Professor Dave here, let’s talk about the Milky Way.
We’ve learned about how stars and galaxies form, and there are hundreds of billions of
galaxies in the observable universe, each with many millions or billions of individual stars.
So which galaxy is our home?
Let’s take a look at this collection of galaxies.
This is called the Virgo Supercluster, containing over a hundred smaller groups and clusters
of galaxies, such as the Virgo Cluster, and the Local Group.
Now let’s zoom in on the Local Group.
This contains more than fifty galaxies, many of which are tiny dwarf galaxies, but some
of which are fairly large, by galactic standards.
The largest of these is called the Andromeda Galaxy, which is a spiral galaxy.
Around half that size, the next biggest is called the Milky Way Galaxy, also a spiral
galaxy, and that’s our home.
It’s a pretty typical barred spiral galaxy, containing somewhere between 200 billion and
400 billion stars.
It is about 100,000 light years in diameter, meaning it takes light 100,000 years to travel
from one edge to the other, whereas it is only about one thousand light years thick.
These are estimates, as there are no well-defined edges to the galaxy.
It is simply a disk with several arms of varying sizes, surrounded by a halo of stars, with
a dense bulge in the middle, most likely harboring a supermassive black hole at the center of
its nucleus, just like other large galaxies.
About fifteen percent of the visible mass of the Milky Way is in the form of interstellar
gas and dust, scattered between all the stars.
Some of these stars form groupings called open clusters, such as the Pleiades, which
is a few hundred stars bound fairly close together.
Beyond the stars in the disk, there are also several objects in more distant orbits called
globular clusters.
These are dense, spherical collections of stars, anywhere from a few hundred thousand
to a few million, and there are a little more than 150 of them traveling around the halo.
These are always filled with population two stars, indicating that they are very old,
probably forming at the same time as the Milky Way.
The Milky Way also has some satellite galaxies, which are smaller galaxies that orbit around it.
The most prominent of these are the Small Magellanic Cloud and Large Magellanic Cloud,
each around a tenth the size of the Milky Way, and observable from the southern hemisphere.
These interact with the Milky Way as they orbit, resulting in disturbances and also
exchange of material.
Andromeda and the Milky Way are also on paths that will cause them to collide sometime around
four billion years from now.
Simulations show that this will be an incredible event that will take several billion years
to play out to completion.
Many stars will be ejected, but the end result will likely be a merging to form a larger,
more elliptical galaxy, that has been dubbed “Milkdromeda”.
So what do we know about the formation of the Milky Way?
Well its rotation provides some information.
Stars outside of the bulge move around galactic center with a speed of around 220 kilometers
per second.
That’s astonishingly fast by earthly standards, but even so, it takes stars in the outer areas
about 250 million years to go once around.
The closer stars are to the center, the less time they take to orbit, just like planets
around their sun.
In fact, this is the phenomenon that is largely responsible for the formation of the spiral
arms in the first place.
This is all typical among spiral galaxies, as we can observe by looking at other galaxies
in our vicinity.
The oldest stars we can see in the Milky Way are very old population two stars, so we believe
the formation of the Milky Way dates back to that early era of galaxy formation, less
than one billion years after the Big Bang, whereby a dense region of gas collected to
form a system of gravitationally bound stars.
Due to the centrifugal force and accentuated by conservation of angular momentum, this
began to spin in the way that it does now, collapsing from a sphere into a disk, and
has presumably been undisturbed by major collisions all these billions of years, as its spiral
shape has been retained.
The older population two stars can be found in the halo, from the time of the galaxy’s
formation, whereas most stars in the disk are younger, more likely to be population
one stars, formed after the galaxy had already begun to take its current shape.
An estimated three to five stars per year continue to form today, from all the interstellar
gas and dust.
Eventually all the stars that can form from the gas and dust will have formed, and many
will die, predicting a grim, dark fate for the galaxy in some several hundred billion years.
But there is much more to talk about regarding the past and present first.
Some time into the existence of the Milky Way, a little less than five billion years
ago, a cloud of gas and dust, full of heavy elements dispersed from the deaths of population
three and two stars, began to accrete to form a very familiar set of objects, our own solar system.
Let’s move forward and learn more about this pivotal event.