Milky Way and other Galaxies

Galaxy- a collection of stars, dust, and gas bound together by gravity -Galaxy contain millions or billions of stars -There are more than 100 billion galaxies -Galaxy contain young stars( found near nebular gas) and older stars( in regions that contain no gas and dust) -Gravity holds galaxies together in clusters Cluster- a group of stars or galaxies bound by gravity

Edwin Hubble used the telescopes to explore galaxies beyond the Milky Way galaxy The Hubble Space Telescope, now probes the depths of the universe from its orbit above Earth

Types of galaxies Spiral Galaxy (side view) Spiral Galaxy (top view) Elliptical galaxy (end view) Elliptical Galaxy (side or top view)

Interstellar matter- gas and dust between the stars Edwin Hubble divided all galaxies into three major types: spiral; elliptical, and irregular We live in Milky Way Galaxy, what is a spiral galaxy, the nucleus is very dense and has many old stars, the arms have both old and new stars Interstellar matter- gas and dust between the stars Irregular Galaxy

Elliptical galaxies have no spiral arms, they contain mostly older stars and have little interstellar matter All other galaxies are irregular galaxies, they don’t have a well-defined structure Milky Way Galaxy, can be seen as a faint, narrow band of light and dark patches across the sky, this band consist in stars, gas and dust in our galaxy

Quasar- quasi-stellar radio sources; very luminous objects that produce energy at a high rate, ant that are thought to be the most distant objects in the Universe Each quasar has a huge center black hole and a large disk of gas and dust around it They can be the central parts of distant galaxies

Origin of the Universe Universe- the sum of all space, matter, and energy that exist, that have existed in the past, and that will exist in the future Most of the Universe is empty space, space is a vacuum with no aer and no air pressure. The Big-Bang theory states that the Universe began with a gigantic explosion 10 billion to 20 billion years ago, the explosion released all of the matter and energy that still exist today

Big-Bang theory- theory that all matter and energy in the universe was compressed into an extremely small volume that 10 to 20 billion years ago exploded and begun expanding in all directions The future of the universe is uncertain: will keep expanding forever, or expansion of the universe will gradually slow down and the universe-will approach a limit size, or the universe will stop expanding and start to fall back in on itself

The fate of the Universe depends on mass: -if is not enough mass, the universe will keep expanding forever if there is the right amount of mass, the expansion will continually slow down but never stops -if is more mass than this right amount, gravity will overcome the expansion and the universe will start to contract

New technologies helps scientist test theories, by making observations to see whether the theories make accurate predictions Astronomers estimate the mass of the universe by measuring , stars, galaxies, and matter in the interstellar medium. There is more matter than what is visible, is called dark matter, may be planets, black holes or brown dwarfs 90% of Universe may be composed of dark matter