The Milky Way Galaxy Contains more than 100

The Milky Way Galaxy • Contains more than 100 billion stars • Is one of the two largest among 40 galaxies in the Local Group • Our Solar System is located a little more than half way from the galactic center to the edge of the galactic disk. • Only one of roughly 100 billion galaxies in the Observable Universe.

Galaxies • Definition: a system that contains millions or more stars. • Galaxy clusters: groups of galaxies with more than a few dozen members • Supercluster: galaxies and galaxy clusters tightly packed together. • Classified using a diagram called “Hubble’s Tuning Fork”

Our Cosmic Address 1. 2. 3. 4. 5. 6. Earth (4. 5 billion years old) Solar System (4. 6 billion years old) Milky Way Galaxy (13. 2 billion years old) Local Group Virgo Supercluster Observable Universe (13. 8 billion years old)

Milky Way Galaxy Structure • Known to look like flat pancake with a bulge in the center. • Named a Spiral Galaxy because of its spiral arms. • Approximately 23, 000 light years away from the nuclear bulge.

Parts of the Milky Way Galaxy • Nuclear Bulge: sphere of stars and other material at the heart of our galaxy. – about 4000 parsecs across (about a fifth of the total spread of the spiral arms of the galaxy. ) – The very center of our galaxy in a super-massive black hole. • 3 -4 million times as massive as the sun. • Appears to be powering a bright source of radio emmision known as Sagittarius A*.

Parts of the Milky Way Galaxy (cont. ) • Disk: Flat, rotating region of stars, dust and gas called interstellar medium around the Nuclear Bulge. – Can obstruct view for observations towards the Nuclear bulge. – Approximately 1, 000 light years in thickness and 100, 000 light years across

Parts of the Milky Way Galaxy (cont. ) • Spiral Arms: well defined bands in the disk because of brilliantly bright stars. • Halo: spherical in shape and contains little gas, dust, or star formation. • Globular clusters: a large compact spherical star cluster, typically of old stars in the outer regions of a galaxy.

Formation of Galaxies • A Protogalactic cloud contains Hydrogen and Helium. • Halo stars begin to form as the protogalactic cloud starts to collapse. • Conservation of angular momentum ensures the remaining gas flattens into a disk. • Billions of years later, the star-gas-star cycle supports ongoing star formation within the disk. The lack of gas in the halo prevents star formation outside the disk. http: //scienceforkids. kidipede. com/physics/space/momentum. htm

Facts about Galaxies and the Universe • Galaxies move apart • Expansion began about 13. 8 billion years ago (the Big Bang) – This is called cosmological redshift. – Proven by observing a faint glow of radiation that is a remnant of heat from the Big Bang. – Early Universe = hotter and denser – Today’s Universe = cooler and less dens because of expansion.

Facts about Galaxies • If you look at a galaxy that is 7 billion light years away you are seeing that galaxy as it appeared 7 billion years ago. • The number of stars in the observable universe is equal to the number of grains of dry sand on all beaches on Earth. • Large Magellanic Cloud and Small Magellanic Cloud orbit the Milky Way at 150, 000 light years and 200, 000 light years.

http: //www. universetoday. com/22828/milky-way-collision/

Nearby Galaxies • Andromeda Galaxy is comparable in size, though slightly larger. • Small galaxies (Sagittarius and Canis Major Dwarf Galaxies) are colliding with us right now! – These small collisions cause ripples in our galaxy and create new stars at points of collision.

Edwin Hubble (1889 -1953) • Studied stars and galaxies outside the Milky Way galaxy. • Designed a period-luminosity relation • Proved that Andromeda is much too far away to be part of Milky Way. • Known for Hubble Law • The more distant the galaxy, the faster its moving away = greater redshift • Galaxies are moving apart in our expanding universe.