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American Museum of Natural History
What is a galaxy?
Galaxies are titanic swarms of tens of millions to trillions of stars, orbiting around their common center of gravity. They also contain interstellar gas and dust. Galaxies show a range of shapes that astronomers group into three basic classes: spiral, elliptical, and irregular.
Spiral galaxies have three visible parts: a thin disk composed of stars, gas, and dust; a central bulge of older stars; and a spherical halo of the oldest stars and massive star clusters.
Elliptical galaxies have smooth, rounded shapes because the orbits of their stars are oriented in all directions. They contain little gas and dust, and no young stars. Like spiral galaxies, elliptical galaxies are surrounded by globular star clusters and dark matter.
Irregular galaxies have a chaotic appearance, and are usually small. Their irregular shapes are probably due to recent disturbances — either bursts of internal star formation, or gravitational encounters with external galaxies.
The Milky Way
Our Milky Way is a barred spiral galaxy containing over 100 billion stars. The spiral disk of stars, gas, and dust is about 100,000 light-years across and 2,000 light-years thick — flatter than a pancake. The central bulge of stars is elongated in the shape of a bar.
The Sun orbits within the disk on the trailing edge of a minor spiral arm, about halfway between the galactic center and the visible edge. The Sun takes approximately 220 million years to circle the galaxy, and it has completed about 20 orbits since the solar system was born.
The Milky Way Galaxy resides in a neighborhood of a few dozen galaxies called the Local Group. They range in size from small dwarf galaxies to the large Andromeda Galaxy. Over time, these galaxies interact with one another, changing their motions and shapes. The long-term evolution of a galaxy is influenced by being part of a group. The Milky Way and the Andromeda Galaxy, our nearest spiral neighbor, are headed toward each other. In about five billion years, they may collide and merge. Eventually, our remote descendants could be living in a large elliptical galaxy.
Want to join the conversation?
- where the spirals come from?(12 votes)
- What is the most kind of common galaxy(5 votes)
- why dose it look like some sort of mold ☠️(4 votes)
- why does it look like a old oreo(4 votes)
- How are the spirals on the galaxy created?(3 votes)
- How will humans live when the galaxies collide?(2 votes)
- its likely that humans will not be around in ~5 billion years when Andromeda collides with the milky way galaxy.(2 votes)
- why does an elliptical galaxy look like one giant star?(3 votes)
- cuz why not
you can be anything you want
jerk(1 vote)
- how is the black hole or a red giant created?(2 votes)
- when the star gets old, it turns into a red giant. when it dies, it either turns into ruins or a black hole.(0 votes)
- can a galaxy hold 1,000,000 to 100,000,000 mi stars(3 votes)
- Yes, because galaxies are mostly made up of stars(1 vote)
- when did astronaughts find out about this(2 votes)
- In 1936, Edwin Hubble used pictures from his telescope to find a way to classify galaxies, grouping them into four main types: spiral galaxies, lenticular galaxies, elliptical galaxies, and irregular galaxies.(0 votes)