【3月16日阅读】

belinda_cao

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.

1）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

2）The primary purpose of the passage is to
A.describe a phenomenon and consider its scientific significance
B.contrast two phenomena and discuss a puzzling difference between them
C.identify a newly discovered phenomenon and explain its origins
D.compare two classes of objects and discuss the physical properties of each
E.discuss a discovery and point out its inconsistency with existing theory

3）it can be inferred from the passage that which of the following is an accurate physical description of typical low-surface-brightness galaxies?
A.They are large spiral galaxies containing fewer stars than do conventional galaxies.
B.They are compact but very dim spiral galaxies.
C.They are diffuse spiral galaxies that occupy a large volume of space.
D.They are small, young spiral galaxies that contain a high proportion of primordial gas.
E.They are large, dense spirals with low luminosity.

belinda_cao

嘻嘻，抢沙发。 ACC