StarDate

Just about everybody has some sort of superstition. Many of us carry good-luck charms or avoid black cats. And football fans will wear the same pair of socks for a month if they think it’ll help their team.

One big superstition is today’s date: Friday the 13th. It’s been considered bad luck for a long time. But no one is sure exactly why it earned such a bad reputation.

The number 13, of course, has had evil connotations for centuries. In Scandinavian mythology, for example, a 13th god crashed a party and killed one of the other gods. And in Christianity, 13 attended the Last Supper, making the number the unluckiest of all.

Friday has had a bad rep for a long time, too. The day is named for Frigg, a Norse goddess. Christians called her a witch, and her day became the Witches’ Sabbath. And in some cultures, Friday was considered a bad day for a wedding or to start a journey.

Linking the bad mojo of Friday and the number 13 seems to have occurred in the 19th century. It was solidified by a 1907 novel, called Friday the 13th. In it, a broker takes advantage of the superstitions about the day to mess with the stock market. And the link got an even bigger boost with the series of “Friday the 13th” movies.

In reality, there’s nothing unlucky about Friday the 13th at all. It’s just another day on the calendar.

One interesting coincidence, though, is that thanks to Leap Year, there’s another Friday the 13th next March — 13 weeks from now.

Script by Damond Benningfield

The planets Jupiter and Saturn are locked in a bit of a battle — a battle over moons. A recent announcement brought the number of Saturn’s known moons to 82 — three more than Jupiter. But the search for moons around both worlds continues, so the lead may change hands many times in the coming years.

Saturn’s new moons were discovered by a team led by Scott Sheppard of the Carnegie Institution — the champions of modern-day moon hunters. Using the Subaru Telescope in Hawaii, the team discovered 20 moons. None of them is much to get excited about, though — they average about three miles in diameter.

Most of the moons follow “retrograde” orbits — in the opposite direction from Saturn’s rotation on its axis. The moons probably are fragments of a single body. The parent was blasted to bits by a long-ago impact with another body.

The other moons orbit in the same direction as Saturn’s rotation. But each of them probably is a chip off another body as well.

None of the moons has yet been named. But a just-completed contest allowed the public to suggest names. The International Astronomical Union will announce the winners next year.

And Saturn itself is about to disappear in the evening twilight. It’s easy to spot over the next few nights, though, because it’s near Venus, the “evening star.” They’re quite low in the southwest shortly after sunset. Saturn stands beside Venus tonight, and will slip to the lower right of Venus over the following nights.

Script by Damond Benningfield

December’s full Moon is known by several names. One is the Cold Moon, while another is the Moon Before Yule — good descriptors of the time of year. And it has another name that’s every bit as obvious — but only when you think about it: the Long-Night Moon. It tells us that the Moon is in view for a longer period than any other full Moon of the entire year.

The moniker becomes obvious when you consider the full Moon is a mirror image of the Sun — offset by six months. So whatever the Sun does today, the Moon will copy six months later.

Around the time of the summer solstice, for example, the Sun is in view longer than at any other time of year. It climbs highest across the sky as well, and rises and sets farthest north. At that time, the full Moon is puny by comparison. Since it rises around sunset and sets around sunrise, it’s not in view for long. And it stays fairly low in the sky.

Now, though, less than two weeks before the winter solstice, it’s just the opposite. The Sun is in view for fewer hours, and it stays low in the sky. But the full Moon shines at its best. It rises early, sets late, and climbs highest across the sky.

From Miami, for example, 10 and a half hours of sunlight will be followed by 14 hours of moonlight — a difference of three and a half hours. From Kansas City, the difference is five and a half hours. And from Seattle, it’s seven and a half hours — providing plenty of moonlight for a cold night.

Script by Damond Benningfield

You don’t need a thermometer to take the temperature of a star — at least not a bright one. All you need is your eyes. Cooler stars look orange or red, hotter stars are blue or white, and in-between stars are yellow.

A star on the cool end of the scale shines near the almost-full Moon tonight. Aldebaran is the eye of Taurus, the bull. It’s to the lower left of the Moon at nightfall, and even closer to the Moon in the wee hours of the morning. Blocking the bright Moon with your hand will help you discern the star’s color.

Aldebaran’s surface temperature is about 7,000 degrees Fahrenheit. By comparison, our yellow Sun is about 10,000 degrees.

Aldebaran is cooler because it’s all puffed up. It’s quite late in life, so it’s undergoing changes in its core. Those changes cause its outer layers to expand. As the gases in those layers spread out, they get cooler — and Aldebaran gets redder.

A couple of other colorful stars align below Aldebaran and the Moon in mid-evening. Both are in Orion the hunter. Betelgeuse is bright orange, so it’s about the same temperature as Aldebaran. But Rigel is blue-white. Its surface blazes at more than 20 thousand degrees.

But those aren’t the limits of stellar temperature. The coolest true stars have a temperature of about 5,000 degrees, so they’re deep orange or red. And the hottest sizzle at more than 100,000 degrees. They look blue-violet — a colorful indication of their temperature.

Script by Damond Benningfield

Sometime in the last million years, Tabby’s Star may have swallowed a planet. One of the planet’s moons survived the encounter, though, entering its own orbit around the star. But the star’s heat is causing the moon to evaporate. That’s leaving a trail of dust around the star. As seen from Earth, clumps of the dust pass in front of the star, periodically causing it to fade.

That’s the scenario presented by a team of astronomers earlier this year. The team was trying to explain the star’s unusual flickering. At times, its light can drop by anywhere from about one percent to more than 20 percent. But there’s no pattern to the drops. That means they can’t be caused by the passage of a planet in front of the star — such passages would follow a regular pattern.

Astronomers have proposed several mechanisms to explain the flickering. They range from swarms of comets to structures built by an alien civilization.

The latest idea doesn’t settle the matter. But several other discoveries may.

One astronomer, for example, found a score of stars that fade and brighten erratically, just as Tabby’s Star does. The stars are of two main types, suggesting there’s a common cause for their flickering. Studies of the entire group could help astronomers find that cause.

Another study found a star whose light faded by 80 percent for about a day. That star may have been eclipsed by a planet with giant rings — just one of the many possible ways to make a star flicker.

Script by Damond Benningfield

It’s lonely out here in the galactic suburbs. The closest neighbors are light-years away — distances that are truly astounding. The distance to the Sun’s closest stellar neighbor, Proxima Centauri, is almost 29 million times the Sun’s diameter. At that same scale, if you lived in a house 50 feet wide, your nearest neighbor would be more than a quarter-of-a-million miles away — farther than the Moon.

That’s a pretty typical gap between star systems in this part of the galaxy — although not necessarily between individual stars. While the Sun is alone, many other stars have close companions. Proxima Centauri, for example, belongs to a system of three stars.

The space between the stars isn’t completely empty. It contains a spray of hydrogen and helium gas, plus a tiny smattering of heavier elements. On average, there’s about one atom for every cubic centimeter of space.

There’s a lot more stuff packed together in the giant clouds known as nebulae. If they have enough gas and dust, they can give birth to new stars. Even so, by earthly standards a nebula still qualifies as a hard vacuum.

The stars are all following their own orbits around the center of the galaxy, so over time, the distances between them varies. In 10,000 years, for example, Proxima Centauri will no longer be our closest neighbor. Instead, that title will belong to Barnard’s Star, at just 3.8 light-years — about half a light-year closer than Proxima Centauri is today.

Script by Damond Benningfield

There’s no formula for predicting just how long a person will live. Genetics, nutrition, exercise, and other factors all play a role. But you can’t just plug in a few numbers and come up with an exact lifespan.

But that’s not the case with stars. A single number determines how long a star will live: its mass.

An example of how it works is in good view on winter nights: Pollux, the brightest star of Gemini. It rises in the east-northeast by mid-evening, below its fainter “twin,” Castor.

Pollux is almost twice as massive as the Sun. And that makes all the difference in its lifespan. Pollux is roughly 20 percent of the Sun’s age. But it’s already finished its “normal” lifetime — the phase the Sun is in right now.

During that phase of life, a star “fuses” hydrogen atoms in its core to make helium. When the hydrogen is gone, the core gets smaller and hotter, so it begins to fuse the helium to make carbon and oxygen. At the same time, its outer layers puff up like a balloon, making the star much brighter. That also makes the star’s surface much cooler, so it glows reddish orange, as Pollux does now.

Over time, Pollux will get even bigger and brighter. Then it will expel its outer layers, briefly surrounding itself with a colorful bubble of gas. That bubble will quickly fade, though, leaving only the star’s small dead core — a hot ember known as a white dwarf.

The same fate awaits the Sun — in several billion years.

Tomorrow: emptiness between the stars.

Script by Damond Benningfield

A discovery that was announced a quarter of a century ago set off an astronomical Gold Rush. Two European astronomers reported finding a planet orbiting a star in the constellation Pegasus. It wasn’t the first “exoplanet” ever found — and not even the first confirmed planet. But it was the first confirmed planet around a Sun-like star.

Michel Mayor and Didier Queloz were studying 51 Pegasi. The star is a little bigger and heavier than the Sun, but the same color. The astronomers discovered a small back-and-forth shift in the star’s spectrum — its individual wavelengths of light. They determined that the shift was caused by the tug of an orbiting planet.

The planet is about half as massive as Jupiter, the giant of our own solar system. Because of that, it’s been given the formal name Dimidium — Latin for “half.” But it’s only a few million miles from 51 Peg, so it’s extremely hot. It became the first of an entire class of planets, known as hot Jupiters. Such planets were completely unexpected, so it took astronomers a while to accept them. And they’re still trying to explain how such planets exist.

The discovery opened a busy new field: planet hunting. It’s yielded more than 4,000 confirmed worlds. And it earned the discoverers a share of this year’s Nobel Prize in Physics.

Under especially dark skies, 51 Peg is just visible to the unaided eye. Pegasus is high in the south at nightfall this month — the home of a prize-winning planet.

Script by Damond Benningfield

The first confirmed exoplanets were shockers. They orbit the remnant of an exploded star. And astronomers are still trying to figure out how they were born.

The star is a pulsar. It formed when the core of a massive star collapsed, causing its outer layers to blast into space. The core is heavier than the Sun, but no bigger than a city. It rotates almost 10,000 times a minute, emitting a beam of radio waves with each turn. From Earth, we see the beam pulse on and off like a lighthouse.

The pulses form an extremely accurate clock — there’s no variation in the timing of the “ticks.” Unless, that is, something else changes the pulses. And that’s what Aleksander Wolszczan and Dale Frail found in the early 1990s. They saw tiny changes in the timing of PSR 1257+12. They concluded the changes were caused by the gravitational tug of orbiting planets.

They found three planets in the system. Two are a few times heavier than Earth, while the third is close to the mass of the Moon. Others have since discovered planets around a few other pulsars.

A supernova shouldn’t leave any planets behind. So astronomers have come up with several ideas to explain pulsar planets. One says they may form from debris blown off the surface of a companion star. Another says they belonged to a companion star but were stolen by the pulsar. However they formed, they’re the first confirmed planets orbiting a star other than the Sun.

More about exoplanets tomorrow.

Script by Damond Benningfield

Gamma Cephei has a couple of distinctions. For one thing, it’s a future North Star. A thousand years from now, it’ll be closer to the north celestial pole than Polaris, the current North Star. In fact, you can find it standing directly above Polaris at nightfall at this time of year.

Another distinction is that it hosts the first confirmed planet beyond our own solar system. Unfortunately for the discoverers, though, it took decades to confirm it.

Astronomers have logged more than 4,000 exoplanets. Just who gets credit for the first one is blurry. But two astronomers will share part of this year’s Nobel Prize in Physics for the 1995 discovery of a planet in the star system 51 Pegasi.

The planet around Gamma Ceph was reported in 1988, by Gordon Walker and Bruce Campbell. They calculated that it was about twice the mass of Jupiter, the giant of our own solar system, and orbited the star once every two and a half years. But others convinced them they were actually seeing pulsations in the star itself. The planet was confirmed many years later.

In 1989, David Latham reported a possible planet around another star. But it could have been a “failed star,” known as a brown dwarf. It was confirmed as a planet a few years ago.

And in 1992, another team reported the discovery of two planets orbiting a dead star known as a pulsar. The finding was controversial, although it’s been confirmed. More about that tomorrow.

Script by Damond Benningfield