Download Exam 2

Practice Exam No. 2
Read the Text:
Planetary Nebulae
Like people, stars grow old and die. The most massive ones live
short lives and terminate in brilliant supernova explosions. The much
more common, intermediate-mass stars like the sun die more gracefully.
As they exhaust their thermonuclear fuel, they swell. In a final spasm
they explode, creating an expanding spherical cloud of gas. The small,
hot remnant of the central star heats the gas, causing it to glow. For a
few thousand years, the dying star is surrounded by a beautiful
gleaming cloud known as a planetary nebula.
Planetary nebulae have a spectacular array of shapes, sizes and
structures. This diversity holds clues about stellar evolution and about
numerous ways in which stars interact with their environments. For
example, some of the heavy elements (such as carbon, nitrogen and
oxygen) that form in a star’s core move into its outer layers. The
abundance of these elements in a planetary nebula can show how
material mixes through the various parts of a star’s interior. The
expanding nebula merges with the interstellar medium – the gas filling
the space between the stars – and enriches it with these heavy elements.
This is the primary process by which matter returns from stars to
interstellar space.
Planetary nebulae cannot be seen with the naked eye. Indeed,
the study of these objects did not begin seriously until long after the
invention of the telescope. Observations of planetary nebulae have been
hampered by the fact that these objects have very low surface
brightness and so tend to fade into the background of the sky. The light
of the Helix Nebula, for example, spreads out over an area as large as
the moon.
In the 1780s the great English astronomer William Hershel
began a detailed investigation and cataloguing of nebulae, fuzzy patches
of light that did not look like stars. In a paper published in 1785, he
invented the term “planetary nebula” to describe a class of objects
whose round shapes vaguely suggested images of the planets. Hershel
wrongly guessed that planetary nebulae represent young objects not yet
condensed into stars.
Early in the 20th century, astronomers discovered that the
spectral lines of planetary nebulae have a distinctive shape, which
indicates that the objects are expanding. Radiation from the
approaching side of the nebula is shifted to a slightly shorter
wavelength, whereas that moving away from the earth is shifted to a
somewhat longer wavelength. Clearly, these objects are not clouds
collapsing into new stars but material ejected from old ones. In 1956 the
Soviet astrophysicist I.S. Shklovsky established the modern view that
nebulae develop out of the eliminated outer layers of aged, red giant
stars and so represent not the beginning but the end of the stellar life
cycle.
Numerous observations have shown that planetary nebulae
expand at velocities ranging from five to 100 kilometers per second; the
5
10
15
20
25
30
35
40
45
average rate of expansion is about 20 kilometers per second. In some
cases, these figures can be checked by visually examining the growth of
the nebula. By evaluating the size of the nebulae and the rate at which
their expansion slows, one can calculate their ages. Their supposed
ages, which range from a few thousand to 30,000 years, have helped
establish models of the evolution of planetary nebulae and their central
stars.
Referents:
What do the following words refer to?
1. Line 4, their _________________________________________
2. Line 6, it ____________________________________________
3. Line 18, this _________________________________________
4. Line 23, these objects _________________________________
5. Line 38, that _________________________________________
Comprehension Questions:
A.Circle the letter before the best completion in questions 2,5.
B.Circle True or False in questions 1,3,4,7,8,9 and justify your answer
by giving the line number/s and quoting the relevant words from the
text.
1. There is an inverse relationship between the mass of a star and
the length of its life.
True/False___________________________________________
___________________________________________________
2. The cloud of gas which is created in the supernova explosion
a. gradually decreases in size.
b. remains luminous for a thousand years.
c. is a product of thermonuclear fuel.
d. is hotter than the central part of the dying star.
3. There is little variety in the dimensions of the planetary nebulae.
True/False
___________________________________________________
___________________________________________________
4. Carbon, nitrogen and oxygen are heavy elements, which
accounts for their stable position in the core of the star.
True/False
___________________________________________________
___________________________________________________
5. Investigation of the planetary nebulae has been made difficult
by
a. The lack of powerful equipment.
b. the intense glow of the surface.
50
c. excessive brightness of these objects.
d. the dispersion of the light over vast areas.
6. Circle True or False before each of the following completions.
No justification is necessary.
It follows from paragraph 4 (lines 27-33) that W.Herschel
a. was the first to use the term planetary nebulae.
True/False
b. noticed the resemblance between stars and planetary
nebulae. True/False
c. made mistaken assumptions about the origin of the
planetary nebulae. True/False
d. systematically arranged the data obtained from the
observations of planetary nebulae. True/False
e. published the results of his research towards the end of
the 17th century. True/False
7. The modern approach to the origin of planetary nebulae was
formed at the beginning of the 20th century. True/False
___________________________________________________
___________________________________________________
8. The rate of the planetary growth reaches up to 20 kilometers per
second. True/False
___________________________________________________
___________________________________________________
9. To determine the approximate age of a planetary nebula it is
sufficient to know its size and location. True/False
___________________________________________________
___________________________________________________
Words in Context:
What do the following words mean in the context of the article?
1. SWELL, line 4
a. a protuberant part.
b. to enlarge in bulk.
c. to grow in amount, degree, force.
2. MATTER, line 18
a. occasion.
b. importance.
c. physical substance.
d. material for thought.
3. DISTINCTIVE, line 35
a. famous.
b. separate.
c. characteristic.