Unit 2: Electromagnetic Radiation and How It is Observed
 
 

OVERVIEW

In this unit of study we discuss electromagnetic radiation and its properties. Astronomers observe electromagnetic radiation using telescopes, and so we also review some of the basic telescope designs, including the advantages of modern telescopes.

LEARNING OBJECTIVES

At the end of this unit you should be able to:

1. Discuss the various types of electromagnetic radiation and their associated properties.

2. Describe the ability (or inability) of various types of electromagnetic radiation to penetrate the Earth's atmosphere.

3. Describe basic telescope designs and how they depend on the type of electromagnetic radiation being observed.

4. Discuss the characteristics of collecting area and resolution for a telescope, why these characteristics are important, and what determines them.

5. Describe modern-day telescopes and the advantages they hold over telescopes of the past.

6. Understand the concept of spectroscopy.

 KEY WORDS electromagnetic radiation

photon

light

gamma rays

x-rays

ultraviolet (UV) radiation

visible radiation

infrared (IR) radiation

microwaves

radio waves

blackbody radiation

Planck radiation

thermal radiation

Wein's Law

Stefan-Boltzmann Law

refracting telescope

reflecting telescope

chromatic aberration

collecting area

resolution

diffraction limit

seeing

interferometry

Hubble Space Telescope (HST)

Gamma Ray Observatory (GRO)

Advanced X-Ray Astrophysics Facility (AXAF)

Space Infrared Telescope Facility (SIRTF)

Keck Telescope

spectroscopy

 WRITTEN NOTES Astronomers study astronomical objects by observing how they emit and absorb electromagnetic radiation.

Another name for electromagnetic radiation is photon(s).

Photons are both particles and wave-like, and they carry energy.

· we know that photons are wave-like because they diffract (spread out) when they pass by the edge of an opaque body.  Think of a water wave bending around the edge of a rock in a pond.  Light of longer wavelengths bends around obstacles more easily than light of shorter wavelengths.  Think of how you can pick up a radio station even if there is a hill in between you and the transmitter, but visible light does not bend around obstacles as much.

· The wavelength of a photon is inversely proportional to its energy (i.e., the shorter the wavelength of the photons, the greater their energy).  Wavelength is the distance between successive crests in the wave.

· The frequency of a photon is proportional to its energy (i.e., the higher the frequency, the greater their energy).  Blue light has higher frequency and more energy than red light.  Gamma rays have much higher frequency and much more energy than radio waves.  Frequency is measured in cycles per second, or Hertz (Hz).
 

 When photons emitted by an astronomical object are separated into their component wavelengths or energies, an electromagnetic spectrum is formed (i.e., a graph or plot which shows the number of photons present at each wavelength). Types of Electromagnetic Radiation There are many types of electromagnetic radiation (photons). In order of long wavelength to short wavelength, they are as follows:
  · Radio Waves. They have the smallest energies and longest wavelengths. Substantial amounts are emitted by plasmas sometimes associated with magnetic fields. Radio waves generally pass through the Earth's atmosphere; however, those at the shorter wavelengths bounce off Earth's ionosphere.

· Microwaves.  These are actually a subgroup of radio waves with shorter wavelengths.

· Infrared (IR) Radiation. Substantial amounts are emitted by cool stars, but most wavelengths of IR radiation are absorbed by water vapor in the Earth's atmosphere.

· Visible Radiation (sometimes called optical radiation or simply light). Moderate temperature (6000 degrees Kelvin) stars like the Sun emit a large fraction of their energy at visible wavelengths. Visible light (which is a very small part of the electromagnetic spectrum) passes through the Earth's atmosphere. The different types of visible light are:

    Red (longest wavelength), Orange, Yellow, Green, Blue, Indigo, Violet (shortest wavelength).
    Some people memorize this by remembering the name ROY G. BIV.

· Ultraviolet (UV) Radiation. Substantial amounts are emitted by energetic processes and hot stars, but UV radiation is also mostly absorbed by the ozone in the Earth's atmosphere.

· X-rays. They are also emitted by very energetic processes and are absorbed by the ozone in the Earth's atmosphere.

·Gamma Rays. They have the greatest energies and shortest wavelengths. They are emitted by very energetic processes, but can be best observed in astronomical objects using space telescopes, because ozone in the Earth's atmosphere absorbs most gamma rays.
 
 

 Thermal Radiation
 

                         ·Blackbody radiation, Planck radiation, and Thermal radiation are three terms
                            used to describe the spectrum emitted by a heated object.  See the textbook, pp.
                            170-172, for a graph of this spectrum.  There are two important rules:
                                            Wien's Law:  The hotter the object is, the shorter the wavelength of the
                                               peak of its spectrum.
                                            Stefan-Boltzmann Law:  The hotter the object is, the more light it
                                                emits at all wavelengths.  (The brighter it is at all wavelengths).
 

Telescopes

Most telescopes are used to collect electromagnetic radiation (a notable exception would be a neutrino telescope).

The design of a telescope depends on what type of radiation you want to collect and what you want to study.

· For example, x-ray telescopes use grazing incidence mirrors to focus electromagnetic radiation.

· UV, optical, and IR telescopes used lenses or mirrors to focus electromagnetic radiation.

· Radio telescopes use wires and metal to focus electromagnetic radiation.

 The `collecting area' of a telescope determines how much electromagnetic radiation from an object can be collected and focused in a given interval of time. By making the collecting area larger, fainter objects can be observed.

The clarity of an image formed by a telescope is determined by the telescope's angular resolution or by atmospheric blurring (called seeing).

· The theoretical resolution or diffraction limit of a telescope (i.e., how well two nearby objects are separated) depends on the wavelength of the radiation and the diameter of the telescope's primary mirror or lens.  Larger primary mirrors or lenses result in sharper resolution than smaller ones.  Observations at shorter wavelengths result in sharper resolution than at longer wavelengths.

· At radio wavelengths, a telescope's resolution is limited by the theoretical considerations.  The atmosphere does not affect the resolution.

· At visible wavelengths, a telescope's resolution is limited by atmospheric seeing, unless it is a space-based telescope, like the Hubble Space Telescope.  The Hubble is a diffraction-limited telescope.


Interferometry is the linking together of multiple telescopes (usually radio telescopes) to act as one.  The angular resolution of the whole system is then the same as if it were one big telescope with a dish the size of the whole array.  So by stretching the array farther apart, you improve the resolution without having to build a larger dish.
 

There are many types of designs for optical telescopes. The design determines the cost, but also determines the feasibility of a particular observation. Some popular standard designs:

· Refracting telescopes which focus light with lenses. These telescopes are costly and often suffer from chromatic aberration (i.e., all the different colors of light from an object are not focused at the same place). In addition, lenses can be very heavy and can only be mounted at their edges, a weakness in a large telescope.  However, refractors can produce superb images if made properly.

· Reflecting telescopes which focus light with mirrors. There are numerous designs (e.g., Newtonian, Cassegrain, Gregorian, Ritchey-Chretien, etc.) which vary in cost. Aberrations (i.e., imperfections) in the image are determined by the design.  They do not suffer from chromatic aberration.  Mirrors can be made lighter than lenses and can be more supported from behind, making them stronger than refracting telescopes.

· Some telescopes focus light with both mirrors and lenses. They are generally expensive. One type is the Schmidt telescope which provides accurate images over a wide field of view.

 Electromagnetic Radiation Collected by a Telescope is Permanently Recorded Astronomers don't simply look through a telescope. Instead they place a detector (some type of scientific instrument) at the telescope's focus which will make a permanent record of the observations. The data collected are later analyzed with the aide of a computer. The type of detector used depends on the electromagnetic radiation being observed.

For example, past observations with optical telescopes often used photographic plates as detectors, but now high-grade CCDs (charge couple devices) similar to the low-grade ones used in home video cameras are often used.

The detector in a radio telescope is an antenna with a receiver.

 Today's Generation of New Telescopes Detectors on modern telescopes are very efficient.

NASA's Great Observatories Program to put telescopes in space will cover all regions of the electromagnetic spectrum not observable from the ground:

· Hubble Space Telescope or HST (UV, optical, and IR) was put in orbit in 1990.

· Gamma Ray Observatory or GRO (gamma rays) was put in orbit in 1991.

· Chandra (originally called Advanced X-Ray Astrophysics Facility or AXAF) (x-rays) was put in orbit in 1999.

· Space Infrared Telescope Facility or SIRTF (IR) will be put in orbit in the next few years.


Large modern ground-based optical telescopes use low-cost designs to achieve large collecting areas. The largest ground-based telescope, completed in 1993, is the Keck 10-m Telescope.

Advanced technology, such as adaptive optics, on ground-based telescopes is beginning to overcome resolution limitations caused by atmospheric seeing effects.

 Modern radio telescopes now being designed, like the Very Large Baseline Array which connects telescopes all over the world, will have very high resolution and sensitivity. Spectroscopy Astronomers often break up UV, optical, and IR light into its component wavelengths or energies using prisms or diffraction gratings. This technique is called spectroscopy and is used to study the details of the spectrum of an astronomical object. [Note that other devices are used to separate electromagnetic radiation into its component energies at other wavelengths.]

The continuum, emission lines, and absorption lines observed in a spectrum of an astronomical object reveal a tremendous amount of information about the physical processes that are occurring (e.g., chemical composition, temperature, pressure, density, magnetic field strength) which can often be used to determine masses and sizes of astronomical bodies.

 READING ASSIGNMENT

Text:

Chapter 7 (except Doppler shift part)

Chapter S2
 

HOMEWORK

Ch. S2, Review Question 16.

OPTIONAL HOMEWORK:

Ch. S2, Review Questions 4 & 6;