2. Aside from stars and the interstellar medium,
what else do galaxies contain?
• Galaxies often have large magnetic fields that run
through and around their disks or bulges. Although at any
particular location, the fields may be weak, the overall
effect of those fields can be tremendous, affecting the
motion of charged particles and interstellar medium
throughout galaxies. Galaxies can also contain
black holes.
3. Does the Milky Way contain a supermassive
black hole?
•It most certainly does. The center of the Milky
Way is in the direction of the constellation
Sagittarius; right at the center, there is an object
called Sag A* (pronounced “Sagittarius A-star”) that
emits much more X-rays and radio waves than
expected for a star-sized body.
4. Does every galaxy contain a black hole?
• There are two general categories of black holes that
have been observed: stellar black holes and
supermassive black holes. Every galaxy that
has contained very hot, luminous stars—stars 20
times or more the mass of the Sun—almost certainly
contains stellar black holes.
5. Does every galaxy contain a supermassive
black hole?
•No, but based on current observations, the
majority of galaxies do contain one. Among
nearby galaxies, more than 90 percent of all
galaxies that have been measured so far
appear to contain a supermassive black hole.
7. What is an “active galaxy,” or an “active
galactic nucleus” (AGN)?
• If a supermassive black hole exists at the nucleus of a galaxy, it may accumulate
matter from the stars and gas that surround it. If this matter is accumulated rapid-
ly—at a rate of a few Earth-masses per second or greater—
tremendous amounts of energy can be generated as the matter falls
toward the black hole. The energy that is released in this way can be
much greater than that of the nuclear fusion of a star. In fact, such a
supermassive black hole system can radiate more energy in a few
seconds than our Sun can produce in thousands or even millions of
years. These systems are called active galactic nuclei, or AGNs.
8. Who first discovered and studied active
galaxies?
• The American astronomer Carl Seyfert (1911–
1960) is credited with the discovery of active
galaxies. Seyfert’s general area of astronomical
expertise was determining the spectroscopic
properties, colors, and luminosities of stars and
galaxies.
9. How many different kinds of active galaxies
are there?
•AGNs can occur in any type or shape of
galaxy—spiral, elliptical, or irregular.
Depending on exactly how the energy
radiates from the AGN, they can have very
different appearances.
10. What determines the luminosity of an AGN?
• The single most important determinant of an
AGN’s luminosity is the rate at which
matter falls toward its central
supermassive black hole.
11. What are radio galaxies?
•Radio galaxies are simply galaxies—
usually very ordinary-looking elliptical
galaxies when viewed by visible light—
that radiate an unusually large amount
of radio waves.
12. What is the unified model of active galactic
nuclei?
• All AGNs have the same basic structure: they hava a QSO
sitting in the middle of a galaxy.
• The QSO will have a different spectroscopic signature from
our point of view. Furthermore,
• The QSO host galaxy could be spiral, elliptical, or peculiar,
and we could be seeing the QSO through a screen of
interstellar dust, or lots of gas, or a lot of stars of differing
colors and luminosities.
13. What determines the luminosity of an AGN?
• The single most important determinant of an
AGN’s luminosity is the rate at which
matter falls toward its central
supermassive black hole.
