StarDate

Early 1771 was a busy time for Charles Messier. The French astronomer was most interested in discovering comets. To make the process faster, he compiled a list of objects that resembled comets: galaxies, gas clouds, and star clusters. He could ignore the objects on the list, reducing the number of false alarms.

And 250 years ago, he recorded four of them. Two of them are clusters in the constellation Puppis: Messier 46 and 47. They stand side by side — so close together that they’re in the same field of view through binoculars.

M47 is the brighter of the two. It’s about 1600 light-years away, and consists of several hundred stars. The cluster is still pretty young, so some of its more massive stars haven’t yet had time to burn out. They’re especially bright, and they shine blue-white, so they overpower most of the rest of the cluster. A couple of massive stars are at the ends of their normal lifetimes. They’ve gotten bigger and cooler, so they look red or orange.

M46 is fainter than M47 in part because it’s about three times as far as M47. But it’s also because the cluster is about three times as old. Its heaviest and most vigorous stars have already expired, leaving it with fewer standouts to draw attention.

The clusters are in the south as night falls. They’re not far to the upper left of Sirius, the brightest star in the night sky. Messier 46 stands close to the left of its brighter neighbor.

Script by Damond Benningfield

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One of the closest stars in the constellation Puppis has a tight family of big planets. All three of them are between 10 and 20 times as massive as Earth. That makes them comparable to Neptune, one of the giants of our own solar system. And all three are much closer to their star than Earth is to the Sun.

HD 69830 is about 40 light-years away. The star is a bit smaller and lighter than the Sun, and less than two-thirds as bright. So unless you have really sharp eyes and really dark skies, you need binoculars to pick it out.

Astronomers discovered its planets 15 years ago.

The closest planet is just eight million miles away from the star — less than a tenth of the distance from Earth to the Sun. It’s the smallest of the planets as well. The farthest planet is about twice as heavy. And it’s still only a little more than half the Earth-Sun distance. That puts the planet near the inner edge of the star’s “habitable zone” — the region that’s most comfortable for life.

And there’s been plenty of time for life to take hold there. Estimates of the star’s age range up to about 10 billion years — twice the age of the Sun. So if life exists there, it could have been around since long before the birth of our own planet.

Puppis is low in the south at nightfall on these early April evenings. HD 69830 is at its northeastern corner. That puts it well to the left of Sirius, the brightest star in the night sky.

Script by Damond Benningfield

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Zeta Puppis is one of the most impressive stars in the galaxy. It’s big, hot, heavy, and bright. Unfortunately, though, astronomers aren’t sure just how big, hot, heavy, and bright it really is. That’s because they can’t agree on how far away it is. Estimates range from roughly a thousand to two thousand light-years.

Zeta Puppis is the brightest star of the constellation Puppis. It represents the poop deck — the elevated deck at the stern of the Argo, the boat that carried Jason and the Argonauts. The constellation is due south as night falls, just above the horizon.

Zeta Puppis is a class “O” star — the hottest and brightest of all stars. Its details vary depending on its distance. In general, though, it’s a few dozen times the mass of the Sun, and up to a couple of dozen times the Sun’s diameter. And it’s probably at least half a million times the Sun’s total brightness.

The star is moving through space in a hurry — far faster than any of the stars around it. That suggests that it’s a “runaway” — a star that got the boot. It could have had a companion star that exploded as a supernova, for example.

Zeta Puppis is quite young — only a few million years old. Because of its great bulk, though, it will live only a few million years longer. When its time is up, its core will collapse to form a neutron star or a black hole. And its outer layers will explode as a supernova — a brilliant ending for a brilliant star.

Script by Damond Benningfield

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The Argo survived many adventures. In the tale of Jason and the Argonauts, it escaped some terrible storms, the Clashing Rocks, a bombardment of huge stones, and other hazards. But it couldn’t survive the whims of astronomers. They split the Argo apart — turning the largest of all constellations into three smaller ones.

The original constellation was known as Argo Navis — the ship Argo. It was created about 3,000 years ago. According to Greek mythology, it was placed in the heavens by Poseidon, the god of the sea.

Its demise began in the mid-1700s. In a sky atlas, French astronomer Nicolas Louis de Lacaille split it into three constellations: Puppis, the poop deck — the raised deck at the back of the ship; Vela, the sail; and Carina, the keel. Lacaille said the constellation was too big and contained too many bright stars to track.

Although other astronomers began using the new constellations, many still used Argo Navis as well. It came to its end in 1930, when astronomers adopted 88 official constellations. So the stellar Argo met its fate not on the high seas, but on a list compiled by astronomers.

Puppis is the largest of the surviving bits of the Argo. It sits atop the southern horizon as night falls. It’s to the lower left of Sirius, the brightest star in the night sky. Vela follows it a little later. But from most of the U.S., Carina stays below the horizon and out of sight.

We’ll have more about Puppis tomorrow.

Script by Damond Benningfield

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From the late 1600s through the early 1800s, European astronomers went on a constellation binge. They drew dozens of new ones, usually in areas of the sky with few bright stars. Many of their creations are still in use. But many others have vanished.

Johannes Hevelius, for example, drew a few constellations in the late 1600s. One was the northern fly. It was adapted from an earlier constellation, the bee. Another was Cerberus — a three-headed snake in the hands of Hercules. Both of those defunct constellations are low in the west at nightfall, well below the bright star Aldebaran.

German astronomer Johann Bode drew seven new constellations in an 1801 atlas. One honored the electric generator, while another honored the creation of the printing press more than three centuries earlier. Its stars were later absorbed into Puppis, the poop deck, which is in the south at nightfall. None of Bode’s creations is still around. But one of them gave its name to the Quadrantid meteor shower.

Another German, Father Maximilian Hell, drew the constellation George’s Harp in the 1700s. It honored King George III of England, the patron of astronomer William Herschel.

Most of the new creations quickly fell out of use. And in 1930, astronomers adopted a list of official constellations, which dropped many of the creations of the previous three centuries.

We’ll talk about the largest obsolete constellation tomorrow.

Script by Damond Benningfield

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The full Moon has a follower tonight: Spica, the leading light of Virgo. It’s below the Moon as it climbs into good view by 9 or 10 p.m., and closer to the lower left of the Moon at first light tomorrow.

Spica is an amazing system. It consists of two stars, not one. But they’re only a few million miles apart — much, much closer than Earth is to the Sun. In fact, they’re so close together that not even the biggest telescopes can see them as individual stars — they form a single pinpoint of light.

At such a tight separation, the gravity of each star distorts the shape of the other. So instead of balls, both stars are shaped like eggs, with the skinny end of each one pointed toward its companion.

The stars bombard each other with radiation and with winds of charged particles. Where the winds ram into each other, they create X-rays. All of that makes the companion-facing side of each star a good bit hotter and brighter than the rest of the star.

Things will get even brighter before too long. The bigger star is at the end of its normal lifetime. It’s so massive that it’s puffing up to become a supergiant. And at the end of that phase of life, it’s likely to explode as a supernova. It will leave behind a small, ultra-dense core: a neutron star. And the blast should send the smaller companion star careening through the galaxy — the runaway survivor of an amazing star system.

Tomorrow: obsolete constellations.

Script by Damond Benningfield

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One of the biggest families of stars yet discovered rotates down the northwestern evening sky at this time of year. It’s one of the two most populous star systems yet found.

AR Cassiopeia consists of seven stars. They’re split into three pairs, with a third star tied to one of the duos. But they’re not exactly a close family — the pairs are trillions of miles apart.

The most impressive member of the family is identified as star Aa. It’s about six times the mass of the Sun, and five times the Sun’s diameter. It’s paired with star Ab, which is also more impressive than the Sun. Star B orbits them once every 545 years.

The members of the other two pairs are a little smaller. And they’re way off on their own. It takes each pair several hundred thousand years to complete a mutual orbit with the central trio.

That’s really about the only way for such a busy star system to remain stable. If you pack a bunch of stars close together, gravitational interactions among them kick some of them out. So the systems need to be widely spread. And it’s best if they form smaller groupings, like the stars of AR Cassiopeia — one of only two septuple star systems yet discovered.

The system is in Cassiopeia the queen, which is in the northwest at nightfall. Some of its stars form a big letter W, which is tilted on its side right now. AR Cas is below the bottom point of the W. Under dark skies, it’s barely visible to the unaided eye.

Script by Damond Benningfield

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When Galileo looked at the heavens through his early telescopes, he saw many cosmic wonders. One of the most astounding was the planet Venus, which went through a cycle of phases just like the Moon’s.

Venus shows phases because its orbit is inside Earth’s orbit around the Sun. Venus is “new” when it crosses between Earth and the Sun, so its nightside faces our way. It’s “full” when it passes behind the Sun, so its dayside faces our way. And in fact, Venus is passing behind the Sun today — a point called superior conjunction.

But although Venus is full, it’s also hidden in the Sun’s intense glare. It won’t return to view for weeks, when it will appear as the “evening star.”

By then, a telescope will show Venus in its gibbous phase — it’ll be daylight across most of the planet’s Earth-facing hemisphere. But Venus will be near its greatest distance from Earth then, so it won’t shine at its brightest.

Instead, the planet is brightest when it’s in its crescent phase. Although most of the hemisphere visible from Earth then is dark, the planet is especially close, so it looks bigger. And at closer range, more of the planet’s reflected sunlight reaches Earth — making the brilliant planet even brighter.

Look for Venus to reappear late next month and climb into better view in the evening sky in May. And if you have a telescope, keep an eye on the planet as it continues its cycle of phases throughout the year.

Script by Damond Benningfield

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Teams of scientists are digging into the Moon again. They’re nowhere near the lunar surface, though. Instead, they’re opening up samples collected by Apollo astronauts that have never been studied.

The astronauts brought back more than 840 pounds of rocks and dirt. Most of those samples have been studied in some way. But a few were held back. Scientists wanted to wait for new instruments and techniques. They started digging in to a few of those samples in late 2019. And they’ll do a few more over the coming months and years.

The first of the new samples is a core tube collected by astronaut Gene Cernan during Apollo 17, the final Apollo mission. Cernan drilled down about two and a half feet, filling the tube with layers of material that had piled up over millions of years. The core was split into two parts. Scientists opened the lower half first.

The new studies have several goals. One is just to learn more about the Moon itself — about how its surface changed over millions of years, for example. Another is to help set goals for new sample-gathering missions in the years ahead. And a third is to evaluate how well the sample storage systems have worked over the past half-century — helping to get ready to dig in to the Moon once again.

Look for the Moon in the east at nightfall, and arcing high across the southern half of the sky during the night. Regulus, the bright heart of Leo, is just a few degrees away.

Script by Damond Benningfield

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A brilliant visitor to the inner solar system zipped by Earth in March of 1996. Comet Hyakutake was closest to us 25 years ago tomorrow — less than 10 million miles away. That was the closest passage by a comet in two centuries.

The comet had been discovered just two months earlier by an amateur astronomer in Japan. It quickly became visible to the naked eye. And by the time of its closest approach, its tail spanned more than a third of the sky. It was moving so fast, though, that it faded from sight in just a few weeks.

The comet itself wasn’t much — a ball of ice and rock less than three miles in diameter. As it approached the Sun, though, solar energy vaporized some of the ice at its surface. That surrounded the comet with a cloud of gas, as well as dust and pebbles that were also released into space. The Sun’s radiation and the solar wind “blew” that material outward, forming a tail. Later observations showed that the tail spanned about 360 million miles — four times the distance from Earth to the Sun.

Hyakutake came from deep space — far beyond the orbit of Pluto. It hadn’t visited the inner solar system in 17,000 years. While it was here, it got a gravitational “boost” from the giant outer planets. That hurled it even farther from the Sun. As a result, the comet won’t visit the inner solar system again for another 70,000 years.

Tomorrow: Digging in to a scientific treasure box.

Script by Damond Benningfield

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