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OG'15 - conventional galaxies

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fiza gupta Master | Next Rank: 500 Posts
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OG'15 - conventional galaxies

Post Sat Oct 15, 2016 7:00 am
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.

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

OA:C

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crackverbal Master | Next Rank: 500 Posts
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Post Sat Oct 15, 2016 9:54 pm
Let us map this passage first -
Note that in general, the first two sentences and the last sentence in each paragraph are important to figure out what the passage is trying to say. Other sentences can be quickly skimmed through.
This is because when a question asks about a detail mentioned in the passage, you will anyways have to read through that portion again.

Para 1 - the author talks about very dim galaxies
Para 2 - these galaxies may help answer the puzzle of missing baryonic mass.

The question talks about baryons and stars' luminosity. This is mentioned in the 2nd paragraph.
Read through the 2nd paragraph now. The first sentence talks about the puzzle of missing baryonic mass. The second sentence states that baryons are the source of luminosity and luminosity can be used to estimate the number of baryons and their mass.
Which answer option states this? Option C. The next sentence talks about the puzzle further.
A - this is mentioned in the first paragraph of the passage and is not relevant to the question. This is a typical "cute phrase" answer choice, wherein the answer option mentions some phrase present in the passage but is not relevant to the question.
B - no reverses the argument given in the passage. The passage clearly states that luminosity is used to determine baryonic mass and not the other way round.
C - Correct answer. since baryons are the source of luminosity, luminosity can be used to estimate their numbers/mass.
D - goes against the argument in the passage. The passage clearly states that the mass calculated through luminosity does not account for all baryonic mass.
E - While this might be true we cannot infer this from the argument. All we know is that baryonic mass can be estimated through luminosity. Note the word 'estimate'. But this answer option talks about 'knowing'. from the 1st paragraph we know that these dim galaxies contain the same number of stars as bright galaxies. So this option stating that bright galaxies contain more baryons might not be true.
Another thing is that the 2nd paragraph does not talk about bright/dim galaxies at all. Eliminate E.

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