The Milky Way is a disk about 120,000 light years across , with a
central bulge that has a diameter of 12,000 light years. The disk is far
from perfectly flat though, as can be seen in the picture below. What
warped it? Two of the galaxy’s neighbors – the Large and Small
Magellanic clouds – have been pulling on the dark matter in the Milky
Way like in a game of galactic tug-of-war. The tugging sets up a sort of
oscillating frequency that pulls on the hydrogen gas (of which the
Milky Way has lots of)
- It has over 200 billion stars
As galaxies go, the Milky Way is a middleweight. The largest galaxy known, IC1101,
has over 100 trillion stars, and other large galaxies can have more
than a trillion stars. Smaller galaxies like the aforementioned Large
Magellanic Cloud, have about 10 Billion Stars. The Milky Way has between 200-400 billion
stars, but when you look up into the night sky the most you can see
from any one point on the Earth is about 2,500. We aren’t stuck with
this many stars forever, though, because the Milky Way is constantly
losing stars – through supernovae – and producing stars, getting about seven stars per year
- It’s really dusty and gassy.
You may not think so by looking at it, but the Milky Way is full of dust
and gas. And when I say full of dust, I mean that we can only see out
about 6,000 light years into the disk of our own galaxy in the visible
spectrum, and the galaxy is about 100,000 light years across! The dust
and gas makes up a whopping 10-15% of the “normal matter” in the galaxy,
with the remainder being stars. The thickness of the dust deflects
visible light, but infrared light can pass through the dust, which makes infrared telescopes like the Spitzer Space telescope
extremely valuable tools in mapping and studying the galaxy. Spitzer
can peer through the dust to give us extraordinarily clear views of what
is going on at the heart of the galaxy and in star-forming regions.
- It’s made up of other galaxies.
The Milky Way wasn’t always as it is today, a beautiful barred spiral.
It became its current size and shape by eating up other galaxies. It’s
still doing so today – the Canis Major Dwarf Galaxy
is the closest galaxy to the Milky Way because its stars are currently
being added to the Milky Way’s disk, and our galaxy has consumed others
in its long history, such as the Sagittarius Dwarf Galaxy
- There is a black hole at the center.
Most galaxies have a supermassive black hole at the center. Ours is no exception. The Center of our galaxy is called
Sagittarius A*
(pronounced “A-star”), and it houses a black hole with a mass of 40,000
Suns that is 14 million miles across (about the size of Mercury’s
orbit). But this is just the black hole itself. All of the mass trying
to get into the black hole – called the accretion disk – forms a disk
that has a mass of 4 million Suns, and would fit inside the orbit of the
Earth. Though like other black holes, Sgr A* tries to consume anything
that happens to be nearby, Star formation has been detected near this black hole.
- It’s almost as old as the Universe itself.
The most current estimate
for the age of the Universe is about 13.7 billion years. Our Milky Way
has been around for about 13.6 billion of those years, give or take 800
million years. The oldest stars in our the Milky Way are found in
globular clusters, and the age of the galaxy is determined by taking the
age of these stars, and then extrapolating the age of what preceded them. Though some of the constituents of the Milky Way have been around for a
long time, the disk and bulge themselves didn’t form until about 10-12
billion years ago, and the bulge may have formed earlier than the rest of the galaxy.
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