221
may emerge in another universe as
“small bang” events. Nevertheless,
as the understanding of black holes
grew, the white-hole explanation
of quasars faded away.
Supermassive black hole
Quasars are too luminous and
energetic to be using nuclear
fusion, the process that powers
stars, to produce their energy.
However, theoretical work on
black holes showed that a region
of material, known as the accretion
disk, would form around an event
horizon. Since this material was
steadily pulled into the black
hole, it would heat up to millions
of degrees. A supermassive
black hole, with a mass billions
of times greater than the sun,
would produce an accretion
disk that matched the output
observed in quasars.
The accretion disk theory also
matched up with the beams of
plasma, known as relativistic
jets, that blasted out in opposite
directions from some quasars.
These are caused by the spin of
the black hole, which creates a
magnetic field and focuses matter
and radiation into two streams.
NEW WINDOWS ON THE UNIVERSE
Superheated plasma blasts out
at close to the speed of light
from each stream.
Today’s understanding of
quasars began to crystallize in the
1980s. The accepted view is that
a quasar is a supermassive black
hole—or perhaps two—at the heart
of a galaxy, that is eating up the
stellar material. A galaxy that
behaves like this is said to have an
active nucleus, and it appears that
quasars are just one manifestation
of these so-called active galaxies.
An active galaxy is detected as
a quasar when the relativistic jets
are angled to Earth’s line of sight.
Therefore, the object is detected
chiefly from its radio emissions. If
the jets are perpendicular to Earth’sline of sight, then they can never
really be detected, and instead
Earth sees a radio galaxy—a galaxy
that is pumping out a loud radio
source. If the relativistic jets are
directed right at Earth, an excellent
view is gained of the active nucleus
in an object, known as a blazar.
Most quasars are ancient
objects, and Earth sees their
activity from when the universe
was young. Unlike in other active
galaxies, the brilliance of a quasar’s
nucleus makes it hard to discern
much of the galaxy around it. It is
thought that young galaxies always
have active nuclei, and that once
there is no material left for their
black hole to swallow they become
quieter places, like the Milky Way
today. However, galactic collisions,
in which one galaxy merges with
another, can activate the nucleus
again. It is likely that the Milky
Way, which is on track to collide
with Andromeda in 4 billion years,
is destined to become a quasar
itself one day. ■Maarten Schmidt Born in Groningen, the
Netherlands, Maarten Schmidt
went to his home city’s university
and studied alongside Jan Oort.
Schmidt earned his doctorate,
before emigrating to the US and
taking up a post at Caltech’s
Palomar Observatory. He became
a leading expert on star formation,
encapsulated by the Schmidt
law, which relates the density
of interstellar gas clouds to the
rate of star formation inside them.
Schmidt also became one of the
chief investigators of quasars.
After a conference on the subject
in 1964, Schmidt and the otherleading figures in the field,
including William Fowler and
Subrahmanyan Chandrasekhar,
were leaving on the same plane,
which experienced a dicey
takeoff. Fowler is said to have
quipped: “If this plane crashes,
at least we’ll get a new start
on this quasar problem.”
Schmidt went on to occupy
several eminent roles in
astronomical institutions.Key work1963 3C 273: A Star-Like Object
with Large Red-ShiftThe Hubble telescope took this
image of the active galactic nucleus
of the elliptical galaxy NGC 4261. The
disk of dust is 800 light-years wide.