NGC253: The Milky Way is like many other flattened, spiral galaxies

However, because the Earth is positioned within the galaxy's disc of stars and obscuring dust fills interstellar space, it is not easy to measure the precise shape of our galaxy.

We do know that our Sun sits about 27,000 light-years from the Milky Way's centre and we can infer the presence of a bar-like structure in the central regions.

And now, by using infrared light to peer through the dust and look at 30,000 stars of known brightness - so-called carbon stars - astronomers can build up a 3D map of the surrounding space.

Markers in the fog

The 2MASS survey, which has identified more than 500 million stars in total, uses two dedicated telescopes, one in the US and the other in Chile, to scan the sky in a co-ordinated way to construct a database of infra-red objects.

The project has spent the past four years collecting near-infrared data. Near-infrared light has wavelengths that are about four times longer than the visible light that human eyes can see, and penetrates the "fog" of dust that obscures much of our view of the Milky Way.

The carbon stars surveyed weigh between one and a few times the mass of the Sun. Their central furnaces are nearly out of fuel, and carbon is one of the products from their final nuclear reactions.

Even though they have run out of hydrogen - their main source of energy - these stars become very bright for a while, making their distances easy to determine. Reconstructing their locations in three-dimensional space yields a picture of the Milky Way as seen from a vantage point well outside the disc.

Evident in the map, and seen directly for the first time, is the cigar-shaped bar toward the centre of the galaxy and the complete outer boundary of the disc of stars.

Future work will attempt to extract more subtle features of the galaxy's structure using hundreds of millions of 2MASS detections rather than just 30,000.