When you look at this photo nearly every object here is identified as a galaxy, and as you can
see there are a variety of shapes that exist. This photo was taken by the Hubble space telescope and
it's called the Hubble Extreme Deep Field and it combines Hubble telescope observations that were
taken over a small patch of sky and it has over 2 million seconds of exposure time, and it's the
deepest look we have into the universe thus far at the time of this video, and this is the region that
the Hubble was photographing, and you can see when comparing it to the size of the Moon it is a very
tiny portion of the sky, and when you're looking at this photo there are over 5,500 galaxies, some
in which scientists have estimated to be over 13 billion years old, and in this video we're going to
look at the types of galaxies that can be observed, how they're classified, and examples of each type.
Welcome to Learn the Sky, my name is Janine and I'll be your guide as we
explore the night sky together one constellation at a time.
Let's begin with the broad definition of what a galaxy is. A galaxy is a system of millions
or billions of stars held together by the gravitational attraction between the gas and dust
within the galaxy, and they're classified by their shape, and if you look here these are the two main
types of galaxies that we see. There's spirals and elliptical galaxies. However, there are variations
of galaxies besides these two categories, and this is Edwin Hubble's fork diagram that it's often
referred to because of how the spiral galaxies can branch off depending if they have a bar a
bar within their structure, or if they're a normal spiral. So let's dig into the different types of
spirals and ellipticals. So here we have a spiral shaped galaxy, an elliptical galaxy, and then I also
included irregular since they are fairly common as well, and spiral galaxies as you can probably
guess from the name they have a spiral pattern and visible component parts, and when we look at these
three main components we have the bulge, which is the center of the galaxy, there's the disc which
are the arms of the galaxy, and then there are there is the halo, the halo contains older stars,
the disc has younger bluer stars, while the center of the galaxy typically has older stars. You're not
observing the halo in this photograph, so I found this one of m-104, it's called the sombrero galaxy
in Virgo, and this area here would be classified as the halo. Here we could see the galactic center,
and then the disc like arms, and this is a cool galaxy because we're looking at it edge on, and
remember we classify spiral galaxies based upon their shape. So those two shapes include being a
normal spiral or a barred spiral. A barred spiral has that bar structure throughout the center, and
then the arms go around the center, where we don't see the barred structure in normal spiral galaxies,
There are a ton of photographs out there of spiral galaxies. Here are some of my favorites,
the Needle galaxy, and then this one is so beautiful you could see the amazing arms
coming out from the center and this is Messier 51, and this is this image is an optical image
but also combines infrared data. So it allows you to see where the heat in the galaxy is along with
the visible bright areas such as the core of the galaxy.
Our next classification of galaxies include elliptical galaxies, and just like spiral galaxies
they're named after their shape. They look like these fat little fuzzy footballs in some
perspectives, and instead of the stars spreading out into a thin disc-like in spiral galaxies,
the stars wrap completely around the galaxy's heart and move out equally in all directions.
So let's look at some of the characteristics of elliptical galaxies. They're spheroidal in shape,
and the stars wrap around the galactic core, and they are equal in all directions. They're
typically composed of older stars and we usually don't see any new star star formation happening
in galaxies such as these, and some of the largest observed galaxies are classified in this shape,
and if we look at our classification fork, Hubble's fork, this is where ellipticals sit in this
area. So right there you can see some are more elongated where some are more circular in shape.
So here's some examples: we have IC 2006, I think that one is really beautiful,
it kind of looks like a fuzzy star but they're just collections of millions or
billions of stars and here is M87, this one has been in the news in the past few years
because we were able to capture a picture of the black hole in the center of the galaxy.
Our next classification of galaxies falls with irregular galaxies, and as you can probably figure
out from its name they have no particular shape or symmetry. They're often small in size and have
younger hotter stars. Typically they have less than a million stars but they're very common
in our universe. Some examples include the small magellanic cloud, the large was once
classified as a regular galaxy but now it is classified as a disrupted barred spiral,
and even though the small magellanic cloud is a dwarf irregular galaxy I have read that
there is a bar structure within here, and this takes us to our next category which are dwarf
galaxies. A dwarf galaxy is a small galaxy composed between 1 million stars or up to a billion stars.
So that again is very small in comparison to our own Milky Way. Here's an example of a dwarf galaxy,
they're very abundant and they have a wide variety of shapes. Here is the dwarf galaxy DDO 68 in the
constellation of Leo and here's the phoenix dwarf galaxy in the phoenix constellation. In the next
portion of this video we're going to take a look at some unique galaxies that exist in our universe.
Ring galaxies are interesting because they are galaxies with the appearance that they have just
a circle of light around them, and within this area there are no spiral arms that are attached
to the core of the galaxy. The ring itself contains younger stars, they're often very blue and bright.
This particular object is called Hoag's object, due to the person who discovered it in 1950 Arthur
Hoag, and it was identified, he thought it was a planetary nebula which it certainly looks like one,
but he it was then identified as a galaxy and what's really peculiar about this object is if we
zoom in if you look in this space right here it looks like there's another ring
galaxy within there that's perhaps farther away, but Hoag's object is estimated to be 60 million
light years away. Another unique galaxy that's observed are starburst galaxies, and starburst
galaxies have an exceptionally high rate of star formation. We see star formation in these galaxies
that are 100 times greater than what we see in our own Milky Way galaxy. Here's an example of
one, these these are the Antennae Galaxies in the constellation of Corvus, and if we were to zoom out
there you could see that's what looks like an antenna perhaps on a bug,
but these two galaxies are interacting with each other and then because they are interacting and
merging together there is a higher rate of star formation. Another example of a starburst
galaxy include IC10, and this one is close to us at only 2.2 million light years away. It's located in
our own local group, and again we just see higher rates of star formation in starburst galaxies.
Lenticular galaxies are galaxies that appear to have a circle-like appearance. The ring
itself in this region typically has younger stars that are blue and bright and these types
of galaxies are fairly uncommon. If we were to look at Hubble's fork, the diagram that helps us
classify galaxies, these are classified right here as lenticular galaxies or S0.
So in terms of, it's a type of spiral galaxy but perhaps very early in its development. Here is
an example of a lenticular galaxy. This is NGC 4866 in Virgo. This one we're looking at
edge on and you can see there's no visible arms in here however there is a galactic core that glows
brightly, and I found this diagram that I thought was interesting. It shows you that these are
lenticular galaxies, so very circular in shape, and it is estimated that their rotation has something
to do with you know how fast it rotates will determine what its arms will be like in the future.
So this is kind of like an evolution of galaxies over time. Here's one more example, I love this one
and this is a lenticular galaxy as you can see almost circular in shape, and it almost looks like
an eye to me and it's located in the constellation of Fornaxm and finally I have this galaxy to share
with you. It's called the Cartwheel galaxy, and it's both a lenticular galaxy and a ring galaxy. So here
you can see it's nearly circular in shape, but it also has that ring around it with the central core.
So it's interesting that you can have galaxies that have characteristics of two different types.
Our next type is ultra diffuse galaxies, and these are peculiar because
they are extremely low luminosity galaxies, and they really they vary so much. Some
lack star-forming gas and typically have older stars, but then it can also be extreme when it
comes to dark matter. Some consist entirely of dark matter, where some are entirely free of dark
matter. So they're very unique and peculiar. Here is Dragonfly 44. This is an ultra diffuse galaxy
in the constellation of Coma Berenices, and this one is 99% dark matter. So very interesting that
they there's a wide variety of of ultra diffuse galaxies but what the thing that they share is
that they just aren't very bright. The final type of galaxy we will examine in this video
are interacting galaxies, and these are ones that are affected by each other because of the
gravitational pull and influence they have from their proximity to each other. It's possible that
some of these interacting galaxies may eventually merge, which can trigger higher star formation.
It creates friction between the gas that exists all throughout the galaxy and astronomers often
find that merging galaxies have higher rates of star formation much like that one starburst galaxy
we looked at earlier. So let's take a look at some examples of merging and interacting galaxies. This
is the Mice galaxies, these two are a pair in Coma Berenices, this right here is NGC 4302
and NGC 4298. It's estimated that these two are interacting with each other, and then the
famous Whirlpool galaxy and NGC 5195. These two are interacting, and then will eventually merge.
So I hope this video was helpful for you to go over the different types
of galaxies that exist in our universe, and I'm really only scratching the surface. There are
other varieties, but these are the ones that are the most common and peaked my interest,
and it should be interesting to see what kinds of images will be produced from the James Webb Space
Telescop,e which has an optical portion but also an infrared portion to this space telescope, and
we've been waiting many years for the James Webb to be in space, and as this video is being recorded
all its instruments are being aligned, and I hope you're as excited as I am to learn what
this telescope and the science it will produce. So I have to give a shout out to all the astronomers,
and I'm sure there's many more people than just the ones here pictured, but this gives
you a size comparison of how big Webb is in comparison to the people that are helped
developing it. So thank you so much for watching! Let me know, have you ever seen a galaxy in the
night sky? I know some of the most common are the Andromeda galaxy of course our Milky Way galaxy,
but thank you so much for watching. I really love learning about different types
of galaxies out there so this was a fun one to make for you guys. Remember it takes time
patience and practice to go out and see the stars, and even though we may not see
objects like this with the naked eye it's really cool to think that we can look at a totally dark
portion of the sky and this is what exists out there, and when we are looking at images like this
we are looking back in time because it takes light so far and so much time to reach us. So just
just something cool to think about. Thank you guys so much for watching, and as always keep looking up!