25.4 Galaxies and the Universe

Objectives: Describe the different types of galaxy Describe the solar system’s position in

our galaxy Describe the universe and its origin

25.4 Galaxies and the Universe

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

Galaxy Messier 101 (also known as NGC 5457 and nicknamed the Pinwheel Galaxy) lies in the northern circumpolar constellation, Ursa Major (The Great Bear), at a distance of 25 million light-years from Earth.

Types of Galaxies

25.4 Galaxies

Spiral GalaxiesSpiral arms of stars and nebulae that wind out from

centerCan be barred – have a central bar of stars with

spiral arms extending out Arms are the location of much star formationMore than half of the observed galaxies are spiral

galaxies.

Have large diameters of 10 000 to 300 000 light-years and contain both young and old stars.

Spiral Galaxies

• This image is of NGC 1300, a spiral galaxy with arms extending from the ends of a spectacularly prominent central bar. It is considered a prototypical example of barred spiral galaxies and lies at a distance of about 65 million light-years, in the constellation of Eridanus (the River).

Spiral Galaxies

• The bright galaxy NGC 3810 image from Hubble.

Types of Galaxies

25.4 Galaxies

Shaped like large 3D elipsesAbout 10 percent of observed galaxies are

classified as elliptical galaxies.

Elliptical Galaxies

Elliptical galaxies range in shape from round to oval.

Contain little gas/dustMay contain mostly old (orange-red) stars

Elliptical Galaxies

• This image from NASA's Hubble Space Telescope shows the diverse collection of galaxies in the cluster Abell S0740 that is over 450 million light-years away in the direction of the constellation Centaurus.The giant elliptical ESO 325-G004 looms large at the cluster's center. The galaxy is as massive as 100 billion of our suns.

Types of Galaxies

25.4 Galaxies

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

Irregular Galaxies• About 3 percent of the observed galaxies have

irregular shapes and are classified as irregular galaxies.

Irregular Galaxy

• The irregular galaxy NGC 1427A

25.4 Galaxies

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

Contain a great deal of gas and dark matterThe Local Group is an example, containing Milky Way,

Andromeda and over 40 other galaxies

Galaxy Clusters

• The Fornax Galaxy Cluster is beyond our own Local Group of galaxies.

• It is at a distance of approximately 62 million light years.

Galaxy Cluster

25.4 Milky Way

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

The sun is about 26 000 light years from the center, on an arm.

Size of the Milky Way

• Radio telescopes reveal that the Milky Way has at least two large distinct spiral arms, with some splintering.

Structure of the Milky Way

The Milky Way

The Milky Way• Image of the night sky

above Paranal on 21 July 2007, taken by astronomer Yuri Beletsky of the European Southern Observatory (ESO). At the center of the image, two bright objects are visible: the brightest is the planet, Jupiter, while the other is the star, Antares. Three of the four 8.2-m telescopes forming ESO's Very Large Telescope (VLT) are seen, with a laser beaming out from Yepun, Unit Telescope number 4. The laser points directly at the Galactic Center.

The Milky Way• The Milky Way arch emerging from the Cerro Paranal,

Chile, on the left, and sinking into the Antofagasta's night lights. The bright object in the center, above the Milky Way is Jupiter, somehow elongated due to the panoramic projection. The Magellanic Clouds are visible on the left side, and a plane has left a visible trace on the right, along the Vista enclosure.

The Expanding Universe

25.4 The Universe

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

Red Shifts

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

Hubble’s Law--1929

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

Raisin Bread Dough Analogy

The Big Bang

25.4 The Universe

The big bang theory developed to explain/understand the expanding universe

States that at one time, the entire universe was confined to a dense, hot, supermassive ball

About 13.75 billion years ago, a violent expansion occurred, hurling this material in all directions

The Big Bang

25.4 The Universe

Supporting Evidence Red shift of galaxies supports the big bang and the

expanding universe theories.

Cosmic microwave background radiation Scientists think that this radiation was produced during the big bang.

Distribution of galaxies

25.4 The Universe

The Big Crunch or the Big Freeze?

• The future of the universe may follow two possible paths:

1. The universe will expand forever and cool.

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.