StarDate

Pegasus has a bit of an identity crisis. The stars that outline its body – the Great Square – don’t all belong to the flying horse. In fact, the brightest of the four stars officially belongs to Andromeda. The Great Square is in the east at nightfall. Its member stars are all bright enough to see even through moderate light pollution. The square is tilted, so it looks more like a diamond than a square. Its brightest member forms the left point of the diamond – the star Alpheratz. It’s about a hundred light-years from Earth. And it consists of two stars, both of which are a good bit bigger, brighter, and heavier than the Sun. In earlier centuries, Alpheratz was considered a member of both Pegasus and Andromeda – it’s a prominent member of the classical outlines of both constellations. But in the early 20th century, astronomers decided to formalize the constellations. In 1930, they adopted a list of 88 constellations and gave them precise boundaries, like the borders of states or nations. That meant that every star could belong to only one constellation. The way the borders were drawn, Alpheratz stayed with Andromeda. There’s no way to not see the star as part of the Great Square. So unofficially, Alpheratz maintains its dual citizenship: the second-brightest star of Andromeda, and the brightest star of the Great Square of Pegasus. We’ll have more about the flying horse tomorrow. Script by Damond Benningfield

The United States and China both plan to land astronauts on the Moon in the next few years. And scientists from both countries are working on the best landing sites – places that are safe and easy to operate from, but that offer some interesting science. One of China’s possible sites is just north of the equator, near the center of the hemisphere that faces Earth. It offers a range of geologic features, spanning almost all of lunar history. The site is known as Rimae Bode. It’s along the boundary between a smooth volcanic plain – the Sea of Vapors – and a more jumbled region in the lunar highlands. It features ancient lava flows, deep channels, layers of volcanic ash, and material blasted out of a nearby crater. Samples from these regions would help scientists piece together more than four billion years of impacts, volcanic activity, and other events. Planetary scientists are especially interested in the layers of ash, which also contain tiny glass beads. The material might have been blasted from deep below the surface billions of years ago. They would provide details about the Moon’s interior – hard-to-come-by insights into our satellite world. The crescent Moon teams up with the planet Venus in the early evening sky the next couple of nights. Venus is the brilliant “evening star.” But they’re quite low as twilight fades, so there’s not a lot of time to enjoy the view before they set. Script by Damond Benningfield

The roster of constellations includes some weird and wonderful creatures. There’s a dragon, a unicorn, and two centaurs. But perhaps the weirdest of all is Capricornus, the sea-goat – it’s half goat and half fish. It’s associated with the god Pan, who was half goat and half man. The story says that he was about to be attacked by the monster Typhon, so he jumped into the water to escape. At the same time, he tried to transform himself into a fish to speed his getaway. But he botched the spell, and turned his human half into a fish, but kept the half that was a goat. The constellation is low in the southeast as darkness falls at this time of year. Its brightest stars form a wide triangle. None of the sea-goat’s stars is all that bright, though, so you need a fairly dark sky to make them out. The brightest forms the left point of the triangle. It’s known as Deneb Algedi – “the tail of the goat.” It’s actually a system of at least two stars. One is about twice as big and heavy as the Sun, and shines several times brighter. The other is a lot like the Sun. The stars orbit each other about once a day. As they do, each one passes in front of the other for a bit. When the fainter star crosses the brighter one, the system’s overall brightness drops by about a quarter. That’s just enough for a skilled observer to notice with the eye alone – a slight flicker in one of the night sky’s oddest creatures. Script by Damond Benningfield

Young star systems are dangerous places. A planet can be pelted by giant asteroids and comets. It can collide with its siblings, and be kicked out of the system by close encounters with other siblings. It might even be swallowed by its parent star. Astronomers in the U.K. recently found evidence of such encounters in several young stars. The researchers zeroed in on stars in three clusters. All of the clusters are young – from about 40 million to 200 million years old. At that age, their star systems might still be chaotic – planets might be getting blasted or destroyed, and they might still be taking shape. Six stars in the clusters showed especially high amounts of lithium. It’s a common planet-building material. But it’s fragile – stars quickly destroy it. So any lithium in the stars must have been added recently. The most likely source is young, rocky planets. The planets could have been kicked inward by the gravity of other planets, or dragged in by gas and dust around the stars. Over time, the remains churn deep into a star’s interior. But some can linger for a few million years – the remains of dead planets. One of the clusters is Blanco 1. It’s 850 light-years away, in the constellation Sculptor. It climbs into the southeast in mid evening, to the lower left of the bright star Fomalhaut. It’s too faint to see with the eye alone, but it’s a decent target for small telescopes. Script by Damond Benningfield

A planet orbiting a dead star 750 light-years from Earth is a real lemon. The planet is shaped like a lemon – a result of the extreme gravity of its parent star. The system is PSR J2322-2650. The star is a pulsar – the crushed corpse of a once mighty star. It’s heavier than the Sun, but only about as wide as a small city. It spins almost 300 times per second, beaming out “jets” of energy. One of the jets sweeps across Earth, so the star appears to “pulse” on and off. The pulsar is so dense that its surface gravity is about a hundred billion times stronger than Earth’s gravity. And the planet is only about a million miles away – about one percent the distance from Earth to the Sun. At that range, the pulsar’s gravity stretches the planet – making it look like a lemon. The planet is about as massive as Jupiter, the giant of our own solar system. And it has a thick atmosphere made mainly of carbon and helium – a combination that scientists can’t explain. Gamma rays from the pulsar heat the planet’s dayside to 3700 degrees Fahrenheit. So you’d need a lot of lemonade to stay cool on this lemon planet. The system is too faint to see with the eye alone, but it’s easy to pick out its location. It’s low in the southeast by mid-evening, close to the left of Fomalhaut – the only bright star in that region of the sky. We’ll talk about another sight close to Fomalhaut tomorrow. Script by Damond Benningfield

The Sun belongs to a rare class of stars: Class G. Members of this class account for only seven or eight percent of all the stars in the Milky Way Galaxy. A star’s class is determined by its surface temperature, which we see as its color. G stars are in the middle of the temperature scale, so they shine yellow or yellow-white. Most class-G stars are in the prime of life – a span that puts them on the main sequence. They’re steadily “fusing” the hydrogen in their cores to make helium. And most of them will stay in that phase of life for 10 billion years or longer. Most of them are within about 15 percent of the mass of the Sun. Any heavier and they’d get hot enough to move up to class F or higher. Any lighter and they’d be cool enough to move down to class K or M. Main-sequence G stars are only a small fraction of the size, mass, and brightness of the top stars. But the stars in higher classes are even more rare than G stars. So the Sun, which is near the top of its class, shines brighter than about 90 percent of the stars in the Milky Way. Not all G stars are on the main sequence, though. Some are bloated and heavy. Such stars are more massive than the Sun. They’ve burned through the hydrogen in their cores, and are fusing hydrogen in a thin shell around the core. But they’re undergoing a transition, so they won’t stay yellow for long – the short-lived monsters of class G. Script by Damond Benningfield

Under a dark sky, far from city lights, the eye alone can see thousands of stars. They come in a variety of brightnesses and colors. Yet only a few of those pinpoints are like the Sun. Stars are classified based on their surface temperature, which is indicated by their color. Hot stars are blue and white, while cool stars are orange and red. The Sun is in the middle, shining yellow-white, with a surface temperature of about 10,000 degrees Fahrenheit. Those traits mean it’s class “G.” From hottest to coolest, stars are classified with the letters O, B, A, F, G, K, and M. Each class is divided into 10 sub-categories. And each star is given a Roman numeral that indicates its size and its stage in life. The Sun is class G2V. That means it’s toward the hot end of class G, and that it’s on the “main sequence” – it’s in the prime of life, converting the hydrogen in its core to helium. A couple of moderately bright G main-sequence stars are in view by late evening. 51 Pegasi is high in the southeast, while Tau Ceti is just climbing into view far below it, close to the horizon. The brightest class G main-sequence star visible from Earth is Alpha Centauri A. It’s the leading light of a triple star system that’s a bit more than four light-years away – closer than any other stars. It’s so far south, though, that it’s visible from only a tiny sliver of the United States. More about G stars tomorrow. Script by Damond Benningfield

To borrow from an advertising campaign, Jupiter is “getting the red out.” Its enormous “eye” – the Great Red Spot – has been shrinking. It’s now just a third as wide as when it was first seen, and it’s getting smaller all the time. The Great Red Spot is one of the most recognizable features in the solar system. But scientists still don’t know that much about it. They don’t know for sure how it formed, why it’s shrinking, or even why it’s red. They have lots of ideas, but no certain answers. The spot was discovered as early as 1831. There were suggestions that it was seen much earlier, but those sightings might have been a different feature. The spot has been tracked on a regular basis since 1878. It’s between two powerful jet streams. They prevent it from wandering to different latitudes. It tops out several miles above the surrounding clouds, and extends at least 150 miles below the clouds. In the late 19th century, the spot was a long oval – about as tall as Earth, but three times as wide. Today, it’s roughly the same size as Earth. And the rate of shrinkage has been increasing. So it’s possible that it could disappear entirely within a few decades. Look for Jupiter close below the Moon at dawn tomorrow. It looks like a brilliant star. The Great Red Spot is visible through a telescope, but whether it’s visible from a particular location depends on the timing. Script by Damond Benningfield

The Moon anchors a prominent triangle in tomorrow’s early-morning sky. The pattern is well up in the east at first light. The stars Pollux and Castor – the twins of Gemini – line up to the upper left of the Moon, with Mars to the upper right. The brilliant planet Jupiter stands well below the triangle. Mars and Pollux are almost exactly the same brightness right now. And they’re the same color: orange. But they achieve that color in different ways. Mars is a planet – a ball of rock and metal that’s smaller than Earth. Its color comes from iron oxide – particles of rust – in the rocks and dust that cover most of its surface. The rust probably formed when iron-rich rocks interacted with liquid water on the surface. But there’s no water on the surface today. So the rocks must have rusted billions of years ago, when Mars was much warmer and wetter than it is today. As the rocks eroded, the Martian winds carried the dust around the globe – enhancing the color of the Red Planet. Pollux, on the other hand, is a star. It completed the prime phase of life, then puffed up to giant proportions – about nine times wider than the Sun. As it expanded, it got cooler. And a star’s color is determined by its surface temperature; cool stars look red or orange. So just by looking at it, we can tell that Pollux is thousands of degrees cooler than the Sun. We’ll talk about the Moon and Jupiter tomorrow. Script by Damond Benningfield

Some of the most imposing features on Mars are its giant volcanoes. The largest is Olympus Mons. It’s more than 13 miles high, and covers an area as big as New Mexico. It’s part of the largest complex of volcanoes on the planet – a region called Tharsis Ridge. The second-largest group is on Elysium Rise. Its largest member is Elysium Mons. It’s the fourth-highest mountain on the planet. It has an elevation of about 10 miles above the Martian equivalent of “sea level,” and it towers about eight miles above the surrounding plains. Like the other major volcanoes, it’s extinct – or at least dormant. It probably hasn’t erupted in hundreds of millions of years. It formed over billions of years, from layers of lava and ash. It’s marred by many craters. Some of them are impact craters, carved by giant space rocks. Others may be volcanic vents, formed by side eruptions of gas or lava. The volcanoes on Mars have grown so big mainly because there are no plate tectonics. Once a pool of magma forces its way to the surface, it just keeps going – the crust above it doesn’t move away. So there’s no “cut-off” valve – the volcano erupts as long as there’s molten rock below to keep feeding it – building some giant mountains on the Red Planet. Mars appears below the Moon in tomorrow’s early morning sky. It looks like a fairly bright orange star. More about the Moon and its companions tomorrow. Script by Damond Benningfield

Little Red Dots might be like Tootsie Roll Pops: colorful on the outside, dark on the inside. They may consist of a glowing cloud of gas and dust encircling a supermassive black hole. And they could be telling us about the birth of the first big black holes in the universe. Little Red Dots were first seen in 2022, by Webb Space Telescope. Since then, it’s discovered hundreds of them. They’re compact but extremely bright. And they’re so far away that we see them when the universe was no more than about one-tenth of its current age. Astronomers have proposed several explanations for them. One is the idea of a black hole surrounded by gas and dust. A recent study looked at a dot that was behind a huge cluster of galaxies. The cluster’s gravity magnified the view of the dot, making it easier to suss out its details. Its heart is a black hole about 50 million times the mass of the Sun. The surrounding cloud is no more than half that mass. As material in the cloud funnels inward, it gets hot, lighting up the rest of the cloud. The gas and dust absorb blue light, so we see only red. Astronomers have pondered the formation of early galaxies for decades. They’ve wondered whether the giant black holes in their hearts formed first, or if the galaxy came first and the black hole formed later. The new finding suggests that, in at least some cases, the black hole came first – born at the heart of a Little Red Dot. Script by Damond Benningfield

In the past decade, astronomers have “heard” almost 400 mergers between black holes. The signals were carried by gravitational waves – tiny ripples in spacetime. They’ve revealed that some of the black holes probably had undergone earlier mergers – making them third-generation black holes. Gravitational waves are produced by the motions of any object. But the waves are extremely weak. So far, the only ones that have been detected were produced by mergers involving black holes or neutron stars – dense, heavy objects that come together in a fraction of a second. The characteristics of the waves reveal the masses of the merging objects. They also reveal how the objects were spinning, and how they were orbiting before the merger. And those details provide hints to the existence of third-generation black holes. One example was discovered in late 2024. A black hole about 20 times the mass of the Sun merged with one about six times the Sun’s mass. Scientists determined that the heavier black hole probably formed from an earlier merger. They even calculated the details of those black holes: about 7 and 13 times the mass of the Sun. Third-generation black holes probably form in places where lots of black holes are jammed close together, such as the hearts of star clusters. That keeps a merged black hole from escaping – setting up the possibility of more mergers ahead. Script by Damond Benningfield

The Moon charges at the Pleiades late tonight. As seen from most of the United States, it will pass especially close to the star cluster – either just skimming its edge or briefly covering some of its stars. The cluster is home to more than a thousand stars of all varieties. The ones that are visible to the eye alone are especially big, hot, and bright. But they’re greatly outnumbered by stars that are much smaller, cooler, and fainter. Many of the stars belong to systems of two stars or more. And the cluster also hosts many “brown dwarfs” – objects that are more massive than planets, but not heavy enough to shine as true stars. Over the decades, there’s been a lively debate about the distance to the cluster. Different telescopes and techniques have provided measurements that vary by many light-years. The best current number puts the distance at about 440 light-years. But that’s the distance to the center of the Pleiades. The cluster actually spans several dozen light-years in all directions. So as you look at the cluster, the light you see from the stars that are closest to Earth headed our way dozens of years earlier than the light from the stars that are farthest – an out-of-sync view of a well-known star cluster. The cluster’s brightest stars form a tiny dipper shape, although it’ll be tough to make out through the moonlight. The cluster will be especially close to the Moon at dawn. Script by Damond Benningfield

The center of the Milky Way Galaxy is packed with treasures: stars, dead stars, gas clouds, and lots of planets. And a new space telescope will spend a lot of its time sorting it all out. That’s one of several big projects for Nancy Grace Roman Space Telescope, NASA’s next big space observatory. The telescope will see the heavens as clearly as Hubble Space Telescope does. But its view will be much wider. And it’ll study the universe mainly in the infrared – wavelengths that are invisible to the human eye. Roman will tackle some of the biggest problems in modern science. As one example, it’ll look away from the center of the Milky Way to study more than a billion other galaxies, and to look for exploding stars. That combination will help us understand how the universe is expanding – a key for deciphering the mystery of dark energy. The telescope’s survey of the center of the galaxy could reveal thousands of new planets. And a special instrument – using a technique proposed by Nancy Roman herself – could provide images of some of those planets. A planet will appear only as a bright dot, with no detail. But breaking down its light will tell us a bit about the planet’s size, temperature, and composition. The center of the Milky Way is in Sagittarius, which is low in the south at nightfall. Its brightest stars outline a teapot. The heart of the galaxy is in the steam above the spout of the teapot. Script by Damond Benningfield

The planet Venus and the star Spica huddle close the next few evenings. They’re quite low in the west-southwest in early twilight. Venus is the brilliant “evening star.” Tonight, Spica stands close above it. Venus will slide to the left of Spica over the following nights. Venus is named for the Roman goddess of love and beauty. It’s the only major planet in the solar system named for a female character. With a few exceptions, all of the features on its surface are named for women as well – from both mythology and real life. No features were named until the Space Age. Venus is covered by an unbroken blanket of clouds, so we can’t see the surface. Radio telescopes on Earth peered through the clouds in the ’60s, and discovered the first known features. All the other features were mapped by spacecraft in orbit around Venus, which scanned the planet with radar. Today, more than 2,000 features have been named – mountains, craters, canyons, plains, and others. Their names have come from cultures around the world and across the ages. One volcano is named Anuket, for an Egyptian river goddess, while another is named La Shen for a Chinese goddess. Other features are named for Anne Frank, Jane Austen, Pocahontas, and Queen Isabella of Spain, along with other writers, artists, rulers, scientists, and women from many other fields – all commemorated on the planet Venus. Script by Damond Benningfield

If you look carefully at pictures of Saturn, you’ll probably notice something odd about the planet – it looks mashed down, like a beachball that a child is sitting on. The planet is more than 7,000 miles wider through the equator than the poles – only a bit less than the total diameter of Earth. That makes it the “flattest” planet in the solar system. Saturn is the Sun’s second-largest planet, after Jupiter – more than nine times the size of Earth. But it’s much less dense than any other planet. It’s a big ball of hydrogen and helium – the two lightest elements – wrapped around a messy core of rock and metal. Despite its size, Saturn spins in a hurry – its day is less than half as long as a day on Earth. That high-speed rotation pushes material outward at the equator – giving Saturn that “squashed” appearance. That shape affects the planet’s gravity. Saturn’s poles are much closer to the center of the planet than the equator is. And anything at the equator is being pushed outward by the high-speed rotation. The combination means that you’d weigh about a third more at the poles than at the equator – perhaps making you feel more squashed on this giant but squashed planet. Look for Saturn close to the lower right of the Moon as they climb into good view, in mid-evening. It looks like a bright golden star. Tomorrow: famous women on the planet Venus. Script by Damond Benningfield

The world’s top tennis players will spend many hours under the lights over the next two weeks. And thanks to some changes made a couple of years ago, almost all of the light will shine down on them – not into the sky. The U.S. Open is played on 17 tournament courts and five practice courts in Queens, New York. Many of the sessions take place at night. But conventional outdoor lighting directs a lot of light into the sky, producing light pollution. To reduce the glow, the venue replaced its lighting in 2024. The new L-E-D fixtures are shaped and shielded so that almost all of their light shines down onto the courts. The change was certified as “dark-sky friendly” by DarkSky International, a group that’s been encouraging better outdoor lightning for 25 years. Cities, parks, and other places get certified by changing their lighting, and enacting policies designed to keep night skies dark. In all, the group has certified more than 270 sites around the world. And since 2019, it’s certified more than 40 sports venues, most of them in the U.S. – from Texarkana to Waukesha, Wisconsin, and from Seattle to Panama City, Florida. Light pollution does more than just ruin the view of the night sky. It can interfere with the migration of birds, sea turtles, and other animals. It wastes energy and money. And it can hurt people’s health. So reducing light pollution is a winning strategy. Script by Damond Benningfield

The Moon will ply the dark waters of the celestial sea the next few nights – a large region of sky that’s populated by constellations related to water. Tonight, the Moon sails from Aquarius the water bearer into Pisces, the fishes. Pisces is so long that the Moon will remain inside its borders until Monday night. The “sea” consists of six major constellations. Together, they cover almost one-eighth of the entire sky. All of them were created thousands of years ago by cultures around the Mediterranean Sea. The constellations probably were associated with water because the Sun passed across them during the rainy season. Like the open ocean on a moonless night, the entire region is dark – most of its stars are quite faint. The brightest of the lot is Fomalhaut. It’s in Piscis Austrinus, the southern fish. The star climbs into view in the southeast in early evening, and swims across the south during the night. It’s bright on its own, but the lack of other bright lights around it makes it really stand out. Pisces is especially dark. Most of its stars are impossible to see from light-polluted cities, or even the suburbs. And with the almost-full Moon passing through, it’s hard to see any of its stars even from sites that are far away from city lights. So as the Moon moves across the constellation, it looks like it’s floating through an ocean of darkness – the cosmic waters of the celestial sea. Script by Damond Benningfield

A season comes to an end today. It has nothing to do with falling leaves, changing weather, or even big-time sports. Instead, it’s a season of eclipses. It didn’t last long – it started on August 12th, with a total solar eclipse, and it ends tonight, with a partial lunar eclipse. The Moon will be almost completely immersed in Earth’s long shadow, so it’ll turn dark. And at least part of the eclipse will be visible across almost all of North America. An eclipse season is governed by the way in which the Sun and Moon align. Most months, the geometry isn’t right – the Moon and Sun don’t line up the right way, so there are no eclipses at all. But every 173 days, they come into the proper alignment. That produces a solar eclipse at new Moon, and a lunar eclipse at full Moon. They can come in either order, a fortnight apart. And a season lasts for about five weeks – a few days longer than the Moon’s cycle of phases. So if there’s an eclipse near the start of the season, it produces three eclipses. This time, the solar eclipse came a few days after the season began, so we’re limited to two eclipses – including tonight’s. The Moon first dips into Earth’s dark inner shadow at 9:34 p.m. Central Time. The eclipse peaks at 11:13, when the shadow covers 96 percent of the lunar disk. It ends an hour and a half later. Script by Damond Benningfield

The full Moon won’t look quite itself for part of tomorrow night. That’s because it’ll pass through Earth’s long shadow, creating a partial eclipse. At its peak, the shadow will cover all but a sliver of the lunar disk. That will darken the surface, with sunlight that filters through Earth’s atmosphere adding an orange or red tint. Lunar eclipses occur only at full Moon, when the Moon lines up directly opposite the Sun. The Moon’s orbit around Earth is tilted a bit, so most months the Moon passes above or below the shadow. But at least twice a year, the geometry is just right, and the Moon plunges into the shadow. This eclipse is only partial, not total. That means the angle isn’t quite perfect, so the Moon won’t be fully immersed in the shadow. But it’ll be hard to tell much of a difference – the shadow will cover 96 percent of the lunar disk. At least part of the eclipse will be visible from almost the entire United States. It gets started at 8:24 p.m. Central Daylight Time, when Earth’s outer shadow first touches the Moon. It’s so faint that you might not even notice it. But you will notice the partial eclipse, when the Moon enters the dark inner shadow. That starts at 9:34 p.m., peaks at 11:13, and ends at 12:52 a.m. on Friday. More about the eclipse tomorrow. Script by Damond Benningfield