Why is 21-cm radiation so important to the study of interstellar matter and the Galaxy?
A) It helps locate the galactic core in Sagittarius.
B) Emitted by hydrogen, it passes through interstellar dust and lets us to map the entire Galaxy.
C) Emitted by carbon monoxide, it passes through interstellar gas and lets us to see places rich in organic molecules around the Galaxy.
D) It is emitted by most stars, enabling astronomers to map the entire Galaxy.
E) It is emitted only in hot regions of star formation, so the pattern of the spiral arms of the Galaxy can be mapped.
B
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A blob of material in outer space moves at 3 m/s when it collides and sticks to a half-as-massive blob initially at rest. Compared with the initial kinetic energy of the 3-m/s blob, the kinetic energy of the coupled blobs is
A) one-third.
B) two-thirds.
C) three-quarters.
D) not enough information
The results of statistical studies of the exoplanet systems discovered to date suggests that what proportion of stars are likely to have a planet less than half the size of the Earth and in an orbit within a star's habitable zone?
A) 10 to 20% B) 50% C) much less than 10% D) more than 90%
Which of the following quantities are conserved in an isolated system?
a. mass c. momentum b. energy d. all of the above
A concave mirror has a focal length of 20 cm. What is the position (in cm) of the resulting image if the image is inverted and four times smaller than the object?
a. 15 b. 25 c. 50 d. 100 e. ?15