This star is RS Puppis, and it isn't just the stellar equivalent of a pretty face; it's absolutely crucial to our understanding of the universe. This is a Cepheid variable star, which in part means its brightness increases and decreases on an exact, predictable schedule. And because it's surrounded by a giant nebula that reflects its pulsating light, we see every detail of its perfectly regular beacon.

RS Puppis โ€” or, somewhat more affectionately, RS Pup โ€” totally dwarfs our star, as it's a good ten times as massive as the Sun and a whopping 15,000 times brighter. And while that high luminosity means we can easily see the star from Earth, it's not the absolute magnitude that really concerns scientists. Instead, it's the subtle, regular changes in brightness and the interplay of its light with the surrounding nebula. Since the speed of light is close to constant in the vacuum of space, we can use the observed time it takes for the light to travel from RS Pup to the nebula to calculate distances far more precisely than we could under normal circumstances, and that in turn is absolutely crucial in establishing benchmarks for the distances of other Cepheid stars. Here's a more detailed explanation of what's going on, courtesy of NASA:

As RS Pup pulsates over a period of about 40 days, its regular changes in brightness are also seen along the nebula delayed in time, effectively a light echo. Using measurements of the time delay and angular size of the nebula, the known speed of light allows astronomers to geometrically determine the distance to RS Pup to be 6,500 light-years, with a remarkably small error of plus or minus 90 light-years. An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup, and by extension other Cepheid stars, improving the knowledge of distances to galaxies beyond the Milky Way.


To see the entire image in its full glory, head over to the NASA website.

Image Credit: Hubble Legacy Archive, NASA, ESA - Processing: Stephen Byrne