Cesium detected in the atmosphere of a hot white dwarf | Hacker News Reader

Cesium detected in the atmosphere of a hot white dwarf

phys.org

59 pointswglb14 comments

What is the significance of this finding? I see something about "Radiative levitation" which means nothing to me. Is the idea to prove that fusion is still happening in these stars somehow?

Look at wikipedia https://en.m.wikipedia.org/wiki/Radiative_levitation

Essentially. Big ions capture photons better.

> The mechanism is that heavier elements have large photon absorption cross-sections when partially ionized (see opacity), so efficiently absorb photons from the radiation coming from the core of the star, and some of the energy of the photons gets converted to outward momentum, effectively 'kicking' the heavy atom towards the photosphere. [0]

I.e. radiation pushes heavier elements to the surface, increasing their visibility to us much higher than their fraction of stellar mass

[0] https://en.wikipedia.org/wiki/Radiative_levitation

I read that and still don't see the significance. Admittedly, I am very far from the topic.

The significance is that these elements detected in a star don't mean they exist throughout the star, but only in the outer layers. So we don't need to somehow adjust theories of stellar evolution to explain how all that cesium got produced somewhere and this star got full of it. There's only a small amount of cesium, which fits within existing theories (produced from past supernovas), it's just unusually visible.

It's the first time this particular element has been observed in this context. Other trans-iron elements have been seen in the atmospheres of hot white dwarfs before, so not that surprising.

I am not a physicist or astronomer, but I would think detectable levels of trans-iron isotopes would call into question some of the assumptions about heavy element production in stars. IIRC anything heavier than iron is not an exothermic nuclear reaction.

I speculate the astronomers are trying to figure out how detectable amounts of a very heavy element are in the outer layers of the star where the emission lines can be detected. Later stage stars are layered by the elements produced in fusion.

I thought that dwarfs resulted from stars lacking the gravitational pressure to create fusion in the core to reach those elements. So how did they get there at all?

And if it's detectable in the halo of the star, that means there is enough in the lower levels (since late-stage stars are basically layers of each atomic number where they are fused) that some of it migrates to somewhere where the lines show up in the star's light emissions?

Maybe it's just stuff left over from a previous star's supernova when the star formed? I can't see that resulting in enough that it would appear.

Anyway, those are my amateurish questions.

It is stuff left over from a previous star's supernova. That's the only way anything heavier than iron and nickel gets produced. It is a small amount, but it's more visible than its raw proportion, because it gets suspended in the stellar atmosphere, because the large atoms get hit by a lot of radiation pressure.

The point of this is that we don't need to rethink heavy element production (or at least not by much), because known processes suffice.

Is it aliens?

It's (almost) never aliens.

Underrated comment

With the barrage of Astronomy articles that have been hitting HN front page of late, perhaps we should start asking, "Is it Aliens yet?"

It's aliens and they are failing do de-cesify their economy.

Just a regular ol' alien nuclear waste landfill star, nothing to see here.

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