The Astronomy Book

185

See also: Spiral galaxies 156–61 ■ Beyond the Milky Way 172–77 ■

Quasars and black holes 218–21

B

etween 1940 and 1942,

American astronomer Carl

Seyfert studied a number

of spiral galaxies that had compact,

particularly bright centers, often

bluish in color. His investigations

revealed that there were distinctive

emission lines in these galaxies’

spectra. He published a paper

describing galaxies of this type,

which later came to be known

as Seyfert galaxies. They are

usually spiral galaxies with nuclei

that produce large amounts of

radiation over a broad range of

wavelengths, often most strongly

in the infrared region, but also

often including visible light, radio

waves, ultraviolet radiation, X-rays,

and gamma rays.

Violent centers

Seyfert galaxies are just one variety

of a class of galaxies called active

galaxies. These have central regions,

termed active galactic nuclei (AGNs),

in which an extraordinary amount

of violent activity occurs. Quasars

are another type of AGN. These are

always a vast distance away and

produce so much energy that they

outshine their host galaxies, which

cannot be seen. AGNs are thought

to be powered by matter spiraling

into massive black holes at their

centers. In addition to emitting

radiation, many AGNs also send

powerful jets of particles into space

from the vicinity of their central

black holes. Some are associated

with vast lobes of material that

emit radio waves—active galaxies

that feature these “radio lobes” are

called radio galaxies. ■

ATOMS, STARS, AND GALAXIES

SOME GALAXIES

HAVE ACTIVE REGIONS

AT THEIR CENTERS

NUCLEI AND RADIATION

IN CONTEXT

KEY ASTRONOMER
Carl Seyfert (1911–1960)

BEFORE
1908 Edward Fath and Vesto
Slipher observe peculiarities
in the spectrum of the nebula
NGC 1068 (now recognized
as a typical Seyfert galaxy).

1936 Edwin Hubble classifies
the shapes of galaxies.

AFTER
1951 Cygnus A, one of the
strongest sources of radio
waves in the sky, is identified
as the first radio galaxy.

1963 Dutch astronomer
Maarten Schmidt coins the
term quasi-stellar radio source
(later “quasar”) for an object
with a starlike appearance
that is actually a distant, very
bright source of radio waves.

1967 Armenian astrophysicist
Benjamin Markarian begins
publishing a list of galaxies with
strong ultraviolet emissions,
many of them Seyfert galaxies.

Spiral galaxy NGC 1068 (M 77) is

the archetypal Seyfert galaxy. It has

an intensely bright active center

surrounded by swirls of ionized gas.