Podcaster: George Bendo
Title: George’s Random Astronomical Object (Object 134: Not Dead Yet)
Organization: Jodrell Bank Centre for Astrophysics, The University of Manchester
Link: https://www.randomastronomicalobject.com
Description: George’s Random Astronomical Object presents the planetary nebula NGC 7094. While this object may look fairly typical in terms of planetary nebula, the star at the center is not quite a dead white dwarf yet.
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 134 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. So, I will now run the random number generator.
The random number generator has returned the coordinates of 21:36:53.0 right ascension and +12:47:19 declination. These coordinates point to the planetary nebula NGC 7094 in the constellation Pegasus (and just as a quick reminder, planetary nebulae are what are produced by stars about the size of the Sun when they die and are not the nebulae surrounding stars that planets are made of). Anyway, NGC 7094 looks kind of roundish for a planetary nebula, although it’s slightly asymmetric on one side because it looks like the expanding gas shell from the nebula collided with something in the interstellar medium. The nebula is also a bit on the faint side, in part because it’s located at a distance of roughly 5400 light years (1655 pc). It can be seen in an amateur astronomy telescope, but you’ll need a fairly hefty telescope if you want to try to find it yourself. Most amateur astronomy references described using 18 inch (46 cm) telescopes or larger. It apparently looks like a faint haze surrounding a bright central star, which sounds like a fairly typical description of any planetary nebula. It also seems to be a popular amateur astrophography target for some reason.
However, while amateur astronomers with big telescopes might be interested in spotting the nebula, professional astronomers are much more interested in the central star. But first, let me quickly review what happens when a Sun-like star dies.
Stars start their lives being powered by the fusion of hydrogen into helium in their cores, but after around 10 billion years or so, the core of a star roughly the size of the Sun will fill up with helium. At this point, the star will expand to become a red giant, and hydrogen fusion will continue in a shell around the core. At some point, enough pressure builds up in the core to trigger the fusion of helium into carbon or oxygen. After a while, though, the star’s core will fill up with carbon and oxygen, but the star won’t have the mass to produce the pressures needed to fuse those elements into heavier things, so the core just sits there while helium fusion continues around the core and hydrogen fusion continues around the helium shell. The star at this stage will not only have expanded a lot but will also begin to blow away its outer gas layers to form a planetary nebula. What should eventually be left is just an inert lump of carbon and oxygen that becomes a white dwarf.
So this describes what’s happened in most planetary nebulae. However, the star at the center of NGC 7094 is not quite dead yet. Spectroscopic observations have identified not only the presence of carbon and oxygen but also hydrogen in this star’s atmosphere. In other words, this star still has a layer of hydrogen gas.
Technically, this star is classified as a hybrid PG 1159-type star, and I could take 10 minutes to try to explain what the heck that name means, but instead, let’s just work with the fact that this star is not yet a completely dead white dwarf. Instead, people have indicated that this star is undergoing its final thermal pulse. The structure is actually fairly simple. It has the inert carbon and oxygen core, a small shell where helium is being fused into carbon and oxygen, and a thick layer of hydrogen on top. That hydrogen layer is undergoing convection, drawing carbon and oxygen up from the lower layers to the surface so that we can see those elements mixed with the hydrogen in the star’s spectrum.
The star in NGC 7094 is one of a very small number of stars at the centers of planetary nebulae that have reached this not quite dead yet stage (or I could say that it’s one of a very small number of hybrid PG 1159-type stars, but I really don’t want to look up where that name comes from or try to explain it). In any case, this star is important to understanding the final stages of how a Sun-like star becomes a white dwarf, and that is why it will continue to be a unique object for astronomers to study.
The location on the Earth’s surface corresponding to the position of NGC 7094 in the sky is bout halfway between South America and Africa in the Atlantic Ocean. Most people’s first thoughts about this location is that it could correspond to the Mid-Atlantic Ridge, but it looks like the ridge manages to avoid this location, instead shifting to the west of the position corresponding to NGC 7094. It’s almost as if the ridge itself is consciously trying to avoid associating with the planetary nebula.
If you do not want to consciously avoid my podcast, you can visit the website for my podcast at www.randomastronomicalobject.com, where you can read information about the astronomical objects, view images of those astronomical objects, look up additional reference information, and send me random feedback. Also, if you are interested, I have accounts on Facebook, Instagram, and Threads for this podcast where you can also connect with my podcast.
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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