首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
THE LIFE CYCLE OF A STAR It has been conservatively estimated that there are some 10,000 billion, billion stars in the unive
THE LIFE CYCLE OF A STAR It has been conservatively estimated that there are some 10,000 billion, billion stars in the unive
admin
2011-02-08
59
问题
THE LIFE CYCLE OF A STAR
It has been conservatively estimated that there are some 10,000 billion, billion stars in the universe. It is difficult to know the exact age of a star (astronomers have identified stars as young as 25,000 years old and others are thought to be over 10 billion years old), but what astronomers do know is that there are many different kinds. How each star is formed, and its mass, influences its type and longevity.
A star is born in a nebula, which is a giant cloud of gas and dust. The larger the amount of matter that is in the nebula, the greater the mass of the star that is created. Inside these nebulae are dense areas of gas, which, due to their density, have a stronger gravitational pull than the rest of the nebula. Gradually, gravity drags the gas in the nebula together and it begins to spin and become increasingly hotter. Once the temperature reaches 15,000,000℃, nuclear fusion occurs in the centre of the cloud, and it begins to glow brightly. It stabilizes at this temperature, contracts slightly and becomes what is known as a main sequence star (an example of this is our own Sun). It can remain in this stage for millions or billions of years.
As it glows, hydrogen in the centre (through the nuclear fusion) becomes helium. Eventually the hydrogen supply in the core diminishes and the core of the star becomes unstable, contracting more. However, the outer parts of the star (which are still mainly hydrogen) expand and cool, and in doing so, the star starts to glow red. It is at this stage that the star becomes a red giant. It is anticipated that it will take the Sun another 5 billion years to reach this stage. By then it will have grown large enough to engulf the three closest planets Mercury, Venus and Earth) and glow 2,000 times brighter than it currently does.
Exactly how a star will react in the red giant phase depends on its mass. Throughout the red giant phase, the hydrogen in the outer parts carries on burning, and the centre gets hotter and hotter. On reaching 200,000,000℃, the helium atoms fuse forming carbon atoms. The remainder of the hydrogen explodes and forms a ring around the core called a planetary nebula.
With medium-sized stars, once the final helium atoms have fused into carbon atoms, the star starts to die. The gravitational pull leads to the last of the star’s matter collapsing inwards and compacting to become extremely dense. A star like this is called a white dwarf. It will shine white-hot until the remaining energy (thermal energy trapped in its interior) has been exhausted after which it will no longer emit light. This can take in excess of several billion years. It is then termed a black dwarf (a cold, dark star, perhaps replete with diamonds) and remains in that stage forever. When the larger red giants (massive stars) collapse, which happens in an instant, so much planetary nebula is created that this gas and dust can be used as building material for planets in developing solar systems. In addition, with massive stars, as the temperature increases, the carbon atoms get pulled together to form increasingly heavier elements like oxygen, nitrogen and finally iron. Once this happens, fusion ceases and the iron atoms begin absorbing energy. At some point in the future, this energy is released in a huge explosion called a supernova. A supernova can have a core temperature of up to 1,000,000,000℃ and the explosion can light up the sky for weeks, outshining an entire galaxy. Astronomers believe that Earth is made up of elements formed from the inside of stars, in particular red giants that exploded as supernovas. These massive stars have an average life span of one million years.
After becoming a supernova, the remaining core of a massive star that is 1.5 to 4 times as massive as the Sun becomes a neutron star. It starts to spin and often emits radio waves. If these waves occur in pulses, the neutron star is referred to as a pulsar. When a massive star has eight or more times the mass of the Sun, it will remain massive after the supernova. It has no nuclear fusion supporting the core and becomes engulfed by its own gravity. This results in a black hole, which sucks in any matter or energy that passes close to it. The gravitational field of a black hole is powerful enough to prevent the escape of light and is so dense that it cannot be measured. The phrase ’black hole’ originated from the physicist John Archibald Wheeler; before this, black holes were known as ’frozen stars’. Wheeler came up with this name two years before the proof of the existence of the first black hole, X-ray binary star Cygnus X-1, in 1971. Astronomers think that there may be a black hole at the centre of each galaxy.
The life cycle of a star is really that-the materials from an exploded star mix with the hydrogen of the universe. This mixture in turn will be the starting point of the next star. The Sun is a case in point, containing the debris from numerous other stars that exploded long before the Sun was born.
its supply of energy has run out ______
选项
答案
E
解析
转载请注明原文地址:https://kaotiyun.com/show/E7VO777K
本试题收录于:
雅思阅读题库雅思(IELTS)分类
0
雅思阅读
雅思(IELTS)
相关试题推荐
Whichofthefollowingexpressestheareaofacir-cleintermsofC,itscircumference?
TheCenterCityLittleLeagueisdividedintoddivisions.Eachdivisionhastteams,andeachteamhaspplayers.Howmanyplay
Intherectangularcoordinatesystem,segmentOPisrotatedcounterclockwisethroughanangleof90°topositionOQ(notshown).
Solveeachofthefollowingsystemsofequationsforxandy.(a)x+y=24x-y=l8(b)3x-y=-5x+2y=3(c)15x-18-2y=-3
Solveeachofthefollowingsystemsofequationsforxandy.(a)x+y=24x-y=l8(b)3x-y=-5x+2y=3(c)15x-18-2y=-3
Inthecoordinatesystembelow,findthefollowing.(a)CoordinatesofpointQ(b)LengthsofPQ,OR,andPR(c)Perimeterof△PQR
Inthecoordinatesystembelow,findthefollowing.(a)CoordinatesofpointQ(b)LengthsofPQ,OR,andPR(c)Perimeterof△PQR
Whenthepositiveintegernisdividedby3,theremainderis2andwhennisdividedby5,theremainderis1.Whatistheleas
DISTRIBUTIONOFWORKFORCEBYOCCUPATIONALCATEGORYFORREGIONYIN2011ANDPROJECTEDFOR2025(a)In2011,howmanycategories
ThePrize:$10MillionATheyareaneliteclubofbillionaires,movieproducers,dotcomwizkidsandtheoccasionalastronaut
随机试题
A.血管损伤B.神经损伤C.骨折延迟愈合D.骨筋膜室综合征胫骨上1/3骨折易导致
A、kPaB、PasC、mm2/sD、cm-1E、μm;物理量的单位符号为运动黏度
案例A市特殊钢B公司是以民营资本为主的股份制企业,职工471人,生产规模140×104t/a。主要产品包括轴承钢、合金钢、模具钢、易切钢、不锈钢等100多个钢号的锻造用钢锭。2007年4月18日7时53分,该公司发生一起钢水包倾覆事故,造成38人死亡、6
企业集团财务公司的服务对象仅限于企业集团成员,但可以向社会吸收存款以及向非成员单位提供服务。()
处理误机事故的准则是()。
一个看电视的孩子经历了一个声音和图像迅速闪过屏幕的过程,其速度之快仅允许让眼睛和耳朵接收这些过程。不像书页,可以按其愿望或快或慢地看。电视画面过快的速度是在打击而不是提高孩子的想象力。上面所表达的推理基于一个假设,下列哪一个最好地充当了那个假设?
非正式群体
OCCD芯片的像素数目和分辨率是数码相机的重要性能指标,两者具有密切的关系。例如,对于一个80万像素的数码相机,它所拍摄相片的分辨率最高为(51)像素。
Theforwardmarketalsoprovidesfacilitiesforforwardcurrencytransactions.Thisisameansofenablingtheimporterorexpor
A、Itchangesthebody’smetabolismthroughrapidweightgain.B、Ittricksthebodyintorespondingasifitwereinadifferent
最新回复
(
0
)