THE UNIVERSE What would we see if we were to look deep into a very black part of the sky at night?

In 1995 a few astronomers wondered this very same question! They wanted to point Earth’s most powerful telescope at an EMPTY part of space. The Hubble space telescope is very expensive to operate, and has many projects waiting so this was a risky idea! The area is equal to about 1 twenty four millionth of the whole sky! 1/24,000,000

…And what did they see?

There are hundreds of billions of galaxies in the Universe Each containing hundreds of billions of stars. There are several basic types of galaxies among the hundreds of billions of galaxies out there

Galaxies 100 Billion Stars! Galaxy - group of stars, dust & gases held together by gravity. Evidence strongly suggests that all galaxies have a supermassive black hole in their center, and studies of their formation continue today! 100 Billion Stars!

Types of Galaxies Spiral Galaxies -About 30% (10% barred spirals, 20% regular spirals) •disk-shaped, greater conc. of stars near their centers - They have large diameters of 20,000 to 125,000 light years, and contain both young and old stars • Milky Way: It is a large, spiral galaxy - disk about 100,000 ly wide &10,000 ly thick at nucleus (we are in an arm). .

Elliptical Galaxies • About 60% -Range from round to oval, does not have spiral arms -Contain Mostly older Stars (very largest known galaxies are elliptical)

 Irregular Galaxies Only 10% of known galaxies Irregular Galaxies contain young stars In addition to shape and size, 1 MAJOR difference among different types of galaxies is the AGE of their stars.

Galaxy Clusters (like stars, galaxies are found in groups) • System of galaxies - contain several to thousands of member galaxies -our own cluster (the Local Group) contains at least 28 galaxies: 3 spiral, 11 irregular, 14 elliptical. -In the local group only 1 galaxy is bigger than ours. Andromeda

Our Place in the Universe