Lesson 25 – Lecture 3 – Stellar Populations and Formation of the Galaxy – OpenStax


Greetings and welcome to the
Introduction to Astronomy. In this video, we are going
to talk about two things we are going to talk
about stellar populations, different types of stars
and how they formed over the course of the
history of the galaxy and that will lead us to looking
at the formation of our galaxy itself. So let’s go ahead
and get started. And what we find with
stellar populations is that there are essentially two
types of stars in the galaxy most stars that we see are
what we call disk stars meaning that they are in the
disk of the galaxy and they orbit along in circular
roughly circular orbits that go around the center of
our galaxy in a flat plane so much as the planets orbit
around the sun in essentially a flat plane so do the
stars in the galaxy. And those are what we
call the disk stars. However, there are
also halo stars that orbit in the
galactic halo instead of following the galactic
plane as do other stars. These go in all
sorts of directions and add all sorts
of eccentricities so sometimes very eccentric
orbits very squashed orbits. So quite different
than the stars that we see in the
galactic plane. And we’re going to see
that this can tell us something about the early
history of our galaxy. So what does this mean
with two types of stars. Well, what we see is
that they were divided into two stellar populations
by Walter Baade when he looked at the Andromeda Galaxy and
he called the stars population I stars those in the disk
and population II stars were the halo stars and we found that
there is a distinct difference between these two populations. Population one stars
are in the disk. They have circular orbits,
they have a very wide range of ages from very old
stars to very young stars, and they have a
high metal abundance and I should take a minute here
to explain what this means. And to remind you that two
metals to an astronomer a metal is anything that is
not hydrogen or helium. So ignoring hydrogen and
helium any other element is considered a metal. So things like carbon
and oxygen and silicon are all metals to an
astronomer or anything that is not hydrogen or helium. Now a population II star here
are in the disk and the halo. They have eccentric
or tilted orbits. They tend to be all
they are all very old. So not a wide range
of ages, and they have a very low metal abundance. They are almost all
hydrogen and helium. Now, of course, this is
an idealized situation that there are too if we
consider the real world. There are really two
really, really a variation between these two
types of objects. But they are convenient
to be able to study. So what does this tell us about
the formation of our galaxy. So looking at the formation of
our galaxy how did the galaxy form in the first place. Is it similar to the
formation of a star but on a much larger scale. That is one way of
thinking about it and essentially, it
means we kind of called the monolithic model
where the galactic cloud. As we see here in figure 1 is
collapsing in to form a disk. So as it collapses
the very first things to form of the globular
clusters once they formed they are actually in orbit. So they remain where they are. They will just continually
orbit around the galaxy. But they do not
continue the collapse. The collapse
continues over time. And the galaxy will
spin faster and faster and material will concentrate
towards the center. So you can think of
this like star formation on a massive scale
that the sun would be what forms at the center. And then the planets around it. In this case, we get
the center of our galaxy a high concentration of stars
and our supermassive black hole and then the rest of the stars
form in the disk around it. So we can look at that. It was old stars and
the globular clusters that formed once they formed
they remained in the halo. It was only gas
clouds the gas that would collapse
down into the disk to form future
generations of stars. So the halo on the outer
regions up here and down here would have had no gas and dust. So things would be it would
undergo a fragmentation that things would
slowly break apart. You’d break apart this gas
into larger pieces that are the size of clusters and
those would go into star sized pieces and those
would eventually go through the types
of star formation that we’ve looked
at in the past. Now, if we look at another
way of thinking about this is what we call the
multiple merger a model that there may have not
been a peaceful formation of the galaxy. Essentially what we
have been finding now is that galaxy
collision collisions are incredibly common. They occur all the time. For example, the Sagittarius
dwarf galaxy and some others are actually being torn apart
by tidal forces of our Milky Way galaxy. So if we look at
our Milky Way here, we see tidal streams which are
left over bits of that galaxy that has been
disrupted by passing to close to our Milky Way. So we have several different
tidal streams that we see here. And those are those
remnants of those galaxies those small galaxies
that have been torn apart and what we find is
that galaxies then grow through collisions
that what starts out is a very small galaxy can
absorb other smaller galaxies and continue to grow in
size eventually becoming the galaxies that we see today. And we see the evidence
of this with multiples of these streams seen in
the halo of our Milky Way. So what are the results of
some of these collisions and what we see is that
the collisions store serve to stir up the stars
and gas in the disk. The disk becomes thicker and
stars form in this thick disk whereas the gas will settle back
down to a thinner disk where the newest stars form. So we start to
get various ranges from the halo with the oldest
stars to the thickest disk down to the thin disk
where the newest stars are forming collisions are coming. So there are more
collisions and what we see is that the Canis Major
dwarf galaxy is gradually merging with the Milky
Way and will eventually become incorporated within
the Milky Way itself on a larger scale. The Milky Way and
the Andromeda Galaxy are on a collision course. And will merge together
in a single galaxy in 3 to 4 billion years. And what we see here is
an artist’s conception of what that would be
we see our Milky Way, which we’re used to seeing here. And as it approaches
billions of years from now, the Andromeda Galaxy will
continue to get larger in size. So not actually
changing its size, but it will becoming
much larger. So the Andromeda Galaxy
here will look much larger in our sky and we’ll see
this gigantic galaxy. That would be approaching
closer and closer and eventually the two
galaxies would collide together and merge into a much larger
probably an elliptical galaxy. So let’s finish up as
we do with our summary and what we find is
that, first of all, we talked about two
different types of stars. We had the population I stars
in the disk and the population II stars in the halo. We think that
galaxies could have formed by a larger version
of the process by which stars formed but also that
collisions are very important. Collisions between
galaxies are really important for explaining
the structures that we see in our galaxy today. So that concludes this
lecture on stellar populations and formation of the galaxy. We’ll be back again next time
for another topic in astronomy. So until then, have a
great day, everyone. And I will see you in class.

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