StarDate PodcastAuthor: McDonald Observatory
02 Dec 2020

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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    Venus in the Claws

    Venus is headed toward a nasty target: a claw of the scorpion. It’ll pass quite close to it in just a few days.

    Venus is the brilliant “morning star.” And the scorpion’s claw is the star Zubenelgenubi. It’s directly below Venus at first light tomorrow, but they’ll stand side by side on Friday morning.

    Zubenelgenubi is the second-brightest star of Libra, the balance scales. But its name pre-dates Libra’s creation. The name means “the southern claw.” The star represented one of the claws of Scorpius, the next constellation over. But thousands of years ago, it was taken away to form part of the new constellation.

    Zubenelgenubi lies just a third of a degree from the ecliptic — the Sun’s path across the sky. So the Sun passes directly in front of the system every year, in early November.

    The Moon and planets also stay close to the ecliptic as they move across the background of stars. So the Moon occasionally passes in front of Zubenelgenubi as well, blocking it from view.

    The planets can pass in front of it as well. But planets cover a much smaller portion of the sky than the Moon does, so it takes a perfect alignment for them to hide Zubenelgenubi. The last planet to do so was Venus, in 1947.

    Venus won’t cover the star this time around, but it won’t miss by much. At their closest, on Friday, they’ll be separated by about a degree and a half — about the width of your finger held at arm’s length.


    Script by Damond Benningfield

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  • Posted on 30 Nov 2020

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    Penumbral Eclipse

    The full Moon will fade a bit in the wee hours of tomorrow morning. The effect isn’t especially obvious, but it should be noticeable.

    The reason is an eclipse. But it’s a penumbral eclipse, which means the Moon will pass through the faint outer ring of Earth’s shadow. None of the Moon will go completely dark, as it does during a partial or total eclipse. Instead, most of the lunar disk will take on a dusky appearance — as though it were covered by a thin layer of clouds.

    Eclipses occur in cycles. Each eclipse in a cycle is separated by 18 years and 11 days. The last eclipse in this cycle, for example, took place on November 20th of 2002. And the next one comes on December 11th of 2038.

    A cycle begins with a series of penumbral eclipses. The Moon then moves deeper into the shadow of Earth, eventually creating a set of total eclipses. It then moves back out of the shadow, ending with more penumbral events. The current cycle is in that phase, so all the remaining eclipses are penumbral, with the last one in 2291.

    Tonight’s eclipse gets underway at 1:32 a.m. Central Time, when the shadow first touches the Moon. It’ll be at its peak at 3:44 a.m., when the penumbra will cover about two-thirds of the lunar disk. The eclipse will end at 5:53, when the Moon exits the penumbra. As a bonus, the Moon has a bright companion all night: Aldebaran, the eye of Taurus, the bull.


    Script by Damond Benningfield

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  • Posted on 29 Nov 2020

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    Moon Watching

    The Moon arcs high across the sky tonight. It rises in the east-southeast a little before sunset, and sets in the west-northwest a little before sunrise.

    The Moon’s rising and setting points move north and south along the horizon. And there’s a big swing between the northernmost and southernmost points every month. Yet the extremes for any given month reflect only a part of the lunar cycle. Over a period of 18.6 years, the northernmost and southernmost setting points vary by quite a bit.

    Many cultures have paid attention to that cycle. They’ve even built structures to mark it.

    One example is the Newark Earthworks in central Ohio. It consists of several giant geometric figures — an octagon, a circle, an ellipse, and a square. They were built a couple of thousand years ago by the Hopewell culture.

    The Hopewell lived in small villages. But they built large earthworks for ceremonial events across the region.

    The Newark site covered four and a half square miles. Its figures were built to a high level of precision. And the circle and octagon were linked. Scientists studied those figures. And they found many alignments to key moonrise and moonset points. The odds of such alignments being random were one in a million.

    The scientists also found alignments between the two figures and nearby hilltops, and between other figures on the site. Together, they suggest that the site was a giant observatory for watching the Moon — 2,000 years ago.


    Script by Damond Benningfield

    Support McDonald Observatory


  • Posted on 28 Nov 2020

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    Galaxy Pair

    Two beautiful spiral galaxies in the constellation Cetus are getting each other all stirred up. The galaxies are about half a million light-years apart — not very far on the galactic scale. At that distance, the gravity of each galaxy tugs heavily at the other. That has warped at least one of the galaxies, and triggered the birth of many new stars in both of them.

    Messier 77 and NGC 1055 are about 50 million light-years from Earth. Both are giant spirals like our home galaxy, the Milky Way. NGC 1055 is about the same size as the Milky Way, while M77 appears to be a good bit bigger.

    We see M77 almost face-on, so we have a good view of its spiral arms and the wide bar of stars in its middle. On the other hand, we see NGC 1055 almost edge-on. We don’t see its middle as clearly, but we have a good view of waves that ripple across its disk.

    The waves probably were created by the interaction between the two galaxies. That interaction has compressed giant clouds of gas and dust. The clouds have collapsed and given birth to new stars. Many stellar nurseries are visible in both galaxies — the birthplaces of millions of new stars.

    The galaxies are in Cetus, the whale or sea monster. Cetus is in the southeast as night falls, to the right and lower right of the almost-full Moon.


    Script by Damond Benningfield

    Support McDonald Observatory


  • Posted on 27 Nov 2020

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    Alpha Persei

    Not many stars have entire clusters named after them. One that does is Alpha Persei, the brightest star of Perseus, the hero. It’s a massive young star that’s nearing the end of its life. And it’s surrounded by a family of hundreds of other stars — the Alpha Persei Cluster.

    Alpha Persei — also known as Mirfak — is the dominant member of the cluster. It’s at least eight times the mass of the Sun, about 70 times the Sun’s diameter, and 5,000 times the Sun’s brightness. That brilliance makes it easy to spot even though it’s more than 500 light-years away.

    Several other big, bright stars are within a few light-years of Alpha Persei. And hundreds of smaller stars lie within a few dozen light-years. The stars are members of the cluster — a family of stars that was born from a single giant cloud of gas and dust.

    Estimates of just when that took place vary, but the cluster’s maximum age appears to be about a hundred million years. At that age, Alpha Persei has already passed the end of its “normal” lifetime. Now, it’s about ready to expire. But just how it will do so isn’t clear. The star’s mass puts it near a dividing line. A star above that line explodes as a supernova. A star below the line dies a bit more quietly, leaving behind a small, hot ember known as a white dwarf.

    Look for Alpha Persei in the northeast at nightfall. It’s far to the left of the Moon, and well above the brighter star Capella.


    Script by Damond Benningfield

    Support McDonald Observatory


  • Posted on 26 Nov 2020

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