ACT Reading Apr. 2017 74F - Passage IV

Questions 31-40 are based on the following passage.

NATURAL SCIENCE: This passage is adapted from the article "Warp Factor" by Charles Liu (©2003 by Natural History Magazine, Inc.).

Astronomers sometimes describe the shape of our
home galaxy, the Milky Way, as a thin-crust pizza with
a plum stuck in the middle. The plum is the slightly
oblong central bulge, protruding about 3,000 light-
5 years above and below the galactic plane, comprised
mostly of older stars; it makes up the core of the Milky
Way, and includes a black hole two and a half million
times the mass of the Sun. The crust of the pizza is the
galactic disk-the source of most of our galaxy's light.
10 Thin and flat, the disk is 100,000 light-years across,
about 1,000 light-years thick, on average, and includes
more than 80 percent of the galaxy's hundred billion or
so stars.

The plum-and-pizza picture works well enough,
15 but like most simple metaphors, it breaks down if you
push it. For one thing,. the galactic disk isn't a rigid
body, but a loose agglomeration of matter streaming
around a common center of gravity. (The swirling pat-
tern of a hurricane far better resembles our spinning
20 galaxy.) For another thing, our galaxy's disk isn't flat;
it's warped. Picture a disk of pizza dough spun into the
air by a skilled chef: our galaxy goes through the same
kind of floppy, wobbly gyrations, though at a rate best
measured in revolutions per hundreds of millions of
25 years.

Why does the Milky Way have such an odd-
looking warp? No definitive answer has emerged. One
thing we do know: when it comes to warps, our galaxy
is hardly unique. About half of all spiral galaxies are
30 warped to some degree. Theoretical and computational
models have shown that a number of physical processes
can warp a galaxy, so it's a matter of figuring out which
scenario applies. An innovative analysis of the problem
by Jeremy Bailin, an astronomy graduate student at the
35 University of Arizona in Tucson, has implicated a small
satellite galaxy, currently being ripped to shreds by the
gravity of the Milky Way.

The Sagittarius Dwarf Spheroidal Galaxy was dis-
covered in 1994. It appears to ·be in a roughly polar
40 orbit around the Milky Way-that is, above and below
the galactic disk-about 50,000 light-years from the
galactic center. That orbit brings the dwarf galaxy far
too close to the huge gravitational tidal forces of the
Milky Way for the dwarf to remain intact. As a result,
45 the Sagittarius Dwarf now looks something like strands
of spaghetti spilling from the front of a pasta-making
machine, the galaxy's matter being drawn out over hun-
dreds of millions of years by intergalactic tides.

Gravitational collisions between small satellite
50 galaxies and big spiral galaxies have long been
regarded as possible culprits in the warping of a larger
galaxy's disk. The best known satellite galaxies orbit-
ing the Milky Way-the Large and Small Magellanic
Clouds-are too far away, and have the wrong orbital
55 characteristics, to have warped our galactic home. The
Sagittarius Pwatf seems a much more likely candidate.
simply because it i.s only a third as far from the center
of the Milky Way as the Magellanic Clouds. But in
astronomy-unlike in real estate-location isn't every-
60 thing; to show a direct connection between warp and
dwatf, the orbital motion of the Sagittarius Dwarf must
be linked to the rotation of the Milky Way's disk.

Bailin 's study is the first to find such a link. His
analysis of the galactic warp is based on angular
65 momentum-a measure of how much a system is spin-
ning or rotating. Just as objects moving in a straight
line have momentum, objects spinning or orbiting
around an axis have angular momentum; and just as the
momenta of two objects combine when they collide, so
70 too do their angular momenta. Imagine two figure
skaters coming together for a combination spin. When
they make physical contact, their individual spiraling
motions combine to produce a single, unified whirl.

Starting with the latest measurements of the struc-
75 ture and spin of the Milky Way, Bailin deduced the
angular momentum of the warped portion of the Milky
Way's disk. He then compared that measure with the
angular momentum of the

 
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