The bright star RS Puppis at the center of this image is surrounded by a cocoon of reflective dust. The super star is ten times more massive than the Sun, 200 times larger, and averages over 15,000 times brighter than the Sun. Averages? Yes, it is one of the most luminous Cepheid variable stars, and its brightness varies rhythmically over a six-week period.
The surrounding nebula flickers in brightness as pulses of light from the Cepheid propagate outwards. By tracking the fluctuation of light in RS Puppis itself and recording the faint reflections of light pulses moving across the nebula, astronomers are able to measure these light echoes and compute the distance to the star with about one percent uncertainty. The distance to RS Puppis is just about 6,500 light-years.
NGC 4603 is a spiral galaxy located a bit more than 100 million light years away in the constellation Centaurus. It’s a pure spiral galaxy with relatively loosely wound arms. During 1999, this galaxy was the subject of an extended study using the Hubble Space Telescope to locate Cepheid variable stars. Over 40 were found, and the measurement of their periodicities gave a net distance estimate of 108.7, +5.5,−4.9 million light years. That was consistent with the distance estimate determined through redshift measurements. As of the date of that study, NGC 4603 was the most distant galaxy for which a distance estimate had been made using Cepheid variable.
This Hubble Space Telescope image resembles a holiday wreath full of sparkling lights. The bright star RS Puppis is at the center of the image and is wrapped in a cocoon of reflective dust lit by the star. RS Puppis is huge, ten times more massive than our sun and 200 times larger. It’s one of the most luminous stars in the class of known as Cepheid variables and brightens and dims over a six-week cycle. Its average intrinsic brightness is 15,000 times greater than our Sun’s.
The surrounding nebula flickers in brightness as pulses of light from the Cepheid move outwards. Hubble has taken a series of photos of light flashes rippling across the nebula in a phenomenon known as a “light echo.” Several can be seen in this picture, most easily the ones moving toward seven o’clock. Even though light travels at around 300,000 km/s, the nebula is so large that reflected light can actually be photographed traversing the nebula. Using these reflections, astronomers are able to measure these light echoes and accurately compute the distance to RS Puppis—6,500 light-years (with a margin of error of only one percent).
This is the spiral galaxy NGC 3021 which lies about 100 million light-years away in the constellation of Leo Minor (The Little Lion).
This galaxy contains several Cepheid variable stars which can be used work out the distance to the galaxy. These stars pulsate at a rate that is closely related to their intrinsic brightness. Measurements of their rate of pulsation and their observed brightness provide information used to calculate the distance to them and, thus, the galaxy itself.
NGC 3021’s Cepheids were also used to calibrate another even brighter distance marker that can be used over greater distances: a Type Ia supernovae. One of these bright explosions was observed in NGC 3021 in 1995.
Spiral galaxy NGC 3370 is roughly the same size and general layout as our Milky Way. It’s about 100 million light-years away in the direction of the constellation Leo. It dominates the foreground of this Hubble image (click it to embiggen) which shows other galaxies scattered in the background. This particular picture is detailed enough for study of individual pulsating stars known as Cepheid variables that can be used to accurately determine the distance to the galaxy. NGC 3370 is also home to a recent type Ia supernova. Combining the known distance to this “standard candle” supernova and the Cepheid data with observations of other supernovae at even greater distances has been useful in refining estimates of the age and expansion rate of the Universe.