186 Chapter 3 The Universe Was Born With a Big Bang
Making a voyage into space means engaging in a challenging struggle with a harsh
environment. Although creating a spaceship on which humans can safely travel is enor-
mously difficult, if the travelers were tardigrades, the planning would be much simpler
because they would be able to travel while in an anhydrobiotic state and then be resusci-
tated many years later at a distant star. If this were done, then in due course, animals that
evolved from tardigrades might be active throughout the universe just as they live every-
where here on Earth.
A Third Method of Measuring the Size of the Universe:
If You Know the Properties of a Star, Can You Figure
Out How Far away It Is?
We have already mentioned that the distance to a heavenly body can be calculated via
triangulation, using annual parallax and the distance between Earth and the Sun. But,
if you recall, the parallax angles are so small that we can only measure those angles for
stars 1,000 light-years away. Since the diameter of the Milky Way galaxy is approximately
100,000 light-years, this distance is less than 2 percent of the distance across our galaxy.
There are over 100 billion galaxies in the universe! What should we do to learn about the uni-
verse beyond this distance?
One simple method is to compare the physical properties of other stars with those of
our Sun.
Although stars like the Sun shine by releasing energy due to a nuclear fusion reaction,
the type of reaction that occurs is determined by the star’s mass (that is, gravity). Therefore,
Contraction of
interstellar gas
Small stars
Giant stars
White Dwarf
Stellar classifications
A Third Method of Measuring the Size of the Universe 187
if the color emitted by stars is the same, the basic brightness (absolute magnitude) of those
stars is also generally the same.
The color of a star is directly related to its surface temperature. This is intuitive if
you consider a flame: a hotter flame burns blue, while a cooler flame burns orange or red.
So a hot star burns blue, while a cooler star burns redder. The luminosity of a star, or the
amount of light a star emits per second, depends on the size of the star but also on the tem-
perature of the star. Therefore, if two stars of the same size appear to be the same color,
then the luminosity of those stars is also generally the same.
The luminosity of a star is an absolute physical value, independent of its distance from
an observer. However, if we were to move away from a star, it would appear to dim—and
the further we moved away from it, the more its brightness would decrease. In fact, if we
doubled our distance from the star, we’d receive only one-fourth of the original amount of
light we measured. Because of this relationship, we can determine how far away a star is by
measuring its apparent brightness and comparing it to the star’s luminosity.
These relationships are shown together in the Hertzsprung-Russell diagram (H-R
diagram). This diagram, which was independently proposed by the Danish astronomer Ejnar
Hertzsprung and the American astronomer Henry Norris Russell, is a distribution diagram
that uses the spectral type (color = surface temperature) of a star for its horizontal axis and
the star’s absolute magnitude for its vertical axis.
note The absolute magnitude of a star is what its apparent magnitude would be if the
star were placed 10 parsecs away from us.
The energy of the light that arrives in this case is 1.
Since the distance is doubled, the energy of the light
that arrives in this case is 1/4.
Since the distance is tripled, the energy of the light
that arrives in this case is 1/9.
The light energy measured by the observer is inversely
proportional to the square of the distance from the observer
to the light source.
Light source
Light source
Light source
Distance measurement according to absolute magnitude

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