StarDate PodcastAuthor: McDonald Observatory
12 Nov 2019

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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    Astronomers use several techniques to discover planets in other star systems. By far the most prolific is the transit method. It looks for a planet to pass in front of a star, causing the star’s light to fade by a tiny bit. Planet-hunting space telescopes have used this technique to discover thousands of exoplanets.

    A transit takes place in our own solar system early today. Mercury, the innermost planet, will pass across the face of the Sun. All or most of the transit will be visible across all of the United States except parts of Alaska.

    You might wonder if astronomers in other star systems could detect Mercury’s transit. It all depends on the sensitivity of their instruments.

    Our transit-hunting telescopes can detect a tiny drop in a star’s light — in many cases, less than a hundredth of a percent. Mercury’s transit will produce a smaller drop than that — about three-thousandths of a percent. So it would take really sensitive instruments to detect such a tiny change.

    If they do see the transit, alien astronomers could use it to determine Mercury’s size, its distance from the Sun, and how long it takes to orbit the Sun. And when combined with other techniques, they could learn Mercury’s mass, density, and much more.

    We recommend that you watch Mercury’s transit online or at a public venue that’s displaying it. Don’t look at the Sun directly, though — you need eye protection to gaze upon this rare astronomical alignment.

    Tomorrow: a “double” foot.

    Script by Damond Benningfield

  • Posted on 11 Nov 2019

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    Morning Treats

    The morning sky offers a couple of treats tomorrow. One is a close conjunction between a bright star and a planet. The other is a transit of a planet across the face of the Sun.

    The conjunction is between Mars and Spica, the brightest star of the constellation Virgo. They’ll be low in the east-southeast during early twilight. Spica is the brighter of the two, with orange Mars close to the left.

    The transit begins shortly after sunrise for those in the eastern third of the United States. And it’ll already be under way at sunrise for those who are farther west. It takes place as Mercury, the Sun’s closest planet, passes directly between Sun and Earth. Mercury will look like a tiny dot silhouetted against the Sun.

    Mercury transits happen 13 or 14 times each century, in either May or November. November transits happen when Mercury is at its closest to the Sun and farthest from Earth. So as seen from Earth, it forms a smaller dot than in May.

    The transit will last about five and a half hours. It’ll end shortly after 1 p.m. as seen from the east, and earlier from more westerly time zones.

    As with any event involving the Sun, never look at it directly — it could damage your eyes. Instead, use eclipse glasses or other protection. And even with protection, Mercury will be only a tiny dot, and quite hard to see. For the best view, watch online, or visit a planetarium or other site that’s tracking it. Then enjoy the transit — the last until 2032.

    Script by Damond Benningfield

  • Posted on 10 Nov 2019

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    Chi Cygni

    When a star is in the prime of life, it can shine steadily for billions of years. As it begins to die, though, it can change in a hurry. A star near the neck of the swan, for example, pulses in and out like a beating heart. That causes its size and brightness to change dramatically.

    Chi Cygni is about 600 light-years away, in Cygnus, the swan. The constellation stands high overhead at nightfall.

    Chi Cygni is in the final stages of life. It’s burned through the hydrogen in its core to make helium. Now, it’s burning hydrogen and helium in shells around the core.

    And that makes the star unstable. Its outer layers fall inward, which makes the surface hotter and brighter. It makes the layers around the core hotter, too. The hotter gas traps energy from the core, which then pushes outward, causing the star to get bigger. But as it does so, it gets cooler, so the star gets smaller. Each pulse takes about 13 and a half months.

    At its biggest, the star is almost 350 million miles wider than at its smallest. And at visible wavelengths, it can get more than ten thousand times brighter.

    Chi Cygni’s overall brightness doesn’t vary nearly as much, though. At its biggest, the star emits mostly infrared energy, which isn’t visible to the human eye. As it contracts and gets hotter, though, it produces mainly visible light. So at its peak, the star is just visible to the unaided eye. And in between, it’s in-visible — hiding along the neck of the swan.

    Script by Damond Benningfield

  • Posted on 09 Nov 2019

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    Mercury Transit

    A tiny black dot will crawl across the face of the Sun on Monday: the planet Mercury. Its passage will be visible across all of the United States except most of Alaska.

    Such a passage is known as a transit. It’s possible only because Mercury is closer to the Sun than Earth is, so it regularly passes between Earth and the Sun. Mercury’s orbit is tilted a bit, though, so the little planet doesn’t transit the Sun on every crossing. Instead, transits occur only when the geometry is just right — when Mercury crosses the plane of Earth’s orbit at the right time.

    Today, such crossings happen 13 or 14 times per century. They occur only within a few days of May 8th or November 10th. November transits happen when Mercury is farther from Earth, so it makes a smaller dot than during May transits.

    All of Monday's transit will be visible from the eastern third of the U.S. For the rest of the country, it’ll be in progress at sunrise.

    It begins at about 7:35 a.m. local time on the east coast, and ends at about 1:04 p.m. The times will be one, two, or three hours earlier as you move westward across the time zones.

    Never look at the Sun directly, though. You need eclipse glasses or other protection to observe it. Even then, Mercury is so tiny that you probably won’t be able to see it. Instead, watch online, or visit a museum or other venue with safe Sun-watching equipment — for a glimpse of a transit across the face of the Sun.

    Script by Damond Benningfield

  • Posted on 08 Nov 2019

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    The starry pictures that decorate the night sky are like a giant mural — they’re a complex canvas that never changes. For most of human history, though, that mural was drawn in different ways. Different cultures connected the starry dots to form their own pictures and tell their own stories. And even cultures that adopted older pictures often rewrote the stories that went with them.

    Consider Aquarius, one of the constellations of the zodiac. It stretches from south to southeast at nightfall, with its brightest stars to the right of the Moon.

    Today, the constellation represents a young man pouring water from a jug. And that’s been the basic depiction for thousands of years. But the story behind it has changed.

    In ancient Babylon, for example, the picture represented the god who ruled over a portion of the Sun’s path across the sky. He held an overflowing water jug that was associated with flooding.

    And in Egypt, the constellation was associated with the annual flooding of the Nile River. The story said the Nile overflowed when a boy dipped a jug into its waters.

    The Greeks adopted many of the constellations from the Babylonians and the Egyptians, so they kept Aquarius. But they changed the story. One version says the boy was Ganymede, a beautiful young man who was stolen away by Zeus, the king of the gods. Whisked to Olympus, he became the cupbearer of the gods, and was immortalized in the stars — where he remains today.

    Tomorrow: Crossing the Sun.

    Script by Damond Benningfield

  • Posted on 07 Nov 2019


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