StarDate

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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