StarDate

The roster of constellations includes some weird and wonderful creatures. There’s a dragon, a unicorn, and two centaurs. But perhaps the weirdest of all is Capricornus, the sea-goat – it’s half goat and half fish.

It’s associated with the god Pan, who was half goat and half man. The story says that he was about to be attacked by the monster Typhon, so he jumped into the water to escape. At the same time, he tried to transform himself into a fish to speed his getaway. But he botched the spell, and turned his human half into a fish, but kept the half that was a goat.

The constellation is low in the southeast as darkness falls at this time of year. Its brightest stars form a wide triangle. None of the sea-goat’s stars is all that bright, though, so you need a fairly dark sky to make them out.

The brightest forms the left point of the triangle. It’s known as Deneb Algedi – “the tail of the goat.” It’s actually a system of at least two stars. One is about twice as big and heavy as the Sun, and shines several times brighter. The other is a lot like the Sun.

The stars orbit each other about once a day. As they do, each one passes in front of the other for a bit.

When the fainter star crosses the brighter one, the system’s overall brightness drops by about a quarter.

That’s just enough for a skilled observer to notice with the eye alone – a slight flicker in one of the night sky’s oddest creatures.

Script by Damond Benningfield

Young star systems are dangerous places. A planet can be pelted by giant asteroids and comets. It can collide with its siblings, and be kicked out of the system by close encounters with other siblings. It might even be swallowed by its parent star.

Astronomers in the U.K. recently found evidence of such encounters in several young stars. The researchers zeroed in on stars in three clusters. All of the clusters are young – from about 40 million to 200 million years old. At that age, their star systems might still be chaotic – planets might be getting blasted or destroyed, and they might still be taking shape.

Six stars in the clusters showed especially high amounts of lithium. It’s a common planet-building material. But it’s fragile – stars quickly destroy it. So any lithium in the stars must have been added recently. The most likely source is young, rocky planets. The planets could have been kicked inward by the gravity of other planets, or dragged in by gas and dust around the stars. Over time, the remains churn deep into a star’s interior. But some can linger for a few million years – the remains of dead planets.

One of the clusters is Blanco 1. It’s 850 light-years away, in the constellation Sculptor. It climbs into the southeast in mid evening, to the lower left of the bright star Fomalhaut. It’s too faint to see with the eye alone, but it’s a decent target for small telescopes.

Script by Damond Benningfield

A planet orbiting a dead star 750 light-years from Earth is a real lemon. The planet is shaped like a lemon – a result of the extreme gravity of its parent star.

The system is PSR J2322-2650. The star is a pulsar – the crushed corpse of a once mighty star. It’s heavier than the Sun, but only about as wide as a small city. It spins almost 300 times per second, beaming out “jets” of energy. One of the jets sweeps across Earth, so the star appears to “pulse” on and off.

The pulsar is so dense that its surface gravity is about a hundred billion times stronger than Earth’s gravity. And the planet is only about a million miles away – about one percent the distance from Earth to the Sun. At that range, the pulsar’s gravity stretches the planet – making it look like a lemon.

The planet is about as massive as Jupiter, the giant of our own solar system. And it has a thick atmosphere made mainly of carbon and helium – a combination that scientists can’t explain. Gamma rays from the pulsar heat the planet’s dayside to 3700 degrees Fahrenheit. So you’d need a lot of lemonade to stay cool on this lemon planet.

The system is too faint to see with the eye alone, but it’s easy to pick out its location. It’s low in the southeast by mid-evening, close to the left of Fomalhaut – the only bright star in that region of the sky.

We’ll talk about another sight close to Fomalhaut tomorrow.

Script by Damond Benningfield

The Sun belongs to a rare class of stars: Class G. Members of this class account for only seven or eight percent of all the stars in the Milky Way Galaxy.

A star’s class is determined by its surface temperature, which we see as its color. G stars are in the middle of the temperature scale, so they shine yellow or yellow-white.

Most class-G stars are in the prime of life – a span that puts them on the main sequence. They’re steadily “fusing” the hydrogen in their cores to make helium. And most of them will stay in that phase of life for 10 billion years or longer.

Most of them are within about 15 percent of the mass of the Sun. Any heavier and they’d get hot enough to move up to class F or higher. Any lighter and they’d be cool enough to move down to class K or M.

Main-sequence G stars are only a small fraction of the size, mass, and brightness of the top stars. But the stars in higher classes are even more rare than G stars. So the Sun, which is near the top of its class, shines brighter than about 90 percent of the stars in the Milky Way.

Not all G stars are on the main sequence, though. Some are bloated and heavy. Such stars are more massive than the Sun. They’ve burned through the hydrogen in their cores, and are fusing hydrogen in a thin shell around the core. But they’re undergoing a transition, so they won’t stay yellow for long – the short-lived monsters of class G.

Script by Damond Benningfield

Under a dark sky, far from city lights, the eye alone can see thousands of stars. They come in a variety of brightnesses and colors. Yet only a few of those pinpoints are like the Sun.

Stars are classified based on their surface temperature, which is indicated by their color. Hot stars are blue and white, while cool stars are orange and red. The Sun is in the middle, shining yellow-white, with a surface temperature of about 10,000 degrees Fahrenheit. Those traits mean it’s class “G.”

From hottest to coolest, stars are classified with the letters O, B, A, F, G, K, and M. Each class is divided into 10 sub-categories. And each star is given a Roman numeral that indicates its size and its stage in life.

The Sun is class G2V. That means it’s toward the hot end of class G, and that it’s on the “main sequence” – it’s in the prime of life, converting the hydrogen in its core to helium.

A couple of moderately bright G main-sequence stars are in view by late evening. 51 Pegasi is high in the southeast, while Tau Ceti is just climbing into view far below it, close to the horizon.

The brightest class G main-sequence star visible from Earth is Alpha Centauri A. It’s the leading light of a triple star system that’s a bit more than four light-years away – closer than any other stars. It’s so far south, though, that it’s visible from only a tiny sliver of the United States.

More about G stars tomorrow.

Script by Damond Benningfield