彻底跪了EX21Q5

HardyNiborUID：3461489

Until recently astronomers have been puzzled by the fate of red giant and supergiant stars. When the core of a giant star whose mass surpasses 1.4 times the present mass of our Sun (M⊙) exhausts its nuclear fuel, it is unable to support its own weight and collapses into a tiny neutron star. The gravitational energy released during this implosion of the core blows off the remainder of the star in a gigantic explosion, or a supernova.
Since around 50 percent of all stars are believed to begin their lives with masses greater than 1.4M⊙, we might expect that one out of every two stars would die as a supernova. But in fact, only one star in thirty dies such a violent death. The rest expire much more peacefully as planetary nebulas. Apparently most massive stars manage to lose sufficient material that their masses drop below the critical value of 1.4 M⊙ before they exhaust their nuclear fuel. Evidence supporting this view comes from observations of IRC+10216, a pulsating giant star located 700 light-years away from Earth. A huge rate of mass loss (1 M⊙ every 10,000 years) has been deduced from infrared observations of ammonia (NH3) molecules located in the circumstellar cloud around IRC+10216.
Recent microwave observations of carbon monoxide (CO) molecules indicate a similar rate of mass loss and demonstrate that the escaping material extends outward from the star for a distance of at least one light-year. Because we know the size of the cloud around IRC+10216 and can use our observations of either NH3 or CO to measure the outflow velocity, we can calculate an age for the circumstellar cloud. IRC+10216 has apparently expelled, in the form of molecules and dust grains, a mass equal to that of our entire Sun within the past ten thousand years. This implies that some stars can shed huge amounts of matter very quickly and thus may never expire as supernovas. Theoretical models as well as statistics on supernovas and planetary nebulas suggest that stars that begin their lives with masses around 6 M⊙ shed sufficient material to drop below the critical value of 1.4M⊙. IRC+10216, for example, should do this in a mere 50,000 years from its birth, only an instant in the life of a star.
But what place does IRC+10216 have in stellar evolution? Astronomers suggest that stars like IRC+10216 are actually ”protoplanetary nebulas“–old giant stars whose dense cores have almost but not quite rid themselves of the fluffy envelopes of gas around them. Once the star has lost the entire envelope, its exposed core becomes the central star of the planetary nebula and heats and ionizes the last vestiges of the envelope as it flows away into space. This configuration is a full-fledged planetary nebula, long familiar to optical astronomers.

5. According to information provided by the passage,
which of the following stars would astronomers
most likely describe as a planetary nebula?
(A) A star that began its life with a mass of
5.5 M⊙, has exhausted its nuclear fuel, and
has a core that is visible to astronomers
(C) A star that has exhausted its nuclear fuel, has a
mass of 1.2 M⊙, and is surrounded by a
circumstellar cloud that obscures its core
from view

pn应该是指那些死的不太猛烈的星体吧，那选一个重量小于1.4的哪里不对呢？据说加黑句可以推出A，因为A的core是pn的central star？这也太拐弯了

小飞行UID：2980207

Since around 50 percent of all stars are believed to begin their lives with masses greater than 1.4M⊙, we might expect that one out of every two stars would die as a supernova. But in fact, only one star in thirty dies such a violent death.  

This implies that some stars can shed huge amounts of matter very quickly and thus may never expire as supernovas. Theoretical models as well as statistics on supernovas and planetary nebulas suggest that stars that begin their lives with masses around 6 M⊙ shed sufficient material to drop below the critical value of 1.4M⊙. IRC+10216, for example, should do this in a mere 50,000 years from its birth, only an instant in the life of a star.

5. According to information provided by the passage,
which of the following stars would astronomers
most likely describe as a planetary nebula?

这道题的定位我觉得还蛮搞笑的，可以从出现那个圈的地方找。题目里就这两个地方。仔细看看还是可以看出些东西的。前面是技术不发达的时期，通过观测发现的那些1.4左右的可能不会如预测那么凶残。

技术发展后，通过测量发现那些6左右的即使外逸很快，但也依旧很稳定不会出现超新星。

相比较而言还是那些初始6左右的更稳定，即使他们消耗到1.4以下也不会出现超新星。而消耗到1.4以下的不一定初始是6左右，可能是3，可能是2.5。那么消耗过程中的稳定度肯定不及6左右的。

我是这么理解的

HardyNiborUID：3461489

小飞行 发表于 2013-9-16 22:23
Since around 50 percent of all stars are believed to begin their lives with masses greater than 1.4M ...

貌似1.4以下就安全了，超新星是指那些不安全死的很惨的那些吧

lanseyyuUID：3458803

! No Chinese Inputting method on my end.
I also got caught by this one and here is my understanding:
there are two standards--begin with around 6M and have a core(implying it contradicts "it is unable to support its own weight and collapses into a tiny neutron star" )

This can be tracked back to " it is unable to support its own weight and collapses into a tiny neutron star" and " suggest that stars that begin their lives with masses around 6 M⊙ shed sufficient material to drop below the critical value of 1.4M"