What is the Hertzsprung-Russell
diagram?
The Hertzsprung-Russell diagram is a
“two dimensional” graph of the mathe-
matical relationship between the absolute
magnitude, luminosity, stellar classifica-
tion, and surface temperature of stars—
all resulting in a diagram of the stellar
life cycle. It was plotted by Danish
astronomer Ejnar Hertzsprung (1873–
1967) in 1911 and independently by
American astronomer Henry Norris Rus-
sell (1877–1957) in 1913.How do astronomers determinethe
distancesto other planets?
Astronomers need mathematics, of course, to determine the distances to the planets
and satellites of our solar system. One of the first astronomers to work this out was
Nicholaus Copernicus (see above) using simple observations of planetary positions.
One of the earliest methods to determine such distances was to use the orbital
period of a planet. This varies as the square root of the cube of the distance from the
Sun: T k r(3/2), in which T is the time for one revolution, r is the distance between
the centers of the Sun and the planet, and k is a constant.How do astronomersdetermine the distances to the stars?
Relatively nearby objects beyond the solar system appear to shift position relative to
more distant objects as the Earth moves from one side of the Sun to the other—a phe-
nomenon called parallax.You can use parallax to determine the distance to stars, as
long as these stellar objects are within a few dozen light years of Earth. (More distant
objects in the sky do not change their position enough as the Earth orbits from one
side of the Sun to the other.) First, measure the position of the star in the sky; then,
measure it again in six months when the Earth is on the opposite side of its orbit. If
the distance aand the angle acare known (as seen in the diagram below), using
trigonometry, ccan be determined as a/ cos (ac).Is it easy to figure out the size of a distant object?
Yes, it is often possible to figure out the size of distant objects, as long as they aren’t
too small. The key is in knowing the distance to the object. For example, if someone
292 holds a nickel at arm’s length and then has someone hold the nickel 200 yards away,
Neptune is the only planet in our solar system that
does not fit the model set by the Titius-Bode Law.
The Image Bank/Getty Images.