StarDate

To modern eyes, the brightest stars of the constellation Sagittarius form the outline of a teapot, with the handle to the left and the spout to the right. To ancient eyes, though, it was the archer — a centaur holding a bow and arrow. It’s tough to pick out the centaur, but the bow is easy. It’s traced by an arc of three stars in the teapot, including the brightest star in the whole constellation.

The three stars are known as Kaus Borealis, Kaus Media, and Kaus Australis — names that mean the northern, middle, and southern points of the bow. Kaus Borealis forms the top of the teapot. Kaus Australis is at the teapot’s bottom right, and is the brightest member of the constellation. And Kaus Media bows outward between them.

All three stars are giants — they’re much bigger and brighter than the Sun. And even though they’re much younger than the Sun, they’re also much later in life. That’s because they’re all heavier than the Sun. Heavy stars consume their original nuclear fuel in a hurry, so they burn out quickly. That process causes their outer layers to puff up, which makes them look much brighter.

Kaus Media is the farthest member of the bow, which means that it’s actually the most impressive of the three stars — it’s more than a thousand times brighter than the Sun.

And if there’s a bow, there must be an arrow as well. It forms the outer point of the spout of the teapot — one more giant star that’s approaching the end of its life.

Script by Damond Benningfield

The Milky Way galaxy boasts more than just a plethora of stars. It also contains lots of gas, which gives birth to yet more stars. And astronomers are making progress in tracing the story of a remarkable cloud of gas that’s crashing into the Milky Way's disk.

Discovered in the early 1960s by astronomer Gail Smith, the cloud is millions of times as massive as the Sun. It’s about 40,000 light-years from Earth, in and around Aquila, the eagle, a constellation that’s well up in the eastern sky at nightfall. Despite its great distance, Smith’s Cloud is so big that if you could see it, it would span 30 times the diameter of the full Moon.

By measuring the cloud’s velocity and its contents, astronomers recently deduced that it originated in the outer regions of the galaxy’s disk about 70 million years ago. Something shot the cloud out of the disk, and now it’s falling back in.

So far, though, no one knows what gave the cloud its original kick. The explosions of large stars have accelerated other gas clouds, but Smith’s Cloud is much heavier than any of them.

The future of the cloud is clearer. In about 27 million years, it’ll crash into the Milky Way’s disk. The disk holds plenty of gas itself, so the collision should cause gas clouds to slam together. That will squeeze the clouds, causing them to give birth to new stars in a galaxy that already abounds with hundreds of billions of them.

We’ll have more about the Milky Way tomorrow.

Script by Ken Croswell, Copyright 2016

The brightest star in the night sky has played a key role in the calendars of many cultures. In ancient Egypt, for example, its first appearance in the dawn sky marked the beginning of a new year, and heralded the annual flooding of the Nile. And in Greece and Rome, that dawn showing ushered in the dog days — the most sweltering time of year. It’s a phrase we still use today.

When the phrase was coined, Sirius first rose into view around the end of June — at the start of summer and a long run of hot, humid days. Sirius is the brightest star of the constellation Canis Major, the big dog. So it’s known as the Dog Star. People associated the star with the climate, so they called that time of year the dog days.

Thanks to an effect known as precession, though, the stars gradually shift position relative to the Sun. So now, Sirius doesn’t peek into view in the dawn sky until August. From the southern states of the U.S., it’s been in view for a few days now, while it’s just emerging from the Sun’s glare from more northerly latitudes. It’s in the east-southeast, almost directly below the line of stars that marks Orion’s Belt.

That shift in Sirius’s position has changed the way we think of the dog days. In early times, the phrase referred to a specific time of year — generally from early July to about mid-August. Today, though, it usually refers simply to the hottest period of the year — the dog days of summer.

Tomorrow: A cloud comes crashing down.

Script by Damond Benningfield

Enceladus is a small moon with a big appeal. Water shoots into space from cracks in the ice around its south pole. That water is fed by an underground ocean that could be a good home for life.

Enceladus is the sixth-largest moon of the planet Saturn. It’s only about 300 miles in diameter, compared to more than two thousand miles for Earth’s moon.

Most moons of that size are little more than chunks of rock and ice, with surfaces marked by countless impact craters — dead worlds that attract little attention.

Enceladus, on the other hand, has a fairly young surface — vast sheets of bright, clean ice. That’s because the gravity of nearby Saturn tugs and twists at the little moon, warming its interior. That creates an ocean of liquid water below the icy crust — an ocean that may offer the right ingredients for life.

Some of the water from below makes its way to the surface, repaving the surface with fresh ice. And right now, it’s squirting out from cracks in the ice at the south pole. Saturn’s gravity constantly pulls at those fissures, preventing the escaping water from freezing and plugging them up — letting geysers of water and ice shoot far out into space.

Look for Saturn to the lower right of our own moon tonight, shining like a bright star. The even-brighter planet Mars is farther along the same line. And the star Antares, the heart of the scorpion, is below Saturn, completing a beautiful grouping in the evening sky.

Script by Damond Benningfield

Tonight should be a busy night for special skywatching events. The evening hours offer a beautiful conjunction between the Moon and three other bright objects. And after that group drops from sight, in the wee hours of the morning, there’s what’s expected to be a pretty good showing for a meteor shower.

As the evening twilight fades away, look for the Moon standing at the top right of a diamond of bright objects. The brightest one after the Moon is the orange planet Mars, which is directly below the Moon. The planet Saturn is a little closer to the lower left of the Moon, with the star Antares below Saturn.

The Moon and its companions set by around 1 or 2 a.m. By then, the Perseid meteor shower should be starting to heat up. As seen from the United States, in fact, it should be at its best in the hours before dawn.

The shower occurs when Earth passes through the trail of debris that was shed by a comet. The grains of comet dust plunge into the atmosphere at high speed. They quickly vaporize, forming the streaks of light known as meteors.

Some experts say that Earth should pass through a pretty good clump of comet dust this year, so you might see up to dozens of meteors per hour.

To really enjoy the late-night fireworks, you need to get away from city lights. But that’s not the case with the Moon and its companions. Weather permitting, they’ll be in good view from just about anywhere — even through the bright lights of the city.
 

Script by Damond Benningfield

The Perseid meteor shower is underway, building toward its expected peak tomorrow night. And the Moon sets early enough to provide several hours of dark skies for watching the fireworks.

Like all meteor showers, the Perseids are caused by debris from a comet slamming into the upper atmosphere. The bits of comet dust vaporize, forming the glowing streaks known as meteors.

The link between comets and meteor showers was first made 150 years ago — a link the Perseids made clear.

The Perseids began perking up in the early 1860s, staging displays of between 100 and 200 meteors an hour. In 1866, Italian astronomer Giovanni Schiaparelli — who later touched off the debate about “canals” on Mars — computed the orbits of the particles that spawned the Perseids. He noticed that they followed the same orbit around the Sun as Comet Swift-Tuttle, which had passed near Earth in 1862. Schiaparelli concluded that the comet was the source of the meteors — the first time anyone had connected a comet to a meteor shower.

After its 1862 encounter, Swift-Tuttle disappeared, and for more than a century no one knew where it was or when it would come back. It finally returned in 1992, reinvigorating the Perseids.

The comet will next visit our part of the solar system in the twenty-second century. But it was 150 years ago that astronomers first discovered that comets cause meteor showers — something to think about as you watch for Perseid meteors.

Script by Ken Croswell, Copyright 2016

Supergiant stars are generally like electoral maps of the states — either red or blue. But third parties pop up every now and then, changing the color landscape.

An example is a star known as Sadr. It connects the body and wings of Cygnus, the swan, which is high in the east as night falls. Sadr is close to the upper right of Deneb, a blue supergiant that’s the constellation’s brightest star.

Sadr’s surface temperature is about the same as the Sun’s — roughly 10,000 degrees Fahrenheit. At that temperature, the star shines yellow-white. But that’s about the only thing that Sadr and the Sun have in common. Sadr may be more than 200 times the Sun’s diameter. At that size, if it took the Sun’s place it would extend all the way out to Earth’s orbit.

More important to the star’s lifecycle, though, it’s probably about a dozen times the mass of the Sun.

It might seem that a heavy star would last a lot longer than the Sun, but that’s not the case. Gravity squeezes the core of such a star much more tightly than a star like the Sun. That makes the core tens of millions of degrees hotter, which revs up the nuclear reactions in the star’s core. So while Sadr is only about three percent of the Sun’s age, it’s already nearing the end of its life.

And when the end comes, it’ll be spectacular. The star probably will explode as a supernova — the same fate that awaits all supergiants — red, blue, or something in between.

Tomorrow: sparks from a comet.
 

Script by Damond Benningfield

Mars is butting heads with the scorpion. The bright planet is just a whisker away from Delta Scorpii, the star in the middle of the head of Scorpius.

Delta Scorpii is an intriguing system. Its main star is only about 10 million years old — a mere infant on the stellar timescale. Yet it’s also about 13 times as massive as the Sun. Such heavy stars “burn” their nuclear fuel in a hurry, so they age much faster than smaller stars. So Delta Scorpii’s main star is already nearing the end of its normal lifetime.

The star is spinning rapidly, so it’s throwing off a lot of hot gas, which cools and condenses to make tiny grains of dust. These grains form a disk that’s at least 125 million miles wide.

The star and a companion follow an elongated orbit around each other. At their closest, they almost touch. That appears to cause the system to flare brightly. In fact, during the two most recent encounters, Delta Scorpii jumped to almost twice its normal brightness.

Over the next few million years, the main star will puff up to enormous proportions, so it’ll shine many times brighter still. After that, it’s likely to explode as a supernova — briefly giving the scorpion a big head.

Mars and Delta Scorpii are in the south as night falls. Mars looks like a brilliant orange star. Delta Scorpii is quite close above Mars. The scorpion’s bright orange heart, the star Antares, is to their lower left, with the rest of Scorpius curving down near the horizon.

Script by Damond Benningfield

Until the invention of the telescope, the science of astronomy consisted of little more than counting objects in the sky and measuring their positions. Yet those basic observations revealed much about how the universe works.

Consider Spica, the brightest star of the constellation Virgo, which stands to the left of the crescent Moon tonight.

Because it’s bright, skywatchers carefully noted Spica’s position in the sky. Some cultures even built temples that aimed toward its rising or setting points at certain times of year — feats that required detailed knowledge of the star’s movements.

In the second-century B.C., Greek astronomer Hipparchus noticed a change in Spica’s position. He compared his measurements of Spica to those made centuries earlier. He found that the star had moved a couple of degrees relative to the Sun’s position in the sky at the autumnal equinox.

Eventually, others discovered that all of the stars undergo this same motion, known today as the precession of the equinoxes. It has nothing to do with the stars, though. Instead, it happens because Earth wobbles like a gyroscope that’s winding down. Over the millennia, that gives us different pole stars. It also causes the entire starry background to shift position relative to the Sun. So thousands of years ago, the Sun was in Libra at the time of the autumnal equinox. But today, it’s moved one constellation over — into Virgo, well to the west of its brightest star, Spica.
 

Script by Damond Benningfield

The scorpion is one of the most unappealing creatures on the planet. The celestial scorpion, however, offers one of the most appealing creatures in the galaxy: the Butterfly Nebula. This amazing structure emerged from its cocoon about 2,000 years ago as seen from Earth. Today, its colorful wings spread across more than two light-years.

The “wings” are big bubbles of gas and dust. They once formed the outer layers of a star that was heavier than the Sun. The star’s hot but dead core sits at the center of the nebula.

Until the last couple of thousand years, the core generated energy through nuclear fusion. Today, though, that process has stopped. The leftover core, which is about two-thirds as massive as the Sun, shines perhaps 2,000 times brighter than the Sun. And its surface is hundreds of thousands of degrees hotter than the Sun’s.

But the dying core may be fading by as much as one percent per year. And it will continue to cool and fade rapidly over the next few millennia. At the end of that time, the core will be a white dwarf — a dense ball not much bigger than Earth. By then, the surrounding butterfly will begin to fade, destroying a bit of beauty in the celestial scorpion.

And look for the scorpion low in the south as night falls, with the bright planets Mars and Saturn close by. The Butterfly Nebula is near the stars that mark the stinger, although you need a telescope to see it.

Tomorrow: A bright companion for the Moon

Script by Damond Benningfield