All in the colours

Although astronomy is often seen to be about pretty pictures, one of the most exciting tools is spectroscopy. This is where you break down the light that you observe into its constituent colours. By identifying particular groups of colours, and the relative intensities of them, you can tell which elements or molecules are responsible. A familiar example is sodium which emits two specific shades of yellow and is the main component of 'orange' street-lights. So, by analysing the light you can tell the chemical composition of distant objects. This turns out to be a pretty amazing technique that can tell you all sorts about the physics of what happens in distant stars or galaxies.

Astronomers using the Ultraviolet and Visible Echelle Spectrograph (UVES) and the European Southern Observatory's Very Large Telescope (VLT), have used spectroscopic techniques to make very detailed measurements of a distant galaxy (redshift 4.224) by watching which colours it absorbs from the light of an even more distant quasar. This galaxy is observed at a time when the Universe was only about 1.5 billion years old i.e. less than 10% of its current age. The observation with the VLT comprised of five exposures which totalled over seven hours of time. With very detailed spectra, ratios of the various elements present within the galaxy could be calculated and this gives implications about its history. For instance, it was possible to deduce that lots of stars about 4-8 times the mass of the Sun must have reached the ends of their lives and dumped their nitrogen into interstellar space. That, in turn, means that there must have been a lot of star formation at least 200-500 million years previously. The observations also showed the presence of molecular hydrogen (H2) absorption lines, setting a record for the most distant object in which these have been measured. Apart from breaking records this is interesting because the measurements help to pin down the temperature of the gas (between 90 and 180 Kelvin) and are even able to suggest that for every two hydrogen atoms joined together as a molecule, there are about 250 hydrogen atoms sat by themselves.

It doesn't stop there though. Using observations of the light from a few more quasars, the authors reckon that the ratio of the masses of the proton and electron may have become 0.002% smaller in the last 12 billion years! If that turns out to be the case (much more evidence will be needed to confirm it) it is pretty exciting for our understanding of physics.

It is pretty neat to be able to work all these things just by looking at coloured light.

Posted in astro blog by Stuart on Tuesday 09th May 2006 (00:14 UTC) | Permalink
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