Thousands of stars comprising Spitzer's Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) have been used to listen for the 'heartbeat' of the Milky Way in terms of the rate at which our Galaxy is producing new stars.

A galaxy's star formation rate provides information on its evolution and projected lifespan. Stars form from the gravitational collapse of gas that rotates around a central core that heats up, eventually igniting nuclear fusion to sustain the star. Leftover gas and dust can accumulate in a disc around the star, and may clump together to create planets. Using Spitzer Space Telescope observations astronomers have tracked down baby stars in our own Milky Way Galaxy, given away by their bright infrared signatures.

Specifically, lead author Thomas Robitaille of the Harvard-Smithsonian Center for Astrophysics, and colleagues, counted young stellar objects (also known as YSOs) located in a slice of sky spanning two degrees by 130 degrees, equivalent to an area covered by 330 full moons. While earlier surveys had captured fuzzy light from tens of thousands of stars, GLIMPSE revealed 100 million stars, of which some 20,000 were YSOs.

"We are seeing forming stars all the way through the Galaxy for the first time," says Robitaille's co-author Barbara Whitney of the Space Science Institute in Boulder.

Applying this observational data to computer simulations of galactic star formation provided a result that suggests our Galaxy has an annual star formation rate of two-thirds to one-and-a-half times the mass of the Sun. Previous research using indirect methods, such as measuring radio waves from hydrogen gas clouds energised by the biggest, hottest stars in the Galaxy and estimating how many smaller stars form for every one of these rare but easily detectable stars, had set an upper limit of five times the mass of our Sun produced each year.

"Measuring the rate of star formation inside the Milky Way with this method is important not just for understanding our Galaxy, but also has implications for measuring star formation rates for all galaxies," says Robitaille.

Our Galaxy hosts around 100 billion stars, so today's low star-formation rate is unlikely a reflection of the past. Instead, after an initial period of intense star formation our Galaxy has settled into the formation rate typical of a middle-aged galaxy, generating stars from gas that older stars have expelled back into the stellar environment having already reached the end of their lifetimes.

The new technique will continue to be refined as more data becomes available, and will help calibrate star-formation rates in other galaxies.