Infrared Astronomy: More than Our Eyes Can See

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In 1800, Sir William Herschel discovered an invisible form of radiation just beyond the red light of the visible spectrum. He named this form of radiation infrared (below red). Herschel's discovery was the first step in establishing what we call the electromagnetic spectrum. Visible light and infrared radiation are just two of the many types of energy produced by objects here on Earth and everywhere in the universe. Only by studying all of these types of radiation can we gain a complete picture of how the universe was formed and is changing.

Studying the Hidden Universe. The heat that we feel from the Sun or from a fireplace is infrared radiation (heat energy). Every object in the universe emits infrared energy, even those that we think of as being very cold, such as ice cubes or the Moon. Measuring this infrared energy is difficult because most is filtered out by our atmosphere, and the atmosphere itself glows brightly in the infrared. That's why astronomers who study the universe by looking at infrared radiation place cooled telescopes in space, above our atmosphere. Looking at infrared radiation from astronomical sources with ground based equipment is as difficult as trying to see stars in broad daylight.

Infrared astronomy involves the detection and study of infrared radiation emitted from objects in the universe. Because infrared radiation can pass through much of the dust and gas which blocks visible light, we can use infrared observations to "see" objects hidden by gas and dust, such as the center of our galaxy and regions of newly forming stars. The heat energy of many objects in the universe that are much too cool and faint to be detected in visible light can be detected in the infrared.

Seeing in Different Ways. The visible and infrared images shown on the front are of the exact same area in the constellation Orion. These images illustrate how features that cannot be seen in visible light show up very brightly in the infrared. Studies of such images give us important information about how stars are born and also on how our own Sun and solar system were formed. The infrared image shows several regions of hot, dense cores within clouds of gas and dust, which scientists believe are newly forming stars. Stars form from collapsing clouds of gas and dust. Embedded in these clouds, new stars are difficult to detect in visible light, but heat reveals their presence in the infrared. Infrared observations also have led to the discovery of a large number of stars which are too cool to be detected by their visible light.

Building New Planets. Astronomers using data from the Infrared Astronomical Satellite discovered flat disks of dust around about two dozen nearby stars. These disks are thought to contain the raw materials from which solar systems of planets are formed; as such, these disks provide the first tantalizing evidence that planets orbiting stars are probably common occurrences.

Building New Stars. The interstellar medium (ISM) is the reservoir of material from which new stars form. The ISM is made up of, primarily, dust and gas; it originates either in violent supernova explosions or in gentler episodes where a star's outer layers are blown off, fueling future generations of stellar birth. These interstellar clouds of dust and gas are best revealed and studied at long infrared wavelengths (about 100 times longer than visible radiation). Since stars are typically born amidst cocoons of dense gas and dust, astronomers depend on observations made at short infrared wavelengths to see through these dusty veils of birth.

Understanding Galaxies. Our visible-light view of most of the distant regions of our own Milky Way Galaxy, including the Galactic Center, is heavily obscured by the ISM. At infrared wavelengths, the Galactic Center is one of the brightest sources in the sky. Infrared observations show that the center of our galaxy consists of a very dense crowding of stars and that gases near the center are orbiting very rapidly (probably due to the existence of a black hole).

Observing the Past. When we observe galaxies billions of light years away, we see them as they were billions of years ago because of the finite speed of light. Moreover, the universe is known to be expanding, producing a shift in the measured wavelength of radiation (from the wavelength at which it was emitted). This shift of visible radiation toward the red end of the spectrum is known as the "cosmological red shift." If the objects are distant (and young) enough, the shift is beyond the visible spectrum into the infrared. Infrared observations thus provide a glimpse of the early universe, an era when stars and galaxies were forming.

 

Observing the Future. Work is now in progress at NASA on several exciting new infrared missions:

 

For more information: http://www.ipac.caltech.edu/

Photo Credits: Akira Fujii, visible-light image Infrared Astronomy Satellite, infrared image