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.
According to the passage, conventional spiral galaxies differ from low-surface-brightness galaxies in which of the following ways
- AThey have fewer stars than do low-surface-brightness galaxies.
- BThey evolve more quickly than low-surface-brightness galaxies.
- CThey are more diffuse than low-surface-brightness galaxies.
- DThey contain less helium than do low-surface-brightness galaxies.
- EThey are larger than low-surface-brightness galaxies.
显示答案
正确答案: B