Infrared Astronomy

Infrared Astronomy

Star-Forming Region

The Flame Nebula, also called NGC 2024, is a region of dust and gas in the constellation Orion. Some of the dust and gas is in the process of condensing into stars. This photograph was taken in the infrared range of electromagnetic radiation. Infrared radiation has wavelenghts slightly longer than those of visible light.

Infrared Astronomy, the detection and study of infrared radiation emanating from objects in outer space. Infrared radiation is another form of electromagnetic energy, similar to visible light and radio waves, but differing by its wavelength (or frequency); all such waves travel at the speed of light in a vacuum. Infrared wavelengths begin around 0.0007 mm just beyond the reddest light that the human eye can detect, this is called the “near” infrared, and grow in size to about 0.35 mm in the “far” infrared. Wavelengths larger than this belong to the sub-millimeter, microwave, and radio parts of the electromagnetic spectrum.

Infrared observations are important in astrophysics for several reasons. Infrared radiation penetrates more easily through the vast stretches of interstellar gas and dust clouds than does visible and ultraviolet light, revealing regions hidden to normal telescopes. Young stars are surrounded by a cocoon of gas and dust which can make them invisible, but their heat warms the dust grains and produces infrared radiation which escapes to reveal their presence. Infrared radiation is also called “thermal” or heat radiation. Many molecules, such as carbon monoxide (CO) and hydrogen (H₂), and tiny solid particles known as dust grains, are best studied at infrared wavelengths. Finally, the expansion of the universe changes (or redshifts) the visible light emitted by the most distant galaxies into red and infrared light.

II

INFRARED TELESCOPES AND DETECTORS

Infrared Telescope

Infrared telescopes detect radiation that has wavelengths longer than the light that humans can see. Infrared radiation enters the telescope and reflects off of a large mirror on the bottom of the telescope, then off of a smaller mirror. Detectors and instruments beneath the mirrors record the radiation. Infrared telescopes must be kept at very low temperatures to prevent their own heat from producing infrared radiation that could interfere with observations.

Infrared telescopes look similar to normal optical telescopes. Light is collected and focused by a large curved mirror onto a smaller secondary mirror and then into the scientific instrument (camera or spectrometer) for analysis. Most often, the secondary mirror is very small and gold-coated because gold offers better reflectance in the infrared than the normal aluminized mirrors do. The secondary mirror is mechanized so that it can tilt back and forth rapidly, up to 20 times per second, allowing the detector in the science instrument to compare the signals from the source plus sky background and the sky background alone. This technique is called “chopping” and is very effective for detecting a faint signal against a very strong background. Infrared telescopes have an open structure, without the black tubes or baffles common to normal telescopes, since these emit infrared radiation. For an infrared telescope in space the baffles can be cooled to reduce infrared emissions. One of the largest telescopes dedicated to ground-based infrared astronomy is the 152-inch (3.8-m) United Kingdom Infrared Telescope (UKIRT) at the Mauna Kea Observatory in Hawaii, but many other telescopes possess infrared instruments, especially for near-infrared work. In recent years, infrared astronomy has been revolutionized by the introduction of tiny imaging devices called “infrared arrays” making it possible to take pictures at these invisible wavelengths and display them on computer screens.

Everything that is warm emits infrared radiation. A star like the Sun emits most of its radiant energy as visible (yellow) light. Redder and cooler stars at half the Sun’s temperature have a peak emission at near-infrared wavelengths. Merely warm objects, like telescopes and Earth’s atmosphere, don’t emit light but they give off profuse amounts of infrared radiation with a peak emission around a wavelength of 0.01 mm. Since they are much closer to the infrared camera or detector, these sources of infrared radiation swamp any signals from distant planets, stars, and galaxies. Likewise, the optical and mechanical parts of the scientific instrument will emit strong infrared radiation. The solution to this problem is to cool the detector, and all the optics, inside a vacuum chamber. Temperatures around that of liquid nitrogen (77 degrees Kelvin (K) above absolute zero) are needed for near-infrared detection whereas temperatures near liquid helium (only 4 K) are required for far-infrared detectors.

To eliminate the large, unwanted background of infrared radiation it would be necessary to cool down the entire telescope and remove Earth’s atmosphere. For a space-based telescope, however, there is no atmosphere and no problem of condensation and ice formation if the telescope itself is cooled. Ground-based infrared telescopes need to be located at very high, dry, and cold sites like Mauna Kea, Hawaii, to minimize the thermal background. This is effective for near-infrared work. Far-infrared observations are best done from space or from high-flying airplanes.

III

THE INFRARED UNIVERSE

Orion Nebula in Infrared

This image of the Orion Nebula was taken in infrared radiation—radiation with wavelengths longer than visible light—and given false visible colors. The Orion Nebula is a cloud of gas and dust that surrounds new stars. The young stars light up the nebula and cause it to glow.

Infrared radiation emanating from the outer planets and their moons reveals much about their temperatures and compositions. Jupiter emits more infrared radiation than expected from absorption of sunlight, indicating that it has an internal source of heat energy. Infrared observations of Jupiter’s moon Io show thermal hot spots caused by the active volcanoes on its surface.

Stars of lower mass than the Sun are less luminous and much cooler. They emit most of their energy at infrared wavelengths. While these red dwarf stars are much more plentiful than the rare and short-lived high-mass stars, the very lowest mass objects—called brown dwarfs—have been hard to find because they are so faint at visible light wavelengths. Infrared astronomy has played a key role in the search for these objects which lie on the boundary between true stars and giant Jupiter-like planets.

Although interstellar space is a vacuum, it is not completely empty. Tiny solid particles called dust grains can be found around and between the stars. These particles are formed from heavier elements (like silicon and iron) produced deep inside an earlier generation of massive stars. Dust grains absorb ultraviolet and visible light, making the most distant stars appear redder and fainter, while also heating the dust slightly above the temperature of space and causing it to glow at far-infrared wavelengths. Dust obscures our view of distant parts of our own Milky Way Galaxy, including the center of the galaxy. The central regions can only be observed at infrared and radio wavelengths. Recent ground-based and space-based observations of the center of the galaxy have revealed for the first time clear evidence of dynamic motion around a massive, invisible source believed to be a black hole with a mass over one million times that of the Sun. Dust also obscures our view of young, recently-formed stars. As a cloud of gas and dust contracts to form a star it tends to flatten into a disk due to rotation. Eventually the disk might dissipate or it may allow planets to form. The disk both hides the starlight and is heated by it. Infrared observations reveal the hidden source and provide knowledge of how stars and planetary systems form.

Spiral galaxies similar to our own contain lots of dust and star-forming regions making them difficult to study except with infrared instruments. Some classes of galaxies are exceptionally luminous in the infrared because they have so much dust and have absorbed so much ultraviolet energy from the central sources within the nucleus of the galaxy. Distant galaxies are fainter and redder than nearby ones. The reddening in this case is not caused by intergalactic dust but by the expansion of the universe discovered by Edwin Hubble. Traveling at 186,000 mi every second (300,000 km/s), it takes light billions of years to reach us from the most distant galaxies, and therefore we see these galaxies as they were when they were young and not as they are now. During this time space itself has expanded and the wavelength of the radiation has been stretched or “redshifted” by such a large amount that the galaxy is no longer readily detectable at visible light wavelengths because all its energy is now in the infrared.

IV

HISTORY AND CURRENT RESEARCH

Saturn’s Atmosphere

This infrared photo of the planet Saturn has been color coded to indicate the cloud level in Saturn’s atmosphere. Violet and blue represent areas in which Saturn’s atmopshere is clear down to the main cloud layer. Green and yellow show layers of haze above the main cloud layer (yellow represents thicker haze). Red and orange indicate the highest level of clouds, thicker than the haze. White areas are areas of the atmosphere with high levels of water vapor. The bright dots at the upper right and lower left of the picture are Saturn’s satellites Tethys and Dione, respectively. The Hubble Space Telescope took this image in 1998.

Modern infrared astronomy began in the 1950s when simple photoelectric detectors made from lead sulphide became available and were used to survey the sky at infrared wavelengths for new sources. Later, germanium detectors were used to open up the study of much longer infrared wavelengths with the aid of rocket, balloon, and airplane surveys. In the late 1970s, several large ground-based telescopes dedicated to infrared astronomy were built which included the 3.8-m United Kingdom Infrared Telescope (UKIRT) and the National Aeronautics and Space Administration (NASA) 3-m Infrared Telescope Facility (IRTF) both on Mauna Kea, Hawaii, at 14,000 ft above sea level. With the launch of the Infrared Astronomical Satellite (IRAS), by the United States, the United Kingdom, and The Netherlands in 1983, infrared astronomy took another leap forward. This mission surveyed the entire sky at wavelengths of 12, 25, 60, and 100 microns (1 micron is a millionth of a meter) until its onboard supply of liquid helium ran out. A short time later infrared astronomy was revolutionized by the first introduction of devices that could take infrared images. The advent of sensitive infrared cameras inspired many traditional observatories to convert for infrared work, especially in the near infrared where city lights don’t cause problems and the thermal background from the telescope is minimal.

Uranus and Its Rings

The planet Uranus rotates on an axis that is tilted nearly horizontal. Other planets in the solar system have axes that are more vertical. This infrared image taken by the Hubble Space Telescope shows Uranus's rings orbiting in the plane of the planet's tipped equator. The colors are not real and are used to bring out details such as clouds in the atmosphere and the shape of the rings. The white disks are moons.

Infrared astronomy from space received two boosts when the Infrared Space Observatory (ISO) was launched in December 1995 and when the Hubble Space Telescope was refurbished in 1997 with NICMOS, the Near Infrared Camera and Multi-Object Spectrograph. Both of these missions used infrared array detectors. Another important satellite which used infrared techniques for part of its mission was the Cosmic Background Explorer (COBE). The Spitzer Space Telescope, launched in 2003, is the largest and most sensitive infrared space telescope ever launched.

Future infrared missions include SOFIA, the Stratospheric Observatory for Infrared Astronomy. SOFIA is a modified Boeing 747 with a 2.5-m telescope on board, due for commissioning in 2004.