StarDate

Billy Henry
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  • Astronomy
  • Updated twice weekly

StarDate, the longest-running national radio science feature in the U.S., tells listeners what to look for in the night sky.

Episodes

  1. 1 day ago

    Moon and Taurus

    The terms that describe the phases of the Moon can be a little confusing. Take tomorrow morning, for example. From most of the United States, the Sun will light up one-quarter of the lunar hemisphere that faces Earth. You might think that would make it a “quarter” Moon – but no. A “quarter Moon” doesn’t describe the Moon’s appearance – it describes the position in its month-long cycle of phases. The cycle begins when the Moon is new. The Moon crosses the line between Earth and the Sun then, so it’s hidden in the Sun’s glare. A bit more than a week after that, the Moon is a quarter of the way through the cycle – it’s at first quarter. It lines up at a right angle to the Earth-Sun line, so the Sun lights up half of the visible disk. A week later comes full Moon. And a week after that comes last quarter – three-quarters of the way through the cycle. Once again, half of the lunar disk is illuminated – but it’s the opposite half from the first-quarter Moon. Then the Moon rolls back around to new – starting a new cycle. The next cycle starts on Wednesday. And this time it brings a rare treat – a total solar eclipse. In the meantime, look for the Moon passing through Taurus tomorrow. Aldebaran, the bull’s bright eye, is to the right of the Moon. Elnath, the tip of one of its horns, is closer to the lower left of the Moon. As a bonus, Mars is below them, and the Moon will pass close to it on Sunday. Script by Damond Benningfield

  2. 2 days ago

    Dog Days

    The Dog Star is just beginning to emerge from the Sun’s glare in the dawn sky. By tradition, that means we’re just getting into the Dog Days of summer. Sirius is the brightest star in the night sky. It’s climbing into view in the morning glow of the brightest star in all the sky – the Sun. Sirius is quite low in the east-southeast shortly before sunrise. It’s best seen from the southern tier of states, although it’ll pull into better view from the northern states over the coming days and weeks. Sirius is also the leading light of the constellation Canis Major, the big dog, so it’s known as the Dog Star. And that’s where we get the name for this part of summer – the Dog Days. Sirius makes its closest approach to the Sun during summer, disappearing for a few weeks in the Sun’s glare. In ancient times, it returned to view in the morning by early July – just as the weather was reaching its hottest. At the time, it seemed logical to associate the bright star with the sweltering weather. So the people of ancient Greece and Rome named that stifling time of year in the star’s honor. Thanks to an effect known as precession, the stars shift position with respect to the Sun over the centuries. So Sirius now returns to view a few weeks later than it did thousands of years ago. So the heat is already in full force at northern latitudes – the Dog Days of summer. Script by Damond Benningfield

  3. 3 days ago

    NGC 6397

    NGC 6397 seems like a pretty ordinary globular star cluster. It’s not especially big or bright, and nothing about its stars is going to strike anyone as odd. Yet it’s been an important astronomical laboratory. Among other things, it’s helped astronomers determine the age of the galaxy, and the minimum “weight” of a star. NGC 6397 is in the constellation Ara, the altar. It’s below the tail of Scorpius, the scorpion. That puts it so far south that it’s visible only from the southernmost United States. Hubble Space Telescope has monitored the cluster for decades. It contains several hundred thousand stars, packed into a ball only a few dozen light-years across. The Milky Way contains about 150 of these clusters, which are the galaxy’s oldest inhabitants. Over the decades, Hubble has monitored many of the cluster’s stars. The observations helped astronomers lock down the minimum mass for an object to shine as a true star – about eight percent the mass of the Sun. Another set of observations helped measure the cluster’s distance – just 7800 light-years – one of the closest of all globulars. That number is important for determining the cluster’s age – 13.4 billion years. That means that NGC 6397 – and its home galaxy, the Milky Way – took shape when the universe was just 400 million years old. Script by Damond Benningfield

  4. 4 days ago

    The Altar

    The gods of ancient Greece weren’t great parents. One of the ancient gods known as the Titans, for example, swallowed his newborn children. Eventually, though, the kiddos escaped and overthrew him. Their victory is commemorated in the stars, as the altar where they swore allegiance to each other. It’s the constellation Ara, and it’s below the tail of the scorpion. In the story, Cronus was the king of the Titans. He’d come to power by defeating his own father. A prophecy said that one of his children would defeat him. To prevent that, he swallowed each of his first five children. But his wife, Rhea, got tired of that plan. When the last child was born, she hustled him away to an island and gave Cronus a rock wrapped in a blanket. That child was Zeus. When he grew up, he returned home and forced his dad to cough up his brothers and sisters. They pledged to defeat the Titans, and won a 10-year war. Zeus became the king of the new gods and ruler of the sky. To celebrate the victory, he placed the altar among the stars. The altar is so far south that most of it’s visible only from far-southern parts of the United States – places like Florida, South Texas, and especially Hawaii. Ara is one of the smaller constellations, with only a couple of moderately bright stars. But it does have some good deep-sky objects, and we’ll talk about one of them tomorrow. Script by Damond Benningfield

  5. 5 days ago

    Desolate Planet

    The formal name for a planet almost 50 light-years from Earth is Kua’Kua – a Costa Rican name that means “butterfly.” But there’s nothing beautiful or delicate about this world. Its surface is a desolate landscape of volcanic rock. The planet is in the system LHS 3844. Its star is a cool, faint ember known as a red dwarf. Kua’Kua – LHS 3844 b – is a super-Earth. It’s bigger and heavier than Earth, and it’s made of rock and metal. The planet is only about half a million miles from the star, and the same hemisphere always faces the star. So the dayside sizzles at more than 1300 degrees Fahrenheit. Every 11 hours, the planet passes behind the star as seen from Earth. Astronomers have watched several passages with Webb Space Telescope. Watching before and after a passage revealed the combined make-up of the star and planet. But when the planet disappeared, that left only the star in view. Comparing the readings revealed details about the surface of the planet. In fact, it’s the first “exoplanet” for which we have such a close look. The surface appears to be made of dark volcanic rock that could have been deposited recently, making it fairly smooth and unbroken. But the surface also could be much older. In that case, a constant pounding by radiation and meteorites could have pulverized the surface. Either way, Kua’Kua is not a good place to go hunting for butterflies. Script by Damond Benningfield

  6. 6 days ago

    Moon and Saturn

    There’s just one place to catch a wave outside Earth: Titan, the largest moon of Saturn. It’s the only other world in the solar system with bodies of liquid on its surface. And the waves there could reach heights of 10 feet. But future surfers will need to dress carefully; the surface temperature is almost 300 degrees below zero. Titan is half-again the size of Earth’s moon, and it has a dense atmosphere. Rain fills its lakes and seas with liquid methane, ethane, and nitrogen. The largest sea covers about 200,000 square miles – twice the area of all the Great Lakes combined. It could be several hundred feet deep. The Cassini spacecraft used radar to measure Titan’s ponds. It saw waves of no more than an inch or so high. But a recent study says the waves could get a lot bigger. Based on 20 years of observations of waves on Lake Superior, scientists built a computer model of how waves might work on other worlds. The model accounted for gravity, the pressure of the atmosphere, and the properties of the liquid. When the model was applied to Titan, it showed that even gentle breezes could whip up some big waves. But the waves would move more slowly than those on Earth – a slow-motion trip across the sea. Saturn looks like a bright star close to our moon the next couple of mornings. It’ll stand to the left of the Moon at dawn tomorrow, and the lower right of the Moon on Tuesday. Script by Damond Benningfield

  7. 1 Aug

    Morning Mercury

    The dawn twilight has a bright visitor the next few mornings – the planet Mercury. It’s farthest from the Sun for its current morning appearance. It looks like a bright star, but it’s so low in the sky that it’s tough to find. Mercury holds an important spot in the history of astronomy and physics. It provided some of the first confirmation of General Relativity – Albert Einstein’s theory of gravity. Mercury’s orbit around the Sun is lopsided, so the planet’s distance from the Sun varies. For a long time, astronomers had seen that the orbit’s closest point shifted a tiny bit over time. Isaac Newton’s laws of gravity explained most of the difference. But there was still a tiny amount that couldn’t be accounted for. Einstein’s theory of gravity held that massive bodies warp the space around them. Since Mercury is the Sun’s closest planet, its orbit is influenced by that “warpage” more strongly than any other planet’s. In fact, general relativity accounted precisely for the shift in the orbit. So Mercury’s orbit provided some of the first evidence to support general relativity – a new way of thinking about gravity. Look for Mercury quite low in the eastern sky during the waxing twilight. It’ll shine a little brighter each day over the next few mornings. But it’ll also drop a little closer to the Sun, so you’ll need a clear horizon to spot it. Tomorrow: catching waves. Script by Damond Benningfield

  8. 31 Jul

    Changing Sun

    The Sun is getting bigger, hotter, and brighter. The change isn’t enough to notice during a human lifetime – or even a thousand lifetimes. It plays out over hundreds of millions of years. And it’s all the result of changes deep inside our star. Like all stars in the prime phase of life, the Sun is “fusing” atoms of hydrogen in its core to make helium. That generates the energy that makes the Sun shine. As the amount of helium builds up, the core gets denser, so gravity squeezes it more tightly. That speeds up the fusion reactions, making the core even hotter. Radiation from the hotter core pushes on the Sun’s outer layers, making the Sun bigger. It also makes its surface hotter. The combination of bigger and hotter makes the Sun brighter. So over its four-and-a-half-billion-year lifetime, our star has grown about 15 percent wider, and perhaps 40 percent brighter. That should mean that the young Earth would have been an iceball. But studies suggest the atmosphere was much thicker when Earth was young. The atmosphere also contained much more carbon dioxide and other greenhouse gases. They trapped more heat, keeping Earth from freezing over. The Sun’s bigger-hotter-brighter trend will continue. In perhaps a billion to two billion years, it’ll be so hot and bright that Earth’s air and oceans will boil away. That will reduce our planet to a bare cinder. Script by Damond Benningfield

  9. 30 Jul

    Standard Candles

    Eta Aquilae is big, bright, and unsteady. Over a bit more than seven days, the star pulses in and out like a beating heart. That causes its brightness to change. How it changes makes the star a good “standard candle” – a type of object that astronomers use to measure the scale of the universe. Eta Aquilae is a Cepheid variable – the first one ever discovered. Such stars brighten and fade in a predictable way. By timing the cycle, astronomers can determine the star’s true brightness. Comparing that to how bright the star looks allows them to calculate the star’s distance. Cepheids are bright enough to see hundreds of millions of light-years away – in galaxies beyond the Milky Way. To go even farther, astronomers rely on another type of standard candle: the exploding stars known as Type Ia supernovas. Like the Cepheids, the way they brighten and fade reveals their true brightness. Some of them appear in galaxies with Cepheids, where we already know their distance. That provides a way to calibrate all of the supernovas, which can be seen from billions of light-years away. Of course, it’s all a little more complicated than that. There are different classes of Cepheids, for example. So astronomers have to understand all the details – making sure that a standard candle really is a good distance marker. Eta Aquilae is high above the Moon in early evening, near Altair, its constellation’s brightest star. Script by Damond Benningfield

  10. 29 Jul

    Morning Dominance

    Saturn feels like it has a big region of the early morning sky practically to itself right now. The giant planet climbs into good view after midnight, and stands high in the south at first light. It looks like a bright golden star. You have to scan a long way in every direction to find another planet or star that rivals it. Saturn is traveling through Pisces, skimming along the border with Cetus. That region of the sky is well below the Milky Way – the hazy band of light that outlines the disk of the Milky Way Galaxy. When we look at the Milky Way, we’re looking into the most heavily populated part of the galaxy. So not only are there a lot more stars in and around that band, there are a lot more bright stars. At the same time, Saturn’s location is a quarter of the way around the sky from the center of the galaxy, which is in Sagittarius. Again, that means we’re looking into more thinly settled parts of the galaxy. It’s like looking toward the suburbs of a major city instead of its busy downtown – there’s just a lot less to see. Saturn is so far from the Sun that it takes the planet about 30 years to make one full circle against the starry background. So the planet will stay in this dimly settled region of the sky for a couple of years – making it especially easy to find as you look into the darkness. Tomorrow: bright “mile markers” for measuring the scale of the universe. Script by Damond Benningfield

  • 10 Episodes

Hosts & Guests

About

StarDate, the longest-running national radio science feature in the U.S., tells listeners what to look for in the night sky.

Information

  • Creator
    Billy Henry
  • Years Active
    2017 - 2026
  • Episodes
    10
  • Rating
    Clean
  • Copyright
    © 2022 The University of Texas McDonald Observatory
  • Show Website
    StarDate

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