12.Practice.Tests.for.the.SAT_2015-2016_1128p

Section 6
790
Practice Test Nine
Questions 16-22 are based on the following passage.
The following is adapted from a history of astronomy
textbook.
Dark matter is the undetected matter that some
astronomers believe makes up the missing mass
of the universe. Astronomers once believed that
Line interstellar spaces were largely empty and quite
(5) transparent. The advent of photography, however,
showed a complicated structure of star clouds and
rifts and holes where there were few or no stars.
As research continued, it was accepted that these
were clouds of obscuring material now known as
(10) dark matter.
Astronomers initially were reluctant to accept
that the bright and dark nebulae within the Milky
Way might be an indication of a substrate of gas
and dust in interstellar space. The consequence of
(15) an absorbing dust, which dims the light of distant
stars and complicates the determination of their
distances and intrinsic brightnesses, was a very
serious obstacle for researchers.
Astronomers were finally convinced of the pres-
(20) ence of dark matter by the patient work of Robert
J. Trumpler. He grouped hundreds of open star
clusters into types according to their structures
and shapes. Once grouped, the relative distance
of the various clusters within one type could then
(25) be measured by two methods. First, the Main
Sequence stars of, for example, spectral type F had
the same absolute magnitude in all the clusters.
Therefore, the greater the difference in the observed
apparent magnitudes of such stars in any two
(30) clusters, the greater the difference in the distances
of the two clusters. This is often referred to as
the "fainter means more distant" method. For
the "smaller means more distant" method, used
for clusters of the same intrinsic size, the angular
(35) diameter of a cluster on the sky was used as a
measure of its distance.
Trumpler's two methods did not give compatible
results: the distances given by the apparent
magnitude method were greater than those given
(40) by the angular diameters, thus showing that the
starlight had been dimmed in its passage to Earth.
The presence of a general absorption at all wavelengths
was thus proven.
Trumpler then investigated the selective absorp-
( 45) tion phenomenon. If interstellar dust particles
are about the same size as the wavelength of light,
then the star would appear to be redder than
its spectral type would suggest. (The setting sun
appears red for just this reason.) This phenome-
( 50) non increases the measured "colored index" of the
star by an amount that is called the "color excess."
It was difficult to measure the quantities exactly
on photographs, but photoelectric methods used
after 1950 could measure stellar magnitudes; color
(55) excess then became a powerful method for correcting
for the effects of the interstellar dust. Trumpler
thus concluded that an absorption was taking
place in the Milky Way.
16. This passage is primarily concerned with
(A) dark matter in interstellar space
(B) the advent of photography in astronomy
( C) star clouds and rifts and holes in space
(D) empty space between stars
(E) astronomy
17. In line 4, "interstellar" most nearly means
(A) in the universe
(B) between Earth and the sun
(C) star clouds
(D) where there are no stars
(E) empty and transparent
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