Researchers have created a chart that depicts the way Milky Way grew from its infancy to the bright spiral galaxy that it is today, Business Standard reports.

"Close to the centre of our galaxy, we see old stars that were formed when it was young and small. Farther out, we see young stars. We conclude that our galaxy grew up by growing out," said Melissa Ness from the Max Planck Institute for Astronomy in Heidelberg in Germany, who led the study. 

"To see this, we needed an age map spanning large distances, and that is what this new discovery gives us," she added. 

For the chart, the researchers used the ages of more than 70,000 stars. The chart extends halfway across the Milky Way to 50,000 light-years away. 

Researchers mapped the galaxy by observing red giants in the inner and outer reaches of the Milky Way. 

"If we know the mass of a red giant star, we know its age by using the fusion clock inside every star. Finding masses of red giant stars has historically been very difficult, but surveys of the galaxy have made new, revolutionary techniques possible," said Marie Martig, co-author of the study. 

The researchers combined information from the APOGEE spectra and Kepler lightcurves and then applied their methods to measure ages for all 70,000 red giant stars sampling all parts of the galaxy. 

"APOGEE is the ideal survey for this work because it can get high-quality spectra for 300 stars simultaneously over a large area of sky," said Steve Majewski, principal investigator of the APOGEE survey. 

The Kepler satellite is a NASA space mission whose main goal is to find planets around stars.