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Science and Society

Light Pollution
ight pollution is an adverse effect of quadrant. Repeat this same exercise when 2. How could photographs of the night
Llight including sky glow, light clutter, there is a full moon, which creates light sky provide qualitative evidence of
and glare. In general, light pollution can pollution conditions. Compare the result light pollution?
destroy our view of the night sky, creating and discuss how light pollution reduces the 3. Is qualitative evidence as good as
problems for astronomers and others who number of stars you can see. Compare what quantitative evidence? Explain.
might want to view or study the stars. you would see near cities with light pollu-
4. How could quantitative and
Go with your group to a dark area on tion and far from cities where there is no
qualitative evidence be used to
a clear, moonless night. Agree upon a plan light pollution.
motivate people to consider light
to consider a part of the sky, such as one-
pollution when installing outdoor
fourth of the sky—a quadrant. All group
QUESTIONS TO DISCUSS lighting?
members should count and record all the
stars they can see in the quadrant. Average 1. What quantitative evidence illustrates
everyone’s count to obtain an overall esti- that light pollution limits viewing a
mate of the number of visible stars in the quadrant of the night sky?

Beginning of the End for Massive Stars neutron star. A neutron star is the very small (10 to 20 km
3
11
A more massive star will have a different theoretical ending diameter), superdense (10 kg/cm or greater) remains of a
than the slow cooling of a white dwarf. A massive star will con- supernova with a center core of pure neutrons.
tract, just as the less massive stars do, after blowing off its outer Because it is a superdense form of matter, the neutron
shells. In a more massive star, however, heat from the contrac- star also has an extremely powerful magnetic field, capable
tion may reach the critical temperature of 600 million kelvins of becoming a pulsar. A pulsar is a very strongly magnetized
to begin carbon fusion reactions. Thus, a more massive star neutron star that emits a uniform series of equally spaced elec-
may go through a carbon fusing stage and other fusion reac- tromagnetic pulses. Evidently, the magnetic field of a rotating
tion stages that will continue to produce new elements until the neutron star makes it a powerful electric generator, capable
element iron is reached. After iron, energy is no longer released of accelerating charged particles to very high energies. These
by the fusion process (see chapter 13), and the star has used up accelerated charges are responsible for emitting a beam of
all of its energy sources. Lacking an energy source, the star is electromagnetic radiation, which sweeps through space with
no longer able to maintain its internal temperature. The star amazing regularity (Figure 14.13). The pulsating radio signals
loses the outward pressure of expansion from the high tem- from a pulsar were a big mystery when first discovered. For a
perature, which had previously balanced the inward pressure time, extraterrestrial life was considered as the source of the
from gravitational attraction. The star thus collapses, then re- signals, so they were jokingly identified as LGM (for “little
bounds like a compressed spring into a catastrophic explosion green men”). Over 300 pulsars have been identified, and most
called a supernova. A supernova produces a brilliant light in emit radiation in the form of radio waves. Two, however, emit
the sky that may last for months before it begins to dim as the visible light, two emit beams of gamma radiation, and one
new elements that were created during the life of the star dif- emits X-ray pulses.
fuse into space. These include all the elements up to iron that Another theoretical limit occurs if the remaining core has a
were produced by fusion reactions during the life of the star mass of about 3 solar masses or more. At this limit, the force of
and heavier elements that were created during the instant of gravity overwhelms all nucleon forces, including the repulsive
the explosion. All the elements heavier than iron were created forces between like charged particles. If this theoretical limit is
as some less massive nuclei disintegrated in the explosion, join- reached, nothing can stop the collapse, and the collapsed star
ing with one another and with lighter nuclei to produce the will become so dense that even light cannot escape. The star
nuclei of the elements from iron to uranium. As you will see in is now a black hole in space. Since nothing can stop the col-
chapter 15, these newly produced, scattered elements will later lapsing star, theoretically a black hole would continue to col-
become the building blocks for new stars and planets such as lapse to a pinpoint and then to a zero radius called a singularity.
the Sun and Earth. This event seems contrary to anything that can be directly ob-
If the core of a supernova has a remaining mass greater served in the physical universe, but it does agree with the gen-
than 1.4 solar masses, the gravitational forces on the remaining eral theory of relativity and concepts about the curvature of
matter, together with the compressional forces of the supernova space produced by such massively dense objects. Black holes are
explosion, are great enough to collapse nuclei, forcing protons theoretical and none has been seen, of course, because a black
and electrons together into neutrons, forming the core of a hole theoretically pulls in radiation of all wavelengths and emits

14-11 CHAPTER 14 The Universe 361