Professor Dave here, let’s smash some galaxies.
We’ve learned a lot about stars.
How they’re born, how they live, and how they die.
So now it’s time to learn some more details about how stars are distributed in space,
from the small scale to the very large, as well as some additional details about how
galaxies behave and interact.
So let’s go back one last time to that period between 150 million years and one billion
years after the big bang, when the first generation of stars formed.
What can we say about these first stars?
First, to get some jargon out of the way, we call these population three stars.
This may seem confusing as they were the first stars around, but this is another relic of
early categorization methods.
Astronomers were assessing the metallicity of stars, which means the proportion of heavy
elements contained in the star.
Those with very high metallicity were deemed population one, intermediate metallicity was
population two, and low metallicity was population three.
We now understand that low metallicity means an older star, because heavy elements were
not fused and dispersed into the interstellar medium until the first generation of stars
died and scattered their ashes, so to speak.
So the first stars, which formed entirely of the hydrogen and helium generated in early
universe nucleosynthesis, are population three stars, whereas population one stars have formed
much more recently, given the possibility of significant amounts of heavy elements being
already present in the cloud of gas and dust that accumulated to form the star.
Next, we want to understand that since collapsing gas clouds typically fragment as they accumulate,
stars are typically born in small groups.
The resulting stars are therefore often gravitationally bound, forming binary systems if only two,
or larger multi-star systems, if more than two.
Even larger systems of stars would be called star clusters.
It is actually the case that most high-mass stars are found in such systems, and only
for lower mass stars like ours is a single star system the more likely scenario.
So in actuality, the famous two-sun dusk that we see on Tatooine in Star Wars might be a
more common sight in the galaxy than what sunset looks like on Earth.
We should note, however, that extremely low-mass stars such as red dwarfs, which are the most
numerous type in our galaxy, are typically isolated.
In terms of the types of stars that can be found in the clusters, these could be of nearly
One interesting pairing involves a binary system with a white dwarf and a main-sequence star.
If these are near enough to one another, and the main sequence star enters a red giant
phase, exceeding its Roche lobe, the white dwarf will begin collecting material from
the other star.
This gas will form an accretion disk, almost like an atmosphere, and this will then heat
up enough to fuse hydrogen, which will produce a small explosion called a nova.
Or, the white dwarf can continue to collect material until the mass of the white dwarf
exceeds the Chandrasekhar limit, which means a supernova will then occur.
If it is a type 1A supernova, meaning that it results from the burning of carbon and
oxygen in the core, there will be no remnant.
But if it is a type two supernova, meaning that it results from the collapse of an iron
core, this will leave a remnant, like a neutron star or black hole.
Zooming out from these smaller star systems, we mentioned that stars are typically bound
together in huge structures called galaxies.
These typically hold anywhere from a few hundred million to a few hundred billion stars, and
the vast majority of stars exist within galaxies, although there are some drifting aimlessly
in the emptiness between them.
So galaxies exhibit a variety of sizes, but they come in a variety of shapes as well,
as first distinguished by Edwin Hubble, whose work we will discuss in detail later.
He noted that some galaxies could be called spiral galaxies, because they are thin disks
of stars rotating slowly around the galactic center, with two or more spiral arms extending
These structures contain the majority of the stars that can be found in the outer sections.
Some are called elliptical galaxies, which are rather smooth, having no distinct features
like a spiral galaxy does.
And lastly, some are called irregular galaxies, and these are ones that don’t really fit
into the other two categories.
So these are the three main categories for galaxies, which we can abbreviate as S, E,
There are also subcategories within them.
Spiral galaxies, can be barred spirals, represented by SB, which are ones whose spiral arms extend
from the ends of a central bar rather than the center of the galaxy.
There are also galaxies comprised of a thin disk but without any spiral arms, and we call
these S zero.
In addition, true spirals can be subdivided into Sa to Sd, depending on how tight or loose
the arms are, and ellipticals can be subdivided into E0, E2, E5, and E7, depending on how
spherical or flat the shape.
Beyond displaying a wide variety of sizes, different galaxies also differ in the types
of stars they contain.
Elliptical galaxies contain predominately older population two stars, whereas spiral
galaxies contain a mixture of population two and population one stars, so many younger
stars can also be found.
This is due to regions of gas and dust of high density in the spiral arms, which allow
new stars to form.
But how do galaxies themselves form?
Well in fact, as we already understand how stars form, galaxies are not really any different.
In the early universe, when clouds of gas began to collect due to gravity, just as certain
patches formed stars and star systems, these were typically found within a much larger
gas cloud, which yielded billions of stars that remained gravitationally bound to one another.
So precisely the same principles are at work for galaxy formation as those for star formation.
In fact, we can even look out towards the edges of the observable universe to see some
of these early galaxies forming, because the light they emanated at that time has taken
the entire age of the universe to get to us.
That’s the beauty of observational astronomy, we can see the universe as it was at nearly
any age just by looking at objects that are more or less distant, taking into account
how long it took the light from that object to get to us.
Some of these galaxies have something at their center called a quasar, short for “quasi-stellar
object”, back when astronomers didn’t know what they were.
These are actually galactic nuclei, and at their center is a supermassive black hole.
That’s a black hole with millions or even billions of solar masses.
This is surrounded by an accretion disk of gas, which as it falls into the black hole,
emits an unbelievable amount of energy, causing quasars to glow thousands of times brighter
than an entire galaxy, which is very helpful for spotting them, since they are so far away.
But this scenario is not limited to young galaxies.
We believe that every single large galaxy in the universe has a supermassive black hole
at its galactic center.
Some are not surrounded by an accretion disk of gas and thus are not quasars, presumably
because the surrounding gas has already fallen into the black hole long ago, but we can still
measure the mass of the black hole by measuring how fast surrounding objects orbit around
it, and calculations tell us that these are supermassive indeed.
This gives us an additional clue as to how galaxies formed, beyond the influence of dark
matter, which we will discuss later.
The largest population three stars in the early universe must have burned through their
fuel very quickly, leaving behind a black hole.
Over time, these must have collided and merged, forming black holes with greater and greater mass.
As this mass increased, surrounding star systems became increasingly gravitationally bound
to them, gradually reinforcing galactic structure.
Over even more time, small galaxies must have collided and merged as well, which eventually
resulted in the distribution of galaxies we see today.
The type of galaxy that formed in each case depended largely on the rotational velocity
of the gas cloud and the magnitude of random motion within it, but we still don’t fully
understand the mechanisms by which specific galaxy types form.
In addition, these shapes will change due to any collisions that may occur.
These can result in distortions of shape in a variety of ways, though we should note that
in such events, no individual stars actually collide, as there is so much space in between
all the stars.
Collisions can also result in total merging, as we mentioned before, which is probably
how elliptical galaxies form, and is sometimes called galactic cannibalism.
Most galaxies are very likely to be involved in at least one of these types of interactions
in their lifetimes, and if they haven’t yet, they may be due quite soon, as galaxies
tend to be collected into groups, clusters, and superclusters, often with an extremely
large elliptical galaxy at the center.
This is confirmed by observation.
We can look at very far away clusters, and see what they looked like billions of years
ago, given the time it has taken their light to get us.
These typically have more galaxies and a higher proportion of spiral galaxies.
By contrast, nearer clusters have fewer galaxies and more elliptical galaxies, supporting the
notion of galactic mergers.
So we now have a solid picture of how all the stars and galaxies we can see today must
have formed, on the basis of a few simple principles.
Clouds of gas and dust collect to form stars within larger clouds that form galaxies, which
are then gravitationally bound into clusters, colliding and merging over time.
We are continuing to see that in astronomy, the driving force of all processes is gravity,
the attraction of matter to all other matter.
Now that we have the big picture regarding the history of the universe up until fairly
close to present day, it’s time to start thinking about us and our home.
There are so many galaxies out there, which one is ours?
Let’s move forward and start to learn about our place in the vast cosmos.