StarDate

The “she-goat” is a lot more than it seems. What looks like a single brilliant star is actually two sparklers. Both of them are much bigger, heavier, and brighter than the Sun. Capella is the brightest star of Auriga, the charioteer. Its name comes from a Latin phrase that means the she-goat. It’s 43 light-years away – just down the block by astronomical standards. Both stars of Capella are about two and a half times as massive as the Sun. And both are more than 70 times brighter than the Sun. But the stars are quite close together – less than the distance from Earth to the Sun. So we can’t see them as individual points of light, even with the biggest telescopes. Astronomers discovered Capella’s dual nature with a technique called spectroscopy. It breaks a star’s light into its individual wavelengths. The spectrum of Capella shows two patterns of light. The patterns move back and forth as the stars orbit each other. The patterns reveal details about both stars – their surface temperature, composition, and more. From that and other details, we know that both stars have moved beyond the prime phase of life. Now, they’re in the giant phase. Both stars have puffed up to about 10 times the diameter of the Sun – two big, brilliant stars for the she-goat. Capella is a third of the way up the northeastern sky at nightfall. It’s one of the brightest stars in the entire night sky, so you can’t miss it. Script by Damond Benningfield

The tales that describe many of the ancient constellations can be romantic, tragic, heroic, or majestic. Some, on the other hand, are just weird. An example is Auriga. The constellation is low in the eastern sky at nightfall, and climbs high across the sky later on. It’s marked by a pentagon of stars. It’s easy to pick out thanks to the brightest member of that figure, Capella – one of the brighter stars in the entire night sky. Although Auriga is described as a charioteer, the character usually isn’t depicted with a chariot. But he is shown with a goat and her two kids on his shoulder. There are several versions of his story. In one, he was an early king of Athens. He was raised by the goddess Athena. Among other things, she taught him how to tame horses. He was so good at it that he became the first person to harness four horses to a chariot, like the chariot that carried the Sun across the sky. Zeus, the king of the gods, was so impressed that he placed the charioteer in the stars. The goat is represented by Capella. It isn’t a part of any of the legends of Auriga from Greek or Roman mythology. It may represent the goat that suckled the infant Zeus, who placed her and her children in the sky in gratitude. The goat and kids may once have formed their own small constellation. Today, though, they ride on the shoulder of the charioteer – who rides on nothing at all. More about the charioteer tomorrow. Script by Damond Benningfield

At the cusp of the 20th century, it seemed like contact with another world was just a matter of time. In fact, the French Academy of Sciences announced a prize for such a feat 125 years ago today. The winner would receive 100,000 francs. There was only one catch: Mars didn’t count. The prize was established by Clara Guzman in honor of her son. He was a follower of astronomer Camille Flammarion, who wrote extensively – and fancifully – about the Red Planet. Guzman excluded Mars from the competition because it seemed just too easy. Percival Lowell had popularized the idea that Mars was crisscrossed by canals – built by Martians to bring water from the poles to the planet’s deserts. Inventor Nicola Tesla had reported hearing possible radio signals from Mars. And many others thought that vast dark areas on Mars were covered with vegetation. Many schemes were proposed to contact the Martians. One suggested creating giant geometric shapes in Siberia. Another suggested digging the shapes into the Sahara Desert, filling them with kerosene, and setting them on fire. None of the schemes ever materialized. And no one ever claimed the prize for contacting another world. So the French academy decided to award the prize for making physical contact. In 1969, it awarded the Guzman Prize to Apollo 11 astronauts Neil Armstrong, Buzz Aldrin, and Michael Collins – the first men to set foot on another world. Script by Damond Benningfield

The planet Mercury is shrinking. It’s contracted by several miles since its birth. And it’s continuing to get smaller even now. Mercury is the closest planet to the Sun. It’s also the smallest major planet in the solar system – a little more than 3,000 miles in diameter – about the width of the 48 states. It has a core of iron and nickel, surrounded by dense layers of rock. And it’s topped by a thin crust. The surface of the planet is marked by lots of impressive cliffs. The biggest is more than 600 miles long and about two miles high. They formed as Mercury lost heat from its interior. As the planet cooled, it shrank. Estimates of how much it’s contracted have ranged from about a mile to about nine miles. A recent study narrowed the range a little bit. It measured the most dramatic features, then scaled that to the surface of the entire planet. The result suggests that Mercury has shrunk by about three to five miles as a result of its cooling. And when you add in some other causes, the total contraction is about four to seven miles. And Mercury is still getting smaller today. This incredible shrinking planet is quite low in the southeast in the dawn twilight for the next few days. It looks like a bright star, but you need a clear horizon to spot it. And because of the viewing angle, it’s easier to spot from more southern latitudes. Tomorrow, the Moon stands to its right or upper right. Script by Damond Benningfield

Here’s what we know for sure about the planet K2-18b. It’s about 125 light-years away. It’s bigger and heavier than Earth. It orbits a cool, faint star once every 33 days. It receives about the same amount of energy from its star as Earth gets from the Sun. And it has an atmosphere. After that, things get muddled. Astronomers aren’t sure about the structure of the planet or the make-up of its atmosphere. And ideas about whether it might be habitable are all over the place. The confusion highlights the challenges of studying planets in other star systems. K2-18b passes in front of its star on every orbit. And as it does so, the chemical “fingerprints” of its atmosphere are added to the starlight. Substracting the starlight provides a profile of the atmosphere. But the profile is hard to read. Many of the fingerprints are subtle, and can be produced by different compounds. Earlier this year, a team announced the discovery of compounds in the atmosphere that could be produced by microscopic life. Follow-up studies by other groups contradicted that finding. But the original study team has stuck by its conclusions. So it’ll take a lot more work to know for sure what’s going on at K2-18b. The K2-18 system is in Leo, which climbs into good view after midnight. K2-18 is to the right of Denebola, the star that marks the lion’s tail. But it’s too faint to see without a telescope. Script by Damond Benningfield