Chapter 28 Stars and the Universe

726 CHAPTER 28: STARS AND THE UNIVERSE
Using Light as a Yardstick
Redshift
Distances in space are so vast that light cannot reach Earth
instantaneously. For example, to reach Earth, electromagnetic
energy takes about 3 seconds to travel from the moon
and 8 minutes from the sun. Light takes more than 4 years
to arrive from the nearest night star, Proxima Centauri. The
most distant object visible to the unaided eye is the Andromeda
Galaxy. Light from the Andromeda galaxy takes
about 2 million years to reach us.
In fact, light provides a good method to measure distances
in the universe. A light-year is the distance that
any form of electromagnetic energy can travel in 1 year:
about 6 trillion miles, or 10 trillion km. Although the light
year may sound like a measure of time, it is a measure of distance.
When astronomers look at distant objects in space, they
see them as the objects were when the light started its long
journey toward Earth. The farther away astronomers look
into space, the farther back in time they see. At present, the
most distance objects visible to astronomers are estimated
to be about 13 billion light-years away. Astronomers can now
look at the universe about a billion years after its origin,
which is estimated to be 14 billion years ago.
Astronomer Edwin Hubble examined the spectra of distant
galaxies in the early 1900s. He compared the dark absorption
lines, or spectral lines, of light from these far away galaxies
to the absorption lines of nearby stars. Nearby stars had
spectral lines similar to those produced in the laboratory. The
light from the distant galaxies did not show the dark lines in
the same colors as the light from the nearby stars. However,
the dark lines in the spectra of distant galaxies were shifted
toward the red end of the spectrum. Hubble reasoned that the
motion of distant galaxies away from Earth causes the redshift
of spectral lines. The redshift of spectral lines is illustrated
in Figure 28-9.