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 any combination. 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 outwards. 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, and Irr. 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.
