49.
What is the Milky Way?
The Earth orbits the Sun in the Solar System, and the Solar System is embedded within a
vast galaxy of stars.
Just one in hundreds of billions of galaxies in the Universe.
Ours is called the Milky Way because the disk of the galaxy runs across the sky as a band
of glowing light, like spilled milk.
Astronomers had suspected the Milky Way was made up of stars, but it wasn't proven until
1610, when Galileo Galilei turned his rudimentary telescope towards the heavens and resolved
individual stars in the band across the sky.
With the help of telescopes, astronomers realized that there were many many more stars in the
sky, and all of the stars that we can see are a part of the Milky Way.
If you could travel outside the galaxy and look down on it from above, you'd see that
the Milky Way is a barred spiral galaxy measuring about 120,000 light-years across and about
1,000 light-years thick.
For the longest time, the Milky Way was thought to have 4 spiral arms, but newer surveys have
determined that it actually seems to just have 2 spiral arms: they are called Scutum--Centaurus
and Carina--Sagittarius.
It's very difficult to figure out what the Milky Way looks like exactly, because we're
embedded inside it.
If you'd never been out of your house, you wouldn't know what it looked like from outside.
But you'd get a sense by looking at other houses in your neighborhood.
Our Sun is located in the Orion Arm, a region of space in between the two major arms of
the Milky Way.
The spiral arms are formed from density waves that orbit around the Milky Way.
As these density waves move through an area, they compress the gas and dust, leading to
a period of active star formation for the region.
Astronomers estimate that there are between 100 and 400 billion stars in the Milky Way,
and think that each star has at least one planet.
So there are likely hundreds of billions of planets in the Milky Way, and at least 17
billion of those are the size and mass of the Earth.
Our Sun is located about 27,000 light years from the galactic core.
At the heart of the Milky Way is a supermassive black hole, just like all of the other galaxies.
This monster is more than 4 million times the mass of the Sun.
Our Sun takes about 240 million years to orbit the Milky Way once.
Just imagine, the last time the Sun was at this region of the galaxy, dinosaurs roamed
the Earth, and the Sun has only made 18-20 trips around in its entire life.
The Milky Way, like all galaxies, is surrounded by a vast halo of dark matter.
Nobody knows what it is, but its mass helps keep the galaxy from tearing itself apart
as it rotates.
It's believed that our galaxy formed through the collisions of smaller galaxies, early
in the Universe.
These mergers are still going on, and the Milky Way is expected to collide with Andromeda
in 3-4 billion years.
The two galaxies will combine to form a giant elliptical galaxy, and their supermassive
black holes might even merge.
The Milky Way and Andromeda are part of a larger collection of galaxies known as the
Local Group.
And these are contained within an even larger region called the Virgo Supercluster.
You might be amazed to know that dung beetles actually navigate at night using the Milky
Way.
If you've never seen the Milky Way with your own eyes, you should take the chance when
you can.
Go to a place with nice dark skies, free from light pollution.
Look up and appreciate the Milky Way, and wave hello to all the neighboring stars who
share our galaxy with us.
Thanks
for watching.