GALAXIES

Unit Overview
This unit will explore the formation of galaxies and the system used by scientists to classify galaxies.

GLOSSARY OF KEY TERMS

universe: is everything that exists, including objects and energy, throughout time and space.

galaxy: a system of about 100 billion stars. Our Sun is a member of the Milky Way Galaxy. There are billions of galaxies in the observable universe. Exactly when and how galaxies formed in the Universe is a topic of current astronomical research.

spiral galaxy: a galaxy consisting of a flattened rotating disk of young stars, a central bulge of generally older stars, and a surrounding halo of older stars and dense clusters of old stars called globular clusters. The disk is prominent due to the presence of young, hot stars in a spiral pattern

dust lane: a lane of dark, obscuring interstellar dust in an emission nebula or galaxy.

primeval: having existed from the beginning; in an earliest or original stage or state.

elliptical galaxy: a galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.

interstellar material: the gas and dust between stars

irregular galaxy: a galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape

astronomer: a scientist who studies the stars and space.

Big Bang Theory: the theory that the Universe began with all matter and energy concentrated to very high density and temperature some 15 billion years ago. The present universe expanded from that epoch and is still expanding..

lenticular galaxy: a galaxy that is shaped like a lens. They are of an intermediate type between an elliptical galaxy and a spiral galaxy

Milky Way Galaxy: the large spiral galaxy in which our Sun and planets reside. Our Sun is one star of several hundred billion in the Milky Way

Print out the Key Term worksheet for extra practice. Key Term Worksheet

Galaxy Formation

One of the most amazing experiences that takes no money and a little bit of time is star gazing. It’s not as easy to do as in the past because of light pollution. Light pollution? That is the light caused by cities, street lights, etc. that can block the beautiful night sky full of stars. But your first assignment in this unit is to take one evening this week and spend some time looking at the night sky. Try to find a place where there is little or no man-made light so that you can have your best view. If you have binoculars or a telescope take them with you, but even with the naked eye, you will see some amazing sights. Look for the brightest lights, look for areas of clusters of light and if you’re lucky enough to be out when the moon is present, take some time to look closely at it too. Stay as long as the weather permits, and then when you go back inside write down your observations and thoughts.

In the days before electricity and video games, night time entertainment was often sky gazing. And man has wondered from the beginning of time a series of questions: “What are those lights?”, “How did they get there?”, “How long have they been there?”, and “How do they affect us?” Early scientists began to write down observations and record the motion of these heavenly bodies. Names were given to the star groups and humans did their best to invent technology to help study and understand outer space. Today, there is a great deal of information that has been compiled and still more is revealed to us daily by new telescopes and space probes. Let’s take a look at the current theories that scientists have about the formation of the universe and the heavenly bodies called galaxies.

The Big Bang Theory: First you must remember that a theory is a well-supported explanation of some aspect of the natural world that explains a certain set of events in nature. The Big Bang Theory is a widely excepted theory that gives an explanation of the origin of the universe. According to the theory there was a primeval explosion approximately 10 to 20 billion years ago that brought all time, matter, and energy into existence. Because of the extreme amount of heat, it is believed that only subatomic particles and radiation existed. As the universe began to cool, atoms of helium and hydrogen began to form. Scientists say that the universe is constantly expanding, which would support the theory of a large explosion and materials moving away from the explosion. There are other theories of the origins of the universe, but the Big Bang Theory is the most complete and widely accepted of the theories.

Galaxy Formation: A galaxy is a large system of stars and interstellar material. Each galaxy contains millions of stars and each star may be millions of times larger than our sun. Scientists estimate that there are over 100 billion galaxies in the universe. There are two major thoughts on how the galaxies formed. Some astronomers believe that the universe built up from small pieces like dust clouds and star clusters to form larger formations like galaxies. Other astronomers believe that the universe formed large chunks or lumps of matter after the Big Bang that eventually broke down into smaller groups of matter like galaxies. Astronomers today still search for the answer and that area of science is still very new. Technology is continuing to grow and gives us new answers and new puzzles. As you look at the picture below, you can see that galaxies come in all sizes and shapes, which is how scientists classify them. It is time to take a look at the different types of galaxies.

Galaxy Classifications

The best known classification system used to organize galaxies was developed by Edwin Hubble. This system classifies the galaxies by their shape into three broad classes. These classes are spiral galaxies, elliptical galaxies and irregular galaxies. Scientists believe that galaxy shape was influenced by their relative gas content, star formation history, and their internal motions.

Edwin Hubble: (1989-1953) Much of Edwin Hubble’s work dealt with galaxies. Before his work, it was the accepted belief that our galaxy was the only galaxy. His discoveries about galaxies and our expanding universe were amazing to the people of the 1920’s and 30’s. He provided some of the first evidence of the Big Bang Theory. The largest space-based telescope is named after Edwin Hubble and this telescope has recorded some of the most amazing photographs in space study history.

Close-Up of Star Formation in Antennae Galaxy

These four close-up views are taken from a head-on collision between two spiral galaxies, called the Antennae galaxies, seen at image center. The scale bar at the top of each image is 1,500 light-years across. The photos were taken by the Hubble Telescope.

In the 1920s Edwin Hubble separated galaxies into four general types according to their appearance elliptical, spiral, barred spiral and irregular and then classified each into subtypes.

Spiral Galaxies: Two major characteristics must be present for a galaxy to be a spiral galaxy. There must be a central disk with a dense concentration of stars surrounded by tightly wound arms and then several spiral shaped arms extending outward. This type of galaxy generally has a rapid rotation. More than half of all observed galaxies are classified as spiral. Spiral galaxies are rich in gas and dust that appears as lanes (dust lanes) or arms when viewed from above. They also appear bluish due to the large amount of new stars they contain.

Our own galaxy is a spiral galaxy and is known as the Milky Way Galaxy. Our solar system is located on one of the arms of the Milky Way. Looking at the night sky, you may see a band of stars and fuzzy light. What you see is the center of our galaxy. It is hard for us to get a clear view because of the distance and the large amount of interstellar dust that blocks the view. This interstellar material provides matter for new stars to form. Below is a chart of the Milky Way galaxy with our Sun labeled on the Orion arm. As you can see, some of the other arms of the galaxy are also named. The arms of a spiral galaxy are very active star forming areas.

Below is a photograph of our view of the Milky Way in the summer sky. The brighter cluster of light in the middle is the center of our galaxy.

A sub-category of spiral galaxies is the barred spiral galaxy. A barred spiral galaxy usually has two arms that extend out from opposite sides of the central disk. This type of galaxy has a straight bar of stars, dust, and gas from which the two arms extend.

Elliptical Galaxies: These galaxies are elliptical (oval) in shape, have no disks, spiral arms, or dust lanes and contain little or no interstellar material. The brightest stars in this type of galaxy are red stars, which indicate that these stars are older and not as hot as blue stars. About twenty percent of the observed galaxies are elliptical galaxies.

Lenticular Galaxies: This type of galaxy is a disk shaped galaxy without any obvious structures. They are believed to be made up of mostly old stars and the galaxy has used up all of its interstellar material. This gives the disk an overall even appearance. They are often misclassified as elliptical galaxies. If you compare the lenticular galaxy below with the elliptical one above, you can see that the one below has fuzzier, less circular edges.

Irregular Galaxies: Irregular galaxies are those that do not fit the characteristics of Hubble’s spiral, elliptical, or lenticular type galaxies. There are various ideas on why they do not have regular shapes, but many scientists believe their shapes are influenced by the gravitational pull of larger galaxies that are close by.

Unit Summary:

Galaxies have formed from dust, gas, and interstellar materials left over from the origin of the universe. The Big Bang Theory is the most widely accepted origin theory. Classification of galaxies is done primarily by shape. The classification system was developed by Edwin Hubble and is used by most scientists today.

Galaxies are categorized into four major groups: spiral, elliptical, lenticular, and irregular. Other characteristics of the groups include relative gas content, star formation history, and the galaxy’s internal motion.