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Thursday, 31 January, 2002, 17:01 GMT
Our galaxy - from the outside
We are in there somewhere
By BBC News Online science editor Dr David Whitehouse

This is our home galaxy as it might look if you could travel outside it and look back.

Astronomers obtained this perspective by analysing half a billion stars measured by the Two Micron All Sky Survey (2MASS).

It features the Milky Way's complete disc and its newly discovered central bar of stars.

The new map will help scientists confirm the existence of hitherto only suspected features in our galaxy.

Our position

There is plenty of indirect evidence to indicate the Milky Way is similar in shape to the many other flattened, spiral galaxies we see scattered throughout the Universe.

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.

See also:

22 Mar 01 | Sci/Tech
Dead stars could be 'missing mass'
20 Sep 00 | Sci/Tech
Scientists look into Milky Way core
19 Dec 00 | Sci/Tech
Journey to the galactic core
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