StarDate PodcastAuthor: McDonald Observatory
24 Nov 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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    John Mitchell was one of the most insightful scientists of the 18th century. He made important contributions to the study of earthquakes, magnetism, and other topics, including astronomy.

    250 years ago, for example, he studied the Pleiades, the star cluster that marks the shoulder of Taurus, the bull. Telescopes of the day revealed a few dozen tightly packed stars. Mitchell calculated that the odds of finding so many unrelated stars in such a tiny volume were just one in 500,000. From that, he reasoned that the stars were bound together by their mutual gravitational pull. That made him the first scientist to suggest that the stars in a cluster are all related.

    Mitchell was right. The Pleiades contains more than a thousand stars. All of them were born more than a hundred million years ago, from a single cloud of gas and dust. And today, they move through the galaxy as a family, bound by gravity.

    That won’t always be the case, though. The gravity of the rest of the galaxy is tugging at the Pleiades, pulling some of its stars away. In about 250 million years, the cluster will have fallen apart, with its member stars orbiting the center of the galaxy on their own.

    For now, though, look for the Pleiades low in the east as darkness falls, above the star Aldebaran, the bull’s orange eye. The cluster’s brightest stars form a tiny dipper. It crosses high overhead around midnight — a big stellar family that won’t stick together forever.


    Script by Damond Benningfield

  • Posted on 23 Nov 2017

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    Rho Cassiopeia

    The heaviest stars in the universe eventually blast themselves to bits. But like a kicker getting ready to try a game-winning field goal, they first get in a few practice swings. These smaller explosions blast out shells of hot gas that expand at a million miles an hour.

    One example is in Cassiopeia, the queen, whose brightest stars form a letter W. It’s well up in the north-northeast at nightfall.

    Rho Cassiopeia is one of the biggest stars in the Milky Way — a yellow hypergiant. If it took the Sun’s place in our own solar system, it would extend past the orbit of Mars.

    Rho Cassiopeia is probably a few dozen times heavier than the Sun. And it’s getting near the end of its life. Not long ago, it most likely was a red supergiant, so it was even bigger than it is now. But as a result of changes deep in its core, its outer layers are shrinking and getting hotter. As a result, the star probably will become a smaller and hotter blue supergiant.

    These changes also trigger explosions on the surface of Rho Cassiopeia. An eruption in 2000 blasted out the equivalent of about 10,000 Earth masses of material — a half-century after a similar explosion.

    More of these eruptions are probably in the star’s future. And eventually, it’ll blow itself apart as a supernova. Its outer layers will blast into space, while its core will be crushed — forming either a super-dense neutron star or an even denser black hole.


    Script by Damond Benningfield

  • Posted on 22 Nov 2017

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    Seeing Deep

    By the time the universe was a billion years old, something had given it a real charge. That “something” could be the first galaxies. And thanks to some help from cosmic magnifying glasses, astronomers have seen enough early galaxies to make that possibility more likely.

    A few hundred thousand years after the Big Bang, electrons and protons began sticking together to make atoms of hydrogen. The atoms had no electric charge. By a billion years later, though, most of the hydrogen atoms had been split apart, giving them a charge. Learning what caused this can help astronomers better understand the early universe.

    In a project known as the Hubble Frontier Fields, Hubble Space Telescope stared at several galaxy clusters for weeks at a time. The gravity of the immense mass in such a cluster acts as a magnifying lens, so it makes the galaxies that appear beyond the cluster look brighter. That revealed galaxies that were just one percent as bright as the galaxies that Hubble could see without it.

    Astronomers at the University of Texas developed a way to remove the bright light from the cluster galaxies from the Hubble pictures. That allowed them to see the faint background galaxies — some of them up to 13 billion light-years away — among the first galaxies in the universe.

    The images showed only a tiny fraction of the early universe. But they revealed enough distant galaxies to make it more likely that they gave the universe a charge.


    Script by Damond Benningfield

  • Posted on 21 Nov 2017

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    Moon and Saturn

    The crescent Moon has a bright companion early this evening — the planet Saturn. It looks like a fairly bright star to the lower left of the Moon as darkness falls. They’re quite low in the sky, though, so any buildings or trees along the horizon will block them from view. They set not long after the sky gets fully dark.

    Saturn is best known for its beautiful rings — thousands of rings in all. Most of them clump together in three wide bands.

    The rings include several interesting features. Some of the rings are braided like a loaf of challah bread. Some aren’t complete rings at all. And at times, one of the main bands of rings shows dark “spokes” — radial features like the spokes of a bicycle wheel.

    The spokes were first seen by the Voyager probes almost four decades ago. They were studied in detail by the Cassini orbiter, which ended its mission in September.

    The spokes aren’t permanent features — they come and go. They disappear during the middle of Saturn’s winter and summer, but return around the equinoxes.

    The spokes aren’t part of the rings themselves — they rotate at a different speed. The most likely explanation is that the spokes are made of tiny grains of dust. The particles acquire an electric charge, so they levitate above the rings, pulled along by Saturn’s magnetic field.

    It’s not certain what causes the spokes to form. Whatever the cause, though, the spokes help make Saturn’s rings both beautiful and intriguing.

    Script by Damond Benningfield

  • Posted on 20 Nov 2017

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    Lacaille 9352

    Our home galaxy boasts hundreds of billions of stars. Some of them emit lots of light, while others are dim. And tonight, the southern sky features a stunning contrast between two nearby stars: one powerful, the other feeble.

    Both reside in the constellation Piscis Austrinus, the southern fish. If you look due south in early evening, you\'ll easily see the bright star, Fomalhaut. This white star is just 25 light-years from Earth.

    Just south of Fomalhaut is another nearby star. In fact, Lacaille 9352 is even closer — a mere 11 light-years away. But you’ll never see it without some help, because the star emits only one percent as much visible light as the Sun does. So, close though it is, Lacaille 9352 is visible only through binoculars or a telescope.

    Despite their vast difference in luminosity, both stars generate energy the same way: Nuclear reactions in their cores convert hydrogen into helium. But Fomalhaut was born with twice the mass of the Sun, so its center is hotter, making it burn brightly. In contrast, Lacaille 9352 was born with only half the mass of the Sun. So it’s faint, cool, and red — what astronomers call a red dwarf.

    Red dwarfs may seem insignificant, but in one way they far surpass their brighter brethren: They account for about three-quarters of all the stars in the galaxy.


    Script by Ken Croswell

  • Posted on 19 Nov 2017


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