StarDate

If you’d like to know how dark your night sky is, then look high in the northeast after the Moon sets this evening for the stars of Camelopardalis, the giraffe. If you can see any of them, then congratulations – your sky is pretty dark. Light pollution wipes out the view for most Americans. The glare of street lights, billboards, and other artificial sources overpowers the stars. None of the stars of Camelopardalis, for example, is brighter than fourth magnitude, which is pretty faint. So unless you’re under dark skies, there’s not much to see. That’s a little misleading, though. The giraffe’s brightest stars are all stunners. They look so faint only because they’re so far away. The giraffe’s brightest star is Beta Camelopardalis – Beta Cam for short. It’s a huge, massive star that shines roughly 1600 times brighter than the Sun. But it’s about 840 light-years away, so it’s a faint dot in the night sky. The next-brightest star is CS Cam. It is a supergiant star that’s perhaps 75,000 times the Sun’s brightness. But it’s 3400 light-years away. And the third-brightest, Alpha Cam, is the most impressive of all: more than 600,000 times the Sun’s brightness. At a distance of 5500 light-years, it’s one of the most remote stars visible to the unaided eye – but only under especially dark skies. More about Alpha Cam tomorrow. Script by Damond Benningfield

Space agencies are talking a lot these days about sending people to the Moon – and even setting up permanent bases there. But you might not want to be on the Moon seven years from today. A space rock that’s half the size of an NBA arena has a slight chance of slamming into the Moon. Asteroid 2024 YR4 was discovered last year, two days after Christmas, when the asteroid had flown just half a million miles from Earth. Early observations gave it more than a three percent chance of hitting Earth on December 22nd, 2032. As astronomers tracked it longer, they ruled out that chance. Instead, they calculated that it’ll pass about 7,000 miles from the Moon. But there’s a 45-thousand-mile margin of error. So there’s a better than four percent chance that it will hit the Moon. 2024 YR4 is so far away right now that we can’t see it. It won’t return to view until 2028. Once it reappears, astronomers will refine their calculations. That probably will rule out the chance of an impact. But for now, we can’t know for sure. The asteroid is about 200 feet in diameter. That’s about the size of the asteroid that gouged the famous meteor crater in Arizona. So an impact on the Moon probably would form a big crater. Debris from the impact could travel hundreds of miles – cosmic missiles crashing across much of the Moon. Script by Damond Benningfield

Today is the December solstice – the start of winter in the northern hemisphere. It’s the darkest time of the year – many hours of darkness for watching the stars. But it’s also a great time for space science in Antarctica, where it’s daylight around the clock. NASA launches high-altitude balloons from a base near McMurdo Station, the continent’s largest settlement. Their payloads can keep an eye on the heavens for weeks as they circle around the south pole. When their work is done, they parachute to the ice. Scientists from the United States, Japan, and other countries hunt for meteorites in Antarctica. There aren’t more meteorites there, but on the ice, there’s a good chance that almost any rock came from beyond Earth. Over the decades, tens of thousands of meteorites have been found there. Astronomers take advantage of the daylight to repair and upgrade telescopes at the south pole. The collection includes instruments that study the “afterglow” of the Big Bang. The instruments can operate even in daylight, but the southern summer is the only time to do most of the maintenance work. The south pole also is home to IceCube – a collection of thousands of light detectors frozen in the ice. They look for neutrinos – particles that tell us about some of the most energetic events in the universe. IceCube can also operate all year – even under the midnight sun at the south pole. Script by Damond Benningfield

If you don’t like winter but you live in the northern hemisphere, then give a little thanks to the laws of orbital mechanics. Because of Earth’s lopsided path around the Sun, winter is the shortest season north of the equator. Earth’s orbit around the Sun isn’t a perfect circle. Instead, it’s an ellipse. It looks like a flattened circle, with the Sun slightly away from the center. Because of that shape, our distance to the Sun changes. And that’s where the laws of orbital motion come into play. Johannes Kepler devised those laws more than four centuries ago. One of them says that if you draw a line from the center of the Sun to the center of a planet, as the planet orbits the Sun that line will sweep out equal areas over equal periods of time. To do that, a planet must move fastest when it’s closest to the Sun, and slowest when it’s farthest from the Sun. Earth is closest to the Sun in early January – the start of winter – and farthest at the start of summer, in July. So Earth moves around the Sun in a hurry during northern winter, making the season shorter. This winter, for example, starts at 9:03 a.m. Central Time tomorrow. That’s the moment of the December solstice, when the Sun stands farthest south for the year. The season ends 89 days later. By comparison, this past summer lasted almost 93 days – a longer season thanks to the science of orbits. More about the solstice tomorrow. Script by Damond Benningfield

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