StarDate

Jupiter is the biggest gas station in the solar system. The planet has provided the energy to propel a half-dozen spacecraft to destinations beyond Jupiter.

The planet itself is a big envelope of gas wrapped around a small solid core. Most of the gas consists of hydrogen and helium. The hydrogen would make a good energy source for rockets. But it’s so far below the clouds that it would take more energy to go down and get it than you’d gain from the hydrogen itself.

Instead, spacecraft get their “gas” in the form of gravitational energy. As a craft approaches Jupiter, it’s accelerated by the giant planet’s gravity. How much acceleration depends on how close it gets to the planet — a closer approach provides a more powerful “kick.” Craft have used that kick to reach Saturn, Uranus, Neptune, and Pluto, and to loop above the Sun’s poles.

These craft “stole” momentum from Jupiter, giving Jupiter an equal kick in the opposite direction. But since Jupiter is so much heavier than any spacecraft, no one will ever notice the difference.

Jupiter itself is quite noticeable at first light tomorrow. It looks like a brilliant star, close to the lower left of the Moon. It’s the brightest pinpoint of light in the sky at that hour, so it’s hard to miss. And the bright star Spica, the leading light of the constellation Virgo, stands almost directly below it, completing a beautiful celestial trio.

More about the Moon and Spica tomorrow.

Script by Damond Benningfield

Some of the brightest stars in the entire night sky wheel into view at this time of year. By late evening, for example, brilliant Capella stands high in the northeast. The colorful stars of Orion stretch across the southeast. And the brightest of them all, Sirius, rises below Orion.

These bright lights are visible from just about anywhere — even the most light-polluted cities. But to see much more than that, you need to find somewhere with darker skies. From the typical suburb, for example, you might see a few hundred stars. And from really dark skies, far away from any streetlights, the stars visible to the eye alone number in the thousands.

The exact number depends on several factors, including the quality of your eyesight and the amount of dust and pollen in the air. With perfect vision and pristine skies, you might see up to about 4,000 stars at any one time.

One thing to keep in mind is that no matter where you are, you can see only half of the universe at once. The other half is below the horizon. The total number of individual stars you can see over the course of a year varies by location. If you spent your year at the north pole, you’d never see more than the northern half of the sky. But if you were on the equator, you’d eventually see the entire sky — and a total of perhaps 9,000 stars.

From the United States, we see only about half to two-thirds of the southern sky — leaving many stars permanently hidden from view.

Script by Damond Benningfield

The next time the Sun launches an especially powerful blast of charged particles toward Earth, the northern Great Lakes could be in trouble. When the blast hits us, the ground and Earth’s magnetic field could conspire to knock out power grids — perhaps triggering blackouts that could last for weeks or longer.

The Sun periodically belches out giant clouds of charged particles. Most of them move away from Earth. And most of the ones that hit us don’t cause major problems.

But the most powerful can cause big headaches. Earth’s magnetic field funnels the charged particles toward the surface, where they can knock out communications and power systems. In 1989, for example, a storm caused a blackout across a large part of Canada.

Studies have suggested that the most powerful storms could leave some regions without power for weeks or months, and cause a trillion dollars in economic losses.

A recent study mapped some of the risk from once-in-a-century storms across parts of the United States. It looked at how the ground conducts electricity, and at how much of a solar storm’s activity is likely to hit a given region.

The study found that northern Minnesota faces the greatest risk, with the surrounding Great Lakes region also with a higher chance of problems.

The study left about half of the U.S. unmapped, though — especially the heavily populated Northeast. So more work needs to be done to assess the risks of a major solar barrage.

Script by Damond Benningfield

If you put your face close to the lamp next to your sofa, it looks pretty bright. But if you move back a few feet, the lamp loses most of its luster. Its bulb isn’t any dimmer — it just looks that way because of the greater distance.

The same thing is true of the stars. Some stars that look faint are actually just as bright as some of the ones that look bright, but they’re farther away.

You can see a good example of this in Leo. The lion’s brightest star, Regulus, stands just a whisker from the Moon as they climb into view after midnight.

A pattern that looks like a backwards question mark stretches to their upper left as they rise. The star at the end of the question mark’s loop is Epsilon Leonis.

It looks about one-fifth as bright as Regulus. But if you could place the two stars side by side, and add up all of their energy, Epsilon would be just as bright as Regulus. Regulus is hotter, so a given surface area emits more energy, but Epsilon is bigger, so there’s more surface to radiate light into space.

Both stars are more massive than the Sun, so the nuclear reactions in their cores take place at a much faster rate. Epsilon is the heavier of the two, so its reactions are fastest. Because of that, the star is closer to the end of its “normal” lifetime than Regulus, even though it’s tens of millions of years younger. It’ll soon shed its outer layers, exposing its hot, dead core — a tiny stellar light bulb with the wattage turned way down.

Script by Damond Benningfield

November evenings are good times to watch the Milky Way — the combined glow of millions of stars in the disk of our home galaxy. It arches from east to west, with a definite dip toward the northern horizon. But you need a dark sky to see it — far from the pesky glow of city lights.

The autumn Milky Way is different from that of spring and summer. During those warmer months, Earth’s nightside faces the heart of the galaxy, so we see vast clouds of stars.

Now, though, the nightside faces the galaxy’s less-populated rim. That makes the Milky Way look thinner and fainter.

Even so, several of its most famous members are prominent in the evening sky now, so it’s well worth a look.

Cygnus, the swan, is dropping toward the western horizon. Cygnus is also known as the Northern Cross, because its brightest stars form the shape of a crucifix. At Christmastime, the cross stands almost straight up from the western horizon around dark.

Cassiopeia the queen stands atop the arch of the Milky Way a few hours after night falls. It looks like a letter M or W.

And one of the brightest stars in the night sky is Capella — a yellow-orange star in Auriga the charioteer. It’s fairly high in the northeast in mid-evening, and shines brightly through the subtle glow of the Milky Way.

Look for the Milky Way after nightfall, and continuing until moonrise, which comes by around midnight tonight, but later on each succeeding night through the end of the month.

Script by Damond Benningfield

The Pleiades cluster is home to thousands of young stars. The brightest form a tiny dipper that’s one of the best-known objects in the night sky.

But photographs reveal that there’s a lot more to the nebula than just its stars. They show wisps of blue around the entire cluster — the glow of a cloud of dust that spans dozens of light-years. It’s known as the Maia Nebula for one of the cluster’s brightest stars.

At first, astronomers thought the nebula was material left over from the birth of the Pleiades. But the cluster is at least a hundred million years old. That’s plenty of time for radiation from the cluster’s hottest stars to blow away any leftover dust and gas. What’s more, measurements show that the nebula is moving in a different direction from the stars. So it’s just a coincidence that they line up in the same direction.

The nebula is known as a reflection nebula because it’s reflecting light from the cluster’s brightest stars. That’s one of three main types of nebula.

A second is known as an emission nebula because it emits light. Ultraviolet energy from hot stars zaps hydrogen gas, splitting the atoms apart. When they link back up, the atoms produce red light.

A third type is a dark nebula. It’s a dense cloud of cold gas. It absorbs the light of the stars beyond it, so it looks like a dark hole in the starry background.

Look for the Pleiades climbing the eastern sky this evening, above Aldebaran, the bright orange eye of Taurus.

Script by Damond Benningfield

The Pleiades is doomed. By the time the star cluster completes another turn around the galaxy, it will have fallen apart, with its stars going their separate ways.

The Pleiades is the most famous of all clusters. Right now, it’s in good view in the east by about 7 p.m., above Aldebaran, the bright orange eye of Taurus the bull. It looks like a small but bright dipper.

In addition to the half-dozen bright stars that are easily visible to the eye, the cluster contains several thousand fainter stars. All of them were born together more than a hundred million years ago, from a huge cloud of gas and dust. Today, they move through space as a family, bound by their mutual gravitational pull.

As the Pleiades moves around the galaxy, though, it’ll begin to fall apart. Close encounters between member stars will fling some of them out of the cluster. And the gravitational field of the rest of the galaxy will slowly pull stars on its outskirts away from its denser core.

None of this happens instantly, though. It’ll probably take about 250 million years for the Pleiades to disintegrate — about the length of a single orbit around the center of the Milky Way.

Even then, most of the stars of the Pleiades will continue to move through the galaxy at the same speed and in the same direction. So even though the family will have fallen apart, it’ll be pretty easy to tell which stars were once members of this impressive cluster.

More tomorrow.

Script by Damond Benningfield

Betelgeuse is one of the biggest and brightest stars in our galactic neighborhood. But the bright orange shoulder of Orion the hunter hasn’t always been such a monster. In fact, it may have reached its current state as recently as 10,000 years ago — the blink of an eye on the cosmic timescale.

Betelgeuse probably was born about 10 million years ago. It was much more massive than the Sun, which made it extremely hot and bright. And it may have had a companion that was even more impressive than Betelgeuse itself.

About a million years ago, the companion exploded, giving Betelgeuse a swift “kick” across the galaxy.

By then, changes in its core were causing Betelgeuse to get bigger and brighter. It became a blue supergiant similar to Orion’s other leading light, Rigel. And not long ago, it got even bigger, causing its outer layers to turn cooler and redder. Today, it’s probably more than a thousand times the diameter of the Sun.

Over the next few hundred thousand years, Betelgeuse may expand even more. Then it may briefly contract, once again turning blue. And that’ll set the stage for its final act: It will blast itself to bits as a supernova. For a while, it may shine as brightly in Earth’s sky as the full Moon — an impressive end for an impressive star.

For now, look for Betelgeuse to the right of the gibbous Moon as they climb into good view by around 9 o’clock. They stick close together for the rest of the night.

Script by Damond Benningfield

The Leonid meteor shower is at its best the next couple of nights. Unfortunately, it’s being cooled off by the bright Moon.

The Leonids take place every November, when Earth crosses the orbital path of Comet Swift-Tuttle. The comet is a big ball of ice and rock. As it comes close to the Sun, the solar heat causes some of its ices to vaporize and spew out into space. That also releases small particles of rock.

Over time, these particles spread out along the comet’s orbital path, leaving a trail of debris. When Earth flies through this path, it sweeps up some of the grains of comet dust. They slam into the atmosphere at tens of thousands of miles per hour. They quickly disintegrate, forming the glowing streaks of light known as meteors.

Most of the particles that form the meteors are no bigger than BBs or perhaps pebbles. But a few are larger, so they form especially bright streaks. They can leave glowing trails that can remain visible for several minutes.

Unfortunately, though, most of the meteors are so faint that they’re not visible from light-polluted cities — or through the glare of an almost-full Moon, which is what we have right now.

To have a chance to see the Leonids, you need to find a dark but safe viewing spot far from the city — something like a state or national park. The Leonids are at their best between midnight and dawn, when your part of Earth turns most directly into the meteor stream.

Tomorrow: a brilliant runaway?

Script by Damond Benningfield

The Moon takes dead aim at the face of the bull tonight. It’s to the upper right of Taurus’s v-shaped head as night falls, and nose-to-nose with the bull at first light tomorrow. Binoculars will help you pick out the stars that outline the bull’s face through the lunar glare.

You shouldn’t need any help to pick out the bull’s eye, though. Aldebaran is one of the most prominent members of the autumn sky. In fact, it’s one of the brightest stars in the entire night sky.

If you keep your eye on the Moon and Taurus tonight, you’ll get a good idea of how the Moon moves across the night sky.

Its orbital path carries it through a full circle against the starry background every 27-and-a-third days. At that rate, it moves a distance equal to its own diameter in less than an hour. So from one night to the next, it moves almost half-again the width of your fist held at arm’s length. That’s not especially fast, but with a bright marker like Aldebaran close by, it’s enough to notice over a period of a few hours.

The Moon’s motion against the background of stars isn’t aligned with its phases, though. Since Earth and Moon are moving around the Sun, it takes a couple of days longer to complete the cycle of phases. So the Moon sometimes passes Aldebaran when it’s about full, as it is tonight. But at other times it’s a crescent or at some other phase — different views for its regular meetings with the bull.

Tomorrow: the Moon and a meteor shower.

Script by Damond Benningfield