1 Galaxies The Andromeda Galaxy - nearest galaxy similar to our own. Only 2 million light years away! Galaxies are clouds of millions to hundreds of billions of stars held together by their mutual gravity. Often galaxies also contain enormous clouds of gas and dust from which new stars can form. Galaxies can have many different shapes and sizes. The distribution of galaxies across the Universe indicates that they generally appear in clusters with very large voids separating clusters from one another.

2 Types of Galaxies Galaxies are classified based on general characteristics and then further subdivided based on more specific characteristics. Spiral galaxies –disk-shaped with spiral arms winding out from the center. –usually have clouds of gas and dust –usually have both young (Pop. I) and old (Pop. II) stars –some spirals have a rectangular- shaped bar through the central bulge and are referred to as Barred Spirals A Hubble Space Telescope view of the Whirlpool Galaxy M51

3 Types of Galaxies Elliptical galaxies are always smooth in appearance. –can be spherical, egg-shaped or flattened in shape –usually have little or no gas and dust –usually only contain old stars Irregular galaxies have a random distribution of stars. –often have large clouds of gas and dust –often contain young stars

4 Galaxy Collisions A Hubble Space Telescope image of the Antennae Galaxies. Large streams of stars and gas are trailing off the galaxies while new stars are being formed near the center. collisions between galaxies appear to have been common in the early Universe collisions can cause bursts of star formation as clouds of gas and dust collapse galaxies may eventually merge together forming large elliptical galaxies

5 Galaxy Clusters Galaxies do not uniformly fill the universe but rather collect into clusters and superclusters spanning millions of light years. The Milky Way is a member of a cluster called the Local Group.

6 Galaxy Clusters and Dark Matter Astronomers find that galaxies often are moving in these clusters with very high speeds. The clusters should be flying apart but there appears to be enough mass to hold the cluster together. This is more mass than can be accounted for just by gas, dust, and stars. This invisible mass holding the cluster together may be several times larger than all the visible mass and is another example of “dark matter”.

7 Gravitational Lenses Light can be bent as it passes close to massive objects. This is what happens in black holes and also happens in galaxy clusters. Light from very distant galaxies can be bent around nearby galaxy clusters and appear as arcs or mirror images of the original galaxy. Astronomers use these gravitational lenses to study galaxies that may have been too far to observe any other way. Hubble Space Telescope image of a gravitational lens formed by a galaxy cluster

8 Distances to the Galaxies Determining accurate distances to galaxies requires knowledge of the properties of stars. From the luminosity of a star and its apparent brightness the star’s distance can be found. Certain stars (called Cepheid variables) show regular patterns of variation in brightness. The period of these variations are directly related to the stars luminosity. So by measuring the time it takes these stars to vary in brightness and their apparent brightness their distance can be found. Cepheid stars are very bright and can be observed in nearby galaxies. Other methods must be used for more distant galaxies.

9 The Expansion of the Universe Astronomers in the early 20th century found that Doppler shifts seen in the spectra of galaxies indicated that almost all galaxies are moving rapidly away from us. Edwin Hubble, using careful determinations of galactic distances, showed that the farther a galaxy is from us the faster it appears to be moving away. He showed that the velocity of the galaxy was simply equal to its distance times a constant. This is known as Hubble’s Law and the constant is Hubble’s constant. The exact value for Hubble’s constant has been one of the great problems in astronomy. It was one of the reasons for building the Hubble Space Telescope. H o = 71 +/- 4 km/sec/Mpc

10 Some galaxies have been found to emit huge amounts of energy and sometimes also jets of fast moving material from very small regions in their nuclei. These are some of the brightest objects in the Universe and because of their brightness are the most distant objects observable. Their small size and rapid variations in brightness tell us about the source of this emission. A supermassive black hole is thought to be the source of energy for these AGNs. Active Galactic Nuclei (AGN)