RS Puppis is a Cepheid variable star. Thanks to the work of Henrietta Leavitt at Harvard, we know there is a relationship between the period of variability and the luminosity (intrinsic brightness) of a Cepheid variable. This means we can use Cepheid as a standard candle to as one way to determine distance to the really far out star or galaxy the stars reside in.
Distances to closer objects can be determined by other means like the parallax but that tends to be less accurate at great distances. One of the reasons the ESA Gaia launch is so exciting is that parallax measurements will be very accurate and to greater distances than we can do currently. Distances to the stars are very difficult to get accurately.
From NASA (links to a full res image and it is amazing!) with acknowledgment to H. Bond (STScI and Pennsylvania State University):
This festive NASA Hubble Space Telescope image resembles a holiday wreath made of sparkling lights. The bright southern hemisphere star RS Puppis, at the center of the image, is swaddled in a gossamer cocoon of reflective dust illuminated by the glittering star. The super star is ten times more massive than our sun and 200 times larger.
RS Puppis rhythmically brightens and dims over a six-week cycle. It is one of the most luminous in the class of so-called Cepheid variable stars. Its average intrinsic brightness is 15,000 times greater than our sun’s luminosity.
The nebula flickers in brightness as pulses of light from the Cepheid propagate outwards. Hubble took a series of photos of light flashes rippling across the nebula in a phenomenon known as a “light echo.” Even though light travels through space fast enough to span the gap between Earth and the moon in a little over a second, the nebula is so large that reflected light can actually be photographed traversing the nebula.
By observing the fluctuation of light in RS Puppis itself, as well as recording the faint reflections of light pulses moving across the nebula, astronomers are able to measure these light echoes and pin down a very accurate distance. The distance to RS Puppis has been narrowed down to 6,500 light-years (with a margin of error of only one percent).
The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Md., conducts Hubble science operations. STScI is operated by the Association of Universities for Research in Astronomy, Inc., in Washington, D.C.