First age-map of Milky Way galaxy created

The first large-scale age-map of the Milky Way shows that a period of star formation lasting about four billion years created the complex structure at the heart of our galaxy, scientists say.

The Milky Way is a spiral galaxy with a bulge at the centre, thousands of light years in diameter, that contains about a quarter of the total mass of stars.

Previous studies have shown that the bulge hosts two components: a population of metal-poor stars that have a spherical distribution, and a population of metal-rich stars that form an elongated bar with a "waist", like a bi-lobed peanut.

However, analyses of the ages of the stars to date have produced conflicting results.

Now, an international team led by astronomers from the European Southern Observatory (ESO) have analysed the colour, brightness and spectral information on chemistry of individual stars to produce the age-map of the Milky Way.

"We analysed the colour and brightness of stars to find those that have just reached the point of exhausting their hydrogen fuel-burning in the core, which is a sensitive age indicator," said researcher Marina Rejkuba.

"Our findings were not consistent with a purely old Milky Way bulge, but require star formation lasting around 4 billion years and starting around 11 billion years ago.

"The youngest stars that we see are at least 7 billion years old, which is older than some previous studies had suggested," said Rejkuba, who presented the research at the European Week of Astronomy and Space Science (EWASS) in the UK.

The researchers used simulated and observed data for millions of stars from the VISTA Variables in the Via Lactea (VVV) infrared survey of the inner Milky Way.

They compared them with measurements of the metal content of around 6,000 stars across the inner bulge from a spectroscopic survey carried out with the GIRAFFE/FLAMES spectrograph on the ESO Very Large Telescope (GIBS).

The results presented are based on the analysis of three areas of the VVV infrared map, which, combined, make up the largest area studied so far in the Milky Way bulge.

In all three areas, the findings on the age range of the stars are consistent, researchers said.