StarDate

The “morning star” and the eye of the bull are staging a brilliant encounter in the dawn sky. They’ll be quite close to each other for about a week, providing plenty of time to see them.

At first light tomorrow, Venus, the morning star, will stand directly above Aldebaran, the star that marks the eye of Taurus. Over the following few days Venus will slide down to the left of Aldebaran, then stand side by side with it. After that, Venus will drop away from the star, with the gap between them getting wider each day.

Although Venus looks brighter, that’s only because it’s much closer to us than Aldebaran is. If you lined them up at the same distance, Aldebaran would look billions of times brighter.

That indicates that they’re completely different kinds of objects. Venus is a planet about the size of Earth. It doesn’t “shine” on its own. Instead, sunlight reflects off the clouds that top its atmosphere. The combination of the clouds and Venus’s proximity to both Earth and the Sun makes it look bright.

Aldebaran is about nine million times farther than Venus is this week. But it’s a star, and a big one at that — about 40 times wider than the Sun, and five thousand times the size of Venus. Nuclear reactions in its core produce energy. By the time it reaches the star’s surface, much of that energy is in the form we see with our eyes — the beautiful glow of a giant star.

Tomorrow: another pairing in the early morning sky.

Script by Damond Benningfield

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A UFO has been spotted in a galaxy more than two billion light-years away — an ultra-fast outflow. It’s a “wind” created by a disk of hot gas around a supermassive black hole in the galaxy’s heart. And soon, it may sweep away all the gas and dust for making new stars.

PDS 456 is in Serpens Cauda — the tail of the serpent. The constellation is a third of the way up the southeastern sky at nightfall. The galaxy is far too faint to see without a telescope.

Even so, it’s the brightest “quasar” within billions of light-years. It’s powered by a black hole that’s a billion times the mass of the Sun. Gas and dust fall toward the black hole, forming a wide disk around it. Material in the disk is heated to extreme temperatures, so it emits a lot of radiation and charged particles. They produce the UFO — a heavy wind that blows at a quarter of the speed of light, and carries as much energy as a trillion Suns.

The wind blows in every direction. And it acts like a galaxy-wide broom — it sweeps gas and dust away from the galaxy’s core. Right now, it’s pushing enough material to make 250 million stars as heavy as the Sun. And in fact, as that material piles up, clumps of it do give birth to some new stars.

That won’t be the case for long, though. At the current rate, the wind from the black hole will sweep all the gas and dust out of the galaxy in a few million years — leaving PDS 456 unable to make any more stars.

Script by Damond Benningfield

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A star in the constellation Serpens appears to be staging a colorful demise. It’s surrounded itself with a cloud of gas and dust that glows bright red — the Red Square Nebula.

The star is known as MWC 922. And astronomers have been piecing together its story since the discovery of the nebula a decade and a half ago.

A study last year, for example, said the system probably consists of two stars. The main star is about 15 times the mass of the Sun. It’s passed the end of its “normal” lifetime, so it’s puffed up to giant proportions. It blows a thick “wind” into space. Its unseen companion helps sculpt that wind into two cones, which flow in opposite directions. We happen to view the system in profile, so we see the outlines of the cones as a bright “X.”

There’s also a thin disk of gas between the cones, and a thicker disk outside them — all glowing bright red.

If the main star really is as heavy as the study says, then its demise is about to get even more dramatic. The star will explode as a supernova. As debris rams into the material in the Red Square Nebula, the entire system will glow even brighter — sculpted into new and beautiful shapes not by a dying star, but by a dead one.

MWC 922 is in Serpens Cauda, the tail of the serpent. It’s about a third of the way up the southeastern sky at nightfall. The star is far too faint to see without a telescope, though — at least for now.

More about Serpens tomorrow.

Script by Damond Benningfield

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Our home galaxy, the Milky Way, gives birth to new stars at a leisurely rate — the equivalent of one Sun per year. But you wouldn’t think the rate was that low if you looked deep into Serpens Cauda — the tail of the serpent. Several stellar nurseries there are busily churning out baby stars — some of which are really big babies.

The nurseries all appear to belong to the Serpens Molecular Cloud. Its gas and dust are cold and dark. But a few million years ago, something rippled through the cloud. That caused knots of gas and dust to begin collapsing to form stars.

Today, that’s taking place mainly in three big clusters. They’re known as Serpens Main and South, and Westerhout 40. In all, they appear to contain at least 2,000 young stars or future stars — some of the objects are so young that they’re just now igniting the nuclear fires in their cores. And some aren’t even that far along — they’re dark knots that are the seeds of future stars.

Most of the stars in these clusters are small and faint. But a few are members of the most impressive classes of stars. One of them, in the center of Westerhout 40, is especially hot and bright. It’s blowing away the nearby gas and dust. That will shut down the birth of new stars close by, but perhaps trigger the birth of stars farther away. And the star has blown a colorful bubble around itself that resembles a butterfly — the result of starbirth in the serpent.

More about Serpens tomorrow.

Script by Damond Benningfield

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Two bright planets accompany the Moon across the sky tonight. Brilliant Jupiter, the solar system’s largest planet, is close to the upper right of the Moon as they climb into good view around 10:30 or 11. And fainter Saturn — the second-largest planet — is about the same distance to the left of the Moon.

Today, scientists can tell you the distance of both planets to within a tiny margin of error. Jupiter, for example, is about 386 million miles away right now, with Saturn about 840 million miles.

But such big numbers are unwieldy. So when talking about distances inside star systems, astronomers typically use the astronomical unit, or AU. That’s the average distance between Earth and the Sun — about 93 million miles. So Jupiter is a little less than 4.2 AU away, with Saturn at nine AU.

The distance from Earth to the Sun isn’t constant, though. Over the course of a year, it varies by about one and a half percent in either direction. Yesterday, for example, we were farthest from the Sun for the entire year: 1.017 AU.

Today, we’re moving back toward the Sun. We’ll be closest to our star on January 2nd, during the dead of winter in the northern hemisphere. The change in distance does little to change Earth’s overall temperature, though. The air and oceans distribute heat around the entire planet — no matter how far we are from the Sun.

Tomorrow: slithering along with the serpent.

Script by Damond Benningfield

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The Moon is full tonight — a brilliant light for the holiday night. Yet it won’t be in view all that long. It’s the Short-Night Moon — the full Moon that’s in sight for a shorter period than any other full Moon of the year.

We see so little of tonight’s Moon because the Sun and the full Moon are like two people on a seesaw — when one is up, the other is down. At this time of year, near the summer solstice, the Sun is “up.” It’s in view for a long time, and it climbs highest across the sky. That means the Moon, at the other end of the seesaw, is “down.” It stays low as it scoots across the sky, and it’s not in view for long.

From Tampa, for example, the Sun will be in the sky for almost 14 hours, with the Moon in view for only 10 and a half hours. From Dallas, it’s more than 14 hours of sunlight, followed by just 10 hours of moonlight. And from Spokane, which is much farther north, the Sun is in view for almost twice as long as the Moon — almost 16 hours versus just eight and a half.

Most years, the Short-Night Moon is in June. This year, though, tonight’s full Moon is closer to the solstice than the one in June, so July takes the honors.

And the Moon is followed across the sky tonight by two short-night planets. Bright Jupiter is to the lower left of the Moon in early evening, with fainter Saturn to the lower left of Jupiter. The three worlds will be much closer together tomorrow night; more about that tomorrow.

Script by Damond Benningfield

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61 Cygni is one of our closest stellar neighbors — just 11 light-years away. And it consists of two stars, not one.

Even so, it’s only barely visible to the unaided eye — a faint dot not far from the graceful outline of Cygnus, the swan. It’s so faint because its two stars are only about half as massive as the Sun. But what the stars lack in flash, they’ll more than make up for in longevity — both will live much longer than the Sun.

In fact, not a single star of that class has had time to die from old age.

A star’s lifespan is determined by its mass.

Heavier stars have more hydrogen to power their nuclear reactions, so you might expect them to live longer. But that’s not the case. A massive star’s gravity is especially strong, so it squeezes the star’s core more tightly, making it hotter. That revs up its nuclear reactions, so the star burns through its hydrogen in a hurry. In fact, the heaviest stars live only a few million years.

Lightweight stars aren’t squeezed as tightly, so they consume their fuel at a slower pace. A star the mass of the Sun will “burn” its hydrogen for about 10 billion years. And a star that’s only about three-quarters of the Sun’s mass will live several billion years longer. Since the universe is less than 14 billion years old, that means that not a single one of these stars has had time to burn out on its own. Only interactions with other stars could snuff out such feeble cosmic lights.

Script by Damond Benningfield

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Antares is a supergiant. It’s hundreds of times the diameter of our own star, the Sun, and millions of times the Sun’s volume. But the star’s true size is still a puzzle. Estimates of its diameter vary by quite a bit.

To determine a star’s true size, astronomers first measure its apparent size — the angle it covers on the sky. They then compare that number to the star’s distance — the farther the bigger. In the case of Antares, though, it’s hard to lock down either number.

Consider its apparent size in the sky, known as angular diameter. The star’s outer layers are quite thin. And Antares blows a thick “wind” of hot gas out into space. That surrounds the star with a cloud of gas. So it’s hard to tell where the surface of the star ends and the surrounding cloud begins.

And Antares changes in both size and brightness — factors that make it even tougher to get a true measurement of its angular diameter.

Then there’s the matter of its distance. The best estimate says it’s about 550 light-years away. There’s a good bit of uncertainty in that number, though — for some of the same reasons as the uncertainty in its apparent size.

When you account for all of that, estimates of the star’s true diameter range from about 700 times that of the Sun, to almost 900 — a giant uncertainty for a supergiant star.

Look for Antares to the right or lower right of the Moon as night falls. The star marks the bright heart of the scorpion.

Script by Damond Benningfield

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The world remembers Julius Caesar for many things: his military campaigns, his affair with Cleopatra, his dramatic death on the Ides of March. We also remember him for setting up the basic calendar that the western world has used ever since. And the Roman Senate named the seventh month in the calendar in his honor: July.

Before Caesar’s reforms, Rome used a calendar that was based on the cycles of the Moon. But such a calendar drifts with respect to the seasons. Extra days were added to try to correct the difference. By Caesar’s time, though, the calendar was a big mess.

So Julius Caesar looked to his newly conquered province of Egypt. Thousands of years earlier, the Egyptians had devised a calendar based on Earth’s orbit around the Sun — the first 365-day solar calendar.

Caesar adopted this system, but he kept the names of the months from the Roman calendar. The seventh month was named Quintillis. The name actually means fifth month, because in the original Roman calendar, the year began with March, not January. After Caesar was assassinated, his successor, Augustus Caesar, thought having a month named for the emperor was a good idea. So the month of Sextillis was changed to August in his honor. Later emperors tried the same trick, but with no luck. Senators realized that they\'d eventually run out of months, so they stopped the practice. But the name of Julius Caesar lives on — in the seventh month of the year.

Script by Damond Benningfield

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If you gaze into a dark night sky, you can see thousands of stars and as many as five bright planets. What you can’t see is the link between them, known as brown dwarfs. Although astronomers have discovered hundreds of them, they’re so faint that not a single one is visible to the eye alone.

A brown dwarf is described as a “failed star.” It’s generally heavier than a planet, but not massive enough to shine as a true star. But the difference between a brown dwarf and a giant planet can be hard to pin down. So some astronomers have suggested they be categorized based on how they were born.

A brown dwarf probably is born in the same way as a star — from the collapse of a cloud of gas and dust. If such a cloud is at least seven and a half percent the mass of the Sun, it begins to “fuse” the hydrogen in its core, so it shines as a true star. At lower masses, though, the core doesn’t get hot enough to start those reactions.

A planet forms from debris around a star. Chunks of ice and rock stick together to build larger bodies. If they get large enough, they can sweep up lots of leftover gas and dust, growing to giant proportions.

Based on the birth process, some planets may be more than a couple of dozen times the mass of Jupiter, the giant of our own solar system. On the other hand, the smallest brown dwarfs may be as little as a half dozen times Jupiter’s mass — muddying the difference between giant planets and failed stars.

Script by Damond Benningfield

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