Milky Way Galaxy. A galaxy is a group of stars, dust, and gases held together by gravity.

Milky Way GalaxyMilky Way Galaxy

AA galaxy galaxy is a group of stars, dust, and gases held is a group of stars, dust, and gases held together by gravity.together by gravity.

• • The Milky Way is a The Milky Way is a large spiral galaxy large spiral galaxy whose disk is whose disk is about 100,000 about 100,000 light-years wide light-years wide and about 10,000 and about 10,000 light-years thick at light-years thick at the nucleus.the nucleus.

Size of the Milky WaySize of the Milky Way

• • The Milky Way is a The Milky Way is a large spiral galaxy large spiral galaxy whose disk is whose disk is about 100,000 about 100,000 light-years wide light-years wide and about 10,000 and about 10,000 light-years thick at light-years thick at the nucleus.the nucleus.

Size of the Milky WaySize of the Milky Way

• • Radio telescopes Radio telescopes reveal that the Milky reveal that the Milky Way has 2 major Way has 2 major spiral arms, with spiral arms, with some splintering. some splintering.

• This diagram shows This diagram shows the possibility of the possibility of five arms and the five arms and the clockwise rotation clockwise rotation of the Milky Way.of the Milky Way.

Spiral Galaxies

• About 30 percent of all galaxies are spiral galaxies.

• They have large diameters of 20,000 to 125,000 light-years and contain both young and old stars.

• About 60 percent of galaxies are classified as elliptical galaxies.

Elliptical Galaxies

• Elliptical galaxies range in shape from round to oval.

• In addition to shape and size, one of the major differences among different types of galaxies is the age of their stars. Irregular galaxies contain young stars.

Irregular Galaxies• Only 10 percent of the known galaxies have

irregular shapes and are classified as irregular galaxies.

• A galaxy cluster is a system of galaxies containing several to thousands of member galaxies.

The Hubble Deep Field (seen as the background on all of the slides) is an image taken by the Hubble Space Telescope which reveals hundreds to thousands of galactic clusters.

• Hubble’s law states that the galaxies are retreating from the Milky Way at a speed that is proportional to their distance.

Hubble’s Law

Hubble’s Law

• To help visualize the nature of the universe, imagine a loaf of raisin bread dough that has been set out to rise for a few hours. As the dough doubles in size, so does the distance between all the raisins. Those objects located father apart move away from each other more rapidly.

The Big Bang

• Red shift, or a Doppler shift toward the red end of the spectrum, occurs because the light waves are “stretched,” which shows that Earth and the source are moving away from each other.

Red Shift

• The red shifts of distant galaxies indicate that the universe is expanding.

• The future of the universe follows two possible paths: 1. The universe will expand forever.

2. The outward expansion will stop and gravitational contraction will follow.

• The view currently favored by most scientists is an expanding universe with no ending point.

• It should be noted, however, that the methods used to determine the ultimate fate of the universe have substantial uncertainties.

• When telescopes led to the discovery of galaxies, astronomers observed many differences.

• In 1926 Edwin Hubble decided to classify the galaxies, grouping them according to shape creating the Hubble Classification Scheme.

• Hubble categorized all galaxies into three main types:

1. Elliptical

2. Spiral

3. Barred Spiral

Bar

• What is the difference between a barred spiral and a spiral galaxy?

– Barred spirals show the same spiral structure as normal spirals, as well as a prominent bar through the nucleus.  The spiral arms emerge from the end of the bar. 

No Bar…

• Hubble created a “tuning fork” diagram to classify the three different galaxy types and he represented each galaxy type with a picture and a letter.

• Classification the following galaxies using Hubble’s “tuning fork” classification diagram.

Galaxy #1

Galaxy #2

Galaxy #3

Galaxy #4

Galaxy #5

Galaxy #6