Like the Sun, Rho Cassiopeia is a class G2 star. That means their surface temperature is the same, so they both look yellow. But that’s just about the only thing they have in common. Rho Cass is one of the most impressive stars in the galaxy. And it faces a much more dramatic fate than the Sun does.

Rho Cass is a yellow hypergiant. If it took the Sun’s place in our own solar system, it would engulf the four innermost planets, including Earth. It’s about 40 times more massive than the Sun, and hundreds of thousands of times brighter. So even though it’s more than 8,000 light-years away, it’s visible to the eye alone – one of the most-distant stars we can see without help.

Rho Cass is nearing the end of its life. That’s made it unstable, so its outer layers puff in and out. The star produces a massive eruption every 50 years or so, and smaller outbursts every 20 years or so. During the last one, Rho Cass blew out enough material to make about 10,000 Earths.

And it may be building up to a huge outburst – a supernova. That would blast away its outer layers, with its dead core collapsing to form a black hole. On the other hand, Rho Cass might be massive enough for the entire star to become a black hole – and vanish from sight.

For now, though, it’s in good view. It’s high in the north at nightfall, to the upper left of the letter M or W outlined by Cassiopeia.

Script by Damond Benningfield

The winter solstice is the shortest day of the year – the shortest gap between sunrise and sunset. After the solstice, the days gradually get longer, all the way until summer solstice, in June. But the weather doesn’t begin to warm up right after the solstice. On average, in fact, the coldest days of the year in the United States come in February.

The weather doesn’t immediately respond to the extra sunlight for several reasons. For one thing, there’s still more darkness than daylight. And for another, the Sun is still hovering at a low angle in the sky. That means the atmosphere blocks more of the Sun’s energy than at other times of the year.

Perhaps the biggest factor, though, is the response of the land and oceans. They absorb the Sun’s energy during the longer, warmer times of the year, then release it back into space quite slowly. It’s like putting a roast in the oven. Even after you turn off the heat, the roast continues to get hotter for a while. And when you take it out of the oven, it doesn’t immediately cool off to room temperature – it slowly radiates its heat into the kitchen.

The land cools off more quickly than the oceans. But both of them continue to “cook” the atmosphere for a while as they gradually radiate their heat – a process that continues for weeks. And just the opposite happens after the summer solstice – it takes extra weeks to crank up the heat, creating the oven-like conditions of summer.

Script by Damond Benningfield

Winter will spring on the northern hemisphere in the wee hours of tomorrow morning – the winter solstice. That makes today and tomorrow the shortest days of the year – the shortest interval from sunrise to sunset. That makes the nights the longest of the year.

Over the centuries, many cultures have held special events to commemorate this changing of the seasons.

In China, for example, the event has been marked by a festival known as Dongzhi – a name that means “winter arrival.” It’s a celebration of the coming victory of light over darkness, as the Sun reverses course and brings longer days.

In ancient Rome, the event coincided with Saturnalia – a festival dedicated to the god Saturn. It was one of the most important events of the year. It began on December 17th and continued for a week. Schools, businesses, and most government activities closed down. People held big banquets, exchanged gifts, and lit their homes with special candles.

Scandinavians held the Feast of Juul. They made sacrifices to the hammer-wielding god Thor. They also lit bonfires to celebrate the returning sunlight, and placed a long-burning log in their hearths – the Yule log.

Many of the features of Saturnalia and the Feast of Juul have been incorporated into the modern celebration of Christmas – another event that roughly coincides with the winter solstice – a dark time with the promise of sunnier days ahead.

Script by Damond Benningfield

Space agencies have been sending missions to the Moon for more than six decades. More than a hundred of them have succeeded. And each success has told us a little bit more about our satellite world.

One of the more recent lessons came from a Chinese mission. Chang’e 5 landed in the Ocean of Storms – a giant volcanic plain that forms one of the dark patches on the lunar surface. It scooped up about four pounds of rock and dirt, and brought them to Earth four years ago this week.

Earlier this year, scientists reported a big discovery from those samples: Three tiny glass beads were produced by a volcanic eruption about 120 million years ago – far later than any previously known eruption. In fact, the finding suggests that the Moon could still pop off today.

Until recently, the best evidence said the last major volcanic activity on the Moon came about two billion years ago. A lunar satellite has photographed some regions that look like they were formed by more recent activity. But that’s hard to confirm without sampling those regions.

The Moon lost most of its interior heat long ago. But regions with large amounts of radioactive elements might generate their own heat. That could power volcanoes even now – a new insight from our continued exploration of the Moon.

The Moon rises in late evening, with Regulus, the brightest star of Leo, close by. They stay close together as they climb high across the sky later on.

Script by Damond Benningfield

If you were camping on Mars right now, you’d see a couple of bright lights low in the west at nightfall. The brighter and higher of the two is Venus. Just as it does for those of us on Earth, it shines as Mars’s brilliant Evening Star. It’s the brightest object in the planet’s night sky other than its two tiny moons.

Just below Venus you’d see the other bright object: our own planet Earth, looking like a blue-white star.

Earth is quickly dropping lower in the Martian sky, and will disappear in the faint twilight within a few days. That’s because Earth is about to pass between Mars and the Sun.

Over the next few Martian evenings, a fainter light will briefly move into view quite close to Earth: the Moon. Tonight, Earth and the Moon will stand so close that you’d probably need binoculars to separate them. But the two bodies are moving apart a bit as viewed from Mars, allowing them to be seen as separate objects with the eye alone – a beautiful sight in the Martian sky.

From here on Earth, Mars is putting in a great appearance almost all night. It looks like a bright orange star, and it climbs into good view in the east-southeast by about 8:30 or 9 o’clock. The Moon rises below it, about 45 minutes later. They climb high across the sky during the night, and are high in the west at dawn – a beautiful sight in Earth’s night sky.

We’ll talk about the Moon and another bright light tomorrow.

Script by Damond Benningfield

It’s generally a good idea to make sure your potatoes are dirt-free before you dig in. If you’re on Mars or the Moon, though, it might be good to mix them with dirt. The combination might make an especially strong form of concrete.

Scientists and engineers have spent years thinking about ways to build habitats on other worlds. It’s too expensive to carry everything from Earth, so habitats will have to be built mostly from local materials. But some ingredients will have to come from home. So scientists have experimented with everything from fungus to urine to cow’s blood. One team even used a ground-up bit of a meteorite to make Lego blocks.

A team in England mixed dehydrated starches with simulated Mars and Moon dirt. They tried several plants – rice, corn, tapioca, and potatoes. And the spuds made the best concrete – a mix they called StarCrete. The blend was about twice as strong as regular concrete. And it could be made from potatoes grown in the habitats.

It’ll be quite a few years before anyone is actually living off-world. So there’s plenty of time to dream up new ways to build habitats on Mars and the Moon.

The two worlds appear quite close together tonight. As they climb into view, in early evening, bright orange Mars is just below the Moon. The Moon will move closer to Mars during the night, and they’ll appear to almost touch in the wee hours of the morning.

We’ll have more about Mars tomorrow.

Script by Damond Benningfield

To the eye alone, the star Castor looks like a single pinpoint of light. It’s one of the “twins” of Gemini. But Castor takes the “twins” business to an extreme. Instead of a single star, it appears to be a system of six stars, consisting of three binaries – three sets of twins.

The sets are known as Castor A, B, and C. The members of each pair are so close together that not even the biggest telescopes can see them as individual stars. Astronomers use special instruments to determine the details of each star.

Castor A and Castor B each consist of a star that’s bigger, heavier, and brighter than the Sun, plus a small, faint companion.

A and B orbit each other once every 450 years or so. Astronomers have been watching the two binaries for hundreds of years. That means they’ve seen enough of an orbit to know for sure that the two binaries are gravitationally bound to each other.

Both of the stars of Castor C are small and faint. The system appears to orbit the other two binaries about every 14,000 years. But astronomers haven’t been watching Castor C as long as the others. So they still can’t be completely certain that Castor C is tied to the others – making one of the “twins” of Gemini a family of three sets of twins.

Look for Gemini near the Moon at nightfall. Castor is to the left of the Moon, with brighter Pollux to the Moon’s lower left. Mars rises well below them, and we’ll talk about it tomorrow.

Script by Damond Benningfield

Call it “The Revenge of the Scrawny.”

The Procyon star system was born with two stars. One was bigger and heavier than the other, so it shined much more brightly. Because of that showiness, though, it burned out much more quickly. Today, all that’s left of the star is its tiny, dead core – while the scrawny companion shines on.

Procyon climbs into good view, due east, by about 9 p.m. That’s just before Sirius, the Dog Star, which rises to the lower right of Procyon. In fact, the name “Procyon” means “before the dog” – an indication that it leads the Dog Star into the sky.

What we see of Procyon is the scrawny survivor, Procyon A. But “scrawny” is relative – the star is bigger and heavier than the Sun.

But when the system was born, the other star, Procyon B, was much more impressive. Because of its greater mass, it “burned” the nuclear fuel in its core much more quickly. That made it much brighter than Procyon A.

It used up its fuel in a hurry, though. It cast its outer layers into space, leaving only its hot but dead core – a white dwarf. It’s more than half the mass of the Sun, but only about as big as Earth. It’ll shine faintly for billions of years.

Procyon A is about to follow the same path. It’s nearing the end of its “prime” phase of life. Soon, it’ll puff up to giant proportions. Then it, too, will cast off its outer layers. That will leave Procyon with two “dead” stars – glowing embers in the cosmic night.

Script by Damond Benningfield

Moon lovers, rejoice! Tonight’s full Moon will be in view longer than any other full Moon of the year – the Long-Night Moon.

The full Moon lines up opposite the Sun, so it does just the opposite of what the Sun does in the daytime sky. The winter solstice – the shortest day of the year – is just a week away, so the Sun is putting in its most feeble appearance` of the year. It rises late, sets early, and scoots low across the south during the day. So the full Moon does just the opposite – it rises around sunset, climbs high across the sky during the night, and sets around sunrise.

The difference is more dramatic as you go farther north. San Antonio, for example, will see about two-and-a-half hours more moonlight than sunlight. But from Seattle, the difference is about seven hours – ten-and-a-half hours of sunlight, followed by seventeen-and-a-half hours of moonlight – a long night to watch the silvery glow of the full Moon.

And as you watch the Moon, look for some bright companions around it. The brilliant planet Jupiter stands to the right of the Moon at nightfall. The star Aldebaran, the bright “eye” of Taurus, the bull, is about the same distance to the right of Jupiter. And the star Elnath, the tip of one of the bull’s horns, is closer to the lower left of the Moon. The Moon will slide especially close to Elnath a few hours later – adding to the beauty of a long night of moonlight.

Script by Damond Benningfield

Friday the 13th brings a bit of bad luck for skywatchers this year. The Geminid meteor shower is at its peak, and it’s generally one of the year’s best, with peak rates of more than a hundred meteors per hour. But the Moon is almost full, so it’ll overpower all but the brightest meteors.

A meteor forms when a bit of debris from a comet or asteroid rams into the atmosphere at tens of thousands of miles per hour. That generates intense heat, so the particle burns up, leaving a streak of light. Most of the streaks vanish in a few seconds. But some of them leave “persistent meteor trains” – glowing trails that can last for a few minutes to more than an hour.

A recent study says that most of these long-lived trains are formed by particles that pass deeper into the atmosphere.

Scientists recorded thousands of meteors above New Mexico. They found that about five percent of meteors left a train that was visible for at least five minutes – a much higher fraction than in earlier studies.

There wasn’t much correlation between the persistence of the train and either the speed or brightness of the meteor. But there was a correlation with the meteor’s altitude. Those that descended to about 55 miles were much more likely to create long-lasting trains.

At that altitude, metals in the chunk of space debris may react with ozone in the atmosphere long after the meteor burns up – creating a glowing trail in the night sky.

Script by Damond Benningfield