Peer deep into the constellation Sagittarius and you will find a spectacular young star cluster and its associated stellar nursery. These are M8 and the Lagoon Nebula.
More than 4300 light years (1320 parsecs) from the Solar System, this is an active star-forming region. Billions of years ago our Sun and its planets formed in a similar structure. Today when we on Earth look at M8 and the Lagoon Nebula we are seeing a vast molecular cloud with newly-formed stars shining through. Newborn stars, perhaps a mere million years old, are ferociously active and their energies are exciting the gases of the nebula causing them to glow. We have become used to admiring beautiful imagery of deep sky objects like this, but how are they created? Let us take a brief look at how the photons emitted from gas and plasma thousands of light years away were turned into this particular image.
The raw data which became this image was captured at the 2.6m VST telescope (VLT Survey Telescope) at the Very Large Telescope (VLT) site at Paranal, Chile in December 2011. You could, I suppose, put an eyepiece to the focus of this telescope in the hope of directly seeing the colourful splendour of the Lagoon Nebula for yourself but you are likely to be disappointed. You would certainly see the nebula and detect its structure, telescopic observers have been mapping it since the 1700s, but something would be missing from your view. Rather than dazzling with vibrant colours, the nebula would appear a pale and drab grey. Its hues are too faint to trigger the colour-sensing cone cells in the retina.
Rather than merely looking down the telescope, astronomers use instruments to capture their data, in this case the OmegaCAM CCD imager mounted on the VST telescope. The OmegaCAM’s field of view of captures an area of the sky roughly 1 degree square at a resolution of 0.21 arcsec/pixel. To put that into perspective, if the OmegaCAM were to be turned towards the Moon (something it is not designed to observe) it could in principle discern features about 400m across. To be more realistic and relevant, the Lagoon Nebula is roughly around 20 000 million million miles away from us. At that distance, each OmegaCAM CCD pixel captures an area in the Lagoon nebula nearly 50 000 million miles (roughly 500 au) across (to give an idea of scale Pluto is currently about 33 au from the Sun) .
The combined VST and OmegaCAM are not a “point and shoot” camera. The image you see had to be developed from 18 separate images taken using three filters (six images per filter), H alpha in red, r-band filter in blue, and i band filter in green. The 18 component images were further processed twice. First they were pipeline processed at the Cambridge Astronomical Survey Unit (CASU) centre at University of Cambridge to correct for various unavoidable instrumental effects. At Armagh Observatory Venu Kalari then used bespoke software to deal with such heavy image processing and combined the multiple images from the different filters to create a single image file.
Venu and his colleagues use images like this to identify individual young forming stars in the Lagoon Nebula and determine their ages. They have discovered that younger stars are still situated close to the gas clouds from which they have collapsed, whereas older stars are more dispersed. This sort of insight into the age and distribution of stars helps astronomers to mentally go back in time to understand the details of how stars emerged in this and other star-forming regions. More than just a pretty picture, images like this help clarify and enhance our vision of how the cosmos came to be.
(Thanks to Venu for sharing this image and describing how it was composed.
Article by Colin Johnston, Science Education Director)