Podcaster: George Bendo
Title: George’s Random Astronomical Object (Object 136: Barium)
Organization: Jodrell Bank Centre for Astrophysics, The University of Manchester
Link: https://www.randomastronomicalobject.com
Description: George’s Random Astronomical Object presents the HD 11397. This Sun-like star might seem ordinary, but it actually contains abnormally large amounts of heavy elements, most notably barium, that it could not have formed itself.
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 episode 136 of 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. And now, I will turn on the random number generator.
The random number generator has returned the coordinates of 01:51:40.5 right ascension and -16:19:04 declination. This episode’s coordinates once again point to an object within the constellation Cetus. As I have previously mentioned, Cetus is a rather lame constellation because it’s supposed to look like a whale or sea monster but really doesn’t. However, Cetus is also the fourth largest constellation in the sky, so despite how stupid the constellation is, it’s going to turn up a lot in this podcast.
The specific object that this episode’s coordinates point to is the star HD 11397, which is located at a distance of 177 light years (54.3 pc). At first, this star might seem like a very ordinary Sun-like star. It’s about the same size, color, and brightness as the Sun, and it’s powered by the fusion of hydrogen into helium in its center like the Sun. It has a somewhat unusually elongated elliptical orbit around the center of the Milky Way, which indicates that it probably formed in our galaxy’s bulge, but that isn’t that unusual.
However, an analysis of spectra of this star published in 2008 by Luciana Pompéia and Dinah Moreira Allen indicates that its outer atmosphere contains excessive amounts of very heavy elements. The most notable element is barium. If you know the periodic table a little well but have not completely memorized it, barium has the atomic number 56 and sits two rows below calcium in the table. It’s used in real life for medical X-ray imaging and for vacuum tube coatings. The other heavy elements that Pompéia and Allen found include strontium, yttrium, zirconium, molybdenum, ruthenium, lanthanum, cerium, neodymium, samarium, dysprosium (which you probably didn’t even know was the name of an element), hafnium, and lead. While I find it fun to read inane lists of elements that include several lanthanides, the real reason why this list of elements measured in the spectra of HD 11397 is very important is because all of them are heavier than iron. A Sun-like star like HD 11397 should not contain excessively large amounts of these heavy elements. Because HD 11397’s outer layers contain so much barium in particular, it has been classified as a barium star, although, just to be clear, it’s still made up of mostly hydrogen and helium gas.
So let’s back up a little bit and talk about where very heavy elements came from. Sun-like stars, including HD 11397, form helium through the fusion of hydrogen in their cores. At later stages in these stars’ lifetimes, when they have gone past the red giant stage, these stars will be able to create elements as heavy as carbon and oxygen in their cores, but they do not have the mass to fuse that carbon and oxygen into heavier elements for energy. Much more massive stars will fuse carbon and oxygen for energy, but it is not possible to produce energy through fusion using any element as massive as or more massive than iron, so these very heavy elements are not produced the through fusion processes that produce power in the centers of stars.
Instead, these elements are produced in nuclear processes that take place the outer layers of stars when they die. These nuclear processes don’t produce energy, so it’s not like what’s taking place in the cores of stars, but they are still very important in forming elements heavier than iron. Even though many people might think of these heavy elements being produced when very massive stars explode as supernovae, when less massive stars about the size of the Sun get close to dying and start to expel their outer gas layers, this expulsion process will also lead to the creation of heavy elements. A few other types of objects can produce very heavy elements, but I’m going to skip over that.
The two common processes that take place in these types of situations are called the s-process (where s stands for slow) and the r-process (where r stands for rapid). In the s-process, neutrons flying through the outer gas layers of the dying object will slowly get absorbed by individual atoms’ nuclei. At first, this will cause the individual atoms to just change into different isotopes, but when any atom’s nucleus gets too many neutrons, the nucleus will become unstable, and one of the extra neutrons will change into a proton. Over time, this process can produce elements heavier than iron. For example, an iron atom nucleus with 26 protons and 30 neutrons can continue to absorb neutrons one by one until one neutron becomes a proton, making the iron atom into cobalt. The cobalt will continue to absorb neutrons until one of those becomes a proton, turning it into nickel, and so on. This process takes place when stars the size of the Sun die.
The r-process is the same except that the neutrons are all added at once. So, for example, 96 neutrons could be added all at once to an iron nucleus, and then 37 of those neutrons could change into protons, making the iron atom into a europium atom (and giving me another chance to mention the lanthanides). The r-process primarily takes place in events like supernovae.
So, back to HD 11397. This star has an abnormal number of elements created by the s-process, but the s-process only takes place when Sun-like stars have passed the red-giant stage and are close to dying and forming planetary nebula. Except for the excessive amounts of heavy elements, HD 11397 looks like an ordinary Sun-like star still at the stage where it’s fusing hydrogen into helium in its core. It’s nowhere near dying. This leads to the question of where did the large amounts of heavy elements come from in both HD 11397 and the very few other Sun-like stars like it that are also classified as barium stars.
One possibility is that these barium stars are in binary star systems with other companion stars that already died, and when these companion stars expelled their outer gas layers, they formed a lot of heavy elements including barium through the s-process and dumped those elements on the outer layers of the barium stars that we see today. The remnants of those dead stars would be white dwarfs that would still be in orbit around the barium stars, but they would be so faint that we would not see them.
The other possibility is that the barium stars originally formed out of interstellar gas clouds where an abnormal number of dying Sun-sized stars were dumping huge amounts of heavy elements into the gas clouds. Consequently, when stars formed out of this interstellar gas, they would have contained abnormally huge amounts of these heavy elements including barium to begin with.
More work is currently being done to both identify more barium stars and to understand exactly how these abnormal stars formed, and HD 11397 is going to be an important part of that research.
So that finishes up my summary of HD 11397, and the location on the Earth’s surface corresponding to the position of HD 11397 in the sky is a hilly, forested area about 110 km south-southwest from Lusaka, Zambia. In a way, it would have been cool if this location corresponded to a barite mine. However, this area is undeveloped wilderness, which is much nicer than a barite mine. Also, Zambia is much better known for its copper exports. People in Zambia just aren’t interested in barium.
The website for this podcast, which may or may not contain barium, is www.randomastronomicalobject.com. You can visit the website to read information about the astronomical objects, view images of those astronomical objects, look up additional reference information, and send me random feedback, and just to remind people, this podcast also has social media pages of Facebook, Instagram, and Threads.
The audio was recorded and edited by George Bendo. The sound effects are from The Freesound Project at www.freesound.org. Thanks for listening.
End of podcast:
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