Radiation: Information from Space

A trickle of "photons" is all that an astronomer has to work with. A photon - a precious "package of energy" - may have traveled for millions of years to reach the eager astronomer. What secrets does it hold? In this first section, you will be introduced to the underlying ideas of light and the electromagnetic spectrum.

Radiation

The term "radiation" has a variety of meanings, some of which may be alarming. You know that uranium or radium, for example, "emit radiation," which may be harmful. But what does this mean, and what is being emitted? Fundamentally, radiation is anything that "radiates" from a source, which means the term can be applied to "particles" or "waves" (Table 5.1).

Light - Wave or Particle?

The nature of light gets at one of the deepest questions in physics. Is light a wave or is it a particle? The answer is it is neither! Depending on how you interact with light, it can display the properties of either waves or particles. Regardless of how you view light, however, remember that light is a form in which energy can be moved through space.

Light as a wave

When we look at light averaged over "long" periods of time (where long could be as brief as a microsecond!), we tend to see wave behaviours. Any wave has two very important defining numbers: Wavelength is the distance between any two successive crests (high points) or troughs (low points) on a wave (see Figure 5.1). The colour we perceive is directly related to wavelength. Blue light has a shorter wavelength than red light. The Greek symbol l ("lambda") is commonly used to denote wavelength. Frequency is the number of waves created per second. Frequency is measured in units of Hertz (Hz).

Figure 5.1: Comparison of light waves of different colours.

Frequency and wavelength are related to one another by the speed of the wave. This can be expressed as , where v is the wave speed, f is the frequency, and l ("lambda") is wavelength. This equation implies the following key connection between frequency and wavelength:

large or high frequency implies smaller wavelength,
low frequency implies longer or larger wavelength

Example 5.1: Visible light travels with a speed of 3 x 10⁸ m/s, and consists of wavelengths between 400 nm and 700 nm. What are the corresponding frequencies of visible light?

Solution: Use the basic equation, , to find the link between wavelength and frequency. Rearrange the equation, and apply it to 400 nm wavelengths to read:

You can confirm this using the applet in Figure 5.3. Visible light has a frequency in the range 4.3 x 10¹⁴ Hz to 7.5 x 10¹⁴ Hz.

Light as a particle

When light interacts with matter at an atomic level, the particle behaviour of light becomes more important. The original idea of light as a "bundle" or quantum of energy is due to Albert Einstein. Eventually, the term photon was applied to the idea of a quantum of light energy.

We often use the term "photon" to refer to light, with the understanding that light is an entity that has both wave and particle characteristics. Regardless of which set of properties we see exhibited, light is a form of energy. We can quantify the relationship between light energy, and the frequency or wavelength of light, with the following formulae:

Energy as a function of Wavelength	Energy as a function of Frequency
Table 5.2: Two different ways to represent the energy of a light wave or photon.

In these expressions, h is Planck's constant, c is the speed of light, f is frequency, and l is wavelength.

The Electromagnetic Spectrum

One of the great triumphs of 19^th century physics was understanding that light is a traveling electric and magnetic disturbance. Figure 5.2 illustrates this idea. The red and blue arrows represent growing and then fading electric and magnetic fields. Any change in the one produces the other, and together they form a wave traveling at the speed of light. For this reason, light is considered to be an electromagnetic wave.

Figure 5.2: Video clip illustrating the idea that light (and any electromagnetic wave) consists of a distrubance of traveling electric and magnetic fields.

Visible light consists of photons with wavelengths between 400 and 700 nanometers; photons of shorter or longer wavelengths bracket the visible region to form a continuum called the electromagnetic spectrum.

The electromagnetic spectrum is also an "energy spectrum" for photons.

Figure 5.3: Applet demonstrating light in either wave or particle form as a part of the electromagnetic spectrum.

Example 5.2: How can Figure 5.3 be modified to show the dependency of photon energy on the position in the spectrum? Which photon has the higher energy: an ultra-violet photon or an infra-red photon?

Solution: Photon energy increases with frequency - Figure 5.4 is a suitable modification of the diagram to reflect this. Ultraviolet photons have higher frequency (shorter wavelengths) than do infrared photons, hence they are higher in energy. Similarily, blue photons are higher in energy than red photons - you can use the applet in Figure 5.3 to demonstrate this.
	Figure 5.4: Rollover image showing how the energy of photons increases toward the higher frequency end of the spectrum.

Spectral Windows

Earth's fragile atmospheric covering is essential to life; BUT - it does complicate the lives of astronomers! Figure 5.5 illustrates the concept of atmospheric spectral windows. Molecules (primarily nitrogen, oxygen, water vapour, and carbon dioxide) have insatiable appetites for most parts of the electromagnetic spectrum (Table 5.3); however, there are gaps, or "windows," through which Earth-bound astronomers can see the heavens!

Immediately prior to the Second World War, a new "era" in astronomy began. Americans Karl Jansky and Grote Weber (independently) began to "look" at the sky in the radio region of the electromagnetic spectrum. Advances in radar technology during the Second World War added to this, and during the 1950s and 1960s, the new and extremely sub-field of Radio Astronomy was born. In the 1960s, with the development of satellite technologies, astronomers continued to enlarge the range over which astronomers could view the universe.

Table 5.3 summarizes atmospheric absorbers, and the part of the spectrum they absorb.

Figure 5.5: Spectral windows and the appropriate technologies used to "see through" these windows. (Diagram courtesy of NASA)

A Gallery of Images of the Universe in Different parts of the Electromagnetic Spectrum

Table 5.4 summarizes the major regions in which astronomic observations are conducted. If an atmospheric window exists, such observations can be conducted from the ground; otherwise, observations are conducted above the atmosphere using an orbiting satellite.

Gamma The most energetic events in the universe will produce gamma rays. Our atmosphere is, fortunately, opaque to gamma rays. For this reason, gamma ray observatories are often orbiting satellites. The HETE (High Energy Transient Explorer) is one example. Another way to observe gamma rays (from the ground) is to observe the effect that gamma rays have on the atmosphere when they are absorbed. The image on the right shows the remnants of a supernova (exploded star) imaged in gamma rays using a ground-based technique using the High Energy Stereoscopic System .	Image courtesy H.E.S.S., Max-Planck-Institut für Kernphysik H.E.S.S. Experiment P.O. Box 86628 (EROS) Windhoek, Namibia
X-ray The image on the right (shown in blue) is a composite of an X-ray image of the core of the nearby galaxy M81. X-rays are very energetic photons produced by very high temperature gases. In this case, it is believed that a supermassive black hole at the core of the galaxy is heating the gas to the temperatures necesssary for the production of x-rays.	(click on the image for an enlarged view) (X-ray: NASA/CXC/Wisconsin/D.Pooley & CfA/A.Zezas; Optical: NASA/ESA/CfA/A.Zezas; UV: NASA/JPL-Caltech/CfA/J.Huchra et al.; IR: NASA/JPL-Caltech/CfA)
Ultraviolet Purple shades in the image of M81 are colour-coded to represent where ultraviolet (UV) radiation is detected. UV is produced by high temperature gas, often associated with extremely bright and hot stars.
Visible This is a Hubble Space Telescope image of the galaxy M81 in the visible wavelength region.             (click on the image for an enlarged view)
Infrared This stunning infrared image was taken with the Spitzer Space Telescope, and shows the central part of the Rho Ophiuchi star forming region. Star forming regions contain large amounts of dust and warm gas that glow brightly in the infrared.       (click on the image for an enlarged view)
Microwave This is a composite image showing the microwave emission from hydrogen gas (coded in dark blue) and the visible wavelength emission from the nearby galaxy M51 is visible in the centre of the image as a bright white-blue galaxy. Notice how much more extensive the system is when viewed in the microwave and radio region.       (Image courtesy of NRAO/AUI and Juan M. Uson)
Radio Looking into the centre of Milky Way galaxy using the radio region of the electromagnetic spectrum.
Table 5.4: Images of the universe, as viewed in different regions of the electromagnetic spectrum.