

Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Community
Ask the community for help and clear up your study doubts
Discover the best universities in your country according to Docsity users
Free resources
Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors
An overview of galaxy classification, focusing on edwin hubble's system and its refinements. Learn about elliptical, spiral, and irregular galaxies, their subdivisions, and the physical differences between them. Discover how interactions, collisions, and mergers influence galaxy appearance and evolution.
What you will learn
Typology: Schemes and Mind Maps
1 / 3
This page cannot be seen from the preview
Don't miss anything!
by Sally Stephens
In 1924, astronomer Edwin Hubble proved that galaxies are very distant "island universes" -- each one a collection of millions or billions of stars bound together by gravity. Within a few years, he had set up a system to classify them that is still used today, albeit with some modifications and additions. Hubble's system divides galaxies into three basic categories: 1) elliptical galaxies, relatively featureless spherical or ellipsoidal (football-shaped) collections of stars; 2) spiral galaxies, with their distinctive arms of stars that spiral out from their centers; and 3) irregular galaxies, a catch- all category for galaxies that don't look like either traditional ellipticals or spirals. Elliptical galaxies, marked by the letter E, are further subdivided depending on how well- rounded they appear. A number is added that ranges from 0 to 7, with completely round ellipticals denoted by 0, and flattened systems (which look like a squashed football) denoted by
across; its disk is only 1000 light years thick, however. (A light year is the distance light travels in one year, that is, 9.5 x 10^12 kilometers.) The Milky Way is a large spiral galaxy, but there are galaxies that are ten times its size, and many that are thousands of times smaller. In fact, astronomers now think that tiny, faint galaxies, called dwarf galaxies, may be the most plentiful galaxies in the universe. Indeed, there are probably so many dwarf galaxies that their combined mass probably exceeds that of all the larger galaxies taken together. But their small size and dimness make them hard to detect and we have only been able to discover them when they are relatively nearby. Hubble based his classification scheme solely on what galaxies look like. His scheme is still used today because it turns out there are significant physical differences between the different types of galaxies, differences that were not known when Hubble first classified them. Elliptical galaxies contain mostly old stars, with very little gas and dust found between stars. Since new stars form from clouds of interstellar gas and dust, elliptical galaxies lack the raw ingredients to make new stars. Spiral galaxies, on the other hand, have a mix of young and old stars. Interstellar gas and dust fill the disks of spiral galaxies, and new star formation continues to take place in their disks. Irregular galaxies appear chaotic, and often have many bright, young stars, the result of recent bursts of intense star formation. For many years, astronomers thought the dissimilarities between galaxy types reflected different conditions present in each when they originally formed. Put another way, this view held that galaxies look like they do because they were “born that way.” In this view, the stars in elliptical galaxies formed very quickly, using up all the interstellar gas and dust before the material had time to settle into a disk. Star formation in spiral galaxies, on the other hand, took place slowly over the galaxy's lifetime, continuing after the interstellar gas and dust had settled into a disk. One problem with this view has been trying to figure out why star formation would occur rapidly in ellipticals, but much slower in spirals. Over the past few decades, however, astronomers have learned that galaxies can change their appearance over time, usually as a result of interactions, collisions, or mergers between galaxies. Interactions between galaxies are common because, relatively speaking, galaxies (especially those found in rich groups) are closer to each other than typical stars are. The distance between the Milky Way and its closest large neighbor (the Andromeda Galaxy) is only about 25 times the diameter of the Milky Way. (And our Galaxy has several smaller neighbor galaxies that are significantly closer than that.) By contrast, the distance between the Sun and its nearest neighboring star is about 30 million times greater than the Sun's diameter. Galaxy interactions can turn one type of galaxy into another. Two or more spiral galaxies, for example, can collide and merge, turning into a giant elliptical galaxy. Mergers and collisions often stimulate intense bursts of star formation in the affected galaxies. As a result, many irregular galaxies are now also thought to be the result of galactic interactions or collisions. Although collisions alter the overall appearance of galaxies, they rarely bother the stars themselves, other than changing their galactic orbits. There is so much empty space between the stars that the stars of two colliding galaxies can pass among each other, like ships on the dark