Video Transcript
Complete the following
sentence. A galaxy is a collection of
billions of blank. Is it (A) stars or (B) planets?
To answer this question, it will be
helpful to consider some facts about galaxies.
First, we can recall that our own
solar system is part of the Milky Way galaxy. The solar system includes Earth,
the seven other planets, and many moons, asteroids, and comets, as well as the Sun,
which is a star. Besides the solar system, we can
see billions of other stars and planets within the Milky Way. And beyond the Milky Way, we know
that there are billions of other galaxies in the universe. Those other galaxies also contain
lots of stars and planets.
So, while it is typically true that
there are many stars and planets in a galaxy, it’s more accurate and useful to
define a galaxy as a collection of stars in particular. To understand why, we can consider
some facts about planets and stars.
We know it’s common for stars to
have planets orbiting them. However, not all stars do have
planets orbiting them. This means that if we see a star
somewhere, it may or may not have planets around it. Also, stars form before
planets. When a star is newly formed, it can
take many millions of years more for planets to form around it. Thus, if we see a galaxy that’s
very young, and maybe not fully formed yet, it’s possible that it contains mostly
stars without planets. However, it is not possible for a
galaxy to contain only planets and no stars. This is because a planet cannot
form without a star nearby.
For these reasons, we define a
galaxy as a collection of stars, rather than a collection of planets. Thus, we can conclude that the
correct answer is option (A). So, completing the sentence, a
galaxy is a collection of billions of stars.
