Universe Organized via Galaxies
Aug 26, 2019 13:10:31 GMT -5
Post by Radrook Admin on Aug 26, 2019 13:10:31 GMT -5
It was once thought that Earth was the center of the universe because the sun and stars appeared to be revolving around it. It was called the geocentric model of the universe. Then it was proposed that the Sun was the center of the uiniverse-the heliocentric model. Then we finally realized that the Earth is just a planet orbiting our Sun and that the sun itself is merely one among millions of stars that make up the Milkey Way Galaxy. .
For a while we assumed that our Milky Way was all there was. Then we finally realized that our Milkey way is merely one among trillions of other galaxies of diverse shapes, such as eliptical, pinwheel, irregular, and dwarf sattelite galaxies as the Major and Minor Magelanic clouds.
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Definition and Discovery
A Galaxy is a system of stars and Planetary systems, dust, and gas held together by gravity. We live in the Milky Way Galaxy, a Spiral galaxy. Tuningfork. The word Galaxy comes from the Greek word for milk. Galileo looked up at the milky streak that divides the night sky and proposed that it might be the aggregate light from millions of stars, very far away.
It wasn't until the early 20th century when people like Vesto Slipher and Edwin Hubble looked closely at "Sky Survey" photographic plates and Spectroscopic data and realized that the "spiral nebulae" were actually galaxies. There must be at least 100 million galaxies in the universe.
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Shapes of the galaxies
Spiral
Spiral galaxies have a nucleus and have arms that spiral away from it
Largish - their diameters range in size from 20,000 light years to 100,000 light years
Examples - M31 and Milky Way
Most consist of at least one billion stars
Barred Spiral
A barred galaxy is a type of galaxy where the spiraling arms start at the ends of a bar that goes through the center of the galaxy.
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Elliptical
Round galaxies without spiral arms
Resemble the Nucleus of spiral galaxies
.Sometimes very small and densely packed... sometimes exceptionally huge
Can be seen from great distances... in other words, some are very old
Irregular
Not spiral or elliptical
Irregular in shape (also asymmetrical)
Various reasons for irregular shape:
Collisions with other galaxies
Gravitational effects of close galaxies
Recently merged with smaller globular cluster
Lenticular
Seen edge-on from our point of view, it is like looking at a cross-section of a lens.
They may be Spiral, Barred Spiral, or a Hoag Object (Elliptical with a surrounding ring). We generally cannot tell.
astronomy.wikia.org/wiki/Galaxy?file=Sombrero-IR.jpg
A Galaxy is a system of stars and Planetary systems, dust, and gas held together by gravity. We live in the Milky Way Galaxy, a Spiral galaxy. Tuningfork. The word Galaxy comes from the Greek word for milk. Galileo looked up at the milky streak that divides the night sky and proposed that it might be the aggregate light from millions of stars, very far away.
It wasn't until the early 20th century when people like Vesto Slipher and Edwin Hubble looked closely at "Sky Survey" photographic plates and Spectroscopic data and realized that the "spiral nebulae" were actually galaxies. There must be at least 100 million galaxies in the universe.
---------------------------------------------------------------
Shapes of the galaxies
Spiral
Spiral galaxies have a nucleus and have arms that spiral away from it
Largish - their diameters range in size from 20,000 light years to 100,000 light years
Examples - M31 and Milky Way
Most consist of at least one billion stars
Barred Spiral
A barred galaxy is a type of galaxy where the spiraling arms start at the ends of a bar that goes through the center of the galaxy.
=
Elliptical
Round galaxies without spiral arms
Resemble the Nucleus of spiral galaxies
.Sometimes very small and densely packed... sometimes exceptionally huge
Can be seen from great distances... in other words, some are very old
Irregular
Not spiral or elliptical
Irregular in shape (also asymmetrical)
Various reasons for irregular shape:
Collisions with other galaxies
Gravitational effects of close galaxies
Recently merged with smaller globular cluster
Lenticular
Seen edge-on from our point of view, it is like looking at a cross-section of a lens.
They may be Spiral, Barred Spiral, or a Hoag Object (Elliptical with a surrounding ring). We generally cannot tell.
astronomy.wikia.org/wiki/Galaxy?file=Sombrero-IR.jpg
We also noticed that such galaxies are not jumbled together in a disorganized fashion but that they are arranged in gravitationally fixed clusters and super-clusters separated by great voids where universal expansion is taking place and distancing such super-clusters from one another.