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Jan Oort Apr 28, 2025

Trillions of icy bodies mark the edge of the solar system. They form a shell that extends one or two light-years from the Sun in every direction. A passing star may sometimes give some of them a nudge toward the Sun. When they get here, they become comets – visitors from the icy deep.

That distant region is known as the Oort Cloud. It’s named for Dutch astronomer Jan Oort, who was born 125 years ago today. He proposed the existence of the cloud in 1950. And today, that’s his best-known accomplishment.

Yet it’s far from his most important work. Early in his career, he confirmed that the Milky Way is a wide, flat, spinning disk. And he showed that, instead of inhabiting the center of the galaxy, the solar system is in the hinterlands – many thousands of light-years outside the heart.

Oort spent most of his career at Leiden University in the Netherlands, including decades as director of Leiden Observatory. When Germany invaded the country, though, he left his post instead of working with the Nazis.

When he returned, after World War II, he became a pioneer in the new field of radio astronomy. He mapped giant clouds of gas and dust throughout the galaxy. Their distribution provided even more proof of his picture of the Milky Way.

Oort continued his research until shortly before his death, in 1992. Scientists have named quite a few things in his honor, including an asteroid – and the icy shell known as the Oort Cloud.

Script by Damond Benningfield

Cor Caroli Apr 27, 2025

Dark spots sometimes dot the surface of the Sun – magnetic storms that can last for days or weeks. But the storms on other stars can make those on the Sun look puny. They can be so monstrous that they can change the star’s brightness by quite a bit.

An example is one of the stars of Cor Caroli – the Heart of Charles. As seen by the eye alone, the star is the leading light of Canes Venatici, the hunting dogs. It’s to the right of the handle of the Big Dipper as night falls, and wheels above the dipper later on.

But a closer look shows that Cor Caroli consists of two stars in a wide orbit around one other. One of the stars is about half again as massive as the Sun, and several times brighter. The other is about three times the Sun’s mass, and more than a hundred times its brightness.

But over a period of about five and a half days, the heavier star’s brightness varies by about 15 percent. And that’s because of the starspots.

The star’s magnetic field is more than a thousand times stronger than the Sun’s. That produces giant spots – far bigger than anything ever seen on the Sun.

The spots are thousands of degrees cooler than the surrounding gas. The combination makes them darker than the rest of the surface. As the star rotates, the spots move in and out of view, causing the star to get fainter and brighter – a big “flutter” for the Heart of Charles.

Tomorrow: an astronomer with a “cloudy” name.

Script by Damond Benningfield

Guardians of the Pole Apr 26, 2025

The stars Kochab and Pherkad serve several roles. They’re part of the body of Ursa Minor, the little bear. They form the outer edge of the bowl of the Little Dipper. And they’re “guardians of the pole” – they circle around Polaris, the star that marks the north celestial pole.

Both stars are giants – they’ve puffed up at the end of the prime phase of life. Kochab is about 50 times the Sun’s diameter, and 450 times its brightness. Pherkad looks fainter than Kochab, but only because it’s almost four times farther. In reality, it’s more than twice as bright.

The stars are so big and bright because they’ve exhausted their original supply of nuclear fuel. That’s triggered changes in the cores of the stars that have caused them to puff up.

In ages past, both stars were much closer to the pole than they are today. In fact, Kochab was the closest bright star to the pole for a millennium. It was the best pole star about 3100 years ago. But it wasn’t nearly as good a marker as Polaris – it never got closer than about seven degrees, which is almost the width of a fist held at arm’s length. Thanks to an effect called precession, it’s moved away from the pole. So Kochab and Pherkad serve as guardians of the pole.

The stars stand to the right of Polaris at nightfall, and wheel high above the Pole Star later on. Kochab is the second-brightest star of the little bear, shining just a touch fainter than Polaris.

Script by Damond Benningfield

Double Destination Apr 25, 2025

A Chinese spacecraft that’s scheduled for launch as early as next month has a double destination: a “quasi-moon” of Earth and an asteroid that acts like a comet.

The first destination for Tianwen-2 is an asteroid, Kamo’oalewa. It’s a chunk of rock no more than the length of two or three football fields. What makes it intriguing is that it weaves around the Sun in a pattern that makes it look like a satellite of Earth.

The asteroid spends half of its time farther from the Sun than Earth is, the other half closer to the Sun. Seen from Earth, it appears to loop around our planet – like a moon. Some research suggests it was a chunk of the Moon that was blasted into space by a big impact.

Tianwen-2 is scheduled to arrive at Kamo’oalewa next year. It’ll spend a year traveling along with the asteroid. It’ll drop off a small lander and rover, and collect a few ounces of rocks and dirt.

The craft will swing by Earth to drop off the samples, then journey to 311P Panstarrs. The object is a third of a mile wide, and orbits the Sun at about twice Earth’s distance.

Observations reveal that Panstarrs is rocky, like an asteroid. But soon after it was discovered it sprouted several long tails, so it was classified as a comet. It may be a loosely bound pile of rocks and dust. If so, it may sometimes lose some of the dust, and sunlight then pushes it away – giving this asteroid the tails of a comet.

Script by Damond Benningfield

Moon and Companions Apr 24, 2025

A beautiful triangle decorates the dawn sky tomorrow. Two of its points are easy to see: the crescent Moon and the planet Venus, the “morning star.” The final point is a bit tougher: the planet Saturn, to the upper right of the Moon. It’s just half of one percent as bright as Venus, but its proximity to the brighter bodies will help it stand out.

Saturn is about as faint as it ever looks right now. One reason is that it’s just emerging from behind the Sun, so it’s about as far as it ever gets from Earth – more than 950 million miles.

The other reason is our view of Saturn’s rings. The rings span almost the distance from Earth to the Moon, and they’re made mainly of small bits of ice, which reflect a lot of sunlight. But the angle at which we see the rings changes – a result of Saturn’s changing seasons.

Saturn is tilted at about the same angle as Earth. At the solstices, one of Saturn’s poles dips toward the Sun, so we see the rings at their best angle. That makes the planet especially bright.

At the equinoxes, though, the rings are “closed” – we see them edge-on. The rings are no more than a few hundred feet thick, so they all but disappear. That makes the planet look much fainter than average.

Saturn will reach an equinox in early May, so the rings have basically vanished. They’ll slowly tilt back into view over the coming months, and reach their greatest angle at Saturn’s winter solstice – in 2032.

Script by Damond Benningfield

Moon and Venus Apr 23, 2025

Two bright crescents team up in the early morning the next couple of days: the Moon and the planet Venus – the brilliant “morning star.” They’re quite low as twilight begins to paint the dawn sky.

The Moon is an obvious crescent. The Sun illuminates only about one-sixth of the lunar hemisphere that faces our way – the crescent. It’s nighttime across the rest of the lunar disk.

It’s not especially dark, though. That’s because that part of the Moon is awash in earthshine – sunlight reflected off of Earth. Earth is much bigger than the Moon, and it’s more reflective. And it’s close to full as seen from the Moon, so the landscape on the nighttime part of the Moon would be much brighter than a moonlit night here on Earth.

Venus is a crescent as well. The Sun is lighting almost a quarter of the planet’s Earth-facing hemisphere. But you can’t see Venus’s phase with the eye alone – you need good binoculars or a telescope to see the shape.

Venus is fairly close to Earth now, so it’s just about as bright as it ever gets. It appears larger than average, and it’s completely covered by clouds that reflect most of the sunlight that strikes them. Over the next few months, Venus will move farther away from us. But the Sun will light up more of the hemisphere that faces Earth. So Venus will remain a bright presence in the morning sky well into fall.

We’ll have more about Venus and the Moon tomorrow.

Script by Damond Benningfield

V404 Cygni Apr 22, 2025

The story of a black hole in Cygnus, the swan, is like the tale of an angler – there’s the star it caught, and the one that got away.

V404 Cygni is about 7800 light-years away. It appears fairly close to the star that marks the intersection of the swan’s body and wings, which is high in the sky at dawn.

The system was first noticed in 1938, when it flared up – a performance it’s repeated several times.

Later, it was discovered that V404 Cygni is a tight binary: a black hole about nine times the mass of the Sun, plus a “normal” star a little less massive than the Sun.

The black hole is pulling gas from the companion. The gas forms a disk around the black hole. Every couple of decades, so much gas piles up that it sets off an explosion. That makes the system shine thousands of times brighter.

Recently, astronomers found that what looked like a background star probably is bound to the other two. It’s more than 300 billion miles from them. It’s bigger and heavier than the Sun, and is puffing up to become a giant.

This third star is about four billion years old – suggesting that the black hole is also that old. The black hole is the corpse of a massive star. Most such stars explode, hurling away any companions. This star must have collapsed completely, without exploding. Since then, it’s consumed at least half of its close companion – but the distant one got away.

Script by Damond Benningfield

Distant Light Apr 21, 2025

The most amazing object visible through a small telescope doesn’t look all that remarkable. In fact, it looks like a faint star. Yet that point of light packs the power of 10 trillion Suns – the power of a quasar.

3C 273 is the first quasar ever discovered. When astronomers first saw it, they thought it was just another star. It looks like a star, and its main ingredient is like a star’s as well. But when they measured its distance, they were astonished: 3C 273 was two and a half billion light-years away. That meant it couldn’t be a star at all. Instead, they classified it as a quasi-stellar object – a quasar.

Astronomers eventually figured out that it’s powered by a black hole at the heart of a giant galaxy. The black hole is about 900 million times the mass of the Sun. Its enormous gravity pulls in huge amounts of gas and dust, and maybe some stars. That material forms a spinning disk around the black hole. The disk is heated to millions of degrees, so it shines brilliantly – bright enough to see from two and a half billion light-years away.

3C 273 is in Virgo. The constellation’s brightest star, Spica, is low in the southeast at nightfall. The quasar stands high above it, about a third of the way up the sky. Despite its great power, it’s too faint to see with the eye alone. But a telescope reveals this deceptive wonder – a monster masquerading as a star.

Script by Damond Benningfield

Lyrid Meteors Apr 20, 2025

The mild nights of spring are good times for skywatching. Only one thing is missing: a great meteor shower. The best showers are clustered in fall and winter, with the Perseids of August sometimes joining the list.

Although the season doesn’t offer a great shower, a pretty good one should reach its peak tomorrow night: the Lyrids. Under a dark sky, you might see up to a couple of dozen meteors per hour between midnight and dawn. The number of meteors increases closer to dawn, as your part of Earth turns more directly into the meteor stream. Unfortunately, by then the waning Moon will be in the sky, so its light will compete with the fainter meteors.

One good thing about meteors, though, is that you don’t have to wait for a shower to see them.

A shower occurs when Earth passes through a stream of small bits of dust and rock shed by a comet or asteroid. There are many showers through the year, but only a few are noticeable.

But bits of rocky debris are scattered throughout the solar system. So on any dark night, you can see several meteors zipping across the sky. And these “random” meteors can come from any direction and blaze across any part of the sky.

So if you have a chance, look for the Lyrid meteor shower in the wee hours of Tuesday morning. If not, then take advantage of just about any clear, dark night to look for meteors flashing across the heavens.

Tomorrow: a steady light far across the galaxy.

Script by Damond Benningfield

More Messier 3 Apr 19, 2025

The Sun is four and a half billion years old – a third of the age of the universe. Compared to the stars in globular clusters, though, it’s a youngster. Those stars were born when the universe was young.

An example is Messier 3 – a family of about half a million stars. The cluster is about 11.4 billion years old. That means its stars were born just a couple of billion years after the Big Bang.

All of the cluster’s remaining original stars are fainter and less massive than the Sun. Anything heavier than the Sun has exhausted its nuclear fuel, leaving only a corpse – a small, dense, faint remnant. And astronomers have seen quite a few of these remnants in M3. But the dead stars are tough to find inside the crowded cluster.

Some of the stars of Messier 3 are more massive than the Sun. But they weren’t born that way. Instead, they’ve grown by cannibalizing companion stars.

Some of them have pulled gas off the surface of a companion. Others have merged with a companion. Either process adds to a star’s mass. That makes the star brighter and bluer – making it look a lot younger that it really is.

This huge cluster of ancient stars is about 34,000 light-years away. It’s in the constellation Canes Venatici, the hunting dogs. M3 isn’t quite bright enough to see with the eye alone, but it’s an easy target for binoculars. It’s well up in the east at nightfall, above the bright star Arcturus.

Script by Damond Benningfield

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