StarDate

The brightest star in all the night sky is clawing its way into prime-time viewing. Sirius is low in the southeast by 10 p.m., and high in the south after midnight. It’s the brightest star of the constellation Canis Major, the Big Dog, so it’s also known as the Dog Star.

Like all the stars that twinkle in the night, Sirius’s brightness is a combination of two factors: its true brightness and its distance.

In fact, Sirius really is a bright star — about 25 times brighter than the Sun. That’s because it’s about twice as massive as the Sun. It “burns” the nuclear fuel in its core in a hurry, so it produces lots of radiation. That energy pushes outward on the surrounding layers of gas, making Sirius bigger than the Sun. It also heats the gas to higher temperatures, so any given area on Sirius radiates more energy into space than the same-size area on the Sun. The combination of the size and surface temperature makes Sirius a real shiner.

The other reason Sirius shines so brightly in our sky is its distance — just 8.6 light-years away. That makes it the fifth-closest star system to the Sun. And it’s moving even closer, so it’ll remain the brightest star in the night sky for tens of thousands of years to come.

Keep an eye on Sirius tonight and throughout the winter — a close and shiny neighbor climbing high across the southern sky.

We’ll talk about an even brighter star in the Big Dog tomorrow.

Script by Damond Benningfield

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Our solar system resides in a giant spiral galaxy — the Milky Way. It’s a thin disk more than a hundred thousand light-years across. Bright, young stars outline spiral arms. The major arms extend from a thick bar of stars in the galaxy’s middle.

It took a long time for astronomers to figure all of this out. In fact, they’re still trying to complete the picture. They’re also still learning about our place in the Milky Way.

The problem is that we’re inside the disk. Giant clouds of dust absorb the light from distant stars, so it’s hard to create a roadmap. It’s like standing in the middle of a forest and trying to map all of its trees.

Fortunately, there are some ways to see through the trees. Radio waves and infrared light shine through the clouds. That allows astronomers to map giant clouds of gas throughout the galaxy, revealing its structure and our place in it.

They’ve discovered that we’re about 27,000 light-years from the center of the galaxy — about half way to the edge. We’re inside a short spiral arm, known as the Orion Arm or Local Arm. And it takes the Sun about 225 million years to complete one full turn around the galaxy — one galactic year.

To see the Milky Way, find a safe site far from city lights. Pick a moonless night, like tonight. At nightfall, the glowing band of the Milky Way arcs from almost due east, high across the north, to due west. It passes overhead around midnight.
 

Script by Damond Benningfield

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A reliable meteor shower is building toward its peak, on Wednesday night. There won’t be any moonlight to spoil the show, so if you can get away from city lights, it should be a good display. And if you want some earlier action, try tonight — some meteor experts say we could get an extra sprinkling.

The Geminid meteor shower occurs every December. That’s when Earth zips through the orbital path of its parent asteroid, which sheds bits of rock and dirt as it orbits the Sun. Earth sweeps up some of this debris. It vaporizes as it hits the atmosphere, forming glowing streaks of light.

At the shower’s peak, it might produce a hundred or more meteors per hour. And while most meteor showers are best viewed well after midnight, the Geminids offer more generous hours — from about 10 p.m. until dawn. Best of all, the Moon will be just past new, so it won’t interfere with the fireworks.

There’s a possibility of some bonus meteors tonight. Earth may pass through the path of Comet 46P/Wirtanen. It doesn’t spawn a regular meteor shower, but it does occasionally provide a few sparks. Some calculations show that we could fly through a trail of its “comet dust” tonight. And even if we don’t get those bonus flashes of light, we will get a few early Geminids — a prelude to the main event late Wednesday into early Thursday.

Tomorrow: mapping our place in the vastness of the Milky Way.
 

Script by Damond Benningfield

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The relationship between stars and the material around them — the interstellar medium — is complicated. Stars are born from this material — wispy clouds of gas and dust. And when stars die, they expel much of their material back into the interstellar medium, enriching it with elements forged inside the stars themselves. And between birth and death, a star can have a powerful influence on the interstellar medium around it.

An example is Alpha Camelopardalis. It’s one of the leading lights of the faint constellation Camelopardalis, the giraffe, which is in the north on December evenings.

The star is about 6,000 light-years away, so it looks faint. But it shines hundreds of thousands of times brighter than the Sun — one of the brighter stars that’s visible to the unaided eye.

Alpha Cam is probably close to 40 times the mass of the Sun. The star blows a strong “wind” of gas into space, forming a big bubble around Alpha Cam.

The star is racing through the galaxy at millions of miles per hour. As it plows through the interstellar medium, its wind squeezes the cooler gas and dust ahead of the star, forming a bow shock — like water piling up in front of a ship. The shock wave spans several light-years.

Over time, the shock wave could help squeeze the interstellar medium enough to give birth to new stars — continuing the complex relationship between the stars and the material around them.
 

Script by Damond Benningfield

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The most-often-observed of all comets should be ready to head back our way. Calculations of its orbit say it should be farthest from the Sun today — more than three billion miles away. That means it’ll start its plunge back toward the Sun. It’ll take a while to get here, though — it won’t get closest to the Sun until 2061.

The first confirmed sighting of Halley’s Comet came in 240 B.C. The comet is a ball of ice and rock a few miles across. When it approaches the Sun, it sprouts a long, bright tail.

Halley orbits the Sun once every 74 to 79 years. It’s easily pushed around by the gravity of Jupiter and Saturn, so no two orbits are the same.

Halley was the first comet that was known to periodically return to view. British astronomer Edmond Halley used Isaac Newton’s laws of gravity to compare the appearances of several comets over the centuries. He discovered that they were repeat appearances of the same comet. Halley calculated that it would return to view in 1758. When it did, the comet was named in his honor.

Comet Halley was last visible in Earth’s night sky in 1986. It was a so-so appearance. But some of its visits have been spectacular. In 1910, it caused widespread fear when astronomers reported that Earth would pass through its tail. People bought bogus “comet pills” to protect themselves from its influence. But there was no danger at all — only a beautiful display in the night sky.
 

Script by Damond Benningfield

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The weekend gets off to a beautiful start tomorrow — if you’re an early riser. The Moon and the planet Venus — the “morning star” — team up before and during dawn. They climb into good view by two or three hours before sunrise. And as twilight paints the sky, they’re well up in the southeast.

The Moon is the major factor that controls ocean tides here on Earth. And a recent study found that it also plays a big role in controlling the health of some mangrove forests.

Mangroves are found along many of the world’s coastlines. They form thickets that prevent erosion during tropical cyclones, and provide shelter for fish, turtles, and other life. They also store carbon dioxide, helping to limit global warming. But they face a lot of threats — from coastal development to higher sea levels.

The study found that the Moon both helps and hurts mangrove forests, over a cycle of 18.6 years. That coincides with a natural “wobble” in the Moon’s orbit. The change in the Moon’s angle can led to especially high high tides, and especially low low tides, at some locations — including a bay along the northern coast of Australia.

Researchers found that the mangroves there were especially verdant at certain times, then especially puny about nine years later, then healthy again nine years after that. So the health of the forests appears to be influenced by the Moon — as it “wobbles” around Planet Earth.
 

Script by Damond Benningfield

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Fictional starships notwithstanding, there’s not much antimatter in the universe. And for us, that’s a good thing. Any time matter and antimatter meet, they cancel each other out in a blaze of energy.

Antimatter is identical to normal matter in almost every way. The only difference is electric charge, which is opposite for the two forms of matter. So there could be a whole galaxy made of antimatter out there and our telescopes wouldn’t see it any differently from a galaxy of normal matter.

Most theories say the Big Bang should have created equal amounts of matter and antimatter. But in the first tiny fraction of a second, something changed that balance. For every billion pairs of matter and antimatter particles, there was one extra particle of matter.

One of the first scientists to consider that imbalance was Andrei Sakharov. He’d helped develop the Soviet hydrogen bomb, but turned away from weapons work. In a paper published more than 50 years ago, he outlined conditions that could create the imbalance.

Sakharov said that protons must decay, but so slowly that it’s almost impossible to detect. Second, he said that the universe must have cooled in a certain way in the moments after the Big Bang. And finally, he said there must be some difference between matter and antimatter.

So far, none of those conditions has been confirmed, so the subject of matter versus antimatter remains a busy topic of research.
 

Script by Damond Benningfield

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A banana is a good source of fiber, vitamin C, and many other goodies. It’s also a source of antimatter. That’s because a banana contains a tiny amount of a radioactive form of potassium. As it decays, it produces positrons, the antimatter counterpart of electrons. They’re no threat, though — there just aren’t enough of them.

Particles of antimatter have the opposite electric charge from normal matter. An electron, for example, has a negative charge, while a positron has a positive charge. When matter and antimatter meet, they annihilate each other, producing pure energy.

Antimatter is rare, but there is some. A tiny fraction of the cosmic rays that strike Earth’s atmosphere, for example, consists of positrons and antiprotons. There’s also evidence that positrons are produced in thunderstorms.

Antimatter is also produced by the decay of radioactive elements, like the potassium in bananas. Antimatter from these reactions is used in PET scans. And research suggests that antimatter could someday be used to treat tumors.

Of course, the most famous use of antimatter is fictional — a power source for starships. And it would be the most efficient power source around. The problem, though, is that the stuff is pricey: it would cost trillions of dollars to make a single gram. So we’re not likely to go warping around the galaxy in starships powered by antimatter anytime soon.

More about antimatter tomorrow.
 

Script by Damond Benningfield

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Astronomers recently found a sizzling “hot dog” in space. It’s not a Nathan’s dog from Coney Island, or a Pink’s hot dog from L.A. Instead, it’s a hot dust-obscured galaxy — one of the most powerful objects in the universe.

Such galaxies are powered by supermassive black holes in their cores. Gas and dust fall toward the black hole, forming a swirling disk around it. It can shine brighter than all the stars in the galaxy combined. That makes the galaxy a quasar — one of the brightest objects in the universe.

“Hot dogs” are even brighter than normal quasars. They contain huge amounts of dust. The dust absorbs a galaxy’s visible light, so it’s not very bright to the eye alone.

But the dust is heated by radiation from the disk around the black hole, so it, too, shines brightly — in the infrared. A hot dog can shine up to a hundred trillion times brighter than the Sun — a thousand times brighter than our entire galaxy.

The newly discovered hot dog is the closest yet seen — just a few billion light-years away. Its black hole is about 250 million times the mass of the Sun. The galaxy is giving birth to lots of new stars. But radiation from the hot disk is blowing away the gas that gives birth to stars. So the rate of starbirth is likely to plunge.

The new hot dog is to the lower right of Cygnus, the swan, which is high in the west at nightfall. Despite its brilliance, the galaxy is far too faint to see without a telescope.

Script by Damond Benningfield

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A planet that orbits a star in the Little Bear has had a traumatic few million years. It either survived a merger of its two stars, or it was born from the debris created by that collision.

Halla orbits the star 8 Ursae Minoris, which is old and bloated. The planet is more massive than Jupiter, the giant of our own solar system.

Halla orbits the star once every three months, at about half the distance from Earth to the Sun. When it was discovered, astronomers thought it must have survived being engulfed by its star, which should have been much bigger in the past.

Scientists continued studying the system with telescopes in space and on the ground. And they found a couple of other possible explanations.

One says the planet originally orbited two stars. The stars merged to make a single star. Perhaps surprisingly, that kept the star we see today from growing as big as it would have otherwise. So Halla kept on orbiting despite the fireworks.

Another possibility is that the merger produced big clouds of debris, and Halla took shape from that material.

The surviving star eventually will puff up even more, then shed its outer layers — another traumatic event for Halla.

8 Ursae Minoris is in Ursa Minor, the Little Bear. Some of its stars form the Little Dipper, which is in the north at nightfall. The star is near the lower right corner of the dipper this evening, but you need binoculars to see it.
 

Script by Damond Benningfield

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