GALAXIES & BEYOND

What is a galaxy? A galaxy is a very large group of stars held together by gravity. Size: 100,000 ly+ Contain Billions of stars separated by large regions of gas, dust, & nothing Very distant objects(over 100,000’s to millions of ly from us) Billions of galaxies in the universe.

Types of Galaxies 1920’s Edwin Hubble developed a classification system based on shape. 3 Types of Galaxies 1. Spiral Galaxies – contain “arms” contains young and old stars. about 5-25% gas and dust 2. Elliptical Galaxies – no arms contains older stars. about 1-5% gas and dust 3. Irregular Galaxies contains young stars. about 50% gas and dust

SPIRAL GALAXIES Some Spirals have tightly wound arms

Some Spirals have loosely wound arms

Some spirals have a “bar” cutting across its nucleus. These are Barred Spirals

Famous Spirals

Andromeda Galaxy 2.3 million ly away Visible in the N. Hemisphere. Closest spiral galaxy

The Pinwheel Galaxy 27 million ly away constellation Ursa Major

Whirlpool Galaxy 23 million ly away Constellation Canes Venatici

The Sombrero Galaxy 30 million ly away constellation Virgo

Structure of Spiral Galaxies Nucleus – central region of densely packed stars. Disk – somewhat flat region of the galaxy that contains the spiral arms composed of gas, dust, & stars Halo - an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component

2. Elliptical Galaxies Round Elliptical Oblong Elliptical Structure: rounded; no arms contains older stars. about 1-5% gas and dust

3. Irregular Galaxies Structure: no distinct shape contains young stars. about 50% gas and dust

Large and Small Magellanic Clouds 150,000 ly years away Closest galaxy to our galaxy Visible in the S. Hemisphere. Small Magellanic Cloud Large Magellanic Cloud

OUR MILKY WAY GALAXY The bright region is the galactic center. Dark areas are dust blocking out light from within.

Structure of the Milky Way Barred Spiral w/ 4 spiraling arms Probable classification: SBb or SBc Total Stars in our galaxy – 200 – 400 billion

Star Density of Milky Way (stars per cubic lightyear) Nucleus - About 10 million stars per ly3 Our location – about.003 stars per ly3 Rotation of the Milky Way Clockwise at about 500,000 mph Sun takes ~250 million years to complete on revolution around the galactic center.

1.The Disk Contain most of the stars, gas and dust. Stars – intermediate to young in age Diameter: ~100,000 ly Sun is located about 28,000 ly from center.

4 arms Norma Sagittarius Centaurus Perseus

2. Nucleus C entral parts of disk thicken Dense swarm of stars and gas. Central region is complex. Small intense energy source called Sagittarius A Mass of 3-4 million suns Supermassive black hole? Image from Chandra x- ray telescope, 2003

View toward galactic center. Green line is the galactic equator

3. Halo Spherical region surrounding spiral galaxies. Contains Very old stars (10-15 billion yrs) Very little gas and dust About ~150 globular clusters Most of the mass of galaxy

Star Clusters smaller groups of stars held together by gravity. Orbit galaxies in groups Two types of Star Cluster 1. Open Clusters Contain few hundred stars. Size: about 7-20 ly across Example: Pleiades (in Taurus)

2. Globular (Closed) Clusters Contain a few thousand to several million stars Size: about ly across. About orbit in the halo of the Milky Way. Example: The Great Cluster in Hercules

Galaxy Clusters A group of galaxies held together by their mutual gravity. The Local Group – contains about 40 members including the MW. Most are faint small dwarf galaxies. Superclusters – group of galaxy clusters held together by gravity. The Local Group is part of the Local Supercluster containing more than a dozen galaxy clusters and stretching out to over a 100 million ly.

Galaxy Collisions & Mergers Collision of Galaxies Some galaxies do collide. Stars very rarely collide. Most will experience several collisions in lifetime

NGC 1512, a starburst galaxy 30 million ly away

The Mice, NGC 4676 The Collision of 2 Galaxies 300 million ly from Earth Result: star formation

The Cartwheel Galaxy – a ring galaxy 500 million ly away

Structure in the Universe? We live in a clumpy universe Light from 1.6 million galaxies The distribution of nearly 63,000 Galaxies in 2 wedges in opposite directions from Earth

ORIGIN OF THE UNIVERSE The Big Bang Theory Billion years ago All matter was contained in a singularity (?) Then a BIG BANG!!! Matter went in all directions. Very hot initially. Billions of degrees F Cooled enough to allow protons & electrons to join and form basic elements of H & He. ½ Billion years later, nebulas formed from which stars, galaxies form.

Evidence for Big Bang 1. Red shift of galaxies Hubble discovered galaxies show a red shift in their spectra

Hubble’s Law Receding velocity of a galaxy is proportional to its distance. (The further the galaxy, the faster its recession speed)

2. Cosmic Background radiation Weak, low temp. microwave radiation detected. Remnant energy from big bang.

END OF THE BIG BANG? Astronomers debate. How will it end? Two views. Based on calculation/estimates on average density of universe. 1. Universe will last forever. Continue to expand. Gravity is not strong enough to slow expansion. 2. Expansion will eventually stop due to gravity. Gravity pulls everything back. All matter will return to the original state that began the Big Bang. Called the Big Crunch.