A subdwarf B star is a blue-hot star smaller than the Sun. A few of these crazy stars pulse in brightness as fast as every 90 seconds! Waves of ionizing iron pulse through their thin surface atmosphere.
What’s up with these weird stars?
Sometimes a red giant loses most of its outer hydrogen… nobody is sure why… leaving just a thin layer of hydrogen over its helium core. We get a star with at most 1/4 the diameter of the Sun, but really hot.
It’s the blue-hot heart of a red giant, stripped bare.
Iron and other metals in the star’s thin hydrogen atmosphere can lose and regain their outer electrons. When these electrons are gone, the metals are ‘ionized’ and they absorb more light. This pushes them further out. Then they cool, become less ionized, absorb less light, and fall back down. This heats them up, so they become more ionized and the cycle begins again.
This happens in standing waves, which follow spherical harmonic patterns. You may have seen spherical harmonics in chemistry, where they describe electron orbitals. The same math is being applied here to a whole star! Now it’s not the electron’s wavefunction that’s pulsing in a spherical harmonic: it’s metals in the atmosphere of a star.
When the star is rotating, spherical harmonics that would otherwise vibrate at the same frequency do so at different frequencies. So, just by looking at the pulsing of light from a distant subdwarf B star, you can learn how fast it’s rotating!
I got the gif of a pulsing star from here:
• White Dwarf Research Corporation.
Pulsating white dwarf stars also oscillate in spherical harmonic patterns, and this website shows how they look.
The figure showing frequency lines is from this cool paper:
• Stephane Charpinet, Noemi Giammichele, Weikai Zong, Valérie Van Grootel, Pierre Brassard and Gilles Fontaine, Rotation in sdB stars as revealed by stellar oscillations, Open Astronomy 27 (2017), 112–119.
This paper says “a κ-mechanism triggered by an accumulation of heavy elements (in particular iron) in the stellar envelope caused by radiative levitation is driving the oscillation.”
So, what’s the κ-mechanism and radiative levitation?
The κ-mechanism causes oscillations when a layer of a star’s atmosphere gets more opaque at higher temperatures. For example, when heavy metals near the surface of the atmosphere get hot they can ionize, and thus absorb more radiation. When the layer of ions falls in it gets hotter, more opaque, blocks more escaping heat, and the star’s pressure goes up… pushing the layer out. But when the layer shoots out it gets cooler, less opaque, blocks less escaping heat, and the pressure drops again. So we can get oscillations!
Radiative levitation can drive heavy metals to the surface of a star. They absorb light, and the light literally pushes them up. This can make
these metals thousands of times more common than you’d expect near the surface.
There’s more that can happen with subdwarf B stars, and you can learn about it here:
• Wikipedia, Subdwarf B stars.
For example, they can simultaneously oscillate in two ways, at two separate rates!
Azimuth 
Very fascinating stuff! I’ve been looking at a citizen science project from ASAS-SN (https://www.zooniverse.org/projects/tharinduj/citizen-asas-sn) which aims at finding more variable stars by collecting light from a star during 5+ years and then presenting the resulting light curve folded with calculated most likely period.
I wonder if this type of variability can be detected by using only the light curve, or you need additional details like mass or spectrum to distinguish sdB from other pulsating variables like RR Lyrae or cepheids, which from the looks of it are pulsing because of the k-mechanism of helium ions, not iron.