In addition to conventional
galaxies, the universe contains
very dim galaxies that until
Line recently went unnoticed by
(5) astronomers. Possibly as
numerous as conventional gal-
axies, these galaxies have the
same general shape and even
the same approximate number
(10) of stars as a common type of
conventional galaxy, the spiral,
but tend to be much larger.
Because these galaxies’
mass is spread out over
(15) larger areas, they have far
fewer stars per unit volume
than do conventional galaxies.
Apparently these low-surface-
brightness galaxies, as they
(20) are called, take much longer
than conventional galaxies to
condense their primordial gas
and convert it to stars—that is,
they evolve much more slowly.
(25) These galaxies may
constitute an answer to the long-
standing puzzle of the missing
baryonic mass in the universe.
Baryons—subatomic particles
(30) that are generally protons or
neutrons—are the source of
stellar, and therefore galactic,
luminosity, and so their numbers
can be estimated based on how
(35) luminous galaxies are. How-
ever, the amount of helium
in the universe, as measured
by spectroscopy, suggests
that there are far more baryons
(40) in the universe than estimates
based on galactic luminosity
indicate. Astronomers have
long speculated that the missing
baryonic mass might eventually
(45) be discovered in intergalactic
space or as some large popu-
lation of galaxies that are difficult
to detect.
--------------------------------------------------------------------------------
The author mentions the fact that baryons are the source of stars’ luminosity primarily in order to explain
A. how astronomers determine that some galaxies contain fewer stars per unit volume than do others
B. how astronomers are able to calculate the total luminosity of a galaxy
C. why astronomers can use galactic luminosity to estimate baryonic mass
D. why astronomers’ estimates of baryonic mass based on galactic luminosity are more reliable than those based on spectroscopic studies of helium
E. how astronomers know bright galaxies contain more baryons than do dim galaxies