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5th century BC – Democritus proposes that the bright band in the night sky known as the Milky Way might consist of stars. 4th century BC – Aristotle believes the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which ...
A disc galaxy (or disk galaxy) is a galaxy characterized by a galactic disc. This is a flattened circular volume of stars that are mainly orbiting the galactic core in the same plane. [ 1 ] These galaxies may or may not include a central non-disc-like region (a galactic bulge ). [ 2 ]
The thin disk of our galaxy began to form when the universe was about 5 billion years old or 9 ± 2 Gya. [15] The Solar System formed at about 9.2 billion years (4.6 Gya), [ 5 ] : 22.2.3 with the earliest evidence of life on Earth emerging by about 10 billion years (3.8 Gya).
The Sculptor Galaxy (NGC 253) is an example of a disc galaxy. A galactic disc (or galactic disk) is a component of disc galaxies, such as spiral galaxies like the Milky Way and lenticular galaxies. Galactic discs consist of a stellar component (composed of most of the galaxy's stars) and a gaseous component (mostly composed of cool gas and dust).
If the template has a separate documentation page (usually called "Template:template name/doc"), add [[Category:Graphical timeline templates]] to the <includeonly> section at the bottom of that page.
The reason is that these galaxy formation models predict a large number of mergers. If disk galaxies merge with another galaxy of comparable mass (at least 15 percent of its mass) the merger will likely destroy, or at a minimum greatly disrupt the disk, and the resulting galaxy is not expected to be a disk galaxy (see next section).
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Image of a dead disk galaxy. A dead disk galaxy is a type of galaxy that is no longer able to form new stars. These galaxies are usually massive yet compact, fast spinning disk shaped galaxies. Most of these galaxies stopped forming new stars billions of years ago when the universe was relatively young in age. [1]