Galaxies them-selves appear to form clusters that are separated by vast expanses
of empty space. As galaxies are discovered they are classified by their differing
sizes and shapes. The most common shapes are spiral, elliptical, and irregular.
Beautiful, full-color photo-graphs of astronomical objects are available on the
Internet, in library books, and in popular and professional journals. It may also
interest students to know that astronomers have inferred the existence of planets
orbiting some stars.
4.b The Sun is a star located on the rim of a typical spiral galaxy called the
Milky Way and orbits the galactic center. In similar spiral galaxies this galactic
center appears as a bulge of stars in the heart of the disk. The bright band of
stars cutting across the night sky is the edge of the Milky Way as seen from the
perspective of Earth, which lies within the disk of the galaxy. Stars vary greatly
in size, temperature, and color. For the most part those variations are related
to the stars’ life cycles. Light from the Sun and other stars indicates that the
Sun is a fairly typical star. It has a mass of about 2
kg and an energy output, or luminosity, of about 4
joules/sec. The surface temperature of the Sun is approximately 5,500 degrees
Celsius, and the radius of the Sun is about 700 million meters. The surface temperature
determines the yellow color of the light shining from the Sun. Red stars have
cooler surface temperatures, and blue stars have hotter surface temperatures.
To connect the surface temperature to the color of the Sun or of other stars,
teachers should obtain a “black-body” temperature spectrum chart, which is typically
found in high school and college textbooks.
4.d The energy from the Sun and other stars, seen as visible light, is caused
by nuclear fusion reactions that occur deep inside the stars’ cores. By carefully
analyzing the spectrum of light from stars, scientists know that most stars are
composed primarily of hydrogen, a smaller amount of helium, and much smaller amounts
of all the other chemical elements. Most stars are born from the gravitational
compression and heating of hydrogen gas. A fusion reaction results when hydrogen
nuclei combine to form helium nuclei. This event releases energy and establishes
a balance between the inward pull of gravity and the outward pressure of the fusion
Ancient peoples observed that some objects in the night sky wandered about while
other objects maintained fixed positions in relation to one another (i.e., the
constellations). Those “wanderers” are the planets. Through careful observations
of the planets’ movements, scientists found that planets travel in nearly circular
(slightly elliptical) orbits about the Sun.
Planets (and the Moon) do not generate the light that makes them visible, a fact
that is demonstrated during eclipses of the Moon or by observation of the phases
of the Moon and planets when a portion is shaded from the direct light of the
Various types of exploratory missions have yielded much information about the
reflectivity, structure, and composition of the Moon and the planets. Those
missions have included spacecraft flying by and orbiting those bodies, the
soft landing of spacecraft fitted with instruments, and, of course, the visits
of astronauts to the Moon
Teachers should look for field trip opportunities for students to observe the
night sky from an astronomical observatory or with the aid of a local
A visit to a planetarium would be another way of observing the sky.
If feasible, teachers should have students observe the motion of Jupiter’s
inner moons as well as the phases of Venus. Using resources in the
library-media center, students can research related topics of interest.