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The Three Types of Galaxies


Using Slooh’s Online Telescope and integrated Quest learning activities, you can capture your own images of each type of galaxy while learning more about their features. The Milky Way's Doppelgänger is one of 60+ curriculum-aligned STEM Quest learning activities on Slooh for students 4th grade to college.



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The Three Types of Galaxies


What is a Galaxy?


Before we explore the different types of galaxies, what actually is a galaxy? In short, a galaxy is a collection of stars, substellar objects, nebulae, interstellar gas, dust, and dark matter, all bound together by gravity. A galaxy can be as small as 3,000 light-years across to as large as millions of light-years, and each one can contain anywhere from a few hundred million to a hundred trillion stars that orbit the galaxy's center of mass. The Milky Way, the galaxy we live in, is just one of an estimated two trillion galaxies in the observable universe.


The Types


Of the galaxies we have discovered, nearly all of them fall into one of the following three major categories. These categories, based on the visual appearance of galaxies, were first proposed by the astronomer Edwin Hubble in 1926.


The first type of galaxy, the simplest that we will explore, is an elliptical galaxy. Elliptical galaxies are typically dominated by older, reddish stars and contain trace amounts of interstellar matter, so they are not able to make new stars. Without new star formation, all of the blue to white stars die off, leaving the reddish stars behind. These galaxies, which are thought to form from collisions between spiral galaxies, have a galactic halo, which is a squashed sphere or disk of star clusters and gas that extends beyond the main part of the galaxy. Although not visible, there is another component to every galaxy halo called the dark matter halo. This structure contains more mass than the luminous matter detected visually, and it plays a vital role in galaxy formation and evolution despite particle physicists still not fully understanding its direct properties. Its presence indirectly affects the speeds at which stars orbit in galaxies, and it heats the intracluster medium of galaxy clusters.


Elliptical galaxies come in a variety of shapes, ranging from being perfectly spherical to squat ellipsoidal. They are among the largest galaxies in the universe, and their stars randomly orbit a supermassive black hole at the galaxy's center, creating the galaxies' large stellar bulges. Elliptical galaxies can be thought of as bulge-dominated, which gives them their overall, indistinct shape.


To review, the three ingredients of an elliptical galaxy are at least one supermassive black hole at its center, a stellar bulge of stars that have random orbits, and a halo of stars and globular star clusters.


Elliptical Galaxy M86 and the Virgo Cluster

The next type of galaxy is a spiral galaxy. This type makes up the majority of observed galaxies. Spiral galaxies contain both newly formed stars and stars formed in the early universe. They are the primary mechanism for the Universe to create new stars! Like elliptical galaxies, spiral galaxies have a central stellar bulge and a halo of stars and globular star clusters. Their central bulges tend to be smaller than those of elliptical galaxies and are usually reddish, dominated by old stars. Beyond these components, spiral galaxies also contain overdense regions of stars and gas orbiting in the same direction as the stars in the stellar bulge. These regions have the appearance of spiral arms. Because new stars are formed there, they are typically a bluish color and are brighter than their surroundings.


The ordered orbits of the stars and gasses in a spiral galaxy cause the structure to flatten, much like a pizza does when the dough is thrown in the air while spinning. As a result, the disks of spiral galaxies are flatter than any elliptical galaxy. Because so much material is pressed into this thin disk, some areas can become very dense.


So, the primary ingredients of a spiral galaxy are a supermassive black hole at the center, a halo, a stellar bulge, and a disk of stars and gas (the spiral arms)!


Spiral Galaxy M33

The third type of galaxy is a barred spiral galaxy. This is the type of galaxy the Milky Way is, and approximately two thirds of all spiral galaxies are barred! Barred spiral galaxies have the same components as a spiral galaxy, but their spiral arms curve directly from either side of a central stellar bar, which straddles the central bulge. Bars are dense regions of stars with oblong orbits that sweep up material and trigger intense star formation, making them very bright. The bar structures are believed to be temporary, meaning that they naturally form and dissipate over time.


Adding to the list of ingredients for a spiral galaxy, the components that make up a barred spiral galaxy are a supermassive black hole at the center, a halo, a stellar bulge, a disk of stars and gas, and a stellar bar.


Barred Spiral Galaxy M109

But what about galaxies that don’t fit into any of these categories? These are known as irregular galaxies. Most irregular galaxies are small, containing only tens of billions of stars. They are home to abundant star formation, and many astronomers think that the more structured elliptical and spiral galaxies were formed from irregular galaxies that merged. Because these galaxies are so small, they are often found in the presence of larger galaxies.


Barnard's Irregular Galaxy

Nearby Galaxies


Now that you know what the primary galaxy types are, check out the chart below to see which of these classifications some of those nearest to us fall in!

Name

Galaxy Classification

Distance to Galaxy's Center (light-years)

Milky Way

Barred Spiral

26,000

Canis Major Dwarf

Irregular

25,000

Sagittarius Dwarf Elliptical

Elliptical

70,000

Large Magellanic Cloud

Irregular

179,000

Small Magellanic Cloud

Irregular

210,000

Andromeda

Barred Spiral

2.5 million


 


More About Slooh's Milky Way's Doppelgänger Quest


Embark on a quest that takes you on a search to capture the Milky Way's doppelganger with Slooh's Telescopes. You will venture far into Deep Space, learn the fundamentals about galaxies, and discover that our galaxy is not alone in the Universe—and it even has a twin!


Learning Objectives


By the end of this Quest, students will be able to answer the following questions:

  • Is the Milky Way a galaxy or is it the entire Universe?

  • What are the three major types of galaxies in the Universe?

  • What kind of galaxy is the Milky Way galaxy and which of your captured galaxies is the same type as the Milky Way?

  • What year was it first determined the Milky Way galaxy was a spiral galaxy?

  • What year was it first determined the Milky Way galaxy was a barred spiral galaxy?

Vocabulary Words

Standards Addressed

NGSS Science & Engineering Practices

​Asking Questions and Defining Problems; Developing and Using Models; Planning and Carrying Out Investigations; Analyzing and Interpreting Data; Engaging in Argument from Evidence

CCSS ELA

​RST.9-10.3, RST.11-12.1, RST.11-12.2, RST.11-12.4, RST.11-12.5, RST.11-12.9, RST.11-12.10, WHST.11-12.2.B,WHST.11-12.2.E, WHST.11-12.2.F

Related Slooh Quests

  1. How Big?

  2. Mystery of the Island Universes

  3. Milky Way Explorer


About Slooh's Astronomy NGSS Aligned Learning Activities


Slooh’s Online Telescope is a learning platform designed to support any educator in teaching astronomy to meet NGSS requirements by collecting and analyzing real-world phenomena. No previous experience with telescopes is necessary to quickly learn how to use Slooh to explore space with your students.


You can join today to access Slooh's Online Telescope and all 60+ Quest learning activities if you are able to make astronomy a core subject of study for the semester or year. If you only have a few weeks to study astronomy, we also have a curriculum designed to fit your busy academic schedule and budgetary limitations. To learn more about our offers, click here.