Galaxies

Milky Way • A galaxy is a large, isolated system of stars

held together by gravitational forces.• Our Solar System is located in the Milky Way

Galaxy.

Structure of the Milky Way

• The Milky Way has a disk-like shape with a central bulge.

Structure of the Milky Way

• The stars, gas, and dust of our galaxy orbit the nucleus, or galactic center, which is a region of very high star density. It is surrounded by a nuclear bulge which sticks out of our galactic disk. Around the nuclear bulge is a spherical region of star clusters called the halo.

Structure of the Milky Way

• The Milky Way has spiral arms extending outward. We are located in a minor arm named Orion.

Structure of the Milky Way

• The halo and bulge are made up of old stars, while the spiral arms and disk contain younger stars and stars that are forming.

Formation of the Milky Way

• Astronomers believe that the galaxy began as a spherical cloud of gas and that the first stars formed when this cloud was still round.

Formation of the Milky Way

• The cloud eventually collapsed under its own gravity and rotation formed it into a disk-like shape. Younger stars are contained in this region.

Galaxies

• Galaxy: a large, isolated system of stars held together by gravitational forces

Galaxies

• There are billions of galaxies in the universe.

Galaxies

• Edwin Hubble classified these galaxies based on their shape. They can be spiral, elliptical, or irregular.

Galaxies

• Most galaxies are in groups. The Milky Way is part of a cluster of galaxies known as the Local Group. There are about 35 known galaxies in this group.

Galaxies

• Clusters of galaxies are organized into even larger groups known as superclusters.

Galaxies

Universe• Based on measurements, we know that all

galaxies are moving away from Earth. In 1929, Hubble discovered that the farther away a galaxy is, the faster it is moving away. The Universe is expanding.

Universe

• By measuring how fast a galaxy is moving away, we can determine the distance to that galaxy.

Universe

• Because it takes light a long time to reach the Earth from distant objects, when we look at objects in the sky we are looking back in time and seeing how they were in the past.

Universe

• Example: It takes light from the Sun 8 minutes to reach the Earth, so we are seeing the Sun as it was eight minutes ago. We are seeing the Andromeda galaxy as it was 2 million years ago!

Universe

• Cosmology: the study of the universe, its current nature, its origin, and evolution

Origin of the Universe

• Big Bang Theory: the universe began as a highly compressed, hot, dense point that has been expanding ever since

Origin of the Universe• The universe began by expanding quickly,

causing it to cool and allowing matter to form. The lighter elements formed first and these clouds of gas began to condense into stars.

Origin of the Universe

• The outward expansion of the universe is slowed by the inward force of gravity from matter in the universe, so that the expansion of the universe was slowed for a while. However, the most recent observations show that the expansion is now speeding up again due to some unknown force that is dominating over gravity.

Origin of the Universe

• The age of the Universe is hypothesized to be about 13.5 billion years.