Objectives:1. Explain current theories of how galaxies form,

and change over time.2. Know the characteristics of the milky way galaxy.3. Compare and contrast different types of galaxies

including; barred, spiral, elptical, and irregular galaxies.

Galaxy Formation and Evolution

Galaxy Formation

• Galaxy formation is hypothesized to occur as a result of tiny quantum fluctuations in the aftermath of the Big Bang.

• They formed as a consequence of the growth of these primordial fluctuations, which are small changes in the density of the universe in a confined region.

Galaxy Formation

• The universe was very violent in its early epochs, and galaxies grew quickly, evolving by accretion of smaller mass galaxies.

• The result of this process is left imprinted on the distribution of galaxies in the nearby universe.

Formation of disk galaxies

• commonly called spiral galaxies.• they are very thin, rotate rapidly, and often

show spiral structure.

merger events in the evolution of galaxies

• Our own galaxy has a tiny satellite galaxy (the Sagittarius Dwarf Elliptical Galaxy) which is currently gradually being ripped up and "eaten" by the Milky Way.

• It is thought these kinds of events may be quite common in the evolution of large galaxies.

Galaxy mergers and the formation of elliptical galaxies

• The most massive galaxies in the sky are giant elliptical galaxies.

• All elliptical galaxies probed so far have super massive black holes in their center.

• In the Local Group, the Milky Way and M31 (the Andromeda Galaxy) are gravitationally bound, and currently approaching each other at high speed.

• Astronomers now see elliptical galaxies as some of the most evolved systems in the universe

The Milky Way Galaxy

• The Milky Way is a barred spiral galaxy 100,000–120,000 light-years in diameter containing 200–400 billion stars.

• The very center is marked by an intense radio source which is likely to be a super massive black hole.

• The rotational period is about 200 million years at the position of the Sun. The Galaxy as a whole is moving at a velocity of 552 to 630 km per second, depending on the relative frame of reference. It is estimated to be about 13.2 billion years old

The Milky Way Galaxy

• The rotational period is about 200 million years at the position of the Sun.

• The Galaxy as a whole is moving at a velocity of 552 to 630 km per second, depending on the relative frame of reference.

• It is estimated to be about 13.2 billion years old

Local Group

• The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way.

• The group comprises more than 54 galaxies, including dwarf galaxies.

Spiral galaxies

• Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge.

• These are surrounded by a much fainter halo of stars, many of which reside in globular clusters.

Barred Galaxies• A barred spiral galaxy is a spiral galaxy with a

central bar-shaped structure composed of stars.

• The bar is thought to act as a mechanism that channels gas inwards from the spiral arms, funneling the flow to create new stars.

dwarf galaxy

• A dwarf galaxy is a small galaxy composed of up to several billion stars.

Irregular galaxy

• An irregular galaxy is a galaxy that does not have a distinct regular shape.

• they are often chaotic in appearance.• Most irregular galaxies were once spiral or

elliptical galaxies but were deformed by disorders in gravitational pull

Elliptical Galaxies

• Appear spherical from all angles, due to the fact that they are not rotating.

• They are the oldest and most stable galaxies in the universe.