19．
Twinkle，Twinkle Little Star is a lovely song for children，but it has two misunderstandings．First，stars aren't little．Some may be Earth-sized，but most are bigger than Jupiter，the largest planet in the solar system．Second，they don't twinkle； they shine．And the brightness and color of the light from each star provide information about its temperature，size and even its age．
Viewing the stars from Earth is a bit like looking at them through a water filter because the air is thick compared to the emptiness of space．What is more，air is always moving，so starlight appears to be moving，too．The air also makes stars appear dimmer（暗淡的） than they would be if we could see them from space．
If you stop and examine the sky on a dark and moonless night，it's easy to find color differences among stars．The color of a star is a signal of its surface temperature．The hottest stars are blue，and the next hottest are white．Yellow stars like the sun are next，while red stars are the coolest of the visible stars．Many red stars are so dim that people can't see them at all，and some stars hardly send out any light at all．One reason that stars differ in brightness is that hotter stars produce more energy than cooler ones，but another important reason is that some are much bigger than others．
Some stars appear brighter to human beings simply because they're closer．Astronomers rank the brightness of stars which are seen form Earth by giving them a name known as magnitude（光度）----the smaller the magnitude，the brighter the object．They have also designed a number that ranks stars according to how bright they are when compared to each other．With a magnitude of minus 26.7，the sun is the brightest object in the sky．
39．From the song quite familiar to children，the writer wants to prove thatC．
A．most stars are smaller than Jupiter
B．Jupiter is bigger and brighter than Earth
C．stars are very large and always shine brightly
D．starlight's brightness and color are the signs of its temperature
40．The underlined phrase"a water filter"in Paragraph 2refers toD．
A．the earth
B．stars
C．the rain
D．the air
41．Magnitude is designed to seeD．
A．how far a star is from Earth
B．how bright a star is compared to other stars
C．how far a star is from other stars
D．how bright a star is seen from Earth
42．Which is the best title of this passage？A
A．What do stars look like？
B．Which song do children like best？
C．How to view the stars from Earth？
D．Why are many stars so dim at night？
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