In early 2009, a team of astronauts visited Hubble to repair the wear and tear of twenty
years of operating in a hostile environment - and to install two new instruments, the
Cosmic Origins Spectrograph, and Wide Field Camera 3 - better known as WFC3.
WFC3 is a combined ultraviolet, visible and infrared camera that dramatically extends
Hubble's ability to image astronomical objects.
With these new capabilities, Hubble is still pushing the boundaries of science after two
decades in orbit.
WFC3 is actually two instruments in one: the ultraviolet and visible-light channel is WFPC2's
replacement, cramming six times as many pixels into a similar field of view.
As well as providing scientists with higher resolution observations than ever before,
the pictures from this part of WFC3 are also Hubble's prettiest yet, revealing details
never seen before through any telescope. But it is WFC3's infrared channel that is the
real breakthrough.
Infrared astronomy is getting a lot of attention right now. It's not just Hubble's new functions
- ESA's Herschel Space Observatory, NASA's Spitzer Space Telescope and the forthcoming
NASA/ESA/CSA James Webb Space Telescope were all designed to work in this part of the spectrum
too.
One of the reasons for this is that studying the sky in the infrared allows astronomers
to look at relatively cool objects that emit little or no visible light. An example of
these are so-called protoplanetary nebulae - a cool gas envelope that gets thrown off
by a certain type of star as its nuclear fuel supply runs low.
Looking at these nebulae through an optical telescope is hard, as they barely emit any
visible light, forcing astronomers to rely instead on faint reflected starlight to see
anything at all. But protoplanetary nebulae shine far more brightly in the infrared part
of the spectrum.
Infrared imaging is also extremely useful for peering through interstellar dust clouds,
which are impenetrable to visible light. The reason for this is similar to why sunsets
are red. Just as particles in the atmosphere scatter blue light more than red, interstellar
dust clouds block visible light more than infrared.
Hubble has become famous for its striking visible-light pictures of huge clouds of interstellar
dust and gas. But sometimes scientists want to know what's happening behind, or inside,
the cloud of dust. Making infrared observations pulls away the veil and reveals the hidden
stars.
Until now, infrared imaging was challenging with Hubble. The Near Infrared Camera and
Multi-object Spectrometer, or NICMOS, did allow astronomers to study objects in infrared
light in ways not possible from the ground, but it forced them to make a difficult choice.
Because its images were small - only about 65 000 pixels in total, similar to a mobile
phone screen - NICMOS could produce the sharpest images only if it concentrated on a very narrow
field of view. Taking in a wider view came at the cost of losing much of the detail.
Along with a much wider field of view and better sensitivity, WFC3's infrared channel
has a million pixels, 15 times better than NICMOS, and similar to what you get on a computer
screen. This means astronomers no longer have to compromise between how much of the sky
they can observe, and how much detail they can study it in.
These improvements mean Hubble is now far better at observing large areas of sky as
well as very faint and very distant objects. These are key for the science of cosmology,
the study of the origins and development of the Universe.
Because the Universe is expanding, light waves coming from distant objects are stretched
as they travel through space, and the waves become longer. The further an object is away,
the more its light is stretched on its journey to us, and the redder the light appears. Hence
the effect is known as redshift.
For really distant objects, the ultraviolet and visible light is redshifted so much it
goes infrared - literally, "below red" - and that is the reason that infrared imaging is
so important for spotting these very distant galaxies.
This is the Hubble Ultra Deep Field, a visible light image taken in 2003 and 4 with Hubble's
Advanced Camera for Surveys. The picture is of a little patch of sky almost a hundred
times smaller than the area of the full moon. It contains no stars visible with the naked
eye - but taking a million second exposure of this little black speck of space reveals
these vanishingly faint faraway galaxies.
Studying the same region with WFC3's infrared photography reveals galaxies more distant
still: some of these are so far away that they have been redshifted out of the visible
spectrum altogether.
We see galaxies here as they were many billions of years ago. When the light from some of
these galaxies started its long journey towards us, our Sun and Earth had not even begun to
form.
But what is really exciting cosmologists about WFC3's infrared imaging of the Hubble Ultra
Deep Field is not just what's in the foreground so to speak, amazing as that is, but the scatter
of tiny, faint specks just visible in the background, beyond these already faraway galaxies.
Some of the flecks of light in this fuzzy image are just anomalies within the light
detectors, but among them are faint impressions of early galaxies. In this photo we are looking
at some of the most remote objects ever seen. They are so distant, and their light has travelled
so far to reach us, that we see these galaxies as they were 13 billion years ago, when the
Universe was only about 5% of its current age.
Discovering and studying these galaxies can tell us a lot about the conditions that prevailed
in the earliest years of the Universe, and confirm - or perhaps refute - our theories
of early galaxy formation. Whatever the case may be, observations like
these certainly take us a few steps closer to understanding the history of our Universe.