StarDate

Mighty Orion the hunter has a mighty resting spot for his tired feet: Cursa, the second-brightest star of Eridanus, the river. The star’s name comes from a longer Arabic phrase meaning “footstool of the central one” – Orion himself. As night falls, the star stands above Orion’s foot: Rigel, the hunter’s brightest star. Cursa is about 90 light-years away. It’s easy to see from that distance because it’s a giant. It’s several times the size and mass of the Sun, and 45 times the Sun’s brightness. Its classification as a “giant” tells us much more than just its size, though. It also tells us about its stage in life. A giant star has puffed up as a result of changes deep in its heart. It’s burned through the hydrogen in its core to make helium, so it’s moved into a new phase. In the case of Cursa, it’s fusing hydrogen in a thin shell around the core. The shell is quite hot, so it produces a lot of radiation. That pushes on the surrounding layers of gas, causing the star to expand. And that makes it brighter. Today, the surface of Cursa is thousands of degrees hotter than the Sun’s. At that temperature, the star shines almost pure white. As it continues to change, though, Cursa may get even bigger and brighter. But its surface will get cooler. So a bigger Cursa will shine redder – an angry-looking footstool for the hunter. Tomorrow: from giant to supergiant. Script by Damond Benningfield

Earth passed by Jupiter yesterday. Now, we’re beginning to leave the giant planet behind. We’ll loop past it again early next year. That passage is known as opposition – Jupiter lines up opposite the Sun in our sky. It’s closest to us then, so it shines brightest for the year. And it’s in view all night. Jupiter is much farther from the Sun than Earth is, so it takes about 12 years to complete a single orbit. Earth follows a much shorter path around the Sun, and it moves faster. So it passes Jupiter every 13 months. As we approach Jupiter, the planet stops its normal eastward motion against the background of stars. For a while, it moves backward – a period known as retrograde. Jupiter itself doesn’t change direction. Instead, the shift is a result of our changing viewing angle. It’s like passing a car on the highway. For a little bit, the other car looks like it’s moving in reverse compared to the background of buildings and trees. As the gap opens, though, it appears to resume its forward motion. Jupiter will reach that point on March 11th – shifting gears as it circles the Sun. Jupiter looks like a brilliant star – brighter than any other planet or star in the night sky now. The twin stars of Gemini are close by. Pollux, the brighter twin, is close to the left of Jupiter at nightfall. Castor is farther to the upper left. The whole group soars high across the south during the night. Script by Damond Benningfield

The closer we look at the worlds of the solar system, the more places we see that could be homes for life. Some of those worlds orbit Jupiter, the largest planet in the solar system. Jupiter itself isn’t on the list. It’s a big ball of gas with no solid surface. There has been speculation that large organisms could float through its skies. But that’s considered a long shot. It’s more likely that life could inhabit some of Jupiter’s moons. The leading candidate is Europa. It’s about the same size as our own moon. A deep ocean of liquid water probably lies below its icy crust. Plumes of hot water may squirt into the bottom of the ocean. The plumes would contain a variety of compounds – perhaps including the chemistry of life. So Europa has the right combination of water, heat, and chemistry to support life – at least microscopic life. Europa isn’t the only Jovian moon with a deep ocean. The largest moon, Ganymede, may have more liquid water than all Earth’s oceans combined. One other big moon may have an ocean as well. But the crusts of these moons are much thicker than Europa’s. So even if their oceans are inhabited, it’ll be much harder for us to find evidence of life. Look for Jupiter in the eastern sky in early evening, and arcing high across the sky later on. It looks like a brilliant star. Through binoculars, its big moons look like tiny stars quite close to the planet. More about Jupiter tomorrow. Script by Damond Benningfield

Jupiter looks like it’s wearing zebra stripes. Bands of clouds that run parallel to the equator alternate between bright and dark – zebra stripes. Each one is thousands of miles wide. The stripes are a result of Jupiter’s composition and its rotation. It’s basically a ball of gas – it’s made almost entirely of hydrogen and helium. And even though it’s 11 times the diameter of Earth, it spins on its axis in less than 10 hours. That forces the clouds that top its atmosphere into bands that stretch from east to west. The bands alternate between belts and zones. The belts are darker – probably because they allow us to see deeper into the atmosphere. The zones are topped by the highest clouds. The clouds are made of frozen ammonia, which looks bright white. The belts don’t have that layer. Instead, we’re seeing clouds in the next layer down. Those clouds are made of water and other compounds, which are darker. The stripes are flanked by jet streams that blow in alternating directions. They can roar at hundreds of miles per hour. They keep the belts and zones separated – maintaining the zebra stripes on this giant planet. Jupiter is at its best this week. It’s in view all night, and it shines brightest for the year. It looks like a brilliant star. It’s low in the eastern sky in early evening, and climbs high across the sky later on. The stripes are easily visible through just about any telescope. Script by Damond Benningfield

If today is your birthday, then Happy Birthday! The next one is just one year away – 365 sunrises and sunsets. If today is your birthday and you happen to be from Jupiter – well, Happy Birthday, and … we’re sorry. Your next one is almost 12 Earth years away – almost 10,500 sunrises and sunsets. The Jovian year is so long for a couple of reasons. First, the planet is more than five times farther from the Sun than Earth is. So its path around the Sun is more than five times longer than Earth’s. The second reason is the laws of orbital motion. The farther a planet is from the Sun, the slower its orbital speed. At Jupiter’s great range, it moves at less than half the speed of Earth. Ergo, one Jovian year lasts almost 12 Earth years. But to get all those sunrises and sunsets, you also have to factor in the length of a Jovian day. Although Jupiter is 11 times the diameter of Earth, it spins in a hurry – a day lasts less than 10 hours. Add it all up, multiply, divide, and carry the two, and – well, it’s a lot of days between birthdays on the Sun’s largest planet. Jupiter is especially vibrant now. It reaches opposition this weekend – it lines up opposite the Sun in our sky. It rises around sunset and is in view all night. The planet is also closest to us, so it shines at its brightest. In fact, in all the night sky right now, only the Moon outshines it. More about Jupiter tomorrow. Script by Damond Benningfield

Stars are born when giant clouds of gas and dust break apart and collapse. And if that’s all there was to it, the Milky Way Galaxy would give birth to a couple of hundred stars every year. Instead, thanks to feedback from the stars themselves, it makes only a few. Feedback is a process that clears away the material for making stars, but can also trigger the birth of more stars. Young stars, for example, produce winds and jets that blow away the gas and dust around them. Since stars are born in clusters, many youngsters can be sweeping away the star-making material at the same time. That pares back the number of stars that can be born in a cluster. Mature stars add to the feedback – not only with winds, but also with radiation. Hot stars generate a lot of ultraviolet energy. It vaporizes tiny particles of dust – eliminating possible building blocks for new stars. The heaviest stars explode as supernovas. These blasts can clear out the space for light-years around, creating big, empty bubbles. And supernovas also accelerate subatomic particles around them to almost the speed of light. These “cosmic rays” help to sweep away the raw material for making more stars. But supernovas can also enhance the birth rate. Their shock waves can cause distant clouds of gas and dust to collapse to form stars. So feedback is a complex process – one that both aids and hinders the birth of new stars. Script by Damond Benningfield

The planet Venus is switching sides today – sides of the Sun. It’s crossing behind the Sun as seen from Earth, so it’s moving from the morning sky to the evening sky. But we won’t be able to see it for several weeks. Venus is the second planet from the Sun, while Earth is third. So Venus crosses both behind the Sun and between Earth and the Sun. It switches between Morning Star and Evening Star appearances each time. Each of these crossings happens every 584 days – about 19 and a half months. The planet spends about eight months in both the morning and evening sky, and disappears from view during the crossings. When Venus passes between Earth and the Sun, it’s closest to us, so it moves across the sky quickly – it’s hidden in the Sun’s glare for only a few days. When it’s behind the Sun, it’s farthest – about 160 million miles. Because of the relative motions of Earth and Venus, it moves across the sky quite slowly. So it remains hidden in the light for three months or so. Depending on your location, Venus could emerge as the Evening Star as early as mid- to late February. It’ll be quite low in the twilight, so it won’t be easy to find. The planet will climb into better view in early March. Venus will reign over the evening sky until October, when it will vanish in the sunlight as it once again switches sides. Tomorrow: slowing down the stellar birth rate. Script by Damond Benningfield

The gibbous Moon soars across the sky tonight. It’s about three days past full, so the Sun lights up about 90 percent of the lunar hemisphere that faces our way. That makes the Moon nice and bright. But it’s not as bright as you might expect. In fact, it’s only about half as bright as the full Moon. There are a couple of reasons for that. One is our viewing angle. The full Moon stands opposite the Sun in our sky, so the sunlight that strikes it is reflected straight back toward Earth. That makes the Moon a more efficient mirror. But the main reason is the shadows. At full Moon, the shadows on most of the visible surface are short. In fact, there are almost no shadows at all across the center of the lunar disk. But as the Moon moves in its orbit around Earth, the angle between the Sun and Moon changes. The Sun drops lower in the lunar sky, so the shadows grow longer as seen from Earth. More shadows mean a darker surface. Despite appearances, none of the Moon is especially bright. It reflects only a bit more than one-tenth of the sunlight. It looks so bright only because it’s a close, big presence – lighting up the night sky. A bright star joins the Moon tonight: Regulus, the heart of the lion. It’s below the Moon as they climb into good view, about 9 or 9:30. The Moon will slide toward the star during the night, and they’ll be especially close as the dawn twilight begins to erase the star from view. Script by Damond Benningfield

There’s no fountain of youth to make people look younger. But there is one for stars. It’s a process that sounds like something from a horror movie – “stealing” life from another star. A good example is in Fornax, the furnace, which is low in the south at nightfall. The constellation has only one moderately bright star, Alpha Fornacis. It’s 46 light-years away. To the eye alone, it’s not much to look at. But binoculars reveal two stars. One of them is bigger and heavier than the Sun. Because of its greater mass, it’s nearing the end of its life, even though it’s almost two billion years younger than the Sun. The other visible star is smaller than the Sun, and its surface is cooler, so it glows orange. Yet it should be even redder than it is. And that’s where the story of rejuvenation comes in. The star is a blue straggler. That means its color has shifted to bluer wavelengths. That might be because it merged with another star. The merger would rev up the nuclear reactions in its core, making it hotter and bluer. On the other hand, it might have changed color by simply stealing gas from a third star in the system. This extra star was discovered in 2016. It’s a white dwarf – a stellar corpse. It’s about half as massive as the Sun, and it’s quite close to the blue straggler. So the straggler might have siphoned away the star’s life – taking some of its gas to “rejuvenate” its own appearance. Script by Damond Benningfield

The Moon sometimes rumbles during “moonquakes.” And according to a recent study in China, those quakes may happen fairly often. The first moonquakes were recorded by instruments left on the lunar surface by Apollo astronauts. Some of the quakes are deep – they’re centered hundreds of miles below the surface. They’re triggered by the tides – the gravitational pull of Earth squeezes and stretches the interior, causing things to clatter about. The other main moonquakes are shallow – they occur much closer to the surface. These quakes are triggered by the Moon itself. Our satellite world is shrinking as it loses its internal heat. It might have shrunk by as much as 150 feet over the past few hundred million years, and continues to contract even today. The Chinese study looked at 74 spots on the lunar surface, on both the nearside and farside. Scientists pored over hundreds of pictures snapped from 2009 to 2024. And they found 41 fresh landslides that happened during that period. They ruled out other causes for about 70 percent of the landslides. That left them with one conclusion: the landslides were caused by shallow moonquakes. So the Moon continues to shake and jiggle long after its birth. The Moon has some prominent companions tonight. It’s flanked by the brilliant planet Jupiter and the star Pollux, the brighter “twin” of Gemini. Castor, the other twin, is to the upper left of the Moon. Script by Damond Benningfield

Nothing symbolizes a cold, moonlit night like the howl of a wolf. The haunting sound can travel for miles. And if you live around wolf territory, you might especially notice it tonight. There’s a full Moon – the Frost Moon, Moon After Yule, or Wolf Moon. Despite what you might think, though, the wolves aren’t actually howling at the Moon. Many cultures have associated wolves and the Moon – ancient Moon goddesses often were depicted hanging with wolves. And biologists say that wolves may howl more around the time of the full Moon. But that’s only because they’re creatures of the night, so they’re more active when there’s more moonlight. Wolves communicate with each other in many ways besides howling. They growl, bark, and whimper. Each method conveys a different type of message. And howls can have different meanings, too – conveyed through changes in pitch, duration, and frequency. The howls help them attract mates, coordinate their hunting, and warn members of other packs to stay away. There’s even a “lonesome” howl when a wolf gets lost. Wolves do tilt their heads up when they howl – as though they were talking to the Moon. But there’s a practical reason – the sound carries farther. So if you happen to hear the lonesome howl of a wolf under the light of the full Moon, enjoy the serenade – just don’t think the wolf is howling at the Moon. Script by Damond Benningfield

At the dawn of the 19th century, the celestial police were on patrol. They were looking for a planet between the orbits of Mars and Jupiter. And on the century’s first day, a future squad member found one – sort of. Later discoveries showed that it wasn’t a planet at all, but the first and largest member of the asteroid belt – a wide band of millions of rocky bodies. Astronomers were looking for a planet because of the numbers. There seemed to be a mathematical relationship between the distances from the Sun to the known planets. But there was a gap between Mars and Jupiter. So one astronomer began organizing a search party: the celestial police. Giuseppe Piazzi, at the Palermo Observatory in Sicily, was on the list of people to invite. But he was already searching on his own. And before he got his invitation, he found something – 225 years ago today. Piazzi originally thought it was a comet – but hoped for something bigger. As other astronomers began studying it, they decided it was the sought-after planet. They named it Ceres, for the Roman goddess of agriculture. Within a few years, though, they’d found several other bodies in similar orbits. So they realized that Ceres wasn’t a planet at all, but just one member of a band of debris – the asteroid belt. Today, Ceres has regained its planetary status – sort of. It’s a dwarf planet – the only one in the inner solar system. Script by Damond Benningfield

By the time the ball drops in Times Square tonight, the people of the Line Islands will be almost a full day into 2026. The islands are in the Pacific Ocean, south of Hawaii. But they’re just across the International Date Line. That makes the islands the first place to see the new year. The Date Line is needed because the time gets an hour earlier for every time zone west, and an hour later for every time zone east. Without a place to reset the date, time just wouldn’t make sense. The line mostly runs down the middle of the Pacific – half way around the globe from Greenwich, England, which is the starting point for the time system. But individual countries can set their own time zones. So the line zigzags between Alaska and Russia. And near the equator, it jumps more than a thousand miles to the east. That extension came three decades ago. The island nation of Kiribati changed its time zones. That made it easier for the country to do business with Australia, which is west of the Date Line. The country’s easternmost extension is the Line Islands. So the date changes there first – making the Line Islands the first places on Earth to ring in the new year. American Samoa is farther west than the Line Islands. But its time zone puts it on the opposite side of the Date Line – making it one of the last places to change the calendar. Script by Damond Benningfield

The Sun and similar stars are losing weight – they blow some of their gas into space through strong “winds.” And at the end, they blow away all of their outer layers of gas. That leaves only their hot, dense cores, known as white dwarfs – tiny remnants of their once brilliant selves. An example is Sirius B, the faint companion of Sirius A, the brightest star in the night sky. Sirius climbs into view in the east-southeast by around 8:30 or 9, and arcs across the south during the night. Sirius B is too small and faint to see without a telescope. But long ago, that wouldn’t have been the case. The star probably was a few times as massive as the Sun, so it would have shined brighter than Sirius A is today. Such a hot, bright star produces a much thicker wind than the Sun does, so it loses mass at a higher rate. And because Sirius B was heavier than the Sun, it burned through the nuclear fuel in its core much faster – it fizzled out in a couple of hundred million years, while the Sun is still only half way through its 10-billion-year lifetime. As it neared the end of its life, Sirius B puffed up like a giant balloon, then ejected its outer layers. Some of that gas probably piled on the surface of Sirius A, increasing its mass. Today, Sirius B is as heavy as the Sun, but only as big as Earth. It still shines because it’s extremely hot. But it’s only a faint reminder of its former glory. Tomorrow: an early new year. Script by Damond Benningfield

Over the centuries, we’ve given all the visible stars many names – proper names, catalog designations, and others. But only one star is best known not by any of its formal names, but by its nickname: the Dog Star. Its proper name is Sirius, and it’s the leading light of the constellation Canis Major, the big dog – hence the nickname. Sirius is so well known because it’s the brightest star in the night sky – its closest competition is only about half as bright. Part of that is because Sirius itself is a couple of dozen times brighter than the Sun. But part of it is because Sirius is one of our closest neighbors – less than nine light-years away. And thanks to the relative motions of Sirius and the Sun, Sirius is moving closer, at about 12,000 miles per hour. It’ll continue to close in for tens of thousands of years. But the distances between stars are so vast that even at that speed, Sirius won’t grow much brighter in our sky. Astronomers discovered the star’s motion toward us by measuring its Doppler shift – a slight change in the wavelength of its light. The Doppler shift also allowed them to measure the orbit of a faint companion – a stellar corpse known as a white dwarf; we’ll have more about that tomorrow. In the meantime, look for Sirius climbing into good view in the east-southeast by around 8:30 or 9. It’s directly below the three stars of Orion’s Belt, so you can’t miss it. Script by Damond Benningfield

The most important thing to know about a star is its mass – how heavy it is. Among other things, the mass reveals how long the star will live and how it will die. Measuring the mass of a single star is tough. It’s a lot easier to get the masses of stars in binary systems – two stars that orbit each other. An example is Menkalinan, the second-brightest star of Auriga. It’s a third of the way up the northeastern sky at nightfall, below the charioteer’s brightest star, Capella. Menkalinan’s two stars are so close together that we can’t see them as individual points. But breaking the system’s light apart reveals the presence of both stars. The stars orbit each other every four days, at about one-tenth of the distance from Earth to the Sun. Combined, those numbers reveal the system’s total mass. A couple of other numbers complete the picture. One is the angle at which we’re seeing the system. In the case of Menkalinan, that’s easy – the stars pass in front of each other, so we see the system edge-on. The other is the orbital motions of the stars. Plugging those numbers into the formula provides a precise mass for the individual stars. The stars of Menkalinan are almost identical. Each is more than twice the mass of the Sun. Each is also bigger and brighter than the Sun. So even though Menkalinan is more than 80 light-years away, it’s easy to see – the combined glow of two big, well-understood stars. Script by Damond Benningfield

Orion is climbing into prominence in winter’s evening sky. The hunter clears the eastern horizon by about an hour and a half after sunset. He’s led by his shield. It’s not as easy to see as his belt or other features. But the shield’s brightest star does stand out. Pi-3 Orion is in the middle of the shield – where Orion’s hand is holding it. The star is a little bigger, heavier, and hotter than the Sun. That makes it about three times brighter than the Sun. There are a couple of ways to look at that brightness: apparent magnitude and absolute magnitude. Apparent magnitude is how bright a star looks. In that scale, Pi-3 shines at about magnitude 3.2 – not especially bright, but bright enough to see under even most light-polluted skies. But that number doesn’t tell you the star’s true brightness. It might be especially bright, but it might also be especially close. So that’s where absolute magnitude comes in. It’s how bright a star would look at a distance of 10 parsecs – 32.6 light-years. If you lined up every star at that distance, you could easily tell which ones are truly bright. Pi-3 is just 26 light-years away. If you moved it out to 10 parsecs, it would shine at magnitude 3.65 – half as bright as it looks now. In fact, if you moved all the stars in the shield to that distance, Pi-3 would be its faintest member – a middling middle for the shield. Script by Damond Benningfield

Not many planetary spacecraft get to shower off. But the Cassini spacecraft did – more than once. It flew through plumes of ice and water vapor from Enceladus, a moon of Saturn. The encounters helped scientists confirm that an ocean hides below the moon’s icy crust. Enceladus is a little more than 300 miles in diameter – roughly the distance from Los Angeles to San Francisco. Its surface is completely coated with ice. That makes it the most reflective large body in the solar system, so it looks bright white. Much of that ice comes from more than a hundred geysers near the moon’s south pole. They erupt from deep cracks in the crust. They contain water vapor, water ice, hydrogen, grains of salt, and other compounds. Much of this material falls back on the surface. The rest of it escapes into space, where it forms a thin ring around Saturn. The geysers erupt from a global ocean. It’s buried about 20 to 25 miles below the surface, and it could be 10 miles deep or more. Hot, mineral-rich water could flow into the ocean through fissures on its floor. So the ocean appears to offer all the ingredients for life: liquid water, minerals, and a source of heat. That makes Enceladus a high-priority target in the hunt for life beyond Earth. Saturn is near our own moon this evening. It looks like a bright star, shining steadily through the lunar glare. But you need a good-sized telescope to pick out Enceladus. Script by Damond Benningfield

A look at the evening sky is a nice way to wrap up your Christmas. It features the Moon, two bright planets, and some of the brighter stars in all the night sky. As twilight drains from the sky, the Moon is well up in the southwest. The Sun lights up more than a quarter of the lunar hemisphere that faces our way, so it’s a fat crescent. It’s waxing toward first quarter, on Saturday. The planet Saturn is to the upper left of the Moon, and looks like a bright star. It shines so brightly for a couple of reasons: It’s the second-largest planet in the solar system – more than nine times the diameter of Earth – and it’s topped by clouds that reflect much of the sunlight that strikes them. The only planet that’s bigger than Saturn is Jupiter, and it climbs into good view, in the east-northeast, by 7 or 7:30. In all the night sky right now, only the Moon outshines it. The “twin” stars of Gemini – Pollux and Castor – stand to Jupiter’s left and upper left. At the same time, the brilliant constellation Orion is off to the upper right of Jupiter. Look for its three-star belt aiming straight up from the horizon, flanked by orange Betelgeuse and blue-white Rigel. Taurus perches well above Orion. It’s marked by its bright orange eye, Aldebaran. And the Dog Star, Sirius, climbs into good view by 8 or 8:30, below Orion’s Belt. It’s the brightest true star in the night sky – a beautiful decoration for Christmas night. Script by Damond Benningfield

For a star, showiness comes with a price. The most massive stars are far brighter than their punier cousins. But they live much shorter lives. An example is Alpha Camelopardalis. It’s the third-brightest star of Camelopardalis, the giraffe. It’s dimmed by its great distance – about 5500 light-years – so you need a dark sky to see it. Even so, it’s one of the most remote stars visible to the eye alone. The star is impressive. It’s more than 30 times the diameter of the Sun, and almost 40 times the Sun’s mass. Because of that great heft, Alpha Cam “burns” through the nuclear fuel in its core in a big hurry. That makes its surface tens of thousands of degrees hotter than the Sun’s, so the star shines blue-white. The combination of size and temperature makes Alpha Cam more than 600,000 times brighter than the Sun. The price for that showiness is a short lifespan. Stars like the Sun live for billions of years. But Alpha Cam will stick around for only a few million years. So even though it’s only about two million years old, its days are numbered. Before long – astronomically speaking – it will expire. Just how it will go out isn’t clear. Its core may collapse to form a black hole, with its outer layers exploding as a brilliant supernova. On the other hand, the entire star may collapse, forming a heavier black hole – a dark ending for a dazzling star. Script by Damond Benningfield