Warm Up 1) 1) Another name for the interstellar matter that will eventually form a star is ____. a.supernovac. red giant b.nebulad. black hole 2) 2) A star is said to be born when ____. a.a dark, cool interstellar cloud begins to contract b.a protostar reaches a temperature high enough for nuclear fusion to begin c.pressure within a protostar becomes so great that a supernova occurs d.a red giant collapses on itself and becomes a black hole 3) 3) Which force is most responsible for the formation of a star? a.interstellar forcec. electromagnetic force b.nuclear forced. gravity Answers: 1) b. 2) b. 3) d.

The Universe Chapter 25, Section 3

The Milky Way Galaxy Galaxies – groups of stars, dust, and gases held together by gravity The Milky Way is a large spiral galaxy whose disk is about 100,000 light-years wide and about 10,000 light-years thick at the nucleus It has at least three distinct spiral arms, the sun lies about 2/3 of the way from the center on one of these arms and orbits the nucleus about every 200 million years There may be more than 100 billion stars in our galaxy alone

The Milky Way Galaxy

Structure of the Milky Way

Concept Check How big is the Milky Way Galaxy? The Milky Way Galaxy is about 100,000 light-years wide and 10,000 light-years thick at the nucleus.

Types of Galaxies Irregular Galaxies – consist mostly of younger stars, appear as clouds of stars Spiral Galaxies – like our galaxy, these galaxies have multiple arms that sweep out from a central nucleus Elliptical Galaxies – do not have spiral arms, makes up ~60% of known galaxies, can range from round to oval In addition to shape and size, one of the major differences among different galaxies is the age of their stars Galaxy Cluster – a system of galaxies containing from several to thousands of member galaxies

Irregular Galaxy

Spiral Galaxy

Elliptical Galaxy

Galaxy Cluster

The Expanding Universe When a light source is moving away from an object, its light appears red Hubble discovered that the light from distant galaxies was shifted to the red spectrum (red shift), implying that these galaxies are moving away from us Hubble’s Law – galaxies are retracting from us at a speed that is proportional to their distance The red shifts from distant galaxies indicate that the universe is expanding

Red Shift

Raisin Bread Analogy

Concept Check What relationship did Hubble discover between red shifts and the distances of galaxies from Earth? Galaxies that have the greatest red shifts are the most distant.

The Big Bang Big Bang Theory – The universe began as a violent explosion from which the universe continues to expand, evolve, and cool The big bang theory states that at one time, the entire universe was confined to a dense, hot, supermassive ball. Then, about 13.7 billion years ago, a violent explosion occurred, hurling this material in all directions Supporting evidence – Hubble’s discovery that the galaxies are speeding away from each other

The Big Bang

Concept Check What evidence supports the big bang? The red shift of distant galaxies and cosmic background radiation support the big bang.

Assignment Read Chapter 25 Section 3 (pg ) Read Chapter 25 Section 3 (pg ) Do Chapter 25 Assessment #1-31 (pg ) Do Chapter 25 Assessment #1-31 (pg ) For Section 3: #’s 9, 10, For Section 3: #’s 9, 10, Turn in Lectures (total of 9) Turn in Lectures (total of 9) Study for the UNIT TEST!!! Study for the UNIT TEST!!!