What are the 4 types galaxies?

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A galaxy is a large group of stars, gas, and dust bound together by gravity. Our solar system resides in the Milky Way galaxy, a spiral galaxy that is part of a group of galaxies called the Local Group.

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There are billions of galaxies in the Universe, but only three outside our Milky Way Galaxy can be seen without a telescope - the Large and Small Magellanic Clouds and the Andromeda galaxy. The Large and Small Magellanic Clouds are about 160,000 light years away and are satellites of the Milky Way. They can be seen from the southern hemisphere. The Andromeda Galaxy is a larger galaxy that is about 2.5 million light years away and can be seen from the northern hemisphere with good eyesight and a very dark sky. The other galaxies are even further away from us and can only be seen through telescopes.

The smallest galaxies may contain only a few hundred thousand stars and be several thousand light years across, while the largest galaxies have trillions of stars and may be hundreds of thousands of light years across. Galaxies can be found by themselves, in small groups and in large clusters. It is very rare to find stars in the space in between galaxies.

Galaxies sometimes collide with each other, with interesting results. These collisions can trigger bursts of star-formation in addition to changing the shapes of the galaxies that collide. However, when galaxy collisions occur, individual stars do not collide, due to the vast distances between them.

Interacting galaxies imaged by the Hubble Space Telescope. (Credit: NGC 4676: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA; Antennae: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration UGC 8334: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University))

Galaxy classification

Galaxies are classified by shape. There are three general types: elliptical, spiral, and irregular.

Perhaps the most familiar kind of galaxy are spiral galaxies. They have a distinctive shape with spiral arms in a relatively flat disk and a central "bulge". The bulge has a large concentration of stars. The arms and bulge are surrounded by a faint halo of stars. The bulge and halo consist mainly of older stars, where spiral arms have more gas, dust and younger stars. Our Milky Way Galaxy is a spiral galaxy.

Some spiral galaxies are what we call "barred spirals" because the central bulge looks elongated – like a bar. In barred spirals, the spiral arms of the galaxy appear to spring out of the ends of the bar.

Examples of spiral galaxies imaged by the Hubble Space Telescope. (Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration)

As their name suggests, elliptical galaxies are round or oval, with stars distributed fairly uniformly throughout. They have a bulge and halo, like spiral galaxies, but don't have the flat disk of stars. The stars in ellipticals tend to be older.

Irregular galaxies have no identifiable shape or structure to them. They are often chaotic in appearance, without a bulge or any trace of spiral arms. The different shapes and orientation of galaxies are a result of their history, which may have included interactions with other galaxies.

Examples of an elliptical galaxy (NGC 1132) and an irregular galaxy (NGC 1472A). (Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration)

Updated: February 2016

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For Educators

• Hidden Lives of Galaxies Booklet

Astronomers are still uncertain about how galaxies have formed. They believe that after the Big Bang, space was made up of mostly hydrogen and helium and as gravity pulled gas and dust together it formed individual stars.

As those star began to pull together with their gravitational forces they created collections of stars that eventually turned into galaxies.

There are other scientists that believe that the mass of the galaxies were pulled together prior to the stars being created.

Galaxies are defined as a large group of gas, dust, and stars that are held together by gravity.

Our solar system is one of many that exist in the Milky Way galaxy, which is a spiral galaxy that is part of a larger galaxy group called the Local Group.

Scientists believe that the universe has billions of galaxies, and for humans, we have been able to view three outside of our Milky Way galaxy just by looking at the night sky, for thousands of years: the Large and Small Magellanic Clouds and the Andromeda galaxy.

To give you an idea of how far these are from Earth, the Large and Small Magellanic Clouds are around 160,000 light-years away. They are considered to be “satellites” of the Milky Way galaxy.

The Andromeda galaxy is quite a bit larger and is around 2.5 million light-years away. All other galaxies are a lot farther away from us.

Types of Galaxies Statistics:

Scientists have classified galaxies into three types: elliptical, spiral, and irregular.

Elliptical Galaxies:

These galaxies are named after their general oval or round shape, and they typically have stars that are speckled evenly throughout.

Like spiral galaxies, they usually have a bulge and a halo, however, they don’t have the flat disk of stars. Elliptical galaxies have stars that seem to be a lot older and have lower masses.

This is due to the fact that there is less of the gasses and dust required to make new stars. They can have as small of a number of stars as a hundred million or as man as a hundred trillion.

Elliptical galaxies can be in a number of sizes as well. Some are a few thousand light-years across, while the bigger ones are as large as a few hundred thousand light-years across.

This type of galaxy is mostly found in smaller groups and galaxy clusters. Scientists believe that all elliptical galaxies have a supermassive black hole at its center. They think that the black hole is related to the galaxy’s mass.

There is also a subgroup of elliptical galaxies call “dwarf ellipticals.” These galaxies have properties that relate to regular elliptical galaxies but also some of the globular clusters, which are a tightknit group of stars.

Lenticular Galaxies:

Named after their lens-shape, lenticular galaxies have thinner rotating star disks. They also have a middle bulge, but no spiral arms.

They share characteristics with elliptical galaxies in that they have very little interstellar matter or dust. Lenticular galaxies seem to exist in some of space’s more densely populated areas.

Spiral Galaxies:

Spiral galaxies may be those that are the most familiar. They have an identifiable spiral shape with extending “arms” and a mostly flat disk that has a bulge in the center.

The bulge is the large concentration of stars, and both the bulge and arms have a halo of stars that can be faintly seen.

The arms contain a higher percentage of gas and dust that are required for new star formation. This is why the spiral arm have the highest quantity of younger stars.

The bulge and arms have the older stars and a lot less of the dust and gas. Our Milky Way is a spiral galaxy.

The most common types of galaxies that exist in the universe are the spiral galaxies. The spiral galaxies are large and have a disk of rotating stars and nebulae that is encircled by dark matter.

The very center or core is called the “galactic bulge” and some spiral galaxies have star clusters and stars below and above the disk that create a halo-effect.

A majority of the spiral galaxies contain a supermassive black hole at their center.There are some spiral galaxies that are in a sub-classification called “barred spirals.”

This is because their bulge looks like a bar. The bulge is more elongated and in these galaxy types the arms usually look like they extend out of the ends of the bar.

There are other spiral galaxy subgroups that are defined by how tightly their arms are wound, their spiral arms, and the bulges.

Irregular Galaxies:

As you could guess by the name, the irregular galaxies don’t really have any structure or shape that makes them identifiable.

These galaxies often look rather chaotic, don’t have any trace that they ever had spiral arms, and don’t have a bulge.

Scientists believe that the appearance of these galaxies is due to the interactions and collisions that they had with other galaxies. Some irregular galaxies may have a bar structure as well as regions that are active in the formation of new stars.

There is a subgroup of irregular galaxies called the “dwarf irregulars,” and they are thought to appear like the earliest galaxies that were formed around 13.5 billion years ago.

History:

For much of the history, astronomers assumed that the Milky Way galaxy was the center of the universe and everything revolved or orbited around it.

This idea continued until the early 1900s when a scientist and Harvard College Observatory head, Harlow Shapley, indicated that the spiral-shaped blobs he was observing were gas and dust that was separate from the Milky Way galaxy. He called these areas “island universes.”

Edwin Hubble identified a number of pulsing stars in 1924 that he called “Cepheid variables.”

He recognized that they were outside of the Milky Way galaxy and were a completely unique set of stars that existed at far away distances.

He began the first creation of an initial classification system for the items that he was identifying.

Gerard de Vaucouleurs took Hubble’s initial classification system and revised it to identify the three main galaxy types and broke them down by the characteristics that included: openness of spirals, extent and size of bars, and galactic bulge size.

As scientists continued to observe galaxies they added additional sub-classifications to include characteristic markers such as the star-formation rate of a galaxy and the age spectrum of the stars in a galaxy.

Structure:

Galaxies exist in all sizes and shapes and can contain a few hundred thousand stars and be quite a few thousand light-years across all the way to being home to trillions of stars and being as large as hundreds of thousands of light-years across.

Galaxies can exist on their own as well as collected together in small groups and even larger clusters. Galaxies are the place where we find the stars and they believe they are the nursery for new stars.

This is one of the reasons that you rarely find stars sitting in space by themselves outside of galaxies.

Galaxies are constantly moving, so it’s no surprise if they crash into each other. When this happens they change their shape and can jumpstart a lot of star-formations.

When galaxies collide their stars don’t collide. This is because there is such a huge amount of distance between the stars.

Interesting Information:

• Almost all galaxies have a center that contains a black hole. Objects that are drawn into the black hole generate huge amounts of energy that can be seen by astronomers using specialize equipment.
• Some galaxies can be relatively small and yet have a black hole that is extremely large.
• The material that circles a black hole can spew out energy in the form of bright jets.
• Some galaxies have quasars at their center, which are the most energetic bodies in the universe.
• The classification of galaxies is based on their shape but also on their evolutionary history.
• The spiral galaxies are the most common and contain huge amounts of dust and gas that create new stars.
• There aren’t as many elliptical galaxies as spiral galaxies, and these have less of the gas, dust and star-making regions. Astronomers think that around half of the universe’s galaxies are elliptical.
• Irregular galaxies make up around three percent of the galaxies. They can have many different shapes and are often rather chaotic looking.

Exploration and Research:

Hubble measured the distances between individual galaxies and continued with his measurements through the use of the “Doppler shift” information.

This is the measurement of the quantity of light from the galaxies that was stretched out due to their motion. Using the Doppler shift he discovered that the galaxies that were around the Milky Way galaxy were traveling at very fast speed and moving away from us.

He also discovered that those in the farthest distance were moving faster than the galaxies closer to the Milky Way galaxy. Using this information Hubble calculated that the universe was expanding.

Astronomers took this data and eventually figured out the speed of the expansion of the universe.

Using the sophisticated and higher technology telescopes on both the ground and in space, astronomers have been able to track galaxies and see how their individual evolution is effected by dark matter.

Dark matter is a mysterious substance that can’t be seen with any type of telescope or technology.

Scientists believe that both dark matter and dark energy make up a majority of the mass and energy in the universe, however, they still grapple with proving their existence.

2017: Astronomers located two large galaxies from the ancient universe that were created with a large quantity of dark matter. The unusually huge size of the galaxies have given scientists the ability to question whether they grew bigger with time or if there is another unknown process that caused their size.

2017: Just a few months after the discovery of the two extraordinarily large ancient universe galaxies, astronomers located a group of galaxies that were orbiting with each other in sync in a pattern that might be later explained as being dark matter.

2017: the Multi Unit Spectroscopic Explorer (MUSE) instrument used at the European Southern Observatory’s Very Large Telescope (VLT) allowed astronomers to view a group of 72 galaxies that were previously hidden due to being so faint.

2018: A research group challenged the theory of dark matter when they found the NGC 1052-DF2 galaxy. This galaxy had nearing four hundred times less dark matter than the predictive models for it size. The results of the discovery remain under debate as it is controversial.

2018: A Hubble Space Telescope image was created that included around 15,000 galaxies in it. This image gave astronomers an incredible amount of information for future studies.

2018: Scientists discovered hundreds of galaxies that were hiding behind a super-energetic black hole.

Facts about Galaxies for Kids:

• The word “galaxy” comes from the Greek word “galaxias” which translates to “milky”. Humans in the past thought that our Milky Way galaxy was the center of the universe, so the word relates to all galaxies.
• Scientists believe that there may be over 170 billion galaxies in the universe that we call “observable,” and there actually could be a lot more.
• The arms of the spiral galaxies have dense molecular clouds of hydrogen gas and dust that are the star nurseries for creating new stars.
• Nearing two thirds of the spiral galaxies are barred-spiral galaxies, with the bars at their center.
• The classification used for galaxies is referred to as the Hubble classification scheme and lists spiral galaxies depending upon how tightly wound their arms are and the center bulge size.
• They include a type “S” with an “a, b or c.” Barred spiral galaxies use the classification symbol of “SB.”
• Scientists believe that spiral galaxies are younger than elliptical galaxies. It’s thought that spiral galaxies burn through the star-forming fuel and eventually become elliptical galaxies.
• The Hubble classification scheme uses the letter “E” to identify elliptical galaxies. There may be an additional number assigned for the ellipticity degree.
• The Hubble classification scheme for Lenticular galaxies uses “S0” for identification.
• The Milky Way galaxy is considered to be a barred spiral galaxy that is around 120,000 light-years in diameter.
• If the night sky looks like it’s full of stars – think about the fact that the Milky Way galaxy has around 400 billion stars and maybe as many planets as stars.
• The name for a galaxy that creates new stars at a faster than normal rate is “starburst.”

https://imagine.gsfc.nasa.gov/science/objects/galaxies1.html
https://www.space.com/15680-galaxies.html
http://www.sciencekids.co.nz/sciencefacts/space/galaxies.html