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Throughout the universe there are enormous groups of solar systems called galaxies. They surround our en8re galaxy, which we call the Milky Way. There are three types of galaxies in the cosmos: Spiral galaxies, Ellip-cal galaxies, and Irregular galaxies. Our Milky Way is a spiral galaxy, which is named a@er its spiral arms that are formed through the mix of cool gas and dust clouds with ionized gas. It is a flat disk shape and its center is yellow and bulges outside of the disk. An ellip8cal galaxy holds less cool gas and more hot ionized gas. These galaxies are longer in one direc8on, hence their name and usually have a redder 8nt. Irregular galaxies are simply clusters of a large amount of solar systems and that is not contained in any shape. Galaxies had to come from somewhere. We all know of the Big Bang Theory, but had happened a@er that. Why is everything in the sky the way it is? What we see today is all thanks to the evolu8on of galaxies. Galaxy evolu-on is the forma8on and development of a galaxy.
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The development of a galaxy has been understood through simple observa8on. Using a telescope, one can look into the sky and as they look further into space they the galaxies that they see will be older due to the speed of light through space. Astronomers will search for similar types of galaxies at different distances and this will result in a step-‐by-‐step development of a galaxy. Galaxies that have been found have been as old as 10 billion years old. This provides a lot of informa8on as to how a galaxy ages. Astronomers cannot record the forma.on of a galaxy, because telescopes cannot see that far into space/8me. The only thing astronomers can do is theorize how these galaxies had formed. Two ideas that they infer are that the en8re universe was mostly filled with hydrogen and helium atoms and that the universe was not evenly distributed. A@er these ideas had been hypothesized, further analysis could be made about how these galaxies started to take form and then how they developed.
Spiral Galaxies • Most common type of galaxy (77% of observable galaxies in universe) • Can be iden8fied with their bright central bulges with a wide spiral disk rota8ng around it. • They have two dis8nct regions. The disk of the galaxy contains the spiral arms. The disk is a region of star forma8on and has a great deal of gas and dust. It is dominated by young, bright blue stars. The central bulge is devoid of gas and dust. As you might expect, the bulge is composed primarily of older, cooler, less luminous stars. • Some have a bright bar running across the length of the galaxy and are called “barred spiral galaxies” • The two most common spiral galaxies are our solar system’s home, the Milky Way Galaxy, and the Andromeda Galaxy, which is about 5 light years away. • The range of masses for spiral galaxies is ~ 109 -‐ 1012 solar masses, with the typical mass being ~ 1011 solar masses. The typical range of luminosi8es corresponds to absolute blue magnitude -‐16 to -‐23, and the typical diameter of the visible disk is 5-‐100 kpc.
• Milky Way • Galaxy Most of the stars that we can see are in the Milky Way Galaxy. The main plane of the Milky Way looks like a faint band of white in the night sky. The Milky Way is about 100,000 light-‐years in diameter and 1,000 light-‐years thick. There are about 2 x 1011 stars in the Milky Way. This spiral galaxy formed about 14 billion years ago. It takes the sun roughly 250 million years to orbit once around the Milky Way. The Earth is about 26,000 light-‐years from the center of the Milky Way Galaxy. • Andromeda Galaxy • The Andromeda Galaxy is the closest major galaxy. It is part of the Local Group, a cluster of galaxies to which the Milky Way belongs. • The Andromeda Galaxy can just be seen with the naked eye in the constella8on Andromeda. Andromeda is the farthest object that can be seen with the naked eye. • It is about 2,500,000 light-‐years from Earth. It is 141,000 light-‐years wide. Recently, the Hubble Space Telescope found that Andromeda has a double nucleus. This second nucleus is probably from an ancient collision with a smaller galaxy.
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Irregular galaxies, as the name suggests, are galaxies that do not fall under either the spiral galaxy form or the ellip8cal galaxy form. – This is due to the stars in the galaxy not having an organized orbit that you find in stars from both ellip8cal and spiral galaxies. These irregular galaxies can take one of two different forms, Type I and Type II. – Type I irregulars are single galaxies that do seem to have some structure and can resemble spiral galaxies. They are considered irregular because their galac8c bulges are not centrally located and the discs show no spiral structure as can be seen in the NGC 55 galaxy. – Type II irregulars are o@en formed through gravita8onal interac8on with other nearby galaxies or when two galaxies collide. • Gravita8onal interac8on can be seen in the Antennae galaxies which are composed of NGC 4038 and NGC 4039, which are currently moving towards a collision. • An example of a galaxy that has already collided is the Cartwheel galaxy which has a dis8nct ring like shape. – This shape was formed because individual stars would rarely actually come into contact with each other during the collision, but rather they would just act on each other with gravity. – It is thought that a smaller galaxy moved into a larger one and the gravita8onal force on the stars and interstellar gas and dust causing it to be pushed out in a wave.
Examples of Irregular Galaxies NGC 55 • -‐ 6 million light-‐years away • -‐ Sculptor Group • -‐ 50,000 light-‐years across • -‐ Considered young and producing new stars Antennae • -‐ 45 million light-‐years away • -‐ NGC 4038 Group Cartwheel • -‐ 500 million light-‐years away • -‐ Sculptor Group • -‐ Diameter of ring is roughly 100,000 light-‐years and composed of newly formed massive stars
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Conclusion • Galaxies are important for our understanding of the universe. Ever since the first astronomers of the world we have been increasing out knowledge of the cosmos. Hubble’s Law has also brought us closer to behering our view of the universe. It has shown us that the universe is con8nuously expanding, but so is our knowledge.
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hhp://cas.sdss.org/dr5/en/proj/basic/galaxies/spirals.asp hhp://csep10.phys.utk.edu/astr162/lect/galaxies/spiral.html hhp://www.enchantedlearning.com/subjects/astronomy/stars/galaxy/spiral.shtml hhp://science.nasa.gov/science-‐news/science-‐at-‐nasa/2012/31may_andromeda/ hhp://www.nasa.gov/images/content/654242main_p1220b3k.jpg hhp://www.jasonsart.com/Galleries/MilkyWayGalaxy-‐Angled2008.jpg hhp://www.wired.com/images_blogs/wiredscience/2012/04/nasa_-‐ _the_andromeda_galaxy_m31_spyral_galaxy.jpg