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Star Date
Daily at 6:19 & 8:19 am, and 4:18 pm.

KMUW is proud to air StarDate, which debuted in 1978, making it the longest-running national radio science feature in the country. The show has been hosted by Billy Henry since July 2019.

StarDate tells listeners what to look for in the night sky, and explains the science, history, and skylore behind these objects. It also keeps listeners up to date on the latest research findings and space missions. And it offers tidbits on astronomy in the arts and popular culture, providing ways for people with diverse interests to keep up with the universe.

StarDate is a production of The University of Texas McDonald Observatory. The feature is available here the day after it airs.

  • Aiello del Friuli is a village in northern Italy, about 75 miles from Venice. It’s home to about 2200 residents and more than a hundred sundials — so many that it’s known as the “village of sundials.” It hosts a sundial festival every year, in late May. The village began earning the nickname in 1994. A resident created a sundial and hung it on the side of his house. It was more than just a pointer and some numbers, though — it was a work of art. So others in the village asked him to make sundials for their homes. He obliged. And before long, other artists got into the act. Today, the sundials are spread all across town. There are more than 20 in a plaza next to a museum, but they’re in all the neighborhoods as well. Like the original, many of the sundials are mostly two-dimensional and are displayed on building walls. But some are more three-dimensional — globes, bowls, and other shapes mounted on their own. The sundials follow many artistic themes. Some have religious or historic significance. Others depict nature or agriculture, or have a more abstract quality. And the bounty keeps on growing. The annual festival includes a contest for the best sundial created during the previous year. All of the sundials also do a job: They track the time as the Sun crosses the daytime sky in the “village of sundials.” Tomorrow: the head of the serpent wriggles into the evening sky. Script by Damond Benningfield
  • Black holes are the darkest objects in the universe — they produce no light at all. Yet they power some of the brightest objects. Known as quasars, these beacons can outshine entire galaxies of hundreds of billions of stars. In fact, the brightest one seen so far emits more light every minute than the Sun will produce in its entire 10-billion-year lifetime. A black hole’s gravity is so powerful that nothing can escape from it — not even light. But before anything disappears into the black hole, it enters a disk that spirals around the black hole at a good fraction of the speed of light. That creates friction, which heats the material to millions of degrees. The disk emits enormous amounts of radiation, so it shines brightly at many wavelengths. Quasars are disks around supermassive black holes in the hearts of galaxies. And the brightest one yet seen is truly a monster. It encircles a black hole that appears to be about 17 billion times the mass of the Sun. The black hole is gulping the equivalent of a star as massive as the Sun every day. That creates a disk that’s hundreds of millions of miles across. It shines thousands of times brighter than our entire Milky Way Galaxy. We see the quasar as it looked more than 12 billion years ago — not long after the birth of the universe. So by now, it’s probably shut down — turning off one of the most brilliant lights we’ve ever seen. Script by Damond Benningfield
  • Stars like the Sun aren’t massive enough to become black holes when they die. But there’s a possible exception to that rule: if the star is born with a small black hole inside it. The idea was first proposed by Stephen Hawking. And it’s supported by a recent study. Normally, a star like the Sun just isn’t massive enough to collapse to make a black hole. But the universe might be sprinkled with black holes created in the Big Bang. Such black holes could be just about any mass — from almost nothing, to as heavy as a star. Such a black hole could be incorporated into a newly forming star. There, it would slowly “eat” the star from the inside. Over time, that process could account for some of the star’s energy production. But the black hole might reach a tipping point where it would gobble the rest of the star — converting the whole thing into a black hole. Measuring the vibrations at the surfaces of Sun-like stars might reveal the presence of black holes inside them — eating them from the inside. The new study says that a black hole up to about one millionth of the Sun’s mass could be at work inside the Sun today. If it’s there, within a hundred million years it could cause the Sun to drop to about half of its current brightness. The Sun then would puff up to many times its current size, and shine brighter for billions of years. After that, it would collapse to form a black hole. More about black holes tomorrow. Script by Damond Benningfield
  • Our brains can see things that aren’t there. They connect points and shapes to create “pictures.” So we might see a dragon in some puffs of clouds, “canals” on the surface of Mars, or a scorpion in the stars. One of the most persistent pictures is the “man in the Moon” — a face created from features on the lunar surface. Stories about the man in the Moon — or, in some cases, the woman in the Moon — go back centuries, from cultures around the planet. In China, for example, the face represented the goddess Chang’e. She was stranded on the Moon after taking too much of a potion that made her immortal. In Germany, the man was a giant who poured water from the Moon to create high tides. And in parts of Europe, the man was banished to the Moon after he stole from a neighbor or worked on the Sabbath. Today, inns and pubs from Tokyo to the Isle of Wight are called Man in the Moon. It’s been the title of several books. And in the first science-fiction movie, in 1902, a rocket from Earth slammed into the man’s “eye,” with messy results. The features that make up the man’s face are a combination of dark, smooth volcanic plains, and lighter areas that are more jumbled. You can look for the face yourself the next couple of nights because the Moon is full. And it has a bright companion: The star Antares is to the lower left of the Moon this evening, but almost touching the Moon tomorrow night. Script by Damond Benningfield
  • The Moon is in a sort of cosmic balance tonight. It’s passing through Libra, the balance scales — the only constellation of the zodiac that doesn’t represent a living thing. But the scales are associated with two living things — Virgo, which represents a goddess, and Scorpius, the scorpion. In various cultures of the Mediterranean, the stars of Libra were attached to one or both of those figures. In ancient Babylon, the stars represented a scale held by Shamash, the Sun god. He was also the god of truth and justice. The scales helped him maintain a “balance” on Earth and in the heavens. And there may have been an astronomical reason for thinking of the stars of Libra as “in balance.” Until about 2700 years ago, the Sun passed across those stars at the fall equinox — a time when day and night are roughly the same length — they’re in balance. Libra also was identified as the claws of the scorpion, which is on the opposite side of the constellation. The names of Libra’s brightest stars still reflect that heritage: Zubeneschamali, the northern claw, and Zubenelgenubi, the southern claw, which is quite close to the Moon tonight. The Greeks maintained the connection to both the gods and the scorpion. Libra wasn’t depicted as a separate constellation until a couple of thousand years ago, in Rome. Yet it maintains the links to its heritage — the scales of justice, and the claws of the scorpion. Script by Damond Benningfield
  • One of the icons of classic western movies is the sunset. Even in black and white, rays of sunlight radiate into the sky like jets of water erupting from a fountain. They add a bit of grandeur to any sunrise or sunset. They’re known as crepuscular rays, from the Latin word for twilight. Technically, the name applies to rays that appear during morning or evening twilight, while the Sun is below the horizon. In modern usage, though, it applies to rays of sunlight shining from behind clouds or other obstacles at any time of day. The bright rays alternate with darker shadow bands, where clouds or mountains block some of the sunlight. The rays appear to radiate in all directions. But that’s an illusion. All of the rays are parallel. They appear to converge on the Sun because of perspective. It’s like looking down a set of railroad tracks. The tracks appear to converge as you look farther away, even though they’re the same distance apart. In the case of crepuscular rays, they all converge at the Sun. We see the rays because they scatter off small particles in the atmosphere, such as grains of dust or pollen. The rays usually look yellow or orange, and for the same reason the twilight sky shows those colors: Air molecules scatter most or all of the blue light, leaving the redder wavelengths to shine through — adding some golden rays to any sunrise or sunset. Script by Damond Benningfield
  • When astronomers look at Spica, they see double. Or at least their instruments do. The system consists of two stars. But they’re so close together that even the biggest telescopes see them as a single point of light. So it took a technique called spectroscopy to “see” the system as a binary. The technique splits the light from a star into its individual wavelengths. Each chemical element imprints its own “barcode” in that spectrum of light. In the case of Spica, there are two sets of those barcodes. And they shift back and forth a tiny bit — the result of the orbital motion of the stars around each other. Careful study of the two spectra has revealed many details about the system. For example, the main star is much bigger and more massive than the Sun. It’s destined to explode as a supernova. The other star isn’t quite as impressive, but still far more impressive than the Sun. It probably won’t explode, but instead will leave a small but heavy corpse. The stars orbit each other once every four days. Their surfaces are only a few million miles apart — so close that the gravity of each star distorts the shape of the other — one more amazing finding about this impressive double star. To the eye alone, Spica looks like a single bright star. Tonight, it’s close to the lower left of the Moon at nightfall. The Moon slides toward it during the night, so they’re especially close as they set, in the wee hours of the morning. Script by Damond Benningfield
  • A horse and rider gallop across the north and northwest on May evenings. They’re in the handle of the Big Dipper, which is high in the sky at nightfall and low in the northwest at dawn. They’re the stars Mizar and Alcor. Mizar is the brighter of the two, with fainter Alcor just a whisker away. They’re so close together that the skywatchers of bygone centuries thought of them as a horse and rider. Mizar is a system of four stars, all of which are a little hotter and brighter than the Sun. Alcor consists of two stars — one of them more impressive than the Sun, the other less impressive. Both systems lie about 80 light-years from Earth. One question that astronomers have asked for centuries is whether the two systems are bound to each other, or if they just happen to line up in the same direction in the sky. They’re both members of a wide-spread cluster. That makes Mizar and Alcor stellar siblings — they formed at the same time, from the same cloud of gas and dust. Just how close their relationship is has remained a mystery. Early observations said the systems were perhaps two or three light-years apart. At that range, they probably would not be gravitationally bound to each other. But observations over the past few years by the Gaia space telescope put the separation at roughly one light-year. That may be close enough for them to be saddled together as a single unit — just like a horse and rider. Script by Damond Benningfield
  • Millions of black holes inhabit our home galaxy, the Milky Way. Most of them probably roam the galaxy alone, so we never see them. Luckily for science, though, many of them have companion stars. That makes it possible for astronomers to “see” and learn about the black holes. An example is a system called Swift J1357. It was discovered in 2011 by Swift, a space telescope that studies the X-ray sky. The system is thousands of light-years away, and appears to be outside the galaxy’s disk of stars. J1357 consists of two objects: a black hole and a small, faint companion star. The black hole’s gravity pulls in hot gas from the companion. The gas spirals around the black hole, heating up and forming a faint disk. The black hole appears to be at least nine times the mass of the Sun. It and the companion orbit each other once every two and a half hours — one of the tighter orbits of any known black-hole binary system. J1357 produces outbursts of X-rays every few years. They may occur when too much gas piles up in the disk. It gets so hot that it causes an eruption of particles and energy — an outburst that reveals more about this intriguing system. Swift J1357 is much too faint to see with the eye alone, but we can spot its location. It’s close to the left of Spica, the brightest star of Virgo. It’s about a third of the way up the southeastern sky at nightfall. Script by Damond Benningfield
  • Jupiter is missing. The solar system’s biggest and heaviest planet is nowhere to be seen. And it won’t return to view for a few weeks. Jupiter is passing behind the Sun as seen from Earth, so it’s hidden in the Sun’s brilliant glare. It’ll be closest to the Sun on Saturday afternoon. That point is known as conjunction, and it happens every 12 and a half months. Jupiter is known as a “superior” planet. That means its orbit is outside Earth’s orbit around the Sun. So Jupiter sometimes lines up directly opposite the Sun in our sky, shining brightly all night long. About half a year later, though, it passes behind the Sun and out of sight. The planet doesn’t necessarily pass directly behind the Sun. This time, for example, it slides about half a degree from the Sun — less than the width of your finger held at arm’s length. Even so, it’s so close to the Sun that astronomers can’t point their telescopes toward it. And radio “static” from the Sun means that flight controllers lose contact with spacecraft at Jupiter for a while. The craft operate on their own, and save their observations to beam to Earth when the Sun is out of the way. For casual skywatchers, Jupiter should return to view in a month or so. It’ll be quite low in the eastern sky during the dawn twilight. It’ll be in better view as we move into summer — climbing farther from the Sun, and shining like a brilliant beacon in the early morning sky. Script by Damond Benningfield