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
In the early years of the twentieth century, astrophysicists turned their attention to a special category of stars, known as Cep
In the early years of the twentieth century, astrophysicists turned their attention to a special category of stars, known as Cep
admin
2010-12-25
64
问题
In the early years of the twentieth century, astrophysicists turned their attention to a special category of stars, known as Cepheid (造父变星) variables. A variable star is one whose apparent brightness changes from time to time. Among some variables, the change in brightness occurs so slowly as to be almost imperceptible; among others, it occurs in sudden, brief, violent bursts of energy. Cepheid variables have special characteristics that make them a useful astronomical tool.
It was Henrietta Leavitt, an astronomer at the Harvard Observatory, who first examined the Cepheid variables in detail. She found that these stars vary regularly in apparent brightness over a relatively short period of time—from one to three days to a month or more. This variation in brightness could be recorded and precisely measured with the help of the camera, then still a new tool in astronomy.
Leavitt also noticed that the periodicity of each cepheid variable—that is the period of time it took for the star to vary from its brightest point to its dimmest, and back to its brightest again—corresponded to the intrinsic or absolute brightness of the star. That is, the greater the star’s absolute brightness, the slower its cycle of variation.
Why is this so? The variation in brightness is caused by the interaction between the star’s gravity and the outward pressure exerted by the flow of light energy from the star. Gravity pulls the outer portions of the star inward, while light pressure pushes them outward. The result is a pulsating in-and-out movement that produces increasing and decreasing brightness. The stronger the light pressure, the slower this pulsation. Therefore, the periodicity of the Cepheid variable is a good indication of its absolute brightness.
Furthermore, it is obvious that the more apparent brightness of any source of light decrease the further we are from the light. Physicists had long known that this relationship could be described by a simple mathematical formula. If we know the absolute brightness of any object—say a star—as well as our distance from that object, it is possible to use the inverse square law to determine exactly how bright that object will appear to be. This laid the background for Leavitt’s most crucial insight. As she had discovered, the absolute brightness of a Cepheid variable could be determined by measuring its periodicity. And, of course, the apparent brightness of the star when observed from the earth could be determined by simple measurement. Leavitt saw that with these two facts and the help of the inverse square law, it would be possible to determine the distance from earth of any Cepheid variable. If we know the absolute brightness of the star and how bright it appears from the earth, we can tell how far it must be.
Thus, if a Cepheid variable can be found irt any galaxy, it is possible to measure the distance of that galaxy from earth. Thanks to Leavitt’s discovery, astronomical distances that could not previously be measured became measurable for the first time.
The primary purpose of the passage is to explain______.
选项
A、the background and career of the astronomer Henrietta Leavitt
B、how and why various categories of stars vary in brightness
C、important uses of the camera as an astronomical tool
D、how a particular method of measuring astronomical distances was created
答案
D
解析
本题的四个选项中,只有D项为正确答案。这可从文中的内容推知,即天文学家是在通过测量出造父变星的亮度周期从而得出其与地球的距离。
转载请注明原文地址:https://kaotiyun.com/show/rOIO777K
0
考博英语
相关试题推荐
Whichchartshowsthecompany’smarketsharethisyear?
Lookatthenotesaboutthequalitiesofgreatmanagers.Someinformationismissing.Youwillhearpartofapresentationbya
Lookatthenotesbelow.Someinformationismissing.Youwillheartwopeoplediscussinganinvoice.Foreachquestion(9-15),
Lookatthenotesbelow.Someinformationismissing.Youwillheartwopeoplediscussinganinvoice.Foreachquestion(9-15),
Youwillhearaninterview.TheintervieweristalkingtoabusinesspersonTomSzaky,whorunsanenvironment-friendlycompany,
Whomakessurethecompany’sproductsaremadeproperly?Whokeepsfinancialrecordsofallbusinesstransactions?
LittletonNewsUpdateTherateofunemploymentinLittleton,asmalltowninthesouthwestofEngland,hasreachedarecord
•Readthefollowingpassagewhichadvisescandidateshowtoanswerdifficultinterviewquestions.•Arethesentences16-22"Ri
•Readthearticlebelowaboutteam-building.•Choosethecorrectwordtofilleachgap,fromA,BorConthenextpage.•For
•Readthearticlebelowaboutteam-building.•Choosethecorrectwordtofilleachgap,fromA,BorConthenextpage.•For
随机试题
以下除哪项以外,均为癫狂的病因
下列属于子宫输卵管造影禁忌证的是
决定能否行烤瓷桥修复时应考虑的因素,除了若右下7缺损3/4,死髓牙,未做根管治疗,根长、大,牙槽骨条件好,则应做哪种处理
A.蠲痹汤B.三痹汤C.独活寄生汤D.六味地黄丸E.虎潜丸
下列关于非银行金融机构的说法错误的是()。
下列说法中正确的是()。
列举两种利用计算机技术和互联网作为教学手段展开美术教学的方式。
将4个不同的小球放入甲、乙、丙、丁4个盒子中,恰有1个空盒的概率为()。
Theword"outsource"(Paragraph1)maybebestreplacedbyJamesQ.WilsonsupportsMaryEberstadt’snewbookbecause
PASSAGETHREEInwhatwaydoesWallStreetdistorttheeconomy?
最新回复
(
0
)