiggles. That’s because the galaxy itself isn’t smooth and even. Instead, it’s “lumpy.” There are clusters of stars, big clouds of gas and dust, and other structures. So as the solar system passes by these objects, it’s tugged by their gravity, slightly changing its orbital path. And we pass through some of the clouds of gas and dust, which also alters the solar system’s path. The solar system has been around for about four and a half billion years. That means it’s completed roughly 20 orbits around the galaxy. And it’ll probably make another 25 or more before the Sun reaches the end of its “normal” lifetime — more high-speed rides on a galactic merry-go-round. We’ll talk about some stellar nurseries in Sagittarius tomorrow. Script by Damond Benningfield

During its first couple of billion years, Venus could have been warm and wet — a paradise for life. Research in recent years says it could have had a global ocean — a perfect place for life to take hold. Today, though, the planet is anything but paradise. The surface temperature is about 865 degrees Fahrenheit, and the surface pressure is more than 90 times greater than Earth’s. The change probably was caused by a runaway greenhouse effect, which began as the Sun got brighter. When it was born, the Sun was a good bit fainter than it is today. That allowed the ocean on Venus to form in the first place. As the Sun aged, though, it got brighter and hotter. The water in Venus’s ocean began to evaporate. The extra water vapor in the atmosphere trapped more of the Sun’s heat, speeding up the process. Radiation from the Sun zapped water molecules in the atmosphere, splitting them apart. The hydrogen escaped into space, while the oxygen combined with carbon to make carbon dioxide. At first, most of it was locked in the rocks. As Venus got hotter, though, more and more was released into the air. The CO2 trapped even more solar energy, cranking up the thermostat even more — turning Venus from paradise to purgatory. Venus is still beautiful to look at, though. Right now, it’s the brilliant “morning star.” It’ll stand quite close to the crescent Moon at dawn tomorrow, and farther to the upper right of the Moon on Sunday. Script by Damond Benningfield

The night sky plays tricks on us. Objects that look like they’re right on top of each other can be separated by thousands of light-years. An example is the star Sadr. It connects the body and wings of Cygnus, the swan, which soars high overhead on August nights. Sadr is a supergiant. It’s at least a dozen times the mass of the Sun, 150 times wider than the Sun, and more than 30,000 times brighter. It’s only about 12 million years old — a tiny fraction of the age of the Sun. Yet it’s already nearing the end of its life. Before long, it will explode as a supernova. A telescope reveals wisps of red around Sadr — clouds of glowing gas. And photographs reveal even more of the clouds. In fact, it looks like Sadr is enveloped by them. Yet they’re not even close. The clouds are about 3,000 light-years farther than Sadr. The clouds glow because they’re “energized” by the radiation from bright young stars that are associated with them. And the clouds are huge — they span more than a hundred light-years. Some of their gas and dust will collapse to give birth to more stars. In time, Sadr will produce its own cloud. When it explodes, it’ll create an expanding shell of gas and dust. The shell will be visible for thousands of years. To future skywatchers, it may look like it’s just part of the larger complex of clouds behind it — another illusion in the night sky. Tomorrow: bright companions at dawn. Script by Damond Benningfield  

The stars tell us a lot about themselves. A detailed analysis of their light reveals their temperature, composition, and motion. It can also show whether they have companions. But interpreting what the stars are saying isn’t always easy. Consider Aldebaran, the “eye” of Taurus, the bull. The bright star is close to the Moon in the wee hours of tomorrow morning. Several faint stars appear quite close to Aldebaran. Most of them aren’t physically related, though — they just happen to line up in the same direction. But one of them could be bound to Aldebaran by their mutual gravity. Despite more than two centuries of observations, though, we’re not sure if that’s the case. The star is known as Alpha Tauri B. It’s tens of billions of miles from Aldebaran. And that’s part of the problem. The two stars appear to be moving through space in the same direction — an indication that they’re linked. To make sure, though, astronomers need to see the orbital motion of the two stars around each other. But with such a wide gap between them, it would take thousands of years to complete even a single orbit. So there hasn’t been enough time to verify a link between them. The other problem is that Alpha Tauri B is a red dwarf — a small, faint cosmic ember. Its feeble light is overwhelmed by Aldebaran’s. So it’s hard to plot its position through the glare. That makes it even harder to know if the stars are related, or just passing by each other. Script by Damond Benningfield

Antares, the bright heart of the scorpion, is one of the youngest stars around — no more than about 12 million years old. And M4, a cluster that appears close to Antares, contains some of the oldest stars — more than 10 billion years old. In fact, Antares and the stars of M4 belong to completely different populations. Antares, like the Sun, belongs to Population I. And the stars of M4 belong to Population II. Population II stars were born when the universe was young. They were made almost entirely of hydrogen and helium, the two lightest elements, which were created in the Big Bang. The stars couldn’t incorporate many heavier elements because there weren’t many around. Most of those elements are forged inside stars, then expelled into space when the stars expire. So stars couldn’t incorporate heavy elements until after the first stars had lived and died. Stars with more heavy elements form Population I. They’re still made mainly of hydrogen and helium, but they have smatterings of carbon, oxygen, and many other elements as well. Most of the stars in the Milky Way belong to Population I. They live in the galaxy’s wide, thin disk. Most Population II stars reside outside the disk, in the galaxy’s halo. Antares and M4 are low in the south at nightfall. Antares shines bright orange. M4 stands close to its right, although you need binoculars to see it — a population of old stars beside a newborn. Script by Damond Benningfield

Venus is stretching its legs right now. The “morning star” stands farthest from the Sun for its current appearance over the next few days. It climbs into view about three hours before sunrise. And it’s more than a third of the way up the eastern sky by the time it fades from view. One of the reasons Venus looks so bright is that it’s blanketed by brilliant clouds. They completely hide the surface from view. The spacecraft that provided our best view through the clouds arrived at Venus 30 years ago today. Over the next few years, Magellan used radar to map about 98 percent of the surface. The images revealed a landscape dotted by hundreds of volcanoes. Most of them are extinct, but Magellan and later missions found evidence that some of them may still be active. Magellan’s maps show that, over the last billion years or so, volcanic processes have repaved Venus’s surface. That could have happened quickly. But recent studies say it probably played out over hundreds of millions of years — and may still be going on today. After its mapping mission was done, Magellan used a technique known as aerobraking to lower its orbit. It dipped into the atmosphere on each orbit, “dragging” it a little farther down with each pass. From the lower orbit, Magellan compiled a map of Venus’s gravity. That gave scientists more information about the planet’s structure and the composition of its crust — painting a better picture of a “hidden” world. Script by Damond Benningfield

Swift-Tuttle is one of the biggest comets yet seen — about 15 miles across. It’s also one of the messiest. Every time it gets close to the Sun, it sheds countless grains of dust. Those grains spread out along the comet’s path. And every summer, Earth plows through the cloud. That creates the Perseid meteor shower, which will be at its best the next few nights. Comets are balls of ice and rock left over from the birth of the solar system. When they get close to the Sun, they heat up. That vaporizes some of their ice, which in turn releases tiny bits of dust and rock. The cloud of debris is thickest around the comet itself. So the Perseid meteors are at the their best when Swift-Tuttle is close to the Sun. Right now, though, it’s headed away from the Sun, and won’t return for more than a century. Even so, the Perseids are still one of the best showers of the year. As Earth flies through the cloud of comet dust, the grains ram into the atmosphere at more than a hundred thousand miles per hour. They vaporize, forming the glowing streaks of light known as meteors or shooting stars. At the shower’s peak, which is expected in the wee hours of Wednesday morning, it might produce several dozen meteors per hour. Unfortunately, the Moon will be in the sky then. Its light will overpower most of the meteors. Yet a few will shine through. The view is best under dark skies, away from city lights — beautiful fireworks from a messy comet. Script by Damond Benningfield

For most of the United States, the winds tend to be pretty calm at this time of year, even in the Great Plains. On the “plains of paradise” of Mars, though, summer is the windiest time of year. As measured by the InSight lander, the winds are about five miles per hour faster in summer than in winter. InSight is designed to study the interior of Mars. It’s using a seismometer to measure “marsquakes.” And it’s been trying to deploy a probe to measure temperatures below the surface. But it also carries a small weather station. The station measures temperature and pressure, plus the speed and direction of the wind. InSight sits on a volcanic plain, known as Elysium Planitia. There are no mountains or canyons around, so there’s nothing to obstruct the winds. InSight’s readings show that the wind is strongest in the morning, not long after sunrise. And it’s weakest not long after sunset. And it’s also stronger in summer — an average of about 13 miles per hour, versus about eight miles per hour in winter. The most powerful wind gust was recorded during a big dust storm — about 70 miles per hour. The Martian atmosphere is so thin, though, that if you were standing on Mars you’d barely feel it — a thin breeze on the Red Planet. And Mars teams up with the Moon tonight. The planet is quite close to the Moon as they climb into good view, around midnight, and just as close at first light. Mars looks like a bright orange star. Script by Damond Benningfield

Like everyone else, astronomers sometimes have to take some time off to care for family members. Few of them are having to defend against accusations of witchcraft, though. But that’s what happened to Johannes Kepler. Four hundred years ago today, his mother was arrested — and charged with 49 counts of witchcraft. Kepler moved his family to her village in southern Germany, where he led her defense — and eventually won the case. Kepler formulated the laws of planetary motion. They provided proof that Earth wasn’t the center of the solar system. Instead, they showed that Earth and the other planets orbited the Sun. Kepler’s mother was first charged with witchcraft in 1615. She was 68 years old and a widow. And she brewed up herbal remedies that were sometimes accompanied by charms. That made her a good candidate for charges of black magic. Witnesses said she’d caused them great pain, passed through closed doors, and even turned herself into a cat. She was one of 15 people indicted for witchcraft at the time — and eight of them were executed. Kepler won her release, and she left town. But when she returned, in 1620, she was arrested again. She was chained to the floor of a cell and threatened with torture if she didn’t confess. She refused. Kepler used science and common sense to refute the charges. After 14 months in prison, she was acquitted — thanks to the efforts of her son, one of history’s greatest astronomers. Script by Damond Benningfield

A strong heart needs strong arteries. And Scorpius has both. The constellation’s “heart” is the star Antares, a bright orange supergiant. And it’s flanked by bright arteries. One is to the upper right of Antares at nightfall, with the other to the lower left. Over the centuries, both stars have been known as Al Niyat — an Arabic name that means “the arteries.” To help tell them apart, the one to the upper right is also known as Sigma Scorpii, while the other is Tau Scorpii. Sigma is a system of at least four stars. At least two of them are similar to Antares. They’re much bigger and heavier than the Sun, and many thousands of times brighter. The two stars are so close together that not even the biggest telescopes can see them as individual stars — their light blurs into a single point. The system’s other two stars are visible. And those stars are impressive as well — though perhaps not as impressive as the tight binary. Tau Scorpii consists of a single star. But it, too, is a monster — about 15 times the mass of the Sun, and about 20,000 times brighter. Antares and its arteries all belong to a big family that includes lots of big, heavy stars. Antares and the Al Niyats are all quite young — no more than about 10 million to 15 million years old. And all of them share an impressive fate: Within a few million years, they’ll explode as supernovae — strong endings for the scorpion’s heart and arteries. Script by Damond Benningfield

A star in Scorpius likes to blow off steam. It stages big outbursts every decade or so. And the next one could happen any day now. U Scorpii is a system of two stars that are only a few million miles apart. One of the stars is at the end of its “normal” lifetime, so it’s starting to puff up. The other has already ended its life, so only its hot, dead core remains — a white dwarf. The white dwarf “steals” gas from its companion. The gas forms a disk around the white dwarf. Some of the gas funnels onto the star’s surface. As it builds up, it gets hotter. Eventually, it gets hot enough to trigger a nuclear explosion. That blasts the built-up gas out into space. U Scorpii erupts about every 10 years. The most recent outburst came in January of 2010. In a few hours, the system grew about 10,000 times brighter than average. It then faded quickly. Within a few days, though, the white dwarf was already pulling in more gas. But observations over the following years showed that it took about 18 months to fully reestablish the disk of gas around the star. Astronomers expect to see the next eruption this year. In fact, it’s possible that it’s erupted since we recorded this program. Whenever it happens, astronomers will keep a close eye on U Scorpii — watching it once again blow off some steam. The system stands above Antares, the scorpion’s bright orange heart, which is in the south as night falls. More about Scorpius tomorrow. Script by Damond Benningfield

There aren’t many constellations that really look like what they’re supposed to depict. And there aren’t many whose ancient names are easy to translate without a dictionary. But one that defies both trends is one of the highlights of summer’s evening skies: Scorpius, the scorpion. It’s low in the south as night falls right now. Under even a moderately dark sky, it’s not hard at all to see the outline of a scorpion. Its “heart” is the bright orange star Antares. The scorpion’s head is formed by a short line of three stars to the upper right of Antares. And its body, tail, and “stinger” curl away to the lower left of Antares. In ancient times, the scorpion was even bigger. Its claws extended far from the head. But more than 2,000 years ago, they were stripped away. Today, they form the constellation Libra, the balance scales. Yet a couple of its stars retain their scorpion names: Zubeneschamali and Zubenelgenubi — the northern and southern claws. One reason that Scorpius shines so brightly is that it outlines one of the spiral arms of the Milky Way Galaxy. Clouds of gas and dust along the arm are giving birth to new stars. And many of those stars are especially big and bright. That makes the scorpion easy to pick out. Many cultures have identified these stars as a scorpion. Many others, though, have seen a different but equally understandable pattern — a fishhook, luring prey across the Milky Way. More about Scorpius tomorrow. Script by Damond Benningfield

A Mars rover came to a dark end in June of 2018. A dust storm blanketed all of Mars, blotting out the Sun. The Opportunity rover couldn’t get enough sunlight to power its solar batteries. So after 14 years and 28 miles of roving, it went dark. The irony is that the dust storm stirred to life because Mars itself was receiving more solar energy than average. The planet was approaching its closest point to the Sun, known as perihelion. And Mars is at perihelion today for the first time since then. Mars’s orbit around the Sun is lopsided. At its closest, the planet is about 27 million miles closer to the Sun than at its farthest. That means it receives about 40 percent more sunlight. That warms the surface. As it warms, frozen carbon dioxide atop the polar ice caps begins to vaporize. It rushes into the atmosphere, making it thicker. It also pulls along plumes of dust that can rise high into the sky. Over weeks or even days, the dust can cover a large area. Some years the storms are as big as states or countries. But sometimes, they cover most or all of the planet. That’s what happened in 2018, in one of the biggest storms yet seen. But such monsters don’t happen on a predictable cycle. They stir up in some years, but not others. And even if there’s a big storm this year, it’s no threat to the only working rover on Mars. Curiosity has a nuclear power source. That allows it to keep going — even when the Sun goes dark. Script by Damond Benningfield

If you want to escape the summer heat by plunging into a cool lake or sea, there are only two places in the solar system to do so: Earth, and Titan, the largest moon of Saturn. The big difference is what’s filling those features. On Earth, it’s water. But on Titan, it’s liquid methane and ethane — hydrocarbons that are chilled to hundreds of degrees below zero. Titan is about half again the size of Earth’s moon. It has a thick atmosphere that’s topped by smog — an orange haze of hydrocarbons. Methane and ethane clouds drift through the sky, sometimes pouring rain on the surface. That fills lakes and seas, and appears to carve riverbeds. Some of the smaller lakes are surrounded by tall cliffs. Those may have formed at a time when Titan was even colder — so cold that liquid nitrogen flowed across the surface. A study last year said that some of the nitrogen could have trickled below the surface, forming underground pools. When Titan warmed up, the nitrogen turned to gas. It expanded, blowing sharp-rimmed holes in the surface. The holes later filled with methane and ethane, forming new lakes — perfect places for a really cool plunge. Saturn is easy to find tonight. The planet looks like a bright star. It’s to the upper right of the Moon as night falls, and leads the Moon across the sky later on. Through a telescope, Titan looks like a tiny star near the planet. The even brighter planet Jupiter is to the right of Saturn. Script by Damond Benningfield

A brilliant triangle slides across the south tonight: the Moon and the planets Jupiter and Saturn. Jupiter looks like a brilliant star above the Moon at nightfall. Fainter Saturn stands to the left of the Moon. The Moon is almost full, so almost the entire hemisphere that faces our way is in sunlight. That reveals the pattern of light and dark that makes up the Moon’s features. One easy-to-spot feature is at the top right edge of the lunar disk in early evening: Mare Crisium, the Sea of Crises. It’s a small dark circle that’s not connected to any other dark features. The Sea of Crises is a plain of volcanic rock that fills a big crater. The crater formed almost four billion years ago, when a giant asteroid slammed into the lunar surface. It blasted a hole almost 350 miles across, and several miles deep. Later, magma bubbled up through cracks in the crater floor. As the magma cooled and hardened it formed a vast plain that bulges upward in the middle. Dense rock from the lunar mantle pushes up below that bulge, so the Moon’s gravitational pull is a little stronger there than in surrounding areas. In 1976, a Soviet robotic lander brought back 170 grams of dirt from the Sea of Crises. The composition wasn’t what had been expected. That may be because the probe landed atop a ribbon of material blasted out of a nearby crater. So the only sample from the Sea of Crises may have come from deep below this prominent lunar feature. Script by Damond Benningfield

If cockroaches ever find their way into habitats on the Moon, they’re likely to thrive. In a study a half century ago, roaches that were fed lunar dirt grew a bit faster than those that weren’t. Before the first Apollo astronauts landed on the Moon, some scientists were concerned they might bring back dangerous microscopic organisms. The odds were low because the lunar surface is bombarded by deadly radiation. But the risk was deemed high enough to take some precautions. The astronauts from the first three missions spent three weeks in quarantine, for example. Samples weren’t sent to researchers until some had been examined for microbes. And several types of Earth life were exposed to the samples. The results of that study were published 50 years ago today. Researchers ground up a few grams of the rock and dirt from Apollo 11, the first lunar landing mission. They mixed it in with the water that was holding fish, shrimp, and oysters. And they fed it to birds, flies, and cockroaches. Scientists kept an eye on the subjects, along with control groups, for four weeks. None of the animals exposed to the samples were any worse off than the ones in the other groups. The roaches actually did a little better with lunar dirt than without, although the difference wasn’t much. The tests confirmed that the lunar surface is lifeless. That allowed scientists to get on with studying the samples — a safe treasure from the surface of the Moon. Script by Damond Benningfield