219
See also: The theory of relativity 146–53 ■ Radio astronomy 179 ■ Nuclei and radiation 185 ■
Quasars and pulsars 236–39 ■ Discovering black holes 254 ■ The heart of the Milky Way 297
startling. The object was 2.5 billion
light-years away, which meant that
it was unimaginably bright. Its
absolute magnitude was –26.7
(the lower the figure, the brighter
the object). The object in Schmidt’s
eyepiece was 4 trillion times brighter
than the sun (magnitude +4.83)—
brighter than the whole of the
Milky Way put together.
Schmidt named the body a
quasi-stellar radio source, which
was later shortened to quasar.
Before Schmidt, the object had been
known as 3C 273. The 3C referred
to the 3rd Cambridge Catalogue
of Radio Sources (produced by
the Radio Astronomy Group) and
273 because it was the 273rd object
to be located in that survey. 3C 273
had been spotted in 1959, although
the first quasar to be identified
(or what would be later termed
a quasar) was 3C 48, which had
been found shortly before.Improving radio astronomy
Radio astronomy had started in the
1930s after the accidental discovery
of cosmic radio sources by Karl
Jansky. Interrupted by World War II,
and helped somewhat by the
development of radar technology,
surveys using radio telescopes did
not start in earnest until 1950. Early
surveys were hindered by the low
frequency of 81.5 MHz (megahertz—
or million cycles per second) used
by early radio receivers. At that
frequency, it was difficult to pinpoint
the location of signals with a low
flux density. (Flux density is a
measure of the strength of a signal,
and is measured in watts per
square meter per hertz, simplified
as the unit jansky [Jy].)NEW WINDOWS ON THE UNIVERSE
In 2001, the Hubble Space Telescope
captured a glimpse of one of the most
distant and luminous quasars ever
seen (circled). It dates to less than
one billion years after the Big Bang.The sky has many powerful radio sources
that seem to be invisible.In 1955, the Radio Astronomy Group
at Cambridge University began a
survey using a radio interferometer,
which picked up signals at 159
MHz. This was better at resolving
faint radio sources, and led to the
discovery of the first two quasars.
The light from both objects was
invisible to the optical telescopes
available to the Cambridge
researchers at the time. However,
their measurements of the flux
density told them that these radio
sources were very compact. ❯❯Understanding [of quasars]
has not developed very much
in 50 years. You only see a
point source; you don’t see
its structure. It’s a difficult
thing to get hold of.
Maarten Schmidt
speaking in 2013It is likely that all
galaxies have a black
hole at the center and
have been quasars
in the past.Quasars are active
galactic nuclei, in which
a black hole is eating
up the galaxy’s stars.These are found to
be distant, bright,
fast-moving, starlike
objects called quasars.