Infrared Astronomy

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Infrared astronomy studies incoming radiation with wavelengths beyond visible red to radio.

Infrared telescopes were first built in the 1960s. They are basically optical reflectors with a special heat detector at the prime focus. Detectors are shielded and cooled to about 2 K to ensure that they register infrared rays from space, rather than stray heat from people, equipment, and observatory walls.

Water vapor and carbon dioxide in the air strongly absorb infrared rays. Large infrared telescopes are located on very high mountaintops where the air overhead is thinnest and driest. Smaller telescopes are lofted in airplanes, balloons, rockets, and spacecraft.

U.S./German Stratospheric Observatory for Infrared Astronomy (SOFIA) ► hu.p:// is an airplane modified to fly a 2.5-m reflecting telescope above 12-km (40,000 feet). U.S. Spitzer Space Telescope (2003- ) ►www.spitzer.caltech.eduM orbits an 85-cm telescope.

Infrared telescopes image invisible sources that are relatively cool or obscured because infrared rays pass through interstellar clouds of gas and dust that block shorter visible rays. You can see false color images of cool stars and galaxies, regions of star and planet formation in giant molecular clouds, comets, and galaxy centers at NASA's Infrared Processing and Analysis Center (IPAC). ► M

What is the main advantage of infrared telescopes?

Answer: They reveal relatively cool objects that may not be visible.


Since the 1960s, ultraviolet, X-ray, and gamma ray telescopes with suitable detectors have been sent above Earth's obscuring air in orbiting spacecraft.

Solar arrays collect and convert sunlight to electricity for instruments and directional control. Insulation protects instruments from the extreme heat and cold, low pressure, and energetic particles and radiation in space. Star trackers and gyroscopes orient space observatories and point them to sky objects on command.

High energy telescopes collect and focus incoming radiation. Detectors record its intensity, energy, duration, and direction of origin. Radio antennas receive commands from mission ground control and transmit data to the ground.

The data are processed and recorded by computer for analysis. They are displayed digitally or as graphs of intensity over time or an energy range to reveal how the source is producing its rays, how bright it is, how long it remains at that brightness, and what kind of object it is. Data can be manipulated to generate spectacular false color images, in which colors are used to show features of invisible objects (not colors you would actually see).

Ultraviolet observations of the Sun, hot stars, stellar atmospheres, interstellar clouds, a hot gas galactic halo, and extragalactic sources abound. The U.S. robot Galaxy Evolution Explorer (GALEX) (2003- ) probes the faintest and most distant sources ever. ►http://galex.caltech.eduM

X-rays and gamma rays shoot right through ordinary mirrors and lenses, so we use alternate ways to image the most energetic objects and violent events in the universe. U.S. robot Chandra X-ray Observatory (1999- ) ►http://chandra.harvard.eduM has nested barrel-shaped mirrors. Incident X-rays that strike them at grazing angles bounce off to a focus and form an image. Gamma ray detectors show the telling spray that appears after they are absorbed or collimated to a collision in a high-density medium (Figure 2.19).

X-ray and gamma ray telescopes reveal sudden, intense bursts of radiation (bursters), possible black holes, active galaxies, and distant quasars.

LAT Instrument

(Large Area Telescope under the AnliCoincidence Detector)


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4x4ArrayofTowers sWeview A

Figure 2.19. Fermi Gamma-ray Large Area Space Telescope (2008- ). Large Area Telescope (LAT) tracker and calorimeter measure direction and energy of incoming gamma rays; outer detector bans other particles. A complementary Glast Burst Monitor (GBM) detects X-rays and less energetic gamma rays. ►fermi.gsfc.nasa.govM

Figure 2.19. Fermi Gamma-ray Large Area Space Telescope (2008- ). Large Area Telescope (LAT) tracker and calorimeter measure direction and energy of incoming gamma rays; outer detector bans other particles. A complementary Glast Burst Monitor (GBM) detects X-rays and less energetic gamma rays. ►fermi.gsfc.nasa.govM

What is particularly interesting about new observations by ultraviolet, X-ray, and gamma ray telescopes?_

Answer: Incoming ultraviolet, X-rays, and gamma rays have much more energy than visible light. They must be generated in extraordinarily energetic processes not yet fully comprehended.


This self-test is designed to show you whether or not you have mastered the material in Chapter 2. Answer each question to the best of your ability. Correct answers and review instructions are given at the end of the test.

1. Explain why looking at stars is a way of seeing how the universe looked many years ago._

2. (a) List the major regions of the electronic spectrum from shortest wavelength (highest energy) to longest wavelength (lowest energy)._

(b) State what all electromagnetic waves have in common.

3. Write the general formula that relates the wavelength and frequency of a wave._

4. Suppose you observe a bluish star and a reddish star. State which is hotter, and explain how you know._

5. List the two windows (spectral ranges) in Earth's atmosphere for observational astronomy._

6. What are the two main parts of a telescope used for stargazing, and what is the function of each?_

7. What are the two main advantages of giant telescopes for research?

Two Telescopes

Type of Telescope


Reflector (1)

Refractor (2)

Diameter of main lens or mirror

2 m

1 m

Focal length of objective

7.6 m

14.6 m

Focal length of eyepiece

5 cm

1 cm

8. Which telescope described in the chart above (1 or 2) has:

(a) greater light-gathering power?

(b) greater resolving power?

(c) greater magnification?

9. What two factors are most important in telescope performance?

10. What is the purpose of a spectrograph?

11. List three advantages of a radio telescope.

12. What is the advantage of sending telescopes up in spacecraft?

13. Match an appropriate innovative tool to the observations.


Faintest and most


Chandra X-ray Observatory.

distant radio sources.


Galaxy Evolution Explorer


Very hot stars and gas.



Visible and relatively


Keck Telescope.

cool sources.


Very Long Baseline Array (VLBA).


X-ray sources.

Compare your answers to the questions on the self-test with the answers given below. If all of your answers are correct, you are ready to go on to the next chapter. If you missed any questions, review the sections indicated in parentheses following the answer. If you missed several questions, you should probably reread the entire chapter carefully.

1. Starlight is radiated by electric charges in stars. Light waves transport energy from stars to electric charges in our eyes. Light waves travel incredibly fast— about 300,000 km (186,000 miles) per second. But trillions of miles separate the stars from Earth, and the journey takes many years. Thus we see the stars as they were many years ago when the starlight began its journey to Earth. (Sections 2.1, 2.5)

2. (a) Gamma rays, X-rays, ultraviolet radiation, visible light, infrared radiation, radio waves. (b) All electromagnetic waves travel through empty space at the same speed, the speed of light—about 300,000 km (186,000 miles) per second. (Sections 2.3, 2.5, 2.8)

4. The bluish star is hotter. The shorter the wavelength at which a star emits its maximum light, the hotter the star, according to Wien's law of radiation. Blue light has a shorter wavelength than red light. (Sections 2.2, 2.10)

5. Optical (visible light) including infrared; radio (Section 2.11)

6. (1) Main mirror or lens (objective): To gather light and form an image. (2) Eyepiece: To magnify the image formed by the main mirror or lens. (Sections 2.12, 2.14, 2.15)

7. Superior light-gathering power and resolving power. (Sections 2.12, 2.19, 2.23)

8. (a) 1; (b) 1; (c) 2. (Sections 2.12, 2.19, 2.20)

9. Size and quality of main mirror or lens. (A stable mount is essential.) (Sections 2.12, 2.17 through 2.23)

10. To separate and record the individual wavelengths in a beam of light. (Section 2.24)

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