Atmospheric opacity
Gamma rays, x-rays, UV, IR and long-wavelength rays are blocked by the atmosphere, while visible light and radio waves can easily pass through. This means that telescopes that are designed to detect gamma ray, x-ray, ultraviolet and infrared light work much better in space, above the Earth's atmosphere. Visible light telescopes and radio wave telescopes work well on Earth's surface.
Different wavelengths of light can also be used to gather different kinds of information about astrophysical objects. We will examine a few representative telescopes and the information they provide us.
Telescopes for various wavelengths of light
- Radio – Very Large Array
- Infrared – Spitzer Space telescope
- Optical, near IR, near UV – Hubble Space telescope
- X-ray – Chandra X-ray Observatory
- Gamma ray - Swift Space telescope
The Very Large Array of radio telescopes, located near Socorro, New Mexico, is a collection of 27 25-meter radio telescopes. They can be oriented to point to the same patch of sky, in effect, acting as one giant telescope. Radio telescopes are useful for detecting hydrogen gas in our galaxy, in other galaxies, and in inter-galactic space. This telescope array was also used to complete the NRAO VLA Sky Survey mapping a huge portion of the sky in radio wavelength light.
The Spitzer Space telescope is an infrared telescope. Infrared images are useful for studying forming stars, detecting protoplanetary dust disks around stars. It is also useful for studying early galaxies and the early universe. The Spitzer telescope has been instrumental in the detection of extrasolar planets, and was the first telescope to directly detect a planet outside our Solar system.
The Hubble Space telescope is probably the most famous space telescope orbiting the Earth today. It takes images in optical, near infrared and near ultraviolet light.
This image of the Butterfly nebula was taken by the Very Large Telescope (VLT) in Chile in 1998. The VLT was the worlds' largest telescope at the time.
This image of the Butterfly nebula was taken by the Hubble Space Telescope (HST) in 2009. The remarkable increase in detail seen by HST provided a wealth of information about objects in our galaxy, and of other galaxies in general. The Hubble telescope revolutionized the way we view space and altered public opinion, renewing interest in astronomy.
If you pointed the Hubble telescope at a region of sky about as big as the eraser of a pencil held at arm's length, and took a total of a few months of light exposure, you would create an image like the one above. It is called the Hubble Extreme Deep Field. With the exception of a couple of stars (with rays showing) every object in this photograph is a galaxy.
This exercise was reproduced in several directions, giving us an unprecedented view of the universe. The above image contains about 5,500 galaxies. The faintest galaxies in the image are one ten-billionth as bright as the dimmest light the human eye can see.
The Chandra X-ray Observatory is Earth's most powerful x-ray telescope. X-rays are extremely high energy light waves that are produced matter is heated to millions of degrees. This kind of light indicates that there is a huge amount of energy involved in its creation. X-rays can be created in very strong magnetic fields, extreme gravity, or very energetic cosmic explosions.
The Swift Space Telescope takes images in gamma ray energy light. This is the most energetic light in the universe. Gamma rays are emitted by such exotic objects as supermassive black holes and core-collapse supernovas. The Swift telescope is largely tasked with detecting gamma-ray bursts, flashes of light that briefly shine as brightly as a billion billion suns.
This image of Andromeda galaxy in visible light shows the galaxy in a way that people find the most familiar. Hundreds of billions of stars blend their light together to form the shape of the galaxy. Dark dust lanes obscure light in the regions where the interstellar matter is the thickest. Individual stars that are seen are actually in out galaxy, visible because we are inside our galaxy looking outward.
This image of Andromeda galaxy is a composite of infrared light and x-ray light. These wavelengths of light are not visible, so the colors are coded according to the energy of the light. The infrared rays are yellow to orange, showing thick rings of dust. Infrared light is long-wavelength light that can pass through dust that obscures shorter wavelength light. The dust lanes are regions of intense new star formation.
The bright blue parts of the image are x-ray light. They show bright stars nearing the ends of their lives. These stars are extremely hot and energetic. The cluster near the center of the galaxy contains shockwaves and debris from exploded stars.
This image of Andromeda galaxy was taken by the Swift telescope in ultraviolet light. It features especially hot, young stars and dense star clusters. Information gleaned from this type of image, combined with images in other wavelengths of light, combine to provide a more complete understanding of the mechanisms of our nearest neighbor galaxy and the birth and deaths of the stars within it.
Not all wavelengths of light pass through Earth's atmosphere, some are blocked. We say that the atmosphere is opaque to some wavelengths and transparent to others. Ground-based telescopes can be used for wavelengths of light that can pass through these transparent atmospheric windows.