StarDate

More than 1.1 billion years ago, a pair of black holes staged a violent merger. As they spiraled inward, the black holes produced an outburst of gravitational waves – “ripples” in spacetime that rang across the universe. Detectors on Earth “heard” those ripples in January of last year. In fact, it was the loudest and clearest detection of merging black holes to date. Analyzing the signal has told scientists quite a bit about black holes, and about the laws of gravity that govern them. The frequency and duration of the gravitational waves revealed details about the black holes. It showed that when they merged, each of them was spinning. And each was about 33 times the mass of the Sun. But the total mass after they merged was only about 62 times the Sun’s mass – less than the combined weight of the individual black holes. The rest of their mass was converted to energy – mainly the gravitational waves. The aftermath of the merger was important as well. The merged black hole vibrated like a ringing bell. As it settled down, the “ringing” faded away. How it faded matched predictions made by General Relativity – the theory of gravity introduced by Albert Einstein and refined by many others over the decades. It was the strongest evidence to date that General Relativity really is the rule that governs black holes – and sends gravitational waves rippling across the universe. Script by Damond Benningfield

For a trip that’s out of this universe, just cross the event horizon of a black hole. Nothing that passes through an event horizon can ever come back out, so we don’t really know what goes on inside a black hole. But we can be pretty sure that it’s like nothing else in the universe. A black hole’s mass is concentrated in a single point, called a singularity. Its gravity is infinitely strong. But as the distance from the singularity increases, its grip weakens. Eventually, it reaches a point where the escape velocity equals the speed of light – the event horizon. Since nothing can travel faster than light, anything that falls through the horizon is trapped. It may be doomed to merge with the singularity. So the event horizon acts like the “surface” of a black hole. But it’s not solid – there’s nothing to ram into. Instead, it’s more of a boundary between the black hole and anything outside it. The distance between the singularity and the event horizon marks the size of the black hole. And as more stuff falls in, the black hole gets bigger. A black hole that’s 10 times the mass of the Sun spans about 35 miles. The supermassive black hole at the heart of the Milky Way spans 13 million miles. And the heaviest black holes yet seen are more than 40 times the size of the orbit of Neptune, the Sun’s outermost major planet – a wide entrance to an out-of-this-universe experience. More about black holes tomorrow. Script by Damond Benningfield

Comet Halley’s loss is Earth’s gain. As the comet orbits the Sun, it sheds a bit of ice and dirt from its surface. That debris spreads out along the comet’s path. Earth passes close to that path twice a year. Some of the solid particles ram into our planet, adding a minuscule amount to Earth’s mass. For skywatchers, the intersection creates two meteor showers, as the comet dust vaporizes in the atmosphere. And one of them is under way now: the Eta Aquariids. The shower’s peak lasts for several nights, centered around tomorrow night. At its best, the shower can produce a few dozen meteors per hour. Halley is a chunk of rock and ice about seven miles in diameter. On average, it orbits the Sun once every 76 years, although that period varies by a few years. It’s been recorded in Earth’s night sky for more than 2,000 years. Edmond Halley linked some of those appearances in 1705, demonstrating that a comet can return to view multiple times. Halley also predicted the comet’s next appearance, in 1758. When it showed up at the time he forecast, the comet was named in Halley’s honor. Over the centuries, the comet’s orbit moves away from Earth a bit. Today, we’re several million miles from that path. As the orbit shifts away, we pick up less and less of the comet dust. That makes the meteor showers less impressive. So over time, the Eta Aquariids will slowly die out. Script by Damond Benningfield

Antares has played a big role in the skylore of many cultures. And it’s not hard to understand why. It’s quite bright, it has a fiery orange color, and it’s near the ecliptic – the Sun’s path across the sky. The Moon and planets are close to the ecliptic as well, so they periodically swing past Antares. In fact, the Moon snuggles quite close to it late tonight. In western skylore, Antares represented the heart of Scorpius, the scorpion. After Orion the hunter bragged that he could kill any beastie on Earth, the angry gods sent the scorpion to sting him to death. They then put Orion and the scorpion on opposite sides of the heavens, so one rises as the other sets. Antares and the surrounding stars also represented a scorpion in the mythology of the Maya and several other cultures. But others saw Antares differently. In China, it was the “fire star” – a description of its color. It and a couple of nearby stars represented the heart of a dragon. And in Hawaii, Antares was part of a fishhook used by the god Maui. The star itself is worthy of its reputation. It’s a dozen or more times heavier than the Sun, hundreds of times wider, and tens of thousands of times brighter – a supergiant star with some supergiant stories. Antares is just a skosh away from the Moon as they climb into good view tonight, by midnight. They’ll still be close as dawn twilight erases the scorpion’s mighty heart from view. Script by Damond Benningfield

The Sun faces a “degenerate” future. That’s not a value judgment – it’s physics. When the Sun can no longer produce nuclear reactions, its core will collapse. How far it collapses is limited by a type of pressure exerted by its atoms – degeneracy pressure. Today, the Sun is “fusing” atoms of hydrogen to make helium. When the hydrogen is gone, it’ll fuse the helium to make carbon and oxygen. But the Sun isn’t massive enough to extend that process, so its nuclear furnace will be extinguished. Fusion releases energy, which balances the pull of gravity. That keeps the Sun puffed up. Right now, it’s big enough to hold a million Earths. When fusion stops, gravity will win out. The core will shrink to the size of Earth itself. But it’ll still be about half as heavy as the present-day Sun. So a chunk the size of a sugar cube would weigh a ton. The dead core won’t shrink beyond that. That’s because the electrons in the core will exert their own pressure – degeneracy pressure. They can be squeezed only so much before they run out of “elbow room” and halt the collapse. That will leave a white dwarf – a dead cosmic cinder – to cool and fade over the eons. The galaxy is littered with white dwarfs, but none of them is bright enough to see with the eye alone. The closest one is a companion of Sirius, the brightest star in the night sky, which is low in the southwest as night falls – a star that faces its own “degenerate” future. Script by Damond Benningfield

For centuries, the people of the British Isles marked the beginning of summer not on the solstice, in June, but on May 1st. It’s a cross-quarter day, which comes about half way between a solstice and an equinox. In Scotland and Ireland, the date was known as Beltane. People built bonfires to celebrate the longer days, and held rituals to protect their crops and livestock. And in England, the date became known as May Day. People celebrated with village fetes, and they danced around the maypole. Dancers grabbed ribbons attached to the top of the pole, then circled around it, getting closer with each circuit. Especially tall maypoles were erected in an area of London known as the Strand. The last of these poles was removed 300 years ago. But it found a new life – supporting one of the world’s largest telescopes. The maypole was acquired by Isaac Newton, who had formulated laws of gravity and motion. In April of 1718, he had the pole moved to a park outside London for use by James Pound, an astronomer and clergyman. Pound had the use of a large lens created by another astronomer. The telescope was created by mounting the lens on the maypole. The eyepiece was on the ground, linked to the lens by a long wire. With that telescope, Pound measured the positions of the moons of Jupiter and Saturn. Newton used those observations to calculate the moons’ orbits – measuring a celestial dance around the maypole. Script by Damond Benningfield

The stars are quite literally desirable. That’s because the roots that make up the word desirable mean “to long for a star, heavenly body, or constellation.” Astronomy has a rich vocabulary: star, planet, galaxy, and many other words. Many of them also have non-astronomical meanings. A “galaxy of stars,” for example, might refer to an auditorium full of actors – though how many of them can be considered “stars” is a matter of opinion. Some words with heavenly connections seem obvious. “Lunatic” refers to the Moon. It comes from an ancient belief that the Moon’s influence could make people behave strangely. And “jovial” – to be full of good cheer – means “of Jupiter;” in ancient astrology, the planet was thought to exert a happy influence. Other words have more surprising connections to the stars. Consider “consider.” Its roots mean “to observe the stars.” “Sider” is from a Latin word that means “star, heavenly body, or constellation.” In fact, many words with some version of the root have a link to the stars – including desire. Disaster also comes from ancient astrology. It meant an unfavorable position for a star or planet. “Aster” was a Latin word for star. The word “influence” appeared in the 14th century. Dictionaries say it meant “streaming ethereal power from the stars when in certain positions, acting upon the character or destiny of men” – a good description of modern-day “influencers.” Script by Damond Benningfield

To the eye alone, Spica is one of the 15 brightest stars in the night sky. And it really is brilliant. At visible wavelengths, it’s about 2,000 times brighter than the Sun. It looks white with a hint of blue. When you look at Spica with special instruments, though, it’s even more impressive. It consists of two stars, not one. Both are much bigger and heavier than the Sun. And when you add up all wavelengths of light, they shine about 20 thousand times brighter than the Sun. Most of that energy is in the ultraviolet – wavelengths that are too short for the human eye. Spica’s two stars produce so much of it because their surfaces are tens of thousands of degrees hotter than the Sun’s. In fact, the type of energy a star emits depends almost entirely on its surface temperature. And so does the star’s color. To the eye alone, the hottest stars look blue. But they emit huge amounts of ultraviolet. The coolest stars look orange or red. They emit huge amounts of infrared light – wavelengths that are too long for the human eye. Stars in the middle are white or yellow. They emit most of their light at visible wavelengths. So with a star like the Sun, we see most of the energy it produces – light that’s just right for the human eye. Spica is quite close to the Moon as darkness falls this evening. The Moon will slide away from the star during the night, but they’ll still be close as they set, around dawn. Script by Damond Benningfield

Aldebaran is like a reverse time capsule. Instead of preserving mementos from the past, the star shows us what we can expect in the distant future – the far, far distant future. It’s in a phase of life that the Sun will pass through in several billion years. Aldebaran marks the eye of Taurus, the bull. It’s low in the western sky as evening twilight fades. It’s a little to the left of Venus, the brilliant “evening star.” The Sun is in the prime phase of life. It’s steadily “fusing” the hydrogen atoms in its core to make helium. That produces the energy that makes our star shine. Aldebaran has already completed that phase. Its core has converted the hydrogen to helium. Now, the star is fusing the hydrogen in a thin layer around the core. This layer is especially hot. Its radiation pushes outward on the surrounding layers of gas. That’s caused Aldebaran to swell to about 45 times the Sun’s diameter. And that’s made the star more than 400 times brighter than the Sun. Over many millions of years, Aldebaran will use up that shell of hydrogen. Nuclear fusion will then fire up in the helium core, briefly making the star even bigger and brighter. After that, fusion in the core will begin to shut down. Aldebaran’s outer layers will blow away, briefly forming a colorful bubble. As the bubble dissipates, only the star’s now-dead core will remain – a final memento of a once impressive star. Script by Damond Benningfield

A giant star in a galaxy more than a billion light-years from Earth died a spectacular death. Then it might have died again – an event that was even more spectacular than the first. The double demise earned it a doubly impressive title: a superkilonova – two powerful explosions from a single star. The system was discovered last August. It produced a huge outburst of gravitational waves – ripples in spacetime. Astronomers first thought it was a kilonova – the violent merger of two super-dense corpses known as neutron stars. Such mergers produce huge amounts of the heaviest elements in the universe, including gold, platinum, and uranium. After a few days, though, the event began to look more like a type of supernova – the explosion of a star much heavier than the Sun. But as astronomers followed the outburst with a dozen telescopes on the ground, one team suggested that it might have been both. The supernova came first. The massive star’s core collapsed to make a neutron star. Its outer layers then blasted into space. But the collapsing core might have split apart to make two neutron stars, not one. Or the second neutron star might have come together from debris around the first one. Either way, the tiny but massive neutron stars quickly spiraled together. That set off the second blast – a kilonova. There are other possible explanations for the object. But for now, a superkilonova tops the list. Script by Damond Benningfield

A pair of ancient but faint constellations flows across the southern evening sky at this time of year. The two of them share a common story, which also involves a third constellation. Corvus and Crater have been around for more than two millennia. Their story, in fact, comes from ancient Greece. According to the myth, the god Apollo sent Corvus, the crow, to fill a cup – known as a crater – with water from a nearby spring. On the way, the crow saw a tree filled with unripe figs. Instead of fetching the water and coming straight back, he waited for the figs to ripen. When they did, he gorged on them. Corvus knew that Apollo wouldn’t be happy with him. So he filled the cup with water, then grabbed a water snake in his talons. He brought both back to Apollo, and blamed the snake for blocking his way. But Apollo wasn’t fooled. Instead, he was angry and vengeful. He cast crow, cup, and snake into the heavens, forming three constellations. As extra punishment, he decreed that the crow would suffer from thirst – with the water-filled cup forever just out of reach. The constellations are in the southeast at nightfall now. Corvus contains four moderately bright stars that outline the shape of a sail. Crater, to the upper right, looks like a goblet – but you need really dark skies to see it. Both of them sit on the back of poor Hydra, the water snake – an innocent victim of the wrath of the gods. Script by Damond Benningfield

As seen from the eastern United States, there’s a “now-you-see-it, now-you-don’t” event in the early evening sky. The Moon will occult Regulus – passing in front of Leo’s brightest star and blocking it from view. The star will remain hidden for a few minutes. But its disappearance is almost instantaneous: Regulus is there one second, then gone the next. It does take a tiny fraction of a second for the Moon to cover the star. Astronomers make precise measurements of that timing. The length of time it takes a star to vanish reveals its apparent diameter – how big it looks in our sky. And that’s how the first good measurement of the size of Regulus came about. In 1933, a French astronomer recorded an occultation of the star on a rapidly spinning photographic plate. That told him how long it took Regulus to disappear. From that, he calculated the star’s apparent diameter. And he was close to the modern value. When astronomers combine that number with a star’s distance, they can calculate its true diameter. Regulus is 79 light-years away – and about four times the diameter of the Sun. Tonight’s occultation is best seen from the eastern U.S. The Moon and Regulus will be in the sky as seen from the rest of the country as well. But at least part of the event will take place during daylight, when Regulus is too faint to see without help. The star will shine close to the Moon after the occultation ends. Script by Damond Benningfield

It’s hard to ask for a better signpost for finding things in the night sky than the planet Venus. Right now it’s the brilliant “evening star,” low in the west as twilight fades. And it points the way to two other wonders: the planet Uranus and the Pleiades star cluster. The Pleiades is fairly easy to find on its own. Its brightest stars form a tiny dipper shape. In fact, the Pleiades is often mistaken for the Little Dipper. But that dipper is in the north, anchored by the North Star. Despite its prominence, the Pleiades is best appreciated with a technique known as averted vision – seeing it from the corner of your eye. And Venus offers a good chance to try it. Look at Venus, then see if you can see the sparkly cluster to its right. They’re separated by the width of a couple of fingers held at arm’s length. Uranus is about one finger width below Venus. It’s the third-largest planet in the solar system. But it’s so far away that it looks tiny and faint. It’s an easy target for binoculars or a small telescope, though. It looks like a faint star. A telescope reveals something interesting about Venus – it doesn’t look quite complete. That’s because it’s in a gibbous phase. If you watch the planet for months, you’ll see it get thinner and thinner. That’s because Venus will cross between Earth and the Sun in late October. Like the new Moon, it’ll be lost in the Sun’s glare no matter how you look at it. Script by Damond Benningfield

Jupiter’s big moon Europa is one of the most likely bodies in the solar system to host life. The moon has a global ocean that holds more water than all of Earth’s oceans combined. The ocean might have sources of energy and chemical compounds that are needed to support microscopic life. But getting to that ocean won’t be easy. It’s covered by a crust of ice. And a recent study says the ice is pretty thick. The Juno spacecraft scanned part of Europa with an instrument that can probe conditions below the surface. It found that the average thickness of the ice is about 18 miles. That’s thicker than suggested by some earlier studies. Juno found many cracks in the ice. But they don’t penetrate anywhere close to the water. So there doesn’t appear to be a good way to get through the ice to study the ocean. That also could be a problem for any organisms in the ocean. Jupiter’s radiation zaps material on the surface, transforming it into possible nutrients. Without any holes or thin spots in the ice, there’s no direct way to flush the nutrients into the water. But another study found that large concentrations of nutrients could make blocks of ice denser than the surrounding ice. Over time, the heavier blocks could sink all the way through the ice – perhaps helping to sustain any life in Europa’s hidden ocean. Jupiter is high in the west at nightfall, and looks like a brilliant star. The twins of Gemini stand above it. Script by Damond Benningfield

In December of 2024, a region on Jupiter’s moon Io blew its top. Several huge volcanoes were erupting at the same time – the most powerful volcanic event ever seen anywhere in the solar system. The outburst covered an area the size of West Virginia. During the hour that a spacecraft was watching, it produced enough energy to power the entire United States for days. Io is by far the most active body in the solar system. It has hundreds of cones, lava pools, and other volcanic features. They’re powered by a constant tug-of-war between Jupiter and some of its other big moons. They pull and stretch Io’s interior, heating it up. The 2024 eruptions were observed by Juno, a spacecraft that’s orbiting through the Jovian system. The region on Io had been quiet when Juno last looked at it, about two months earlier. So the eruptions must all have started at about the same time. That suggests they were powered by the same source of magma below the surface. The magma must have traveled through a network of underground plumbing, allowing it to power several eruptions at once. So Io’s interior might be like a sponge, with lots of open spaces – that are sometimes filled with molten rock. Jupiter appears just above our moon tonight. It looks like a brilliant star. Through binoculars, Io and Jupiter’s other big moons look like tiny stars quite close to the planet. We’ll talk about one of Jupiter’s icy moons tomorrow. Script by Damond Benningfield

Two planets cross paths in the evening sky this week. One is brilliant, the other a little too faint to see without some help. The brilliant one is Venus, the “evening star.” In all the night sky, only the Moon outshines it, so you can’t miss it. But you can miss Uranus. It’s a giant, but it’s so far away that it’s not easy to see. Several factors control how bright a planet looks: the planet’s distance from both Earth and the Sun, its size, and how much sunlight is reflected from its surface. The clouds that blanket Venus reflect much more sunlight than the clouds of Uranus do. And while Uranus is about four times the diameter of Venus, right now it’s almost 14 times farther. That makes it look smaller in our sky. The distances are also important in another way. The farther an object is from the Sun, the feebler the Sun appears. At their average distances from the Sun, each square foot of Venus receives more than 700 times more sunlight than the same size patch of Uranus. At the same time, the farther an object is from Earth, the less of its light we receive. When you put it all together, Venus looks more than seven thousand times brighter than its giant sibling. Venus blazes into view as twilight fades. Uranus is a couple of degrees to its upper left, and it’s an easy target for binoculars. The two worlds will stand almost side by side on Thursday, just a fraction of a degree apart. Script by Damond Benningfield

A modest meteor shower should be at its best the next couple of nights. You need dark skies to see it – the glow of city lights will erase it from view. And even at its peak, the shower produces no more than a dozen or so meteors per hour. But the Moon won’t get in the way, so if you have good weather and a good viewing spot, it’s worth a look. The Lyrid shower occurs at this time every year as Earth passes through a trail of comet dust – debris from Comet Thatcher. The comet last visited the inner solar system in 1861, and it won’t return for almost three centuries. But each time it approaches the Sun, it sloughs off bits of rock and dirt. They spread out along the comet’s orbital path. When Earth flies through that path, some of the grains ram into the atmosphere at a hundred thousand miles per hour. They vaporize, forming the glowing streaks of light known as meteors. The shower is named for the constellation Lyra, the harp. That’s because its meteors all appear to “rain” into the sky from near Vega, Lyra’s brightest star. They can fly across any part of the sky, though, so you don’t need to be looking at Lyra to see them. The best view comes after Lyra climbs into good view, after midnight. The Moon sets a little later, making the skies nice and dark. That will provide several good hours to watch the meteors – reminders of a comet that’s billions of miles from Earth. Script by Damond Benningfield

The Moon passes through the bull tonight. The bull’s “eye” – the star Aldebaran – is off to the left of the Moon. The bull’s face and shoulder are even closer, represented by a pair of star clusters – the Hyades and the Pleiades. For the most part, you can’t tell the distance to an astronomical object just by its appearance. Something that looks quite bright might be close, but it might also be far away and especially bright. But you can tell something about the distances to the objects around the Moon tonight by their appearance. The Pleiades looks like a tiny dipper close below the Moon. It contains hundreds of young stars, some of which are hot and bright. But the cluster’s small size is a good indication of its distance – almost 450 light-years. The Hyades looks bigger. It forms a letter V that outlines the bull’s face. It looks a good bit more spread out than the Pleiades. But that’s largely because it’s only a third as distant. Aldebaran stands at the top left point of the V. It outshines all the other points. In part, that’s because it’s less than half as far – just 65 light-years away. So these prominent features really do tell us something about their distances. One other bright light stands directly below the Moon in early evening, and it’s the brightest of all: Venus, the “evening star.” Right now, it’s closer to us than anything else except the Moon. Script by Damond Benningfield