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.
It can be inferred from the passage that which of the
following is an accurate physical description of typical
low-surface-brightness galaxies?
- AThey are large spiral galaxies containing fewer
stars than conventional galaxies.
- BThey are compact but very dim spiral galaxies.
- CThey are diffuse spiral galaxies that occupy a
large volume of space.
- DThey are small, young spiral galaxies that contain
a high proportion of primordial gas.
- EThey are large, dense spirals with low luminosity.
显示答案
正确答案: C