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StarDate
Weekdays at 6:32 p.m. - after the 6:30 p.m. newscast

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 debuted in 1978, making it the longest-running national radio science feature in the country. It airs on more than 300 radio stations. It has been hosted by Billy Henry since July 2019.

StarDate is a production of The University of Texas McDonald Observatory, which also produces the Spanish-language Universo Online website and the bi-monthly StarDate magazine. More information can be found on their program website.

  • Most people keep their hands as far from snakes as possible. But handling a snake was the job for the mythological serpent bearer. He’s represented in the stars as the constellation Ophiuchus, which is in the east and southeast at nightfall. The head of the snake is above Ophiuchus, with its tail below. In one story, the constellation represented a serpent god. In another, it was a god wrestling with a snake. And in still another, it was an ancient healer who learned the secret of life by watching snakes. His left hand — the one holding the snake’s head — is represented by two stars: Yed Prior, which leads the way across the sky, and Yed Posterior. In our sky, the stars are separated by less than the width of your finger held at arm’s length. But they’re not related. One is a little more than a hundred light-years away, while the other is about 160 light-years. Both stars are more massive than the Sun. So even though they’re much younger than the Sun, they’ve already moved past the prime of life. They’ve burned through the hydrogen fuel in their cores. That’s caused their outer layers to expand to giant proportions. So they’re many times bigger and brighter than the Sun. Yed Prior and Yed Posterior are about a quarter of the way up the east-southeastern sky at nightfall, with Yed Prior slightly higher. Under even moderately dark skies, they’re easy to see — outlining the hand of the serpent bearer. Script by Damond Benningfield
  • The giant families of stars known as globular clusters are like carnival rides: They do a lot of bobbing up and down. And they may lose a bit of themselves with every gyration. An example is the cluster Messier 12. It’s probably more than 16,000 light-years from Earth. It contains a couple of hundred thousand stars, all packed into a ball about 75 light-years across. That means the stars are much closer together than the stars in our part of the galaxy. Like all globulars, M12 travels at an angle to the Milky Way’s disk. So it periodically passes through the disk. During each pass, the gravity of the disk may pull away some of the stars on the outskirts of M12. A study a couple of decades ago found a lack of lower-mass stars in the cluster. Heavier stars tend to congregate in a cluster’s core, where they’re held fast by the gravity of the other stars around them. Less-massive stars migrate to the outskirts, where they’d be easy to pull away. Today, M12 contains about 200,000 stars. But it could have lost several times that number over its 13-billion-year lifetime. So as many as a million of the cluster’s stars might now be orbiting the center of the galaxy on their own — pulled away from their birthplace. Messier 12 is in Ophiuchus, the serpent bearer, which is in the east and southeast at nightfall. But you need a telescope to see this possibly vanishing cluster of stars. Script by Damond Benningfield
  • The Sun is about four and a half billion years old, so it’s been around awhile. Compared to some of the galaxy’s oldest stars, though, it’s a youngster. Some stars have been around since shortly after the universe was born. In the Milky Way Galaxy, many of those ancient stars reside in globular clusters — giant balls of stars that may have formed as the Milky Way itself was taking shape. An example is Messier 5. It’s in the southeast at nightfall, in the constellation Serpens. It’s a bit too faint to see with the eye alone, but through binoculars it looks like a fuzzy star. M5 is about 25,000 light-years away. It contains several hundred thousand stars. Together, they form a slightly flattened ball that spans about 160 light-years. Most of the stars in M5 formed at about the same time, from a single giant cloud of gas and dust. Those stars are about 12 billion years old, and perhaps older. Almost all of its stars are fainter and less massive than the Sun. Only a few are more impressive — stars that are in the final stages of life, or that’ve been “rejuvenated” by encounters with other stars. Any stars that were born heavier than the cluster’s current population have either exploded or shed their outer layers to expose their hot, dead cores — the remnants of some of M5’s most ancient stars. Script by Damond Benningfield
  • The head of the serpent slithers into the early evening sky this month, with its tail twisting along a bit later. Serpens is the only constellation that’s split apart. The two halves are separated by Ophiuchus, the serpent bearer. The snake’s head rises first. It’s in the east and southeast at nightfall, marked by a serpentine trail of faint stars. The tail, which is below Ophiuchus, climbs into view about an hour later. The brightest of the stars of Serpens is Unukalhai — an Arabic name that means “the serpent’s neck.” The star also is known as Alpha Serpentis, indicating its ranking as the constellation’s leading light. The star is in the final stages of life. It converted the hydrogen fuel in its core to helium, causing the core to shrink and get hotter. That triggered the next round of nuclear reactions, with the helium being converted to carbon and oxygen. The changes in the core have caused the star’s outer layers to puff up like a balloon. That’s made the star about 14 times wider than the Sun. The expansion also made the star’s surface much cooler, so it shines yellow-orange. In time, all the reactions in the star’s core will stop, and the outer layers will puff out into space. For a while, that expanding cloud will form a colorful bubble. As the bubble cools and dissipates, though, only the star’s dead core will remain — depriving the serpent of its bright neck. More about the serpent tomorrow. Script by Damond Benningfield
  • 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