Video Transcript
Approximately how long does it take
the solar system to make one full orbit around the center of the galaxy? Is it (A) hundreds of years, (B)
thousands of years, (C) tens of thousands of years, (D) several hundred thousand
years, or (E) several hundred million years?
To answer this question, we must
consider the size of the Milky Way and how quickly the solar system is orbiting its
center.
The Milky Way is incredibly
vast. It’s roughly 100,000 light-years
across. Recall that a light-year is the
distance that a beam of light would travel in one year. The approximate position of the
solar system in the Milky Way is halfway from the center of the galaxy to its
edge. The solar system orbits the center
of the Milky Way, keeping roughly the same distance from the center of the galaxy
throughout its orbit.
We can tell from this that the
distance traveled by the solar system in a complete orbit must be huge, at least
100,000 light-years. Recall that nothing can travel
faster than the speed of light. This means that for anything to be
able to travel the distance that the solar system travels in one complete orbit, the
time taken must be at least 100,000 years. We can see that options (A), (B),
and (C) all describe lesser time intervals than 100,000 years, so we can eliminate
these options.
We instead must choose either
option (D), several hundred thousand years, or option (E), several hundred million
years. Which of these options is correct
depends on whether the orbital speed of the solar system is close to the speed of
light or much less than the speed of light. If the solar system travels at
close to the speed of light, then option (D) is correct. And if the solar system travels
significantly slower than the speed of light, then option (E) is correct.
Actually, the solar system orbits
the center of the galaxy at less than 0.1 percent the speed of light. Therefore, we can say that the
correct answer option must be (E). The solar system takes several
hundred million years to complete one full orbit. An interesting fact about this is
that the Sun has completed about 20 orbits around the galaxy since its
formation.
