StarDate PodcastAuthor: McDonald Observatory
29 May 2017

StarDate Podcast

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

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    More Northern Crown

    A star can “die” more than once. And the second death can be just as impressive as the first. It can produce a brief outburst that’s easily visible across billions of light-years of space.

    Such an outburst was detected three years ago in Corona Borealis, the northern crown. The constellation is in the east as night falls at this time of year, and crowns the sky a few hours later.

    A space telescope detected a gamma-ray burst — a flash of the most powerful form of energy. For a couple of seconds, it appeared brighter than any other single object in the universe. But follow-up observations from space and the ground saw nothing — no exploding star or anything else that might have produced the gamma rays.

    The brightness and brevity of the blast, and the lack of a visible counterpart, suggested a source: the merger of two dead stars — either two neutron stars, or a neutron star and a black hole. Both kinds of objects form when a massive star explodes as a supernova, crushing its now-dead core. Such objects are no more than a few miles across, but several times as massive as the Sun.

    When the two dead stars in Corona Borealis collided, they produced a brief but brilliant burst of gamma rays. The rays were “beamed” into space in narrow jets. We saw the outburst only because one of the jets happened to beam in our direction. If it hadn’t, we wouldn’t have seen anything — and the “re-death” of these two stellar corpses would have remained hidden.


    Script by Damond Benningfield

  • Posted on 28 May 2017

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    Northern Crown

    A pretty little semicircle of stars crowns the sky on spring and summer nights: Corona Borealis, the northern crown. It’s in the east as night falls right now, but stands high overhead a few hours later. In a couple of months, it’ll be overhead at nightfall.

    Most of the semicircle isn’t very bright — you need pretty dark skies to see it. It stands out because of the tight pattern, with a fairly bright star at its center: Alphecca, “the bright one.”

    Alphecca’s actually a binary — two stars locked in a gravitational embrace. The heavier of them is about three times as massive as the Sun, thousands of degrees hotter, and dozens of times brighter. Its companion is a little smaller, cooler, and fainter than the Sun.

    The stars are quite close together — an average of about half the distance between the Sun and its closest planet, Mercury. They orbit each other once every 17 and a half days.

    And they’re lined up in such a way that we see the fainter star eclipse the brighter one. When that happens, Alphecca dims by a few percent. That’s not enough for most of us to notice with the eye alone, but it’s an easy catch for astronomical instruments.

    Instruments also detect a disk of debris around the stars. It extends billions of miles out into space. The disk consists mainly of small grains of dust — material left over from the formation of Alphecca itself.

    Tomorrow, we’ll talk about another pair of stars in Corona Borealis that blew itself up.


    Script by Damond Benningfield

  • Posted on 27 May 2017

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    Heading North

    While the Moon orbits Earth and Earth orbits the Sun, the Sun isn’t exactly standing still. In fact, it’s racing around the center of the Milky Way galaxy, carrying Earth and the other planets with it.

    The solar system inhabits the most luminous part of the Milky Way: its disk, which includes the beautiful spiral arms and most of the galaxy’s stars. It’s shaped like a pancake, except this pancake is enormous: at least a hundred thousand light-years across and a couple of thousand light-years thick.

    We’re about 27,000 light-years from the galaxy’s center — nearly halfway to the edge of the disk. And we’re near the midplane of the disk — the line that divides it into northern and southern halves. Estimates vary, but the solar system is probably between 40 and 90 light-years north of the midplane. That’s not very far when compared to the thickness of the disk.

    Still, with every passing minute, we’re moving higher. As the Sun orbits the galactic center, it slowly bobs up and down, like a horse on a merry-go-round. And for millions of years, we’ve been moving up. Eventually, we’ll reach a peak altitude and start to head back down. But we’ll never venture beyond the bounds of the Milky Way’s starry disk.

    And under dark skies, that disk is visible as the hazy band of light known as the Milky Way. It’s quite low in the sky this evening, but arcs high overhead before dawn tomorrow. It’ll be in better view in the evening sky as we head into summer.


    Script by Ken Croswell, Copyright 2017

  • Posted on 26 May 2017

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    Johannes Hevelius saw things that no one else did. Perhaps that was because he had a vivid imagination. Or perhaps it was because he consumed a lot of the famous beer he brewed. But whatever the reason, in the late 17th century he drew 10 new constellations. All of them filled in regions where no constellations had existed before — relatively dark areas of the sky with almost no bright stars.

    And seven of those constellations are still with us today. An example is Lynx, which is in the west and northwest at nightfall. Although it’s faint, you can find its location with the help of three bright stars below it: Pollux and Castor, the twins of Gemini, which are almost due west; and even-brighter Capella, the leading light of the charioteer, to their lower right.

    Hevelius, who didn’t use a telescope, counted 19 stars in that region, between Capella and the Big Dipper. He linked some of those stars in a zigzag pattern. And showing that he had a sense of humor, he called the new constellation Lynx, after the wild cat. He didn’t pick the name because the pattern looked like a lynx — it was because the viewer needed the eyes of a lynx to see it.

    And thanks to light pollution, it’s even harder to see today. To pick it out, you need a nice, dark sky, far from city lights. From the suburbs, you might make out a few stars, especially the brightest one, Alpha Lyncis. It’s an orange giant that’s 200 light-years away — the brightest “spot” of a faint cat.


    Script by Damond Benningfield


  • Posted on 25 May 2017

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    Radio Power

    A project known as Breakthrough Starshot hopes someday to use a powerful beam of energy to propel a tiny probe to another star. A recent study says that other civilizations might already be doing that, but on a much larger scale — and that we might have seen some of those beams.

    Two researchers at Harvard considered the objects known as fast radio bursts. Astronomers have seen about a score of them, all in other galaxies. They’re intense outbursts of radio waves that last only a tiny fraction of a second. They could be powered by exploding stars, collapsing neutron stars, or some other exotic objects.

    But the Harvard astronomers wondered if the bursts could have an artificial origin. And they concluded that it’s possible.

    Their idea is that a civilization would build a starship propelled by a giant sail. A beam of radio waves would “push” the ship just as the wind pushes sailing vessels here on Earth. Occasionally, the radio beam would sweep past Earth — producing a radio burst.

    If the radio beam were powered by solar energy, it would require a collector that’s about twice the diameter of Earth. Building and operating such a system wouldn’t be easy — but it’s within the realm of possibility. And the payoff could be big: it could propel a million-ton ship to a good fraction of the speed of light.

    The researchers aren’t saying that fast radio bursts really are produced by other civilizations — only that it’s a possibility worth checking.


    Script by Damond Benningfield

  • Posted on 24 May 2017


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