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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.
当前版本由VERNA更新于2018-05-09 17:43:47 感谢由VERNA对此题目的解答所做出的贡献。
都是有点儿相反的感觉,A 和 D 的最大区别是是否有 fact;E 的 gathering data 木
有提;答案 A。
325秋梨膏
采花大盗123
Optimism
[数学 ]【新版】冲分救命题 -11998
The integers a, b, and c are positive, and a>b>c. When a, b, and c are divided by 3, the remainders are 0, 1, and 1, respectively.
[数学 ]【新版】冲分救命题 -11997
For all positive even integers n. n* represents the product of all even integers from 2 to n, inclusive. For example, 12*=12×10×8×6×4×2. What is the greatest prime factor of 20*+22*?
[数学 ]【新版】冲分救命题 -11996
[数学 ]【新版】冲分救命题 -11995
[数学 ]【新版】冲分救命题 -11994
In a game, the cards in a certain deck are distributed to players one at a time until each player has the same number of cards and there are not enough cards left in the deck to distribute one more card to each player. The number of cards in the deck is between 60 and 100. If the cards in the deck are distributed o 5 players, 2 cards will be left in the deck. If the cards in the deck are distributed to 6 players, 4 cards will be left in the deck. If the cards in the deck are distributed to 7 players, how many cards will be left in the deck?
[数学 ]【新版】冲分救命题 -11993
Let a be the greatest integer such that is a factor of 1,500, and let b be the greatest integer such that is a factor of 33,333,333. Which of the following statements are true? Indicate all such statements.
[数学 ]【新版】冲分救命题 -11992
A set consists of all three-digit positive integers with the following properties.Each integer is of the form JKL,where J,K,and I are digits;all the digits are nonzero;and the two-digit integers JK and KL are each divisible by 9.How many integers are in the set?
[数学 ]【新版】冲分救命题 -11991
[数学 ]【新版】冲分救命题 -11990
The number of children in a certain family is a prime number less than 10. The number of boys in the family is greater than the number of girls, and the number of boys is a prime number. If at least 1 of the children in the family is a girl, which of the following could be the number of boys in the family?Indicate all such numbers.
[数学 ]【新版】冲分救命题 -11989
[数学 ]【新版】冲分救命题 -11988
x and y are integers.
[数学 ]【新版】冲分救命题 -11987
x and y are integers.
[数学 ]【新版】冲分救命题 -11986
a and b are distinct odd prime numbers.
[数学 ]【新版】冲分救命题 -11985
When the integer n is divided by 33, the remainder is 24. Which of the following must be a divisor of n?
[数学 ]【新版】冲分救命题 -11984
4.What is the length of an edge of the smallest solid cube that can be made by placing together solid rectangular blocks of size 7 by 6 by 3?
[数学 ]【新版】冲分救命题 -11983
Light travels at approximately 186,000 miles per second.Which of the following is closest to the number of miles that light travels in 3 hours.
[数学 ]【新版】冲分救命题 -11982
The number n is the least positive integer for which 108n is the square of an intege
[数学 ]【新版】冲分救命题 -11981
The number n is the least positive integer for which 108n is the square of an intege
[数学 ]【新版】冲分救命题 -11980
If x=2y+1, and y=2w, where w, x, and y are integers, which of the following must be an odd integer?
[数学 ]OG -11979
If x=2y+1, and y=2w, where w, x, and y are integers, which of the following must be an odd integer?
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