Top Five Mysteries of the Galaxy That Have Not Yet Bn Explained! 109
A black hole is a region in which the force of gravity is so great that even light cannot
escape from it. Some scientists propose that a supermassive black hole is at the center of
the galaxy.
How Are Supermassive Black Holes Formed?
Although there are smaller black holes with approximately the same mass as a star, black
holes like the one hypothesized to be in the center of the galaxy are enormous, having a
mass from several million to several hundred million times the mass of the Sun. It is still
not known how or why these supermassive black holes are formed.
A stellar-mass black hole is believed to be formed from the remnants of a star. When
a supernova explosion of a large star at least 20 times the size of the Sun occurs, the core
that remains continues to be compressed by gravitational collapse, creating a black hole. It
seems reasonable that a coalescence of smaller-sized black holes or other heavenly bodies
would lead to a larger black hole.
Since such intermediate-sized black holes (ISBH) have not been found so far, scientists
aren’t sure how they are created. However, an explanation may not be far off, since X-ray
sources from nearby galaxies seem to be ISBHs. The existence of these ISBHs cannot be
confirmed, though, until a mass measurement has been made using the gravitational effect
the ISBH has on surrounding bodies.
What Is the Galaxy Made Of?
Although the mass of the galaxy is thought to be equivalent to 600 billion to 1 trillion Suns
(based on motion analysis), all of the heavenly bodies that can be observed by telescopes
and radio telescopes combined do not account for more than 10 percent of it. This is the
same for other galaxies and galaxy groups, and astronomers currently think that more than
90 percent of what forms the universe is dark matter that cannot be observed because it
does not emit or reflect light.
Opinions on what dark matter is made of range from neutrinos (a type of elementary
particle) or some unknown elementary particle to black holes. The discovery of the composi-
tion of dark matter will surely merit a Nobel Prize.
Incidentally, the actual condition of more than 70 percent of the energy that exists in
the universe is unknown, and this is referred to as dark energy.
What Will Happen When We Collide with the Andromeda
Galaxy?
The Andromeda galaxy is close to the Milky Way and is known to be approaching at a
speed of 100 km per second (62 miles per second). Since its current distance is approxi-
mately 2.52 million light-years away, if it continues approaching at the current rate, the
two galaxies should collide in 7 to 8 billion years. What will happen at that time is not well
understood.
Of course, individual heavenly bodies are quite far apart even inside a galaxy,* and
although it seems unlikely that stars will actually collide, various predictions have been made
as to what might occur. These include that the two galaxies will combine to form one new
galaxy and that there will be enormous gravitational effects accompanying the collision.
* The closest nearby star, Proxima Centauri, is 4.2 light-years away from Earth (approximately 25 trillion miles).

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