Welcome back, astronomers! In a previous lesson, we learned about how the universe is arranged.
Today, we will dive deeper into one of the most fascinating structures that we use to
organize the universe: galaxies. By the end of this lesson, you will be able to describe
the size and shape of the Milky Way galaxy and classify galaxies into one of the three types.
As you may remember, a galaxy is a giant collection of stars, solar systems, and
everything in between. Astronomers estimate that there are somewhere between 100 and 200 billion
galaxies in the universe. With that many galaxies, how can astronomers possibly keep track of them?
That's where one of the founding fathers of modern astronomy comes in: Edwin Hubble. In 1924, Hubble
discovered that there were other galaxies besides our own. Within just a few years, astronomers had
discovered so many galaxies that they needed a way to sort them all. That's when Hubble proposed
the classification system that astronomers still use today, called the Hubble sequence.
Let's take a closer look at the Hubble sequence and see how galaxies are classified.
Galaxies are sorted into three categories based on their shape: elliptical, spiral, and irregular.
Before we take a closer look at each type, try your hand at classifying like an astronomer.
Pause the video here and write down what you notice about the elliptical galaxies from
the Hubble sequence. Make observations about their shape, brightness, color, and features.
Excellent! What observations did you make about elliptical galaxies? If you mentioned their oval
shape, uniform brightness, and red color, you nailed it. Elliptical galaxies are classified
as round or ovular galaxies with no internal structure. Most stars in elliptical galaxies are
old, which is what gives them their reddish color and dim brightness compared to other galaxies.
Next up are spiral galaxies. They are split into two subtypes called normal spiral and
barred spiral galaxies. Time to put on your astronomer hat again. Pause the video here
and write down your observations about the two types of spiral galaxies in your guided notes.
Again, include observations about their shape, brightness, color, and features.
Awesome! What observations did you make about spiral galaxies? Did you mention the cluster
in the middle that is brighter than the spiral arms twisting around the outside?
How about the bar that the arms connect to in the "Barred Spirals" section?
Spiral galaxies are classified as a rotating disc with spiral arms that curve out from a
dense central region. As you may have noticed, there are two subcategories of the spiral galaxy:
the normal spiral and the barred spiral. Let's take a closer look.
Normal spiral galaxies are classified by their central mass of stars surrounded by a disc with
spiral arms extending out from the center. Since many stars are clustered around the center of
normal spiral galaxies, the middle is very bright. The long curving arms are made up of more stars,
gas, and dust. These arms are where new stars within the galaxy are born. Barred spiral
galaxies share most of their characteristics with normal spiral galaxies. The difference is clear:
barred spiral galaxies have a huge bar-like structure running through their center.
This bar is composed of stars, gas, and dust and usually extends out to the ends of the
spiral arms. Our own Milky Way galaxy is a spiral galaxy, but it actually wasn't until
recently that astronomers discovered it was a barred spiral instead of a normal spiral.
You may remember that galaxies are absolutely humongous, spanning thousands of light years.
I wonder how big our Milky Way galaxy is.
Astronomers estimate the bar structure in the center of our galaxy is about 27,000 light-years
across. As gigantic as that is, that's not even close to how big the Milky Way is in full.
Astronomers have estimated that our galaxy has a diameter of about 100,000 light-years. Crazy!
The last category of galaxy is called irregular galaxies.
Let's take one more shot at classifying like an astronomer. Look at the full Hubble sequence
again. Now pause the video and write down your observations of irregular galaxies.
Spectacular! What observations did you record about irregular galaxies?
If you had a little bit of trouble with this one, that's okay. These are the most
difficult galaxies to classify because they're basically the leftover galaxies. Poor little guys.
Irregular galaxies are classified as galaxies with no clearly defined shape or structure.
So, in other words, these are the galaxies that don't really fit well into the other
classification types. They're not really elliptical, and not really spiral. They are often
not symmetrical, relatively small in size compared to other galaxies, and have low brightness.
It's amazing that astronomers have discovered over 100 billion galaxies,
all with varying shapes and structures. But, they all fit into these three main categories.
Astronomers estimate that about 10 percent of galaxies are elliptical,
10 percent are irregular, 60 percent are normal spiral, and 20 percent are barred spiral.
Remember, we are constantly learning and expanding our observations of the universe every day. So,
in the future, as we learn more and develop better tools, these numbers could be very different.
You did an amazing job classifying galaxies like a true astronomer today. Let's recap what we
learned. You now know that galaxies are classified into three categories based on their shape and
structure: elliptical, spiral, and irregular. The Milky Way galaxy is a barred spiral galaxy
that is over 100,000 light-years in diameter. Thank you for joining me today to learn all
about the different types of galaxies and how astronomers classify them.
Be sure to complete the practice questions and extension activities that go with this lesson as
you continue your journey into the universe. And, as always, remember: you are out of this world!