StarDate

There’s a beautiful conjunction between the Moon and the planet Venus early this evening. Venus is the “evening star” – the brightest object in the night sky after the Moon. The Moon is a thin crescent – the Sun illuminates only a sliver of the lunar hemisphere that faces Earth.

We can’t see it, but the Moon is moving farther from us – by about an inch and a half per year. It’s been moving away since it was born, when Earth was young. In fact, that shift was one of the clues that led to the leading theory of how the Moon was born.

In the chaotic conditions of the early solar system, Earth was walloped by a planet about the size of Mars. That blasted debris into orbit around Earth. Much of that material quickly coalesced to form one or more moons. Today’s Moon is the only survivor.

The collision caused Earth to spin much faster, so a day was much shorter than it is now. Gravitational interactions between Earth and Moon have slowed us down. But they’ve also caused the Moon to slide farther away. The process isn’t smooth – the Moon speeds up and slows down. And it won’t stay smooth in the future.

Given enough time, the Earth-Moon system would reach a point when the same hemisphere of Earth would always face the Moon, and the Moon would stop moving away. But that time may never come. It could be so far in the future that the Sun will have expired – perhaps destroying Earth and its slip-sliding Moon.

Script by Damond Benningfield

Many astronomical discoveries have come in stages – a series of “aha” moments where we learn more about the nature of an object.

A good example is Messier 13, the Great Hercules Cluster. Under especially dark skies, it’s just visible to the unaided eye, so people have known about it forever. It looks like a faint, hazy star. But during the 1700s, the cluster was “discovered” several times.

The first discovery was made by Edmond Halley. Using a small telescope, he came across it in 1714. He described it as “a little patch.” Charles Messier saw it a half-century later. He described it as “round, beautiful, and brilliant.” But, he wrote, “I am sure it doesn’t contain any star.” He made it the 13th object in his catalog.

In 1779, though, William Herschel contradicted Messier. M13 “is a most beautiful cluster of stars,” he wrote.

Many other discoveries have followed. They’ve told us that M13 contains hundreds of thousands of stars packed into a tight ball. And the cluster is ancient – 12 billion years old or older.

Messier 13 is 25,000 light-years away. In early evening, look in the east-northeast for the Keystone of Hercules – a lopsided “square” of stars. M13 is between the two stars at the top of that pattern, a bit closer to the one on the left – a giant cluster that’s still producing amazing discoveries.

Script by Damond Benningfield

Most of the stars in the Milky Way orbit the center of the galaxy in the same direction as all the other stars around them, and at about the same speed.

But a few follow their own paths. An example is a star at the tip of the Guitar Nebula. The nebula is a bubble of gas with an outline that resembles a guitar.

It’s in Cepheus, which is low in the north at nightfall. The king’s brightest stars form an outline that resembles a child’s drawing of a house. Don’t look for the nebula, though – it’s so faint that it wasn’t discovered until 1992.

The guitar was sculpted by a pulsar – the crushed corpse of a mighty star. It spins once every two-thirds of a second, emitting a beam of energy that sweeps past Earth on each turn.

The pulsar was born when the star exploded as a supernova. The explosion must have been off-center, so it gave the dead core a powerful kick. The pulsar is plowing through clouds of gas and dust at almost two million miles per hour. It leaves an expanding wake behind it, like a ship traveling across the ocean.

That wake is what we see as the Guitar.

But there’s more to the nebula than meets the eye. X-ray telescopes in space reveal a long, high-speed “jet.” It’s firing away from the tip of the nebula at a right angle to the nebula itself. The jet most likely is powered by the pulsar’s magnetic field, which funnels charged particles away from the pulsar – an interesting note from a celestial guitar.

Script by Damond Benningfield

The Milky Way Galaxy is home to a few hundred billion stars. And on average, it gives birth to a couple of Sun’s-worth of stars every year. But a much smaller galaxy about 12 million light-years away puts the Milky Way to shame. It is spawning about 10 times as many stars per year.

Like the Milky Way, Messier 82 is a thin disk, with spiral arms wrapping around a dense core. It’s less than half the size of the Milky Way.

M82 is a starburst galaxy. It had a close encounter with another galaxy within the past hundred million years or so. That caused huge clouds of gas and dust to collapse, triggering the starbirth.

The new stars are concentrated in the center of the galaxy, where astronomers have cataloged more than a hundred super star clusters. Each one contains hundreds of thousands of stars. Many of the stars are especially hot and massive, which makes the clusters especially bright.

A strong “wind” of hot gas races away from that region. It squeezes the surrounding clouds, giving birth to more stars. But within another hundred million years, all the gas and dust will have been used up. Then, M82 will settle down to the same quiet life as the Milky Way.

M82 is in Ursa Major. As night falls, it dangles below the upside-down bowl of the Big Dipper. It’s an easy target for small telescopes. We see it edge-on, so it looks like a small, bright slash.

Script by Damond Benningfield

The brightest star of the Southern Cross is like a whole episode of “Dancing With the Stars.” It consists of perhaps six or more stars. They’re all twirling through their own ballroom, linked by the strong hands of gravity.

Alpha Crucis is 320 light-years away. To the eye alone, it looks like a single point of light – the 13th-brightest star in the night sky. But binoculars or a telescope show two stars. Both of them are at least a dozen times as massive as the Sun, and thousands of times brighter. They’re so far apart that it takes about 1300 years for them to complete a single orbit around each other – a slow turn across the dance floor.

But one of those stars is actually two stars on its own. They’re so close together that not even the biggest telescopes can see them individually – the second star reveals its presence only to special instruments. But it’s also bigger, heavier, and brighter than the Sun. The two stars are dancing to a faster tempo – one turn around each other every 76 days.

Those three stars might have three more companions. They’re a long way from the first trio, and they’re not as impressive. But they appear to share a common motion through the galaxy with the brighter trio. That means the two groups could be gravitationally bound to one another – dancing a waltz that would require a hundred thousand years to complete one turn across the floor.

Script by Damond Benningfield

The farther north you live, the less of the universe you can see. Earth itself blocks the view of a large swath of the southern celestial hemisphere. That’s the half of the sky that’s south of the celestial equator – the projection of Earth’s equator. So those of us in the United States miss out on at least part of the southern sky.

One of the treasures we miss is Crux, the southern cross. It’s the smallest of the 88 constellations. But it’s also one of the prettiest and most prominent.

Four of its stars are fairly bright, and they do form a shape that looks like a cross. If you include one more star in the pattern – the faintest of the five – the pattern looks more like a kite. It points the way to the south celestial pole.

Not surprisingly, that pattern has played a big role in the skylore of many southern-hemisphere cultures. Several saw the cross as the footprint of a big bird. Others saw it as a stingray, the anchor of a giant canoe, or some other prominent object or animal.

Today, Crux is featured on the flags of Australia, New Zealand, and Brazil. It’s also on the flag of the European Southern Observatory – which has a great view of the southern cross.

From the United States, Crux is barely visible from the Florida Keys, far-southern Texas, and Hawaii. At this time of year, it’s quite low above the southern horizon in early evening – pointing the way to the celestial pole.

More about Crux tomorrow.

Script by Damond Benningfield

Today, Saturn and its system of moons and rings look calm and peaceful. But things might have been much more chaotic in the fairly recent past. A collision between two moons might have destroyed one of them, changed the orbit of the other, and led to the birth of yet another moon and the planet’s rings.

Researchers have been trying to explain some oddities in the Saturn system. The planet itself is tilted far more than it should be, for example. The biggest moon, Titan, follows a more lopsided orbit than expected. And the moon is moving away from Saturn by about four inches per year.

A few years ago, a team proposed that Saturn once had another big moon, which the team called Chrysalis. Interactions between the moons might have kicked Chrysalis so close to Saturn that it was ripped apart, forming the rings.

But this year, another team came up with a slightly different scenario. It, too, involves a second moon. It collided with Titan a few hundred million years ago, changing Titan’s orbit. Debris from the impact formed the present-day moon Hyperion.

The activity caused two other moons to ram together as well. Both moons quickly re-formed, with the leftovers spreading out to form the rings as recently as 50 million years ago. This model explains many of the system’s oddities – bringing order to a chaotic arrangement.

Look for Saturn near our moon at dawn tomorrow. The planet looks like a bright star, low above the horizon.

Script by Damond Benningfield

Venus might be feeling a bit neglected. The last dedicated mission to the planet wrapped up its work two years ago. A couple of spacecraft have scanned the planet since then, but Venus wasn’t their main target. They were using the planet’s gravity to fling them toward their intended targets.

But Venus exploration could tick up over the next few years. Several missions are being developed. Most of them are big and complicated, so they won’t be ready until the next decade. But a craft the size of a beachball could head for Venus as early as this summer. It’ll probe the planet’s clouds for signs of organic compounds – the chemistry of life.

Venus Life Finder is a project of Rocket Lab and MIT – the first commercially developed mission to the planet. It’s a small, blunt cone. When it arrives at Venus, it will plunge through the planet’s clouds, shining a laser on the way down. The reflected light will reveal details about the cloud particles. Bits of organic matter might be set aglow.

Some recent observations have hinted that the clouds could contain microscopic life. Life Finder won’t actually search for life, but it could tell us if the building blocks of life lurk inside the planet’s clouds.

Venus is the “evening star.” It’s sneaking up on the star Elnath, at the tip of one of the horns of the bull. Tonight, the star is a little to the upper right of Venus. The planet will slip past it during the week.

Script by Damond Benningfield

The eyes of the dragon shine a third of the way up the northeastern sky at nightfall. Eltanin is the brightest star of Draco, with third-ranked Rastaban just above it. They circle high across the north during the night, and stand in the northwest at first light.

Although Eltanin looks brighter than Rastaban, that’s only because of their different distances. Eltanin is about 150 light-years away, while Rastaban is at 380 light-years. So if you lined them up side by side, Rastaban would shine about twice as bright as the dragon’s other eye.

Rastaban is only about 65 million years old, compared to four and a half billion years for the Sun. Yet it’s already passed the end of its “prime” lifetime.

That’s because it’s about six times as heavy as the Sun. Such massive stars “burn” through their nuclear fuel at a frantic rate. So Rastaban has already converted the hydrogen in its core to helium. Now, it’s probably getting ready to ignite the helium to make carbon and oxygen.

That’s caused the star to puff up – it’s about 40 times the diameter of the Sun. That’s the main reason it looks so bright – there’s a lot of surface area to beam light out into space.

Eventually, the nuclear reactions in the core will stop. The star’s outer layers will blow out into space. That will leave only the dead core – a cosmic ember as heavy as the Sun but only as big as Earth – closing one of the dragon’s bright eyes.

Script by Damond Benningfield

A pair of eyes stares down from the northeast as night falls now – the eyes of Draco, the dragon. They’re to the upper left of brilliant Vega, one of the night sky’s most prominent stars.

The brighter eye is the star Eltanin, the dragon’s leading light. The name means “the serpent,” because the star once represented the entire dragon. Eltanin is an orange giant. It’s a good bit bigger, heavier, and brighter than the Sun. That makes it easy to see even though it’s more than 150 light-years away.

The star should get a lot easier to see in the coming millennia. That’s because Eltanin and the Sun are moving closer together. In about one and a half million years, they’ll be at their closest – just 28 light-years apart. Assuming Eltanin hasn’t changed much by then, it’ll be the brightest star in the night sky – about as bright as the current champ, Sirius.

The other eye, Rastaban, is just above Eltanin. Its name means “head of the serpent.” It’s more than twice as far as Eltanin. It, too, is a giant, but it’s much bigger and brighter – a thousand times as bright as the Sun. So it looks only a little fainter than Eltanin despite the extra distance.

The rest of Draco curves to the left and above the dragon’s eyes, and curls around Polaris, the North Star. The eyes stare in the opposite direction – toward Hercules, who killed the dragon before both of them were placed in the heavens.

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

You might want to have the butter and the Mrs. Butterworth’s handy for this one – it’s all about pancakes. Some of them are as big as a major city. There are only two problems: They’re made out of dense volcanic rock, and they’re on the planet Venus.

Venus is covered with many thousands of volcanic features – lava plains, cone-shaped mountains, and structures that look like crowns and spiders. Most of the features are old, but there are hints that the planet is still volcanically active today.

The list of features includes pancake domes. There are scores of them – some by themselves, but many in groups. They’re almost perfectly round and flat. They can be up to a few dozen miles across, and more than half a mile tall. And their edges are steep – almost-sheer cliffs.

The domes probably formed when thick molten rock bubbled to the surface. It spread out in all directions. And it continued to spread well after the lava spigot was turned off.

A study published last year said that some of the pancakes dented the surface below them – perhaps one reason they’re so flat. That dimple created a moat around one of the domes, with a raised rim around the moat – a good arrangement for catching all that butter and syrup.

Venus is the brilliant “evening star.” It’s quite close to the crescent Moon this evening. The Moon will stand above the planet tomorrow night; more about that on our next program.

Script by Damond Benningfield

There’s a frustrating meeting of planets in the early morning sky right now. It’s frustrating because the planets are quite low in the sky in the dawn twilight, so they’re hard to see.

The participants in this meet-up are Mercury, Mars, and Saturn. Tomorrow, they’ll form a tight triangle. They’ll form a straight line on Monday before they begin to separate. Mercury is the brightest of the trio, followed by Saturn, then Mars.

The planets aren’t actually anywhere close to each other – they just happen to line up in the same direction. Mercury is the closest, at a distance of a bit more than a hundred million miles. Mars is more than twice that far. And Saturn is farther still – almost a billion miles.

Mercury is making a small loop across the dawn before dropping back into the solar glare. It’s the closest planet to the Sun, so it never moves far from the Sun in our sky. That means our chances to see it are limited.

Mars and Saturn are farther from the Sun than Earth is, so they move all the way across the sky. Both are slowly working higher into the dawn. As the months pass, they’ll rise earlier and remain in view longer.

For now, Mercury, Mars, and Saturn are quite low in the east as twilight paints the sky. They’re tough to see, especially from farther north. From the U.S., the best views are from places like Miami and Honolulu. The best place to watch the meet-up is the southern hemisphere.

Script by Damond Benningfield

Every time that two or more planets congregate in the night sky, fear mongers crank up the volume on their predictions of doom. They say the combined gravity of the planets will cause everything from earthquakes and storms to boils and hangnails.

Don’t listen to them.

All of the planets are so small or so far away that their short-term effects on Earth are negligible.

Jupiter, the largest and heaviest planet in the solar system, is only one-tenth of one percent as massive as the Sun. And, on average, it’s about five times farther. When combined, those numbers tell us that Jupiter’s gravitational tug on Earth is just one-25,000th as strong as the Sun’s. The pull of the other planets is even weaker. So even if you lined up all of the planets in the same direction from Earth, their combined pull would be insignificant.

That’s not the case on longer terms, though. The gravity of Jupiter and Venus change the shape of Earth’s orbit and the planet’s tilt on its axis. Mars may play a role as well. That influence creates cycles of warmer and colder climate. But the cycles play out over tens of thousands of years or longer – not over days, weeks, or even centuries.

Planetary alignments are common. In fact, there’s one right now. Mars, Saturn, and Mercury are close together in the dawn twilight. But they’re so low in the sky that they’re tough to see. We’ll have more about their alignment tomorrow.

Script by Damond Benningfield

The Coma galaxy cluster is like a cosmic iceberg. What you see is impressive. But what you don’t see is even more impressive.

The cluster is centered more than 300 million light-years away, and it spans 25 million light-years. It contains thousands of individual galaxies. Many of them are far bigger and heavier than our own galaxy, the Milky Way.

But in the 1930s, German astronomer Fritz Zwicky found something odd. He measured the motions of individual galaxies within the cluster. They were zipping along much too fast to be held in check by the gravity of the visible galaxies – they should all fly away from each other.

Zwicky concluded that something else was acting as a sort of gravitational “glue.” He called it dark matter – matter that couldn’t be seen, but that exerted a gravitational pull on the visible matter around it.

It took decades to confirm that finding. And even today, we don’t know what dark matter really is. The leading idea says it’s some type of subatomic particle. But despite many years of searching, no such particle has been found. All we know for sure is that dark matter accounts for about 85 percent of all the matter in the universe – the vast hidden depths of the cosmic iceberg.

The Coma Cluster is in Coma Berenices. The constellation is in the east at nightfall. It’s above brilliant Arcturus, the brightest star of Bootes, and to the lower left of Leo, the lion.

Script by Damond Benningfield

Astronomers love star clusters. All the stars in a cluster were born at the same time, from the same cloud of gas and dust. So any differences in the stars are the result of their evolution, which is a result of their mass. That makes it easier to learn what’s going on inside the stars.

One problem, though, is identifying which stars belong to a cluster. It takes detailed measurements of motion and brightness to separate members of the cluster from stars that just happen to line up in the same direction.

An example is the Coma star cluster, in Coma Berenices. The constellation is in the east at nightfall. Under dark skies, the cluster is a good target for binoculars.

The cluster is about 280 light-years away. But it spans dozens of light-years, so its stars are spread out. That makes it harder to pick out its members. And it takes big telescopes to pick out its fainter stars.

So despite decades of study, astronomers are still locking down the census of stars in the Coma cluster. A study about a decade ago confirmed eight small, faint members – the first of their kind known to belong to the cluster. And another study found that about a quarter of the stars in the cluster are binary or multi-star systems.

These discoveries bring the total number of stars in the Coma cluster to several dozen, with a few dozen more possibilities – members of a wide-spread stellar family.

More about Coma Berenices tomorrow.

Script by Damond Benningfield