StarDate

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

As the roster of known planets in other star systems grows, astronomers learn a lot more about planets and stars in general. One key finding is that almost all stars are still infants when their planets form.

A planet is born from a disk of gas and dust around the newborn star. Solid particles stick together to form larger and larger bodies. They eventually form a chunk of rock and metal that can be much more massive than Earth. If these embryos are far enough from the star, where conditions are cold, they may sweep up vast amounts of leftover gas to form giant worlds.

Observations of infant stars and planets suggest that most planets take shape within 10 million years or so. By then, the star’s radiation and winds have cleared away most of the raw ingredients for making planets. That leaves only the planets and some leftover “building blocks.” The gravity of the giant planets kicks many of the building blocks out of the system. But others remain, forming asteroids and comets.

The planetary system isn’t in its final form by then. Gravitational interactions can cause a planet to move toward or away from the star, change the angle of its orbit, or even get kicked out of the system. And the asteroids and comets can pound the young planets – perhaps giving birth to moons.

So while planets form quickly, they continue to evolve throughout the lifetime of their star system.

Script by Damond Benningfield

Of the thousands of planets that have been discovered in other star systems, only a few orbit stars that are easy to see with the eye alone. Most of the stars are so faint or so far away that they just fade into the firmament.

One exception is Hamal, the brightest star of Aries, the ram. It climbs into view, in the east-northeast, by mid-evening. It’s about 66 light-years away, but it’s bright enough that it’s easy to see from all but the most light-polluted cities.

Hamal is so bright because it’s completed the “prime” phase of life – a phase known as the “main sequence.” Now it’s in the next phase, as a giant. It’s puffed up to about 15 times the diameter of the Sun, making it much brighter.

Hamal has one confirmed planet – Hamal b. It, too, is a big ‘un – it’s almost twice as massive as Jupiter, the giant of our own solar system. But the planet is only a little farther from Hamal than Earth is from the Sun. So every square inch of the surface receives about 10 times more energy than the same-sized patch of Earth.

As a result of that bombardment, Hamal b is extremely hot. That makes it highly unlikely to host any form of life – at least not like any life on Earth. And if it ever did have life, it probably perished as Hamal expanded and brightened. So Hamal b isn’t a good place to go hunting for neighbors.

We’ll have more about planets in other star systems tomorrow.

Script by Damond Benningfield