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What is a galaxy?

A Hubble Space Telescope image of the Sombrero galaxy, M104 (credit: NASA)

A Hubble Space Telescope image of the Sombrero galaxy, M104 (credit: NASA)

Galaxies are huge collections of stars, dust and gas. They usually contain several million to over a trillion stars and can range in size from a few thousand to several hundred thousand light-years across.

There are hundreds of billions of galaxies in the Universe.

Galaxies come in many different sizes, shapes and brightnesses and, like stars, are found alone, in pairs, or in larger groups called clusters.

Galaxies are divided into three basic types: spirals, ellipticals and irregulars.

– CoolCosmos, NASA,  IPAC (Infrared Processing and Analysis Center)

What types of galaxies are there?


Where is our solar system, within our galaxy?

When we look up at night, what part of our galaxy do we see? Why does it appear that way?
(to be added)

Since our solar system lies in one of the spiral arms, we live in the flat plane of the Milky Way. We can actually see the dense plane of the Milky Way stretch across the sky in dark places that do not have a lot of surrounding light pollution. The diffuse light is the combined light from millions of stars. Some of the light from these stars are obscured by large clouds of dust, which is why there are dark patches. Dust and gas are necessary to form stars, and most stars are formed within the spiral arms. Note that we can’t really see the center of the galaxy with our eyes because there is dust in the way!

– http://www.astro.ucla.edu/~ghezgroup/gc/journey/journey_intro.html

Interstellar travel

How long would it take a spaceship to get from one star to another? To get to the center of our galaxy? (to be added)

Galaxies moving through the universe

Gravitational repulsion and the Dipole Repeller

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