ACT Reading Apr. 2016 73E - Passage IV

Questions 31-40 are based on the following passage.


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
5 ends, 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
10 white 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
15 heed 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
20 core 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
25 fuel 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-
30 nomical 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
35 He 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,
40 the 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-
45 on 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
50 just 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
55 Wyoming. 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
60 1,200 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
65 powering 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
70 1.5 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
75 will 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
80 of 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.
85 Maybe 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.