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Galaxies. What are Galaxies?. A galaxy is a massive, gravitationally bound system consisting of hundreds of billions of stars, stellar remnants, of gas and dust, and dark matter, an important but poorly understood component.
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What are Galaxies? A galaxy is a massive, gravitationally bound system consisting of hundreds of billions of stars, stellar remnants, of gas and dust, and dark matter, an important but poorly understood component. From the Hubble Extreme Deep field it is estimated there are at least 176 billion galaxies in the observable Universe and possiblytrillions in the total Universe.
What are Galaxies? • At the turn of the 20th Century, a great debate was raging in astronomy. The debate concerned faint, fuzzy objects called "nebulae." Some astronomers believed nebulae were small clusters of stars in our own galaxy. Others saw some of them as vast, distant collections of stars, some larger than the Milky Way itself.
What are Galaxies? • Finally, in 1924, American astronomer Edwin Hubble measured the distance to what was then called the Andromeda Nebula. He found it to lie over 2 million light years from Earth. It was the first object to be recognized as another galaxy. • Hubble's discovery totally changed our view of the Universe. The alreadyvast distances between stars weredwarfed by the incomprehensibledistances between galaxies. The universe was suddenly a muchlarger place than anyone imagined.
Classifying Galaxies – Part 1 Pretend that you are an astronomer working shortly after Edwin Hubble made his startling discovery. Now that you know the "nebulae" are actually other galaxies like our own, you must come up with a way to classify the galaxies. Exercise 1: Look at the galaxy pictures. Divide them into groups based on features they have in common. There is no set number of groups.
Classifying Galaxies How many galaxy classes do you have? What distinctions did you use to create each class?
The Hubble Tuning Fork After he discovered what galaxies really were, Edwin Hubble became the first person to classify galaxies. Astronomers use his system, called the "Hubble Tuning Fork," even today. First, Hubble divided the galaxies into two general categories: elliptical and spiral galaxies.
Elliptical Galaxies “E” Elliptical galaxies are made of a central bulge characterized by a smooth, ball shaped appearance. They contain contain old stars and • possess little gas or dust. They are classified by the shape • of the ball, which can range from oval (E7) to round (E0) • (baseball-shaped to football-shaped). The stars in elliptical galaxies do not revolve around the center in an organized way. The stars move in randomly oriented orbits within the galaxy, like a swarm of bees.
Elliptical Galaxies Elliptical galaxies, such as M87, have very little gas and dust. Because gas and dust are found in the clouds that are the birthplaces of stars, we should expect to see very few young stars in elliptical galaxies. In fact, elliptical galaxies contain primarily old, red stars. Elliptical galaxies also vary widely in size. Both the largest and the smallest known galaxies are elliptical. Very large elliptical galaxies can reach 300 million light years in diameter. Dwarf ellipticals, which are very common, may contain only 1/100,000th as many stars as the Milky Way!
Spiral Galaxies “S” or “SB” Spiral galaxies consist of a flattened disk containing spiral (pinwheel-shaped) arms, a central bulge, and a halo. Spiral galaxies have a variety of forms, and they are classified according to the size of the central bulge and the tightness and appearance of the arms. Type a galaxies have their arms wound very tightly and have largecentral bulges. Type c galaxies havevery their arms would loosely andhave small central bulges. A barred spiral galaxy is a spiral that has a bar-shaped collection of stars running across its center. The arms of a barred spiral are attached to the bar.
Spiral Galaxies “S” or “SB” The spiral arms, which wrap around the bulge,contain many young hot, blue stars and lots of gas and dust. Stars in the central bulge where there is not a lot of gas and dust tend to be older and redder. Yellow stars like our Sun are found throughout the disk of a spiral galaxy where there are young and middle aged stars and lots of gas and dust. This is the main region of star formation. Spiral galaxies rotate somewhat like a hurricane or a whirlpool.
Lenticular Galaxies “SO” and “SBO” Some galaxies are a transition type betweenthe elliptical and spiral galaxies, labeled S0 on the tuning fork. These are called "lenticular galaxies." They have a central bulge and a disk but no spiral arms. Lenticular galaxies like M85 have a centralbulges and disks, but no spiral arms. If thedisk is faint, it is easy to mistake a lenticulargalaxy for an EO galaxy. There is a second type of lenticular galaxy called a barred lenticular galaxy. Barred lenticular galaxies have bars, much like the barred spirals, and so they are denoted SB0.
Irregular Galaxies “Irr” Galaxies that do not fit into either the spiral or elliptical classes are called irregular galaxies. Irregular galaxies, such as M82, have a wide variety of shapes and characteristics. They are frequently the result of collisions between galaxies or gravitational interactions between galaxies. M82 has also been called "The Exploding Galaxy." It is believed to have recently passed close to another galaxy called M81. M81's gravity warped M82, and the warping caused a burst of new stars to form. Today, M81 and M82 make a beautiful sight through binoculars.
Evolution of Galaxies Hubble believed that galaxies started at the left end of the diagram and evolved to the right. He called the elliptical galaxies "early galaxies" and the spirals "late galaxies." We now know that he was wrong: galaxies do not move down the forks of the diagram as they evolve because spiral galaxies rotate quickly while elliptical galaxies do not. There is no way that an ellipticalgalaxy could spontaneouslybegin rotating, so there is noway an elliptical galaxy couldturn into a spiral galaxy. Instead, galaxies evolve fromspiral to elliptical.
Classifying Galaxies – Part 2 Exercise 2: Look at the galaxy cards and classify them to create the Hubble Tuning Fork. Irregular E0
Glossary • Galaxy- a massive, gravitationally bound system consisting of hundreds of billions of stars, stellar remnants, of gas and dust, and dark matter. • Hubble Tuning Fork- Hubble’s classification system for stars. • Elliptical Galaxy-shaped like ellipses, and vary in shape from circular to oval.. • Spiral Galaxy-shaped like spirals, with arms winding in to a bright center. • Lenticular Galaxy-transition galaxy between elliptical and spiral galaxies • Irregular Galaxy-no definite shape; do not fit in any other class.
