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
47
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.
According to the passage, the absolute brightness of a Cepheid variable______.
选项
A、depends upon its measurable distance from an observer on earth
B、may be determined from the length of its cycle of variation
C、changes from time to time according to a regular and predictable pattern
D、indicates the strength of the gravitation force exerted by the star
答案
B
解析
本题的四个选项中,只有B项为正确答案。这可从文中第三段的“…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”推知,即星体从最亮点到最暗点又到最亮点所需的时间,与其内在亮度也就是绝对亮度相对应。
转载请注明原文地址:https://kaotiyun.com/show/uOIO777K
0
考博英语
相关试题推荐
•Lookatthenotesbelow.•Someinformationismissing.•Youwillbearawomantalkingaboutanewproductrange.•Foreach
•Lookatthenotesbelow.•Someinformationismissing.•Youwillbearawomantalkingaboutanewproductrange.•Foreach
Lookatthenotesbelow.Someinformationismissing.YouwillheartheChiefFinancialOfficerispresentingtheannualreport
ComputersoftwareAsmoreandmorecompaniesareusingtheInternettodobusiness,MarkWilliamsexplainshowanewsoftwa
MANUFACTURER’SNOTICEPleasereturnanygoodsdamagedduringtransportationwithintwoweeksofdelivery,forimmediate
BacktotheshopfloorotAttwood,ChiefExecutiveofAttwoodPublicity,hasattractedtheattentionofhisindustrybygoingto
•Readthefollowingpassagewhichadvisescandidateshowtoanswerdifficultinterviewquestions.•Arethesentences16-22"Ri
•Readthearticlebelowaboutteam-building.•Choosethecorrectwordtofilleachgap,fromA,BorConthenextpage.•For
CustomerTargetingFindingandkeepingloyalandprofitablecustomersisessential.Theroutetolong-termsuccessistofin
Accordingtothelatestreport,consumerconfidence______abreathtaking15pointslastmonth,toitslowestlevelin9years.
随机试题
汽油机电子控制系统的主要优点哪些?
简述五四以前新文化运动的局限。
曲线y=x3-3x上切线平行于x轴的点是()
A.相加作用B.协同作用C.独立作用D.拈抗作用E.综合作用两种化学物质的毒性比单独化学物质的毒性大,但引起动物中毒失望不是两种毒物类加的结果称为
心绞痛发作的典型部位在
某基金A在持有期为12个月、置信水平为95%的情况下,若计算的风险价值为一5%,则下列说法中,正确的是()。
甲市卫生局在一次突击检查中,认为乙药店销售的丙药厂生产的A药品不符合质量要求,作出了没收乙药店库存的全部A药品并罚款1万元的行政决定,乙药店不服,拟申请行政复议。根据行政复议法律制度的规定,下列有关说法中,不正确的是()。
企业的技术改造可以分为表层、内层和深层三个层次,内层的技术改造主要包括()。
下列标准对话框JOptionPane类创建的对话框中,综合了其他3个对话框类型特点的是()。
Thebooksonmedeskarenotmine:they’re______.
最新回复
(
0
)