108 Chapter 2 From the Solar System to the Milky Way
Top Five Mysteries of the Galaxy That Have Not Yet Been
Astronomers and astrophysicists are still exploring our solar system and galaxy.
What Is the Galaxy’s Shape, and How Did It Form?
Previously, the Milky Way was considered a spiral galaxy. However, American astronomers
analyzed observational data from the Spitzer Space Telescope, which was launched by NASA
in 2003, and concluded that a bar structure approximately 27,000 light-years long runs
through the center of the Milky Way. A barred spiral galaxy is a type of spiral galaxy but one
that looks as though the spiraling arms have been moved to the end of a bar of stellar and
interstellar matter that goes right through the center of the galaxy.
Therefore, the theory that the Milky Way is a barred spiral galaxy is now dominant.
Some research suggests that the barred shape may be an indicator of more mature galax-
ies; however, we still don’t know why or how spiral galaxies and barred spiral galaxies are
What’s at the Center?
Our solar system is an aggregation of heavenly bodies gravitationally collected by the star
known as the Sun. So what is at the center of the galaxy, the thing that gathered it all
together? The answer is still not definitively known. The enormous mass of the galaxy,
which is between 300 billion and 3 trillion times the mass of the Sun, cannot be accounted
for by normal stars, so the galaxy must contain some kind of extremely heavy but small
heavenly body at its center. Currently, the only plausible solution is a black hole.
Barred Spiral Galaxies
Types of galaxies