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Podcaster: George Bendo

Title:  George’s Random Astronomical Object 70: The Uncredited Star

Organization: Jodrell Bank Centre for Astrophysics, The  University of Manchester

Link: http://www.jb.man.ac.uk/~gbendo/home.html

Description: George’s Random Astronomical Object presents the star EtA Aquilae.  Although Eta Aquilae was the first Cepheid variable ever discovered, this class of stars was named after Delta Cephei instead.

Bio: George Bendo is an astronomer who specializes in studying interstellar dust and star formation in nearby galaxies.  He currently works at the Jodrell Bank Centre for Astrophysics at the University of Manchester, and his primary role is to support other astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA).  He has been creating biweekly episodes of George’s Random Astronomical Object since 2019.

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Transcript:

Welcome to George’s Random Astronomical Object.  Every episode, I run a random number generator to select random astronomical coordinates in the sky and I then search for an astronomical object near those coordinates and talk about what makes that object so interesting to astronomers.  So, let’s now turn on the random number generator.

So a screw came loose in the random number generator and began rattling around in one of the rotating metal parts.  That was actually supposed to happen.  The coordinates from the generator are 19:52:28.4 right asecension and +01:00:20 declination.  These coordinates point to the star Eta Aquilae.

This is a yellowish star located at a distance of about 880 light years (or 270 pc), although astronomers seem to have had difficulty accurately measuring that distance).  The constellation Aquila is supposed to represent an eagle, and, unlike some constellations, it actually looks somewhat like what it’s supposed to represent, with the brightest star in the constellation as the head, two more bright stars in a line below the brightest star representing the body and tail of the eagle, and two other bright stars to the left and right of the body that represent the wings.  Eta Aquilae is located in the bottom part of the left wing of the constellation.

What’s special about Eta Aquilae is that its brightness is variable.  Its magnitude changes from 3.5 to 4.3 over a time period of about 7 days and 4 hours.  This change in brightness is big enough and short enough that it’s possible to look at the constellation over a period of several days and witness the star changing brightness relative to the other stars in the constellation.  This is something that you can even try yourself if you have enough patience.

Eta Aquilae was also one of the first stars that was ever identified to periodically change in brightness.  The British astronomer Edward Pigott discovered that the star was variable in 1784.  The scientific paper announcing the discovery of Eta Aquilae’s variability is so old that many of the s’s look like f’s and Edward Pigott is referred to as “Edward Pigott, Esquire”.  Eta Aquilae turned out to be the first star identified as belonging to a new class of variable stars that would later be named Aquilids.  No, just kidding.  They were named Cepheids after Delta Cephei, which was the second star to be identified as belonging to this class of variable stars.  I feel kind of sorry for Eta Aquilae, which really seemed to miss out on a golden opportunity.

Anyway, Cepheids like Eta Aquilae are giant yellow stars several times the mass of the Sun that have evolved past the stage where they fuse hydrogen into helium in their cores.  They could be at the stage where they are fusing helium into carbon in their cores or at a point when their cores have filled up with carbon and they are fusing helium into carbon in shells around these cores.  What makes them vary in brightness is that their outer atmospheres are unstable and tend to pulse regularly with a period ranging from about a day to several weeks.  At the beginning of one of these cycles, the atmosphere might be as compact as possible, but it would also be relatively hot and opaque.  This would cause the outer gas layer to expand, but as it expands, it becomes more transparent and cools.  When that happens, the outer gas layer contracts, but it then turns opaque and gets hot again, which causes it to expand again.  This continues for a very long time while the star is at this stage in its evolution.

What is notably special about Cepheid variables is that their pulsation periods are directly linked to their luminosities.  This means that, if someone spots a Cepheid variable somewhere, they can use the pulsation period to estimate how much light the star produces and then compare that to how much light reaches the Earth to estimate the distance to the star.  This has been very important for measuring the distances to other galaxies, and it is still the most reliable technique for making such measurements.

So, astronomers these days will spend a lot of time observing a lot of different Cepheid variable stars, but they still pay extra attention to Eta Aquilae.  I would like to think that this is because some astronomers feel guilty about how Eta Aquilae missed out on having a class of stars named after itself and are trying to give it some love and attention, but the real reason is because it is one of the closest and brightest Cepheids as seen from Earth, so it is a good target to observe if astronomers want to understand the details of how Cepheids work.

In fact, Eta Aquilae has attracted a lot of attention because astronomers have determined that it might have at least one and possible more than one star orbiting it.  If this is actually the case, astronomers could use measurements of these orbiting stars to estimate the mass of Eta Aquilae, which would be very helpful for creating accurate scientific models of this star as well as other Cepheid stars.  However, astronomers have struggled to determine exactly what’s happening in the Eta Aquilae star system.  Both the Hubble Space Telescope and the Very Large Telescope found a yellowish star to the east that could be in orbit around Eta Aquilae, but not enough measurements have been made to actually show that this other yellow star doesn’t actually just lie somewhere in front of or behind Eta Aquilae by chance and doesn’t have anything to do with the Cepheid.  Meanwhile, a bunch of other observations imply that a much fainter but really hot blue star should be closely orbiting Eta Aquilae, but no one has yet found definitive proof that this hot blue star actually exists.  In any case, astronomers will be spending much more time looking at Eta Aquilae to try to figure out exactly what’s going on in the star system.

So that is my summary of why astronomers have spent so much time watching Eta Aquilae vary in brightness, and remember, this star is bright enough that you too can spend time watching it change in brightness.  The location on the Earth’s surface corresponding to the position of Eta Aquilae in the night sky is located in Parque Nacional da Serra da Mocidade in northern Brazil.  While this park is named after mountains that lie in the north part of the park, the location corresponding to Eta Aquilae lies in lowlands in the south that are prone to flooding.  I would imagine that the area is popular with capybaras.  Too bad I can’t find a good capybara sound effect.

The website for this podcast is www.randomastronomicalobject.com.  You can visit the website to download episodes of the show, read information about the astronomical objects, view images of those astronomical objects, look up additional reference information, and send me random feedback.  You can also find this podcast of Facebook and Twitter.

The audio was recorded and edited by George Bendo, and the sound effects are from The Freesound Project at www.freesound.org.  Thanks for listening.

End of podcast:

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