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
39
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
Questions16-22•Lookatthenotesbelow.•Someinformationismissing.•Youwillhearamantalkingaboutjobhun
Howmanyaretherequalitycontrolinspectionpoints?Atgoodsstage,wemakesurethat
MERTONTRAININGSHORTRESIDENTIALCOURSESA.AdvancedPresentationSkillsB.BetterTimeManagementC.
•Lookatthenoticebelow.ItshowsthecompanieswithofficesintheRoyalCommercialbuilding.•Forquestions6-10,decidewhi
BritainoccupiedJavaduringtheNapoleonicWars.BoththeBritishandlatertheDutchtriedtocentralizeandreformJava’sadm
Childrenwhoaredescribedasmildlymentallyhandicappedareoftenupsettohearthemselvesdescribedassuch,andsuchtermsa
Whentelevisionfirstbegantoexpand,veryfewofthepeoplewhohadbecomefamousasradiocommentators【31】effectiveontelev
Asshehasakindly,outgoingnature,sheisa______hostessandhasawidecircleofverygoodfriends.
Telecommutingisanewformofwork______towork,suchasfatherswithchildren,thechancetoworkwhileremainingathome.
WearesorrytosaythatMaryisnottheverypersonwhocanbe______witheithermoneyorsecretinformation.
随机试题
风痰哮证的代表方宜选用( )寒包热哮证的代表方宜选用( )
寒滞胃脘证与寒凝肝脉证均可见的表现有
流行性乙型脑炎病毒传播环节中最重要的中间宿主是
某8年前建成交付使用的建筑物,建筑面积是120m2,单位建筑面积的重置价格为800元/m2,建筑物残值率为6%,年折旧率2.2%,计算该建筑物的现值是()元。
下列经济业务中,会使资产和权益总额同时增加的有( )。
下列情形中,可以申请开立异地银行结算账户的有()。
中共十七届四中全会提出了深化干部人事制度改革,在培养和选拔干部的时候,必须坚持的用人标准是()。
[*]
Evidenceofthebenefitsthatvolunteeringcanbringolderpeoplecontinuestorollin."Volunteershaveimprovedphysicalands
Theauthorismostprobablya______.Whichofthefollowingbestexplainsthereasonwhytheauthorstakesoutanareaofhis
最新回复
(
0
)