StarDate

The bull is charging into the evening sky. Taurus is in full view by about 11 o’clock, low in the east. He stands high in the south before dawn. He’s rising earlier each night, and will be in view all night long by about Thanksgiving. All the stars rise four minutes earlier each night – a result of Earth’s motion around the Sun. Earth makes one full turn on its axis against the background of distant stars every 23 hours and 56 minutes. So, if you looked at the sky every 23 hours and 56 minutes, and you could see through the daytime glare, you’d always see the same stars in the same position. But during that period, Earth moves along its orbit around the Sun. The distance it covers means the planet has to turn four extra minutes for the Sun to reach the same position in the sky. That makes a day 24 hours long. And it also means that the background stars rise and set four minutes earlier on our 24-hour clock. As a result, every star and constellation is in prime evening view at different times of the year. For Taurus, it’s fall and early winter – the time the bull charges across the evening sky. For now, look for Taurus beginning in late evening. Its brightest star is Aldebaran, the bull’s eye. His face is outlined by a V-shaped pattern of stars to the upper right of Aldebaran. And his shoulder is the sparkly little Pleiades star cluster, well above Aldebaran. More about Taurus tomorrow. Script by Damond Benningfield

The Moon is full tonight, and it’s especially bright as well. And to top things off, it’s the most famous full Moon of them all – the Harvest Moon. Harvest Moon is the full Moon closest to the fall equinox, so most years it falls in September. But once every five years or so it skips into October. This year, September’s full Moon came 15 days and 10 minutes before the equinox, which took place on the 22nd. This month’s full Moon comes 14 days, 9 hours, 29 minutes after the equinox, so it barely takes Harvest Moon honors. The Harvest Moon was important in earlier times because it shined over the fields when crops were ready to be brought in. Its light allowed farmers to work into the night. And because of the angle of the Moon’s path at this time of year, the full Moon rises only a few minutes later each night as seen from more northerly latitudes. So it’s almost like having a full Moon for several nights in a row. People often think that the Harvest Moon must be especially bright, but that isn’t usually the case. This year, however, it is. That’s because it comes less than a day and a half before the Moon is closest to Earth for its current orbit – roughly 15,000 miles closer than average. That provides some especially bright nights for farmers – and the rest of us, too. Tomorrow: the bull charges into the evening sky. Script by Damond Benningfield

It’s pretty easy to measure the length of a day on Mars or most other solid bodies. Just pick a feature on the surface and see how long it takes to spin back into view. It’s not so easy for planets that don’t have a solid surface. We can track bands of clouds, but different bands can move at different speeds. That’s been an especially tough problem for Saturn, the second-largest planet in the solar system. Scientists have been trying to pin down its rotation rate – the length of its day – for centuries. When the twin Voyager spacecraft flew past Saturn in the 1980s, they measured the planet’s magnetic field to reveal the rotation rate of its interior. But when the Cassini spacecraft orbited Saturn decades later, its observations showed the day was about six minutes longer. At the end of its mission, Cassini flew between Saturn and the inner edge of its rings. Measuring waves in the rings and tiny changes in the planet’s gravitation field produced yet another length: 10 hours, 33 minutes, and 38 seconds. That’s not necessarily the final answer. Scientists continue to study the giant planet to know how to set their Saturn clocks. And Saturn is in great view tonight. It looks like a bright star quite close to the lower right of the Moon at nightfall, and below the Moon as they set, before dawn. Script by Damond Benningfield

Saturn and Venus bracket the pre-dawn sky now. As Saturn drops from view in the west, Venus nudges into view in the east. Saturn looks like a bright star, while Venus is the brilliant morning star. The planets are both sliding eastward against the background of distant stars. Saturn lined up opposite the Sun a couple of weeks ago. For a few months around that point, the planet looks like it’s “backing up” against the background of stars – a result of the relative motions of Saturn and Earth. Earth is closer to the Sun than Saturn is, so our planet moves faster. It overtakes Saturn every 13 months, making Saturn appear to shift into reverse. It’s actually still moving in its usual direction – only our viewing angle is changing. It’s like passing another car on the highway. For a while, the other vehicle looks like it’s moving backward against the background of buildings and trees. When you move far enough past it, though, it appears to resume its normal forward motion. Saturn will end its backward motion and shift back into forward at the end of November. Venus, on the other hand, is about to pass behind the Sun as seen from Earth, so it’s dropping closer to the Sun every day. That’s also a result of the orbital motions of the two planets. Venus will disappear in the twilight in December, and cross behind the Sun in January – depriving us of the “morning star.” More about Saturn tomorrow. Script by Damond Benningfield

Scientists don’t know what dark matter is. But they have some ideas of what it isn’t. And they took a big step in ruling out some possibilities with the release of a study last year. Dark matter produces no energy – the reason it’s described as “dark.” But we know it’s there because its gravity pulls on the visible matter around it. In fact, it appears to make up about 85 percent of all the matter in the universe. The leading idea says dark matter consists of some kind of subatomic particle. A top candidate is called a WIMP – a weakly interacting massive particle. Although dark matter almost never interacts with normal matter, it might occasionally do so – ramming into the nucleus of a normal atom. That would produce a tiny spark of light, which detectors might see. One experiment is LUX-ZEPLIN. It’s in a former gold mine, almost a mile below the town of Lead, South Dakota. The rock above it blocks other types of particles from reaching the experiment. Its detectors are inside a vat filled with about 8,000 tons of liquid xenon. The hope is that a WIMP will hit a xenon molecule and trigger that spark of light. Project scientists conducted 280 days of observations. And they didn’t find any indication of WIMPs. But their test was the most sensitive yet for certain types of WIMPs. So the experiment rules out some candidate particles – narrowing the possibilities for dark matter. Script by Damond Benningfield

At first glance, the dwarf planet Ceres doesn’t seem like a friendly home for life. It’s small, dark, and scarred by impact craters. Yet a deeper look presents a more optimistic picture. It has more water than any body in the inner solar system besides Earth. It has an abundance of organic compounds – the chemical building blocks of life. And it should be warm enough below the surface to sustain microscopic life. Ceres is the largest member of the asteroid belt – a wide band of debris between the orbits of Mars and Jupiter. It’s about a quarter the diameter of the Moon. It probably consists of a dense core and mantle surrounded by an icy crust. The Dawn spacecraft studied Ceres from orbit a decade ago. It saw big patches of bright, salty minerals. It also saw mountains, including one that’s three miles high; if you scaled Ceres to the size of Earth, the mountain would be 40 miles high. And the craft discovered that much of the surface consists of minerals that formed in a wet environment. So Ceres has water, heat, and organic compounds – the basic ingredients for life in what looks like an unfriendly world. Ceres is at a point called opposition – it lines up opposite the Sun in our sky. That means it rises around sunset and is in view all night. It’s also closest to us at opposition, so it shines at its brightest. Even so, you need binoculars or a telescope to pick it out, in the constellation Cetus. Script by Damond Benningfield

The constellations are well armed. Several of the star patterns that depict people or gods are carrying weapons. And some of them are in good view at this time of year. As darkness falls, look low in the west for the brilliant star Arcturus. It stands at the base of Boštes the herdsman. Like many of the ancient star figures, Boštes has different stories, and is drawn in different ways. In most depictions, he’s holding something long and straight against his right side. In some cases, it’s a staff. But in others, it’s a spear. Well above Boštes is Hercules, marked by a lopsided box of four stars. He’s wrestling the multi-headed hydra. And in some depictions, he’s holding up a club. In the south, look for Sagittarius. To modern eyes, it forms the outline of a teapot. But to the ancients, those stars formed an archer. The star at the outer edge of the spout is the point where he’s gripping both bow and arrow. And low in the northeast there’s a figure with a unique weapon. Perseus the hero is holding the head of Medusa. In mythology, anyone looking at Medusa was turned to stone. Perseus managed to sever the head, then used it to save the princess Andromeda from a monster. And if you’re stargazing before dawn, there’s another armed figure, well up in the south: Orion the hunter. He has two weapons. He’s holding a club in an upraised arm, with a sword strapped to his belt – a heavily armed figure in the stars. Script by Damond Benningfield

The star Fomalhaut is a bit of a disappointment. Almost two decades ago, astronomers announced the discovery of a giant planet orbiting the star – the first exoplanet actually seen at visible wavelengths of light. Almost from the beginning, though, other astronomers questioned the discovery. And they were right. It wasn’t a planet at all, but a big clump of dust – the aftermath of a giant collision. Fomalhaut is about twice as big and heavy as the Sun, and quite a bit brighter. It’s encircled by wide bands of dust. Most of the dust is at least a hundred times the distance from Earth to the Sun. Fomalhaut is only about one-tenth the age of the Sun. Even so, it’s old enough that it should have blown away most of the dust. The fact that the belts are so prominent – especially the outer belt – means that they’re being renewed. The most likely source is collisions between large comets or asteroids. As those bodies are destroyed, they spew dust out into space. One estimate says it would take the destruction of 2,000 comets that are one kilometer in diameter every day to keep the belts going. The would-be planet was the result of a collision between two even larger objects – briefly creating the illusion of a giant planet around this bright star. Fomalhaut is low in the southeast at nightfall, and climbs across the south later on. Script by Damond Benningfield

The southern evening sky is pretty bare at this time of year – lots of dark, empty spaces, but few bright stars. The one notable exception is Fomalhaut. It’s the brightest star of Piscis Austrinus, the southern fish. It’s low in the southeast at nightfall, and arcs across the south later on. The star we see as Fomalhaut is 25 light-years away. It’s about twice as big and heavy as the Sun, and more than 15 times brighter. It’s young – about 10 percent the age of the Sun. And it’s encircled by wide bands of dust, which may contain planets; more about that tomorrow. Fomalhaut has two companion stars – bound to it by their mutual gravitational pull. Both stars are smaller, cooler, and fainter than the Sun. One of them is barely visible to the eye alone, but you need a telescope to see the other. Both stars are a long way from Fomalhaut itself. One is almost a light-year away, while the other is two and a half light-years. Astronomers know they’re bound to Fomalhaut because they’re moving in the same direction and at the same speed. Their composition is similar to Fomalhaut’s as well, and so is their age. Fomalhaut itself will shine for another few hundred million years. But the companions will last much longer – billions of years for the larger one, and hundreds of billions of years for the other. So they’ll still be shining across the galaxy long after the demise of their showy companion. Script by Damond Benningfield

The Andromeda Galaxy, M31, is encircled by dozens of satellites – smaller galaxies in orbit around it. One of the larger satellites is something of an oddball. Of the three-dozen brightest, it’s the only one that lines up on the far side of Andromeda as seen from our home galaxy, the Milky Way. M31 is the closest giant galaxy to the Milky Way – just two-and-a-half million light-years away. Messier 110 is a couple of hundred thousand light-years farther. It’s a few thousand light-years in diameter, and contains about 10 billion stars – a tiny fraction the size of Andromeda. Astronomers have spent years watching M31’s entourage with Hubble Space Telescope. They recently reported that 36 of the 37 brightest members line up on the side of M31 that faces the Milky Way. And that’s hard to explain. The study said there’s only a tiny chance that the alignment is a coincidence – there must be a reason for it. But no one knows what that reason might be. It’s not a result of the Milky Way’s gravitational pull – it’s not strong enough. So there’s no obvious explanation for why M110 is an oddball – lurking on the far side of M31. M31 is low in the northeast at nightfall. Under dark skies, it looks like a hazy slash of light about as wide as the Moon. Through a small telescope, M110 looks like a bright star close by. Script by Damond Benningfield

Earth has only one moon – one large natural satellite. But it might travel with an entourage of Moon chips – bits of the Moon blasted into space by impacts with asteroids. Some of the chips may share Earth’s orbit around the Sun. Others become “quasi”-moons. They weave around the Sun in a way that looks like they’re orbiting Earth. Astronomers have catalogued a dozen or more quasi-moons in recent years. The smallest is the size of a house. The largest is about three miles across. A recent study looked at how easy it would be to make a quasi-moon as the result of an impact. The study team simulated tens of thousands of impacts across the entire Moon, at different lunar phases and with different ejection speeds. The results showed that it’s pretty darned easy. Almost seven percent of the simulations produced objects that share Earth’s orbit. And two percent became quasi-moons. They can remain in stable orbit near Earth for thousands of years before they’re kicked away. A Chinese spacecraft is scheduled to visit one of the quasi-moons next year. It’ll collect a few ounces of dirt and pebbles and return them to Earth for study. That should tell us whether the object is a chip off the ol’ Moon, or an interloper from elsewhere in the solar system. The Moon has a bright companion tonight: Antares, the brightest star of Scorpius. It’s close to the right of the Moon as they drop down the western sky in early evening. Script by Damond Benningfield

The closest giant galaxy to the Milky Way is Messier 31, the Andromeda Galaxy. It’s two-and-a-half million light-years away. But it’s getting closer – by about 250,000 miles every hour. For more than a decade, in fact, it’s looked like the two galaxies were on a collision course. But a recent study says there’s only a 50-50 chance of a collision and merger. And if it does happen, it’ll take place billions of years later than previous estimates. The new study used years of observations by two space telescopes – Hubble and Gaia. Researchers plugged those observations into simulations that also considered the gravitational effects of two smaller galaxies. The results indicated that one of them tends to push Andromeda and the Milky Way together, while the other tends to pull them apart. The researchers ran a hundred thousand simulations. In half of them, Andromeda and the Milky Way flew past each other and went their own ways. In the other half, they eventually spiraled together and merged – but not for at least 10 billion years – twice as long as earlier estimates. The simulations aren’t the final word – there are just too many uncertainties. But for now, it seems likely that the two giants will stay apart for a long, long time. M31 is in the northeast at nightfall. Under dark skies, it’s visible as a hazy patch of light. Binoculars make it easier to pick out. Script by Damond Benningfield

Messier 31, the Andromeda Galaxy, is the largest and most-distant object that’s easily visible to the unaided eye. Under dark skies, it looks like a skinny cloud about as wide as the Moon. Right now, it’s about a third of the way up in the northeast at nightfall. M31 is two-and-a-half million light-years away. In other words, the light you see from the galaxy tonight began its journey across the cosmos two-and-a-half million years ago. The galaxy is roughly 150,000 light-years across – bigger than the Milky Way – and may contain a trillion stars. It’s also the hub of its own galactic empire – it’s orbited by more than three dozen smaller galaxies. And a recent study revealed many new details about the satellites. Astronomers spent years looking at them with Hubble Space Telescope. And they supplemented the new observations by going through older ones. They found that most of the stars in the smaller galaxies had been born by about 12 billion years ago – when the universe was about one-tenth of its present age. And star formation had all but stopped by about eight billion years ago. Galaxies that are bigger and farther from M31 gave birth to stars a little longer than those that are small and close. One of the bigger satellites might have rammed through M31 a few million years ago. That stirred things up throughout the empire surrounding big, beautiful M31. More about M31 tomorrow. Script by Damond Benningfield

People become astronomers for many reasons: They’re interested in the workings of the stars, or the quest to find life in the universe, or the fate of the universe itself. Geoffrey Burbidge joked that he became an astronomer because he married one. He and his wife, Margaret, were astronomy’s power couple. And they co-authored one of the most important studies of the 20th century. Burbidge was born 100 years ago today, in the English village of Chipping Norton. He first studied history, but switched to physics. After earning his undergraduate degree, shortly after World War II, he developed bombs for a while. Back in academia, he married Margaret, and they hopped around England and the United States over the next few decades. Burbidge contributed to many areas of astronomy theory. But he’s best known for a single paper, known as B-squared-F-H for the names of its authors – the two Burbidges, William Fowler, and Fred Hoyle. In it, they explained how stars forge most of the elements in the universe. Many elements are created in a star’s core during its long life; others, in the violent deaths of stars. Some of the elements are expelled into space, where they can be incorporated into new stars. The newer generations make even more elements – eventually creating the chemistry we see in the universe today. So the paper showed that we’re all made of “starstuff” – elements created in the stars. Script by Damond Benningfield

Neptune is one of the giants of the solar system. But it’s so far away that it’s tough to study. We know little about its interior. And much of what scientists think they know comes from lab experiments and computer models. Neptune is the Sun’s most remote major planet. So although it’s almost four times Earth’s diameter, it’s a tiny target for telescopes. And only one spacecraft has ever visited the planet – Voyager 2, in 1989. From those observations, along with those from telescopes on the ground and in space, scientists have developed a model of how Neptune is put together. It probably has a dense, rocky core, surrounded by an “ocean” of super-heated water, ammonia, and methane. The pressure squeezes this layer so tightly that the compounds act like ice. Around that is a layer of hydrogen, which is topped by a methane-rich atmosphere. The methane absorbs red light, giving the planet a blue-green color. It’ll be decades before another mission can approach Neptune. Until then, we’ll have to rely on a lot more calculations to understand this remote giant. Neptune is at its best right now. It’s in view all night and it’s brightest for the year. Even so, you need a telescope to spot it. But you can easily spot its location. As night falls, look for Saturn, which looks like a bright star, low in the east. Neptune is to the left of Saturn, by a bit more than a finger held at arm’s length. Script by Damond Benningfield

Earth “falls” into a new season today – astronomically speaking. It’s the September equinox, when the Sun crosses the equator from north to south. It marks the start of autumn in the northern hemisphere, and spring in the southern hemisphere. On the equinoxes, neither the north pole nor the south pole tips toward the Sun, so night and day are roughly the same length in both hemispheres – about 12 hours between sunrise and sunset. We say “roughly” because there are a couple of caveats. One is the way we calculate the times of sunrise and sunset. For the days to be truly equal, we’d have to mark the times when the Sun is bisected by the horizon – half in view, half still hidden. But we don’t. Instead, sunrise is the moment when the Sun first peeks into view, and sunset is the moment when the limb of the Sun drops from view. That adds a couple of minutes to the day. The other correction factor is Earth’s atmosphere. It “bends” the sunlight above the horizon. So when we see the Sun standing just atop the horizon, it’s actually a little below it. That combination adds a few minutes to the equinox “day.” So at the equator, daylight lasts for 12 hours plus six and a half minutes. At 30 degrees north – the latitude of Austin – it’s 12 hours and eight minutes. And at 60 degrees – roughly the latitude of Anchorage – it’s 12 hours and 16 minutes – an extra dose of sunlight as we fall into autumn. Script by Damond Benningfield

Officially, Saturn has 274 known moons. Un-officially, it has billions upon billions of them – the bits of ice and rock that make up the planet’s rings. They range from the size of dust grains to giant boulders. All of them orbit the giant planet like tiny moons. The system consists of three main bands, which are easy to see. Together, they span about three-quarters of the distance between Earth and the Moon. But there are some thinner, fainter bands as well. One is closer to Saturn than the main bands, while the others are farther. Despite their great span, the rings are quite thin – generally no more than a few dozen feet thick. Individual rings are held in check by the gravity of some of Saturn’s moons and “moonlets” – bodies no more than a few hundred feet in diameter that orbit inside the ring system. In some cases, they force the rings to intertwine like the braids in a loaf of challah bread. Scientists are still debating the age of the rings. Estimates range from a hundred million years to more than four billion. Either way, the rings are constantly replenished with fresh supplies of ice and dust – sustaining one of the most amazing features in the solar system. Saturn is at its best for the entire year. It looks like a bright star, low in the east at nightfall and climbing high across the south during the night. Telescopes reveal the planet’s beautiful rings. Script by Damond Benningfield

Happy Saturn’s Day – the day of the week named for Saturn, the second-largest planet in the solar system. And the name is especially fitting today, because the planet is at its best for the entire year. It looks like a bright star, shining all night long. The seven-day week was created in ancient Babylon. The days were named for the seven known “planets.” The list included the Sun, Moon, and the five true planets that are easily visible to the naked eye. The day was split into 24 hours, with each hour named for a planet. The planets were ranked by how long it took them to cross through the background of stars. Saturn takes the longest, so it was number one on the list. Each day was named for the planet that came up in the first hour of that day. So “Saturn’s Day” was the first day of the week. That changed later on, especially in the Christian era, when the week began a day later, on the Sun’s Day – Sunday. Other than Saturn and the Sun and Moon- Saturday, Sunday, and Monday – old English adopted the planet names from the Norse pantheon of gods. Tuesday is named for Tiw – the representation of Mars. It’s followed by Woden, Thor, and Freya – Mercury, Jupiter, and Venus – celestial names for the days of the week. Saturn is low in the east at nightfall, and looks like a bright star. It climbs high across the south later on, and sets around sunrise. We’ll talk about Saturn’s rings tomorrow. Script by Damond Benningfield

These are the sounds of Mars: a dust devil … a rover trundling across the surface … the steady sigh of the wind. All of these sounds were recorded by the Perseverance rover – the first craft to carry microphones to Mars. Scientists have used the recordings to learn more about how sound carries on Mars. The planet’s atmosphere is less than one percent as thick as Earth’s atmosphere, so it’s much quieter on Mars. It’s especially quiet around noon, when sound waves are bent upward, away from the ground. The atmosphere is also much colder than Earth’s, and it’s made mainly of carbon dioxide. Combined with the air’s low density, on average, sound travels about 30 percent slower on Mars. And there’s a big difference in both the speed and distance at which different frequencies travel. Higher frequencies die out more quickly, and they move slower. So if you wanted to carry on a conversation – if you could survive without a spacesuit, of course – you’d want a nice, deep voice. Mars is disappearing in the evening twilight. From the northern part of the country, in fact, it’s probably too low in the twilight to see at all. From south of about Dallas, it looks like a moderately bright star quite low in the west-southwest as twilight begins to fade – silently dropping from sight. Script by Damond Benningfield

There’s an extraordinary conjunction in tomorrow’s early morning sky – a tight grouping of the Moon, the planet Venus, and the star Regulus. They’re quite low at first light, so you may need a clear horizon to spot them. Venus is the brilliant “morning star,” just a fraction of a degree from the Moon. Regulus is a bit farther from the Moon. It’s much fainter than Venus, but its proximity to the brighter bodies will make it pretty easy to pick out. This beautiful meeting is possible because all three bodies lie near the ecliptic – the Sun’s path across the sky. Regulus, which marks the heart of the lion, is “fixed” in position just half degree a from the ecliptic. It does move through the galaxy, but it’s so far away that it takes centuries to notice any change. Venus’s orbit around the Sun is tilted by about three degrees – about one and a half times the width of your finger held at arm’s length. The planet crosses the ecliptic during each orbit, so it’s always close. On rare occasions, it can even cross in front of Regulus, blocking it from view. That last happened in 1959, and it’ll happen again on October 1st, 2044. The Moon’s orbit around Earth is tilted by about five degrees. So, like Venus, the Moon moves back and forth across the ecliptic. Tomorrow, it’ll be just about one degree from that path – setting up a beautiful conjunction in the dawn sky. Script by Damond Benningfield