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 early radio
astronomy was that because of the wavelength of
the observed radio-light was so long, the
positions of the sources were exceedingly hard to pinpoint.
So 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 these cosmic objects were doing in the optical as
well as the radio regime. One of the techniques used to
identify some of the 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 of temporarily
the earth bound radiation. By precise timing of the disappearance
and reappearance of the source, accurate positions could be obtained.
Two such sources located were 3C48, and 3C273.
The 3C stands for the Third Cambridge Catalogue
of Radio Sources. Cambridge University in England was a
pioneer in the radio astronomy field, and the numbers after
3C, were ordered by right ascension of the objects looked at.
When Allan Sandage of Caltech saw the spectrum in visible light of
3C48, he said, and I quote, the thing was exceedingly weird, unquote.
Indeed it was an object unlike any previously seen.
It's optical appearance was extremely blue.
And although it looked like a star, it's 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 seem to be gone.
Instead, other lines in the spectrum seemed to emerge at
odd wavelengths corresponding to nothing 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 wave lengths
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 QSO's or simple quasars for short.
Let's look at the optical spectrum of 3C273.
The three strong lines seen in the quasar spectrum are those of hydrogen,
marked H delta, H gamma, and H beta. At rest on
the Earth they correspond to the following wavelengths.
H beta equals 486.1 nanometers.
H gamma, 434 nanometers. H delta
410.2 nanometers. If you go back to the set of spectra
we looked at when we studied stellar spectra, the A1 star
shown here has for its most prominent features exactly these lines.
You can also find a copy of this figure posted
in the supplementary materials section of the course navigation bar.
These lines and some others are identified
in the comparison spectrum below the quasars.
This comparison spectrum is taken in the observatory, at
rest. And represents what a mixture of gases
looks like when nothing is moving, with respect to the telescope.
The nm here, stands for nanometers, and represents a unit of length
equal to ten to the minus nine meters or ten to the minus seven centimeters.