题目材料
In addition to conventional galaxies, the universe
contains very dim galaxies that until recently went
unnoticed by astronomers. Possibly as numerous
as conventional galaxies, these galaxies have the
same general shape and even the same
approximate number 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 larger areas, they have far fewer stars per unit
volume than do conventional galaxies. Apparently
these low-surface-brightness galaxies, as they 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.
These galaxies may constitute an answer to the
long-standing puzzle of the missing baryonic mass
in the universe. Baryons-subatomic particles 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
luminous galaxies are. However, the amount of
helium in the universe, as measured by
spectroscopy, suggests that there are far more
baryons in the universe than estimates based on
galactic luminosity indicate. Astronomers have long
speculated that the missing baryonic mass might
eventually be discovered in intergalactic space or as
some large population of galaxies that are
difficult to detect.
The author implies that low-surface-brightness galaxies
could constitute an answer to the puzzle discussed in
the second paragraph primarily because
- Athey contain baryonic mass that was not taken
into account by researchers using galactic
luminosity to estimate the number of baryons in
the universe
- Bthey, like conventional galaxies that contain many
baryons, have evolved from massive, primordial
gas clouds
- Cthey may contain relatively more helium,
and hence more baryons, than do galaxies
whose helium content has been studied using
spectroscopy
- Dthey have recently been discovered to contain
more baryonic mass than scientists had thought
when low-surface-brightness galaxies were first
observed
- Ethey contain stars that are significantly more
luminous than would have been predicted on
the basis of initial studies of luminosity in lowsurface-brightness galaxies
显示答案
正确答案: A