Light Echoes From A Variable Star

[drhoz]

Timelapse of Cepheid variable star RS Puppis taken by the Hubble Space Telescope.  Light echoes ripple through the surrounding nebula as the star pulses in a 41 day cycle.

When I saw this GIF I suspected it was photoshopped, mostly on the grounds that those light echoes are, or so I thought, too far apart for a 41 day cycle. Perhaps I'm overly suspicious, because it turns out these are real images, and the reason that they are so fasr apart is the geometry of the echo, which can appear to move at faster-than-light speeds, as was the case with V838 Monocerotis, where a small star was engulfed by a larger one, and the resulting light echo spread out to 7 light years across in a matter of months. Light echoes can also be used to study centuries-old supernovae, even ones that were never seen from Earth because of dust clouds in the way.

In the case of RS Puppis, the star pulses - Cepheid variables are famously useful because of this, because the speed at which they pulse, and the speed at which they pulse depends on how intrinsically bright they are. Thus, by measuring how bright they appear from Earth, you can determine the shape of the galaxy, the position of the sun above or below the galactic disc, and the distances to the nearer galaxies. The actual mechanism by which they pulse is called the Eddington Valve. Hot gas in the star's outer layers becomes ionised, and doubly ionised helium is more opaque than singly ionised helium. The doubly ionised helium gets hotter and hotter, and the star's outer layers expand, cool, recapture their lost electrons, cool faster now the gas is less opaque, and collapse back down under gravitational pressure. Then the cycle repeats.