Friday, October 13, 2006

Seeing through dust

The Hubble Space Telescope is the public face of astronomy, but despite the beautiful pictures it produces it alone can only show use a tiny fraction of the Universe - specifically those parts that can be seen in the visible part of the electromagnetic spectrum (although its vision does extend into the near ultra-violet and near infra-red, unlike human vision).

But even a small change in the wavelength of light can make a big difference - what appears opaque to the human eye can be completely transparent to IR or X-ray radiation.

Astronomers need telescopes covering the full range of the EM spectrum. Without that coverage, even with a Hubble or JWST, we're really handicapped. Try to image medicine without X-rays, CAT scans, PET, MRIs and just relying on one technique, say ultrasound.

A native Hawaiian was leading a tourist trip up Mauna Kea that I was on last year asked "Why do these astronomers need all these telescopes? Why can't they all just get together and share one?" Its a fair question [particularly if it is your sacred mountain that is being built on] - one answer is one type of instrument can only detect a limited range of wavelengths, and hence you need multiple observatories to be able to cover as much of the spectrum as possible.

If we astronomers were limited to just the optical it wouldn't really save money, it actually be a waste of money as we could not fully understand and make use of the limited data we'd get. Unfortunately the public, and politicians, don't understand this. They tend to think that if they scrap a billion dollar mission they've saved a billion dollars (which can then be spent on a pork-barrel bribes to the public), but in fact they've just reduced the bang-for-buck of the remaining observatories.

As an example of what a small change in wavelength does, compare the picture at the top (the Hubble optical image of the Eagle Nebula) to this one here: a near IR image taken with the VLT ISAAC. I've tried to scale and rotate the VLT image to match the HST image (as closely as is possible by eye).

Seen in the near-IR the once imposing "Pillars of Creation" have half vanished. Look at the left-hand-most pillar, for example. Background stars are visible through the region that appeared almost black in the Hubble image. Only the densest conglomerations of dust and gas still block the background star (IR) light near the tips of the pillars.


In fact it is these dense clumps that create the pillar-like structures by shielding the less-dense gas in their shadow from the destructive UV radiation of the young massive stars in the cluster (they're outside the field of view to the upper right). In between the dense clumps the atomic gas has been photo-dissociated and effectively evaporated away.

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