Have you ever seen an echo? Maybe if you’re a bat, which can form a mental picture of its environment using echolocation. Well, then, allow me to introduce you to the star RS Puppis in the constellation Puppis the Ship’s Stern. Unlike the steady sun, RS Pup pulsates like a beating heart, becoming brighter and fainter by a factor of five about every 40 days. Instabilities in the star’s core brought on by switching from hydrogen to helium fuel cause regular expansions and contractions – actual changes in the star’s size – that we see as cyclic changes in its brightness.
Time-lapse video using observations from the Hubble to show the variable star RS Puppis. What appears to be an expanding gas cloud are really echoes of light reflecting from a gas cloud around the star.
Lucky for us bat types, the star is swaddled in nebulosity, a swirly cloud of gas and dust. As RS Pup brightens, a pulse of light travels from near to far across the expanse of the nebula, reflecting off gas and dust to create a light echo. Watching the video might make you think you’re seeing an expanding gas cloud. That’s an illusion. Instead, light moves across the nebula like someone pointing a flashlight in a cave, the beam touching nearby walls first and then reaching farther back into the deeper recesses. Every 5-6 weeks another pulse echoes across the murky gas clouds.
We hear an echo when sound bounces off objects in the environment and reflects back to our ears; echoes around RS Puppis happen the same way except with light instead of sound.
Interestingly, as the reflected light bounces off the nebula’s nooks and crannies, it takes a variety of different paths through the gas and arrives at Earth after light that travels straight from star to telescope. This helps reinforce the ‘expanding gas cloud’ illusion.
Time-lapse video of the light echo around V838 Mon photographed multiple times between 2002 and 2006 by the Hubble Space Telescope.
An even more amazing light echo blossomed into view when the star V838 Monocerotis underwent a tremendous outburst in early 2002, suddenly becoming more than 600,000 times brighter than the sun. Over the next few years the eruption illuminated whorls and eddies of interstellar dust around the star, producing the most spectacular light echo ever recorded. Even modest-sized telescopes showed the light echo as a small, comet-like haze.