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Galaxy Classification: Hubble's System and Evolution, Schemes and Mind Maps of Astrophysics

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

  • What physical differences exist between elliptical and spiral galaxies?
  • How do astronomers classify elliptical galaxies based on their shape?
  • What are the three main categories of galaxies according to Hubble's system?

Typology: Schemes and Mind Maps

2021/2022

Uploaded on 09/27/2022

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GALAXY SORTING
by Sally Stephens
Background Information on Galaxy Classification
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 s ystem to classify them that is still used today, albeit with
some modifications and additions.
Hubble's sys tem divides galaxies into three basic categories: 1) elliptical galaxies, relatively
featureless spherical or ellipsoidal (football-shaped) collections of stars; 2) s piral 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
7. An E5 galaxy, for example, is not very spherical, appearing twice as long as it is wide.
Spiral galaxies are divided into two main type s: regular spirals, denoted by the letter S, in
which the arms spiral outward from the galaxy center, and barred spirals, marked SB, in which
the arms wind outward from the ends of a straight "bar" of stars that passes through the center.
About two-thirds of all spiral galaxies have some kind of bar. Indeed, astronomers have recently
seen evidence of a bar in our own Galaxy's center, making the Milky Way a barred spiral. Both
regular and barred spiral galaxies are defined by a spherical bulge of stars at their center, which is
surrounded by a thin, rotating disk of stars that contains the spiral arms. You can tell the
direction in which the disk rotates by looking at the spiral arms; they trail behind the direction of
rotation, much like the coattails of a runner or water thrown out by a twirling lawn sprinkler.
In addition, both regular and barred spirals are subdivided according to how tightly wound
the spiral arms are and how prominent the central bulge of stars appears. In Sa galaxies, the spiral
arms are tightly wound and the central bulge appears bright, whereas in Sc galaxies, the arms are
more loosely wound and the central bulge is much less prominent. Sb galaxies fall in between the
two. Similar criteria apply to barred spirals, which are denoted by SBa, SBb and SBc. Irregular
galaxies have no subdivisions.
There is also a class Hubble called S0 galaxies, which have characteristics of both
ellipticals and spirals, showing a large central bulge and a disk, but no obvious dust lanes or spiral
structure.
Regardless of the ty pe, galaxies come in all different sizes. It's impossible to say how
large a galaxy is based solely on its photograph. A ver y large galaxy that is also very distant
might look like it is the same size as a much smaller one that's nearby. You have to know its
distance to know a galaxy's true size. Our own Milky Way Galaxy is roughly 100,000 light years
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GALAXY SORTING

by Sally Stephens

Background Information on Galaxy Classification

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

  1. An E5 galaxy, for example, is not very spherical, appearing twice as long as it is wide. Spiral galaxies are divided into two main types: regular spirals, denoted by the letter S, in which the arms spiral outward from the galaxy center, and barred spirals, marked SB, in which the arms wind outward from the ends of a straight "bar" of stars that passes through the center. About two-thirds of all spiral galaxies have some kind of bar. Indeed, astronomers have recently seen evidence of a bar in our own Galaxy's center, making the Milky Way a barred spiral. Both regular and barred spiral galaxies are defined by a spherical bulge of stars at their center, which is surrounded by a thin, rotating disk of stars that contains the spiral arms. You can tell the direction in which the disk rotates by looking at the spiral arms; they trail behind the direction of rotation, much like the coattails of a runner or water thrown out by a twirling lawn sprinkler. In addition, both regular and barred spirals are subdivided according to how tightly wound the spiral arms are and how prominent the central bulge of stars appears. In Sa galaxies, the spiral arms are tightly wound and the central bulge appears bright, whereas in Sc galaxies, the arms are more loosely wound and the central bulge is much less prominent. Sb galaxies fall in between the two. Similar criteria apply to barred spirals, which are denoted by SBa, SBb and SBc. Irregular galaxies have no subdivisions. There is also a class Hubble called S0 galaxies, which have characteristics of both ellipticals and spirals, showing a large central bulge and a disk, but no obvious dust lanes or spiral structure. Regardless of the type, galaxies come in all different sizes. It's impossible to say how large a galaxy is based solely on its photograph. A very large galaxy that is also very distant might look like it is the same size as a much smaller one that's nearby. You have to know its distance to know a galaxy's true size. Our own Milky Way Galaxy is roughly 100,000 light years

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