Stellar Tiramisu?

The press release from ESO compares a red giant to tiramisu—because, as Luca Pasquini puts it, “There is cocoa powder only on the top!” Hmmm.

(The cocoa powder analogy has to do with the distribution of heavy elements in stars with planets. We know that extrasolar planets are preferentially seen around stars with high iron content, but do the planets form around stars with a lot of iron distributed throughout, or do planets sprinkle iron, cocoa-like, on the stars’ surfaces?)

The image above does a bang-up job, I must say. It possesses clarity, first and foremost, comparing apples to apples and balancing the diagrammatic and the photorealistic with aplomb. I like the clear labels (with caveats to be addressed below), and the two stars even show limb darkening. Most especially, I must express my deepest appreciation for the inclusion of a small figure (in the lower right) to communicate scale! Yes! Fantastic! Super! Well done!

I would not go so far as to suggest that the diagram is flawless in its execution, however. Aside from a slight irrelevance to the topic at hand, the main liability I can detect is the inconsistency between the left- and right-hand images: “radiative zone” gets labelled only on the left, while ”burning shell” appears on the right. Something of a fumble in the home stretch…

Stars, Planets, and Dwarfs

The above, rather featureless image of the Sun comes from a Google video that shows the scale of spherical objects in the Universe. It begins with an image of Mercury (N.B. not Pluto) and proceeds through all the planets of our solar system, from smallest to largest, then through a selection of stars of increasing size. The snapshot shows the transition from the planets (those itty, bitty blobs on the right, which are just the giant planets of our solar system) to the stars…

It’s cute. I like it. But here’s the caveat: some stars are smaller than the planets pictured. So the sequence gives the impression that there’s a much greater distinction between the two categories (of “star” and “planet”) than actually exists. In fact, an continuum exists that gives astronomers a bit of a headache. (Check out a comparison chart not dissimilar from the above from the educational resources at the Dwarf Archive.)

Communicating the scale of the Universe is a tricky thing, but as the recent Pluto controversy has shown us, classifying objects (even spherical ones) can be even trickier!

Tip of the hat to my friend who pointed this out.