The Star Garden
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Andromeda Galaxy in ultraviolet Image credit: NASA
As human beings, we are used to seeing the universe through a very narrow range of light waves, known as the visible part of the electromagnetic spectrum. The light that we see is composed of photons, 'bundles' of energy travelling with wavelengths of about 390 to 750 billionths of a metre. We perceive different wavelengths as different colours.
Image credit: Antonine Education
Whilst some animals can see ultraviolet light, humans have had to invent machines in order to map the universe beyond the visible spectrum and they have found that things can look vastly different. The pictures below show the Earth in infrared, visible, ultraviolet and x-ray light.
Infrared image credit: Imperial College London, other images from NASA
The infrared image of the Earth is not dissimilar to the visible one, the land, ocean and clouds can all be seen, but this changes when we look at light with shorter wavelengths. When viewed in ultraviolet, the land and oceans disappear but the difference between night and day is still evident. On the side of the Earth that faces the Sun, most ultraviolet light is reflected. The bands on the dark side are caused by charged particles which travel along the Earth's magnetic field lines, causing aurora - the northern and southern lights. These are the only feature that is evident when the Earth is viewed in the x-ray spectrum.
The Earth is not the only object to look very different when viewed at different wavelengths. The pictures below show elliptical galaxy Centaurus A in radio, visible and x-ray light. The jets in the radio and x-ray spectrums are caused by a supermassive black hole in the centre of the galaxy. Most galaxies, including the Milky Way, are expected to contain supermassive black holes, which are at least a million times as massive as the Sun. The hot active centre of the galaxy is best viewed in the x-ray spectrum. The stars visible around the centre are most likely x-ray binary stars.
Image credit: NSF/VLA/Univ.Hertfordshire/M.Hardcastle, ESO/WFI/M.Rejkuba et al, NASA/CXC/CfA/R.Kraft et al from Chandra
X-ray binary stars make up some of the brightest x-ray sources in the sky. They are composed of a dense star, like a black hole or a neutron star, and a larger star that is still powered by nuclear fusion. Matter is accelerated as it moves towards the black hole or neutron star and is rapidly decelerated when it reaches the surface, converting energy into x-ray radiation.
Artist's impression of an x-ray binary star Image credit: NASA
In order to gain a full understanding of the universe, we need to observe it across all wavelengths. The future of radio astronomy is fairly secure. The Netherlands Foundation for Research in Astronomy (ASTRON) is currently building the largest ever radio telescope, The Low Frequency Array (LOFAR). LOFAR combines many small telescopes which are placed far enough apart to produce the equivalent of a thousand kilometre wide dish. LOFAR will be succeeded by the Square Kilometre Array (SKA) which should be operational in 2020 and will be at least fifty times more sensitive.
The future of space based telescopes is less secure. NASA's Great Observatories were designed to cover the rest of the electromagnetic spectrum. The Spitzer Space Telescope (SST) observes in infrared. The Hubble Space Telescope (HST) works in near infrared, visible and near ultraviolet light. The Chandra X-ray Observatory (CXO) observes low energy x-rays and the Compton Gamma Ray Observatory (CGRO) observed high energy x-rays and gamma rays. Although the HST and CXO are still operating far passed their originally intended lifetimes, the SST now only works in a very narrow range of wavelengths and the CGRO fell back to Earth in a controlled crash in 2000. There are a number of campaigns to launch telescopes to replace them but their futures are uncertain.
The James Webb Space Telescope (JWST) would potentially replace the SST, if NASA can continue to support it. The JWST would allow us to look further than before and therefore see objects as they were when the universe was younger than we have ever seen it. The Advanced Technology Large-Aperture Space Telescope (ATLAST) could potentially replace Hubble if the product can gain support from NASA. If ATLAST were launched then we would be able to detect 'biosignitures' like molecular oxygen, ozone, water and methane in the atmospheres of exoplanets.
The International X-ray Observatory (IXO), a collaboration between NASA, the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) would have become a successor to the CXO and CGRO, however NASA pulled out in 2011. The ESA is currently trying to salvage the mission with the ESA Advanced Telescope for High Energy Astrophysics (ATHENA) which will tell us more about supermassive black holes and the large scale structure of the universe.
When these telescopes are eventually launched, we will be able to see things that have never been seen before and this will almost inevitably lead to many new revelations about the universe.
9th October 2011 0 Comments