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Section I: Introduction
Section II: 6 Activities
Section III: Summary

Education Activities To Accompany Chandra Data Analysis Software

The Discovery of the Quasars

After World War II, radio astronomy surged to the forefront of the astronomical frontier. Vast numbers of radio sources were discovered in the sky, which added greatly to our understanding of the processes that go on in our universe. But one of the problems with radio astronomy is that because the wavelengths of the observed radio "light" are so long, the positions of the sources are exceedingly hard to pinpoint. It was very difficult to correlate the newly discovered radio emissions with known optical counterparts in the sky. But by 1960, many positions were able to be refined, and we began to see what cosmic objects were doing in the optical as well as the radio regime. One of the techniques to identify some sources involved looking at lunar occultations. If a radio source just happened by chance to be along the path of the Moon's orbit, the Moon would pass in front of the object, thereby shutting off temporarily the earthbound radiation. By precise timing of the disappearance and reappearance of the source, accurate positions could be obtained. Two such sources located in this fashion were 3C48, and 3C273. (The "3C" stands for third Cambridge catalog of radio sources; Cambridge University in England was a pioneer in the radio astronomy field; and the 273rd source). When Allan Sandage of Caltech saw the spectrum in visual light of 3C48, he said: "The thing was exceedingly weird." Indeed, it was an object unlike any previously seen. Its optical appearance was extremely blue, and although it looked like a star, its spectrum was very strange indeed. None of the known elements appeared to be there! The well studied fingerprints of hydrogen, calcium, and other stellar constituents seemed to be gone. Instead, other lines in the spectrum seemed to emerge at odd wavelengths, corresponding to nothing that we knew about in the laboratory.

Then, in 1963, the Dutch astronomer Maarten Schmidt realized that the pattern of lines in the spectrum of 3C273 were identifiable, but they corresponded to wavelengths red-shifted by an astounding amount. Never was a "star" like this seen before. Thus, the objects (which now number in the thousands) were dubbed quasi-stellar-objects, or QSOs or simply quasars, for short. Let's look at the optical spectrum of 3C273…

3C273 Optical Spectrum
Optical Spectrum of 3C273

The three strong lines seen in the quasar spectrum are those of hydrogen, marked Hd, Hg, and Hb. At rest, on the Earth, they correspond to the following wavelengths:
  • Hb = 4861 A
  • Hg = 4340 A
  • Hd = 4102 A

These lines, and others, (at 3889 A, 5016 A, and 6030 A) are identified in the comparison spectrum below the quasar's. This comparison spectrum is taken in the laboratory (at rest), and represents what a mixture of gases looks like when nothing is moving with respect to the telescope. (The "A" stands for Angstroms, and represents a unit of length equal to 10-8 cm.

Activity 2: Get the velocity and distance to 3C273!
Print out the spectrum above, and measure the positions of several lines in the comparison spectrum with a ruler.
  1. We need to figure out how many Angstroms on the spectrum corresponds to one millimeter on the paper. This is called the dispersion, or scale, of the spectrum. Measure several pairs of lines, and take the average for a more accurate value. (NOTE: The answers shown here will be different from your results because of variations in printing of the page from computer to computer). For example, if the lines of 6030 A, and 3889 A are separated by 134.7 mm, the plate scale will be: 15.89 A/mm
  2. Now, we can choose one of the hydrogen lines in the quasar spectrum, and see how far (in mm) it is from the corresponding rest wavelength. For example, if the Hb line appears in the quasar spectrum a distance 41mm to the right of the laboratory position, its wavelength is: 4861 + 41 x 15.89 = 5512.5 A.
  3. Now we can derive the velocity: v/c = Dl/l. If our line "moved" to 5512.5 A, the numerator here is 651.5 A. With the denominator of 4861 A, we get v = 0.13c, where c equals the velocity of light. Do this for the other hydrogen lines as well, and get an average value for increased accuracy. (v = velocity; c = velocity of light; Dl = displaced line in quasar spectrum; l = line at rest)
  4. Get the distance to the object. If v=Hr, then with v= 4 x 104 km/sec, r = 800 Mpc, or about 2.5 billion light years.
So when you see this object through a telescope, you are viewing it the way it appeared 2.5 billion years ago, when only bacteria and algae were present on the Earth. Mammals were still over 2 billion years away from existence on our planet!

Thus, these objects, though they look like stars, are so distant that they must be more luminous than the brightest galaxies. At 2.5 billion light years and more (since 3C273 is the closest quasar), they are over 1000 times more distant than the Andromeda galaxy, M31. Yet even at such gargantuan distances, many (including 3C273) are bright enough to be seen in small telescopes.

Just how bright they really are, we shall soon see [next] [back]

Last updated: 11/08/02


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