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.
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