17. The primary purpose of the passage is to

(A) describe what results when a supernova triggers the creation of chains of star-forming regions

No. this is used as a premise to introduce a new theory

(B) propose a modification in the most widely accepted theory of spiral-galaxy structure

No. Because the new theory is exciting new. So, no modification

(C) compare and contrast the roles of clouds of gas and dust in two theories of spiral-galaxy structure

No contrast. In both theories, the compression clouds are part of the same theories

(D) describe a new theory of spiral-galaxy structure and contrast it with the most widely accepted theory

Correct. Blue is the contrast

A computer simulation based on this theory has reproduced the appearance of many spiral galaxies without assuming an underlying density wave, the hallmark of the most widely accepted theory of the large-scale structure of spiral galaxies.

(E) describe a new theory of spiral-galaxy structure and discuss a reason why it is inferior to the most widely accepted theory


No inferior. It is a new way of thinking about the formation of spiral galaxies

----------------------------------------------------------------------------------------------------------------

18. The passage implies that, according to the new theory of spiral-galaxy structure, a spiral galaxy can be created by supernovas when the supernovas are

(A) producing an underlying density wave

the wave produces supernovas

(B) affected by a density wave of spiral form

the spiral is the final result

(C) distributed in a spiral pattern

we observe the supernovas in a spiral pattern. That does not mean they are in a spiral formation

(D) located in the central plane of a galaxy

they are widespread throughout

(E) located in a differentially rotating galaxy

Correct

If many such chains were created in a differentially rotating galaxy, the distribution of stars would resemble the observed distribution in a spiral galaxy.

---------------------------------------------------------------------------------------------------------------------------------


19. Which of the following, if true, would most discredit the new theory as described in the passage?

(A) The exact mechanism by which a star becomes a supernova is not yet completely known and may even differ for different stars.

We do know how they form

(B) Chains of star-forming regions like those postulated in the new theory have been observed in the vicinity of dense clouds of gas and dust.

The gas and dust are inside and form the stars

(C) The most massive stars formed from supernova explosions are unlikely to evolve into supernovas.

Both theories imply the formation of novas and supernovas thanks to the compression of clouds of gas and dust. Which encompass that a nova becomes a super

(D) Computer simulations of supernovas provide a poor picture of what occurs just before a supernova explosion.

No information if we do have a poor look.

(E) A density wave cannot compress clouds of gas and dust to a density high enough to create a star.

Is the contrary.

Hope this helps