ACT Reading Practice Test 94: NATURAL SCIENCE

DIRECTIONS: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.


NATURAL SCIENCE: This passage is adapted from the article "Not Dead Yet: A Dying Star Is Caught Flaring Briefly Back to Life" by Charles Liu (©2005 by Natural History Magazine, Inc.).

About a billion years before a sunlike star "dies,"
or stops generating energy via nuclear fusion, it
becomes a red giant, growing dramatically to a hundred
times its original diameter. Then, as the red-giant phase
5ends, the star blows off its outer layers, giving rise to an
expanding gas cloud called a planetary nebula. The
planetary nebula, in turn, swells in size and -drops in
density for at most another 100,000 years, exposfug the
remaining stellar core at its center. That core becomes a
10white dwarf-the most common celestial cadaver visi-
ble in the sky. The white dwarf usually radiates its left-
over heat into space for billions of years, and it slowly
fades to black.

Some soon-to-be white dwarfs, however, seem to
15heed the counsel of poet Dylan Thomas: "Do not go
gentle into that good night." According to the theory of
stellar evolution, the temperature in the stellar core can
fluctuate wildly, and sometimes spikes as high as tens
of millions of degrees. For a little while at least, the
20core may even flicker back into stellar life as a giant
star; generating new energy with new. flares of nuclear
fusion.

Alas, such a giant can't last long, because the core
is, in essence, running on fumes. Without a substantial
25fuel source to sustain fusion, a nuclear re-ignition of
this kind runs out of gas within a few centuries, and the
star heads back toward white dwarfhood. But during its
brief return to'fusion-powered life, its interaction with
the surrounding cloud of gas creates a fascinating astro-
30nomical laboratory for the study of stellar and interstel-
lar processes.

The star FG Sagittae, a highly variable .star in the
constellation Sagitta, seems to be a case in point. FG
Sagittae lies at the heart of a planetary nebula called
35He 1-5. In the past thirty years the star's temperature
has. dropped from more than _J0,000 degrees Fahrenheit
to less than 10,000 degrees, though its brightness has
changed erratically from year to year. As with an old,
grease-choked diesel engine struggling to start back up,
40the star's efforts to restart nuclear fusion create puffs of
hick smoke-carbon atoms coughed up from the
fading stellar core. The smoke absorbs the star's radiat-
ing heat and periodically obscures the visible light it
emits. To see through the haze and examine the goings-
45on near the star's surface, astronomers must look at its
radiation in less obscured wavelengths, such as infrared
light.

A research team led by Robert A. Gehrz of the
University of Minnesota in Minneapolis has now done
50just that. Recently the team published the results of
twenty years of monitoring the infrared properties of
FG Sagittae with three telescopes equipped with
infrared photometers-in effect, photon counters. One
instrument is in Minnesota, one in Arizona, and one in
55Wyoming. Gehrz and his colleagues discovered that,
though the star's overall brightness and temperature
have changed dramatically through the years, carbon
dust from the surface of FG Sagittae has been shining
more or less steadily at a temperature of about
601,200 degrees F (650 degrees Celsius). That's roughly
hot enough to melt aluminum, but substantially cooler
than the core of any star undergoing active nuclear
fusion. Gehrz and his colleagues conclude that, besides
giving rise to clouds of obscuring gas, FG Sagittae is
65powering a strong stellar wind peppered with this
arbon dust. They think this dust has been glowing con-
tinuously for the past decade. On the basis of the mea-
sured amount of emitted infrared radiation, Gehrz's
team estimates that the wind is carrying between
701.5 and 7 .5 quadrillion ( 1.5 to 7 .5 x I 015) tons of stellar
material away from FG Sagittae each second-or about
eight to forty Earth masses each year.

Sooner rather than later the current burst of new
nuclear fusion will cease, and the dusty stellar wind
75will cease. The stellar core, no longer obscured by a
thick, dusty blanket, will turn once more into a hot
white dwarf. If, as theoretical models predict, the stellar
renaissance of FG Sagittae lasts a few hundred years,
the wind will deposit thousands of Earth-masses' worth
80of carbon-rich matter into the star's surroundings. The
carbon atoms, as they cool down, could become seeds
for the buildup of interstellar dust grains-which, in
turn, could seed the formation of asteroids, moons,
planets, and perhaps eventually even life as we know it.
85Maybe the astronomers of the twenty-fourth or twenty-
fifth century will look toward FG Sagittae and see, in
its surroundings, the potential makings of a new and
distant earth.

1. The passage indicates that one difference between a sunlike star and a dying star is that a sunlike star:

A. has more fuel for nuclear fusion.
B. has a larger planetary nebula.
C. is more often studied with a photometer.
D. has more dramatic fuctuations in brightness.

2. The main purpose of the first paragraph is to:

F. describe how some stars flicker back to stellar life before becoming white dwarfs.
G. use FG Sagittae as an example of a star currently heading toward the white dwarf phase.
H. explain the dying process of stars from the red giant phase through the white dwarf phase.
J. contrast the longevity of a red giant with that of a white dwarf

3. The author quotes a Dylan Thomas poem in lines15-16 mainly to introduce the passage's point that:

A. astronomers often feel nostalgic about the stars they have studied.
B. a dying star is sometimes a danger to the stellar matter around it.
C. scientists often refer to white dwarfs as being in a gentle stage of a star's life.
D. some stars flare back into life before fading to black.

4. According to the passage, one reason the brightness of FG Sagittae has appeared to change erratically from year to year is that:

F. the star's light is periodically blocked by another stellar body.
G. smoke sometimes obscures the star's light.
H. wind is distorting the planetary nebula.
J. white dwarfs don't produce visible light.

5. According to the passage, compared to the temperature of the core of a star undergoing nuclear fusion, the temperature of the carbon dust from the surface of FG Sagittae is:

A. about the same.
B. slightly cooler.
C. much cooler.
D. much hotter.

6. The passage states that when studying the wind pow-ered by FG Sagittae, Gehrz's team used the measured amount of emitted infrared radiation to estimate the:

F. likelihood of stellar material produced by FG Sagittae affecting Earth's mass.
G. probability of FG Sagittae becoming a red giant by the twenty-fifth century.
H. reason for the decrease in the amount of dust gen-erated by FG Sagittae.
J. amount of stellar material carried away from FG Sagittae per second.

7. Based on the passage, FG Sagittae's efforts to restart nuclear fusion would best be characterized as:

A. smooth.
B. labored.
C. sudden.
D. impossible.

8. According to the passage, Gehrz's research team used infrared light to study FG Sagittae because infrared light allowed the team to:

F. identify the individual gases that compose the planetary nebula.
G. calculate the exact number of years the star will spend in the white dwarf phase.
H. view the activity close to the surface of FG Sagittae.
J. determine the precise location of FG Sagittae in He 1-5

9. At the end of the passage, the author muses that the current regeneration of FG Sagittae may eventually result in:

A. the formation of new life in the universe.
B. FG Sagittae's return to the beginning of its life cycle.
C. the death of a distant star.
D. the onset of the red giant phase in a neighboring star.

10. According to the passage, if theoretical models are accurate, the stellar renaissance of FG Sagittae will last for a period of time best described as:

F. a few hundred years.
G. between 10,000 and 30,000 years.
H. approximately 100,000 years.
J. billions of years