European Space Observatory astronomers claim to have successfully measured the distance to a Cepheid star inside our galaxy, with a precision of about 1 percent, making it the most accurate stellar distance measurement ever. The observations have been conducted at ESO's New Technology Telescope at La Silla by lead author of the paper, Pierre Kervella.

Cepheid stars are being routinely used by astronomers all over the world to measure stellar and galactic distances. They represent a particular type of variable stars, with extremely tight correlation between the period of variability and absolute luminosity, which makes them ideal as standard candles, astronomical objects with known luminosity from which distances can be derived by implying several methods.

They are usually represented by population I giant yellow stars with luminosity varying from 1,000 to 10,000 times that of the Sun. The unique pulsing in luminosity is given by an instability in the star which determines it to expand and contract rapidly. Luminosity variation is triggered by an ionization cycle of the atmosphere of the star, composed mostly of helium gas. When helium is ionized, it suddenly becomes more opaque to light, meaning that less light is emitted into space.

Distance measurements using Cepheid star light are usually extremely simplistic as they are not directly related to any physical aspect or property of the stars themselves, and are mostly reduced to a geometrical problem. This is why they have been used by astronomers as distance indicators for nearly one century.

Kervella writes that, in their study, the team has mostly relied on observations made on the Cepheid RS Pup, a variable star located in the Puppis constellation, 6500 light years away. RS Pup is easily visible on the night sky with the help of a pair of binoculars or a small telescope, and is 10 time more massive than the Sun, has a volume about 200 larger and can experience an absolute brightness at least 15,000 time higher than our Sun. Once in 41.4 days, RS Pup decreases its brightness by a factor of five.

What makes distance measurements so accurate on RS Pup is the fact that it is located in a large nebula, which reflects part of the light emitted by the star in the surrounding areas and towards our direction. By calculating the distinctive pattern of light emitted by the star and that reflected by the massive cloud of dust and gas, astronomers are able to measure the distance to the star with enhanced precision.

Kervella explains: "The light that traveled from the star to a dust grain and then to the telescope arrives a bit later than the light that comes directly from the star to the telescope, thus if we measure the brightness of a particular, isolated dust blob in the nebula, we will obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time."

The phenomenon is known in scientific vocabulary as 'light echo'. Knowing that distance is speed multiplied by time, the speed of light, the delay of the light echo and the distance between the star and the nebula, astronomers calculated that RS Pup lies 6500 light years away from Earth, with an error of 90 light years, which is about a quarter of the distance between the Sun and the center of the Milky Way. The RS Pup Cepheid star is located in the galactic matter disk, in an area of the Galaxy populated by a large number of stars.