Observing in other wavebands

5.13 Observing in other wavebands

5.13.1 Infrared

Many large optical telescopes are also used to observe in the infrared region of the spectrum, but there are some dedicated telescopes, such as the United Kingdom Infrared Telescope (UKIRT) located at a height of 4194 m on Mauna Kea, Hawaii (Figure 5.29). The UKIRT has a 3.8-m mirror and is equipped with a new mid infra- red eschelle spectrometer (Michelle); it has been used to observe young stars which were previously hidden in cocoons of the dust and gas from which they formed. Observations at ‘near infrared’ wavelengths cannot penetrate the surrounding dusty material, but Michelle’s view at longer ‘mid infrared’ wavelengths reveals the young stars within.

UKIRT has also signifi cantly advanced our understanding of brown dwarfs, mysterious objects sometimes referred to as ‘failed stars’. They are more massive than gas giant planets like Jupiter, but are not quite massive enough to shine like normal stars.

5.13.2 Submillimetre wavelengths

Submillimetre wavelengths lie between infrared light and radio waves on the wavelength scale. This largely comes from cold material in the universe, such as the clouds of gas and dust found between the stars that form the ‘interstellar medium’. It is out of this dust and gas that new stars are born, and into which stars disperse material as they explode at the end of their lives.

Figure 5.29 The Eskimo planetary nebula imaged in visible light by the Hubble Space Telescope (left) and in the infrared by the United Kingdom Infrared Telescope (right). Image: Hubble Space

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Figure 5.30 The James Clark Maxwell Telescope. Image: Science and Technology Facilities Council.

The James Clark Maxwell Telescope (JCMT) is the largest submillimetre telescope in the world having a 15-m diameter dish and is located on Mauna Kea, Hawaii (Figure 5.30). (You may have noticed that it is pretty crowded up there!) As is the case for infrared, water vapour in the Earth’s atmosphere absorbs submillimetre waves – which is why the high, dry site of Mauna Kea is one of the best places on the planet for astronomy. The JCMT is equipped with a detector array called SCUBA, the world’s most powerful submillimetre wave camera which takes pictures showing the faint heat radiation of interstellar dust grains. These fi ne particles, like soot or sand, are at temperatures below ⫺240°C. To be able to detect such faint radiation, SCUBA itself is kept within a jacket of liquid helium at a temperature of less than 1/10th of a degree above absolute zero! The JCMT also has ‘heterodyne receivers’ which detect light from gas molecules in space; their submillimetre radiation tell us about the temperature, density, and motion of the gas.

In the nearby massive star formation region, the Orion Nebula, SCUBA’s maps have revealed bright knots where stars are being born and has shown a complex region of shells, fi laments, and clouds at the centre of our Galaxy hidden in visible light by intervening dust. SCUBA can also observe galaxies, enshrouded in dust, more than 10 billion light years away and give information about star birth in the

Observing the Universe

5.13.3 The Spitzer space telescope

At wavelengths between 3 µm and 180 µm, most of this infrared radiation is blocked by the Earth’s atmosphere and cannot be observed from the ground so NASA has launched the Spitzer Space Telescope which has a 0.85-m telescope and three cryogenically cooled science instruments. Spitzer is the largest infrared telescope ever launched into space and, like SCUBA in the JCMT, the telescope must be cooled to near absolute zero so that it can observe infrared signals from space without interference from the telescope’s own heat. To protect the telescope from the heat of the Sun and infrared radiation from the Earth, Spitzer carries a solar shield and its orbit, trailing that of the Earth, places Spitzer far enough away from the Earth to allow the telescope to cool rapidly without having to carry large amounts of cryogen (coolant).

5.13.4 Ultraviolet, X-ray and gamma-ray observatories

To observe at these very short wavelengths, the telescopes have to be in space and NASA has launched two space observatories, the Compton Gamma-ray Observatory and the Chandra X-ray Observatory, to observe at the shortest wavelengths, whilst the HST can observe at ultraviolet wavelengths,

Ultraviolet

There are currently no dedicated ultraviolet observatories in orbit, but the HST carries out signifi cant observing at ultraviolet wavelengths, but it has a fairly small fi eld of view. From 1978 until September 1996, the International

Ultraviolet Explorer (IUE) was operating to observe ultraviolet radiation. There have, however, been some ultraviolet space missions where a set of three wide fi eld telescopes, called Astro, were carried into orbit in the bay of the space shuttle.

Extreme ultraviolet observatories

Astronomers have been somewhat reluctant to build space telescopes to observe at extreme short ultraviolet wavelengths since theories suggests that the interstel- lar medium (the tiny traces of gases and dust between the stars) would absorb much of this radiation in this part of the spectrum. However, when the Extreme Ultraviolet Explorer (EUVE) was launched, observations showed that the region around the Sun is relatively sparse of gas and dust – it appears to be located within

a bubble in the local interstellar medium – and this enabled the EUVE instruments

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The ultraviolet/X-ray boundary

An array of low energy X-ray imaging sensors (ALEXIS) was launched in 1995. Though its name implies that it is an X-ray observatory, the range of wavelengths observed by ALEXIS is at the very lowest end of the X-ray spectrum and often considered to be extreme ultraviolet.

X-rays – The Chandra X-ray observatory

NASA’s X-ray observatory is named in honour of Subrahmanyan Chandrasekhar who, at the age of 19, determined the mass limit for white dwarf stars. It was launched and deployed by the Space Shuttle Columbia in 1999 for a nominal 5-year mission.

Mirrors cannot be used to focus X-ray radiation as X-ray photons would be absorbed by normal mirror surfaces. Instead X-ray telescopes use nested cylindrical paraboloid and hyperboloid surfaces coated with iridium or gold, deployed so that the X-rays are incident at very low grazing angles. (An analogy is the fact that a water surface refl ects light well when it falls on it at a very shallow angle.) Chandra uses four pairs of nested iridium mirrors to give a resolution of 0.5 arcsec.

Chandra observations have revolutionized the fi eld of X-ray astronomy: the image of the supernova remnant Cassiopeia A, gave a fi rst glimpse of the compact object at its centre, probably a neutron star or black hole (Figure 5.31). X-ray

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emission has been seen from the region surrounding the super-massive black hole, Sagittarius A*, at the centre of the Milky Way and Chandra has made a measurement of Hubble’s constant giving a value of 76.9 km s ⫺1 Mpc ⫺1 . Recently, Chandra’s observation of colliding superclusters has found strong evidence for the existence of dark matter.

Gamma rays – The Compton Gamma-ray observatory

The Compton Gamma-ray Observatory, named after Dr Arthur Holly Compton, a Nobel Prize winner for work involved with gamma-ray physics, was launched in April 1991 and was deployed in low earth orbit at 450 km in order to avoid the Van Allen radiation belt.

It carried a complement of four instruments to cover gamma-ray energies from

20 keV to 30 GeV. A key use of the telescope was the detection of transient events, called gamma-ray bursts (GRBs). It detected, on average, 1 GRB per day with a total of 2700 detections. A GRB observed in 1999, one of the brightest bursts recorded, was the fi rst GRB with an optical afterglow that allowed astronomers to measure a redshift of 1.6 for the object giving rise to the gamma rays which corresponds to a distance of 4.5 Gpc. It appears that GRBs result when a massive star explodes in what is called a hypernova or perhaps when two neutron stars coalesce.

Following the failure of one of its control gyroscopes, the observatory was deliberately brought out of orbit and its debris fell into the Pacifi c Ocean in June 2000.