Types of Galaxies

Types of Galaxies - be a regular spiral where the arms come...

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Types of Galaxies Edwin Hubble divided the galaxies into three basic groups: ellipticals, spirals, and irregulars. The ellipticals are smooth and round or elliptical, the spirals are flat with a spiral pattern in their disk, and the irregulars have stars and gas in random patches. Most galaxies are small and faint so only the luminous galaxies are seen at great distances. These spectacular galaxies tend to be either the elliptical or spiral type, so they are the ones often displayed in astronomy textbooks. In 1936 Hubble put these groups onto a two-pronged sequence that looks like a tuning fork because he thought that the galaxies started out as ellipticals, then changed to spirals and then to irregulars. In this scenario, a galaxy could take one of two paths. It could take the top prong and
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Unformatted text preview: be a regular spiral where the arms come right out of the galaxy center, or it could take the bottom prong and be a barred spiral with the arms starting from the ends of a bar of gas and stars going through the center. The ellipticals are sub-divided by how round they are and the spirals are sub-divided by how loose their arms are and how big their nucleus is. Astronomers now know that it is NOT an evolutionary sequence because each type of galaxy has very old stars. The oldest stars in any galaxy all have about the same age of around 13 billion years. This means that spirals form as spirals, ellipticals form as ellipticals, and irregulars form as irregulars. However, the ``tuning fork'' diagram is still used to classify galaxies because of its convenience....
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