Copyright © Michael Richmond.
This work is licensed under a Creative Commons License.
Random cool astronomy news item: Impact of lunar meteorite captured on video
We'll begin by discussing galaxies in a general way today, focusing on the several systems astronomers use to classify galaxies. I'll mention some of the most common terms used to describe galaxies. The second half of the class will be used to start an activity in which you will classify some galaxies yourself; we'll complete the activity later.
Galaxies are complex structures, consisting as they do of billions of individual stars, giant clouds of gas and dust and pervasive yet invisible amounts of dark matter. It should be no surprise that different astronomers have chosen to classify them in different ways.
In the image below, the central galaxy NGC 5982 is an elliptical, flanked above (NGC 5981) and below (NGC 5985) by spirals.
Image by Giovanni Benintende ,
thanks to
Astronomy Picture of the Day.
In his book The Realm of the Nebulae, Edwin Hubble organized galaxies into a sequence which splits spirals into normal and barred families, yielding the familiar "tuning fork diagram".
Image courtesy of
The Las Cumbres Observatory
The criteria used to place galaxies on this diagram are
Ellipticals are given a designation of the letter "E" plus a digit, where the digit represents the ellipticity of the galaxy's outer isophote, multiplied by 10. In the equation below, a is the diameter of the galaxy's major axis, and b the diameter of the minor axis.
Image of M87 courtesy of (and modified from)
Canada-France-Hawaii Telescope, J.-C. Cuillandre (CFHT), Coelum
The designations given to spiral galaxies are less easy to quantify; they rely on visual inspection of the galaxy and a long experience of classifying galaxies. The general idea is to assign a letter between a and d to each spiral galaxy, with Sa meaning "large bulge, tightly wound continuous arms"
Image of M81 courtesy of
NASA, ESA, and The Hubble Heritage Team (STScI /AURA)
and "Sc" or "Sd" meaning "small bulge, loosely wound flocculent arms".
Image of NGC 7793 courtesy of
ESO
Perhaps the best explanation of the procedure can be found in The Hubble Atlas of Galaxies (see pages 7 - 26).
The simple schematic "tuning fork" diagram shown above suggests that the difference between, say, an SBb and SBc galaxy should be easy to see. In real life, galaxies tend to be messy, as the diagram below indicates; it shows real images of galaxies.
Image courtesy of
Los Cumbres Observatory.
In 1981, the Second Reference Catalog of Bright Galaxies (RC2) by deVaucouleurs et al. provided a means to put all galaxies onto a semi-quantitative scale. The "Hubble Stage" parameter T is an integer value ranging from -5 (for ellipticals) to +10 (for irregulars).
when applied to stars
Taken from
The Observatory, vol 38, p 379 (1915).
Some scientists believed that this same sequence, from hot to cold, described the evolution of individual stars over time. In this way, the term "early" came to be applied to the hot O and B stars, and "late" to the cool K and M stars.
By the nineteen-twenties, this view had fallen out of favor, due to increases in the understanding of stellar structure. Nonetheless, the words "early" and "late" continued to be applied to stars following the earlier convention. This annoyed some people
but failed to sway the IAU, as this correspondence shows.
when applied to galaxies
As a result, the terms "early" and "late" have stuck to galaxies, with the sense that "early-type" galaxies are centrally concentrated and redder, while "late-type" galaxies are more diffuse and bluer.
Note the internal inconsistency: early-type galaxies are dominated by late-type stars, and late-type galaxies by early-type stars. Sigh.
One choice for a numerical measure of concentration is based on ratios of the radii which contain specific fractions of the total light of a galaxy. Let us define
Now, these values will certainly change if we move a particular galaxy closer to or farther from the Earth; but the ratios of these quantities might not. One can use the concentration indices
to describe the degree to which light in a galaxy is collected near the center. Large ratios means that the light is highly concentrated, while small ratios mean that the light is diffuse.
Early work showed hints that high concentrations were found among elliptical galaxies, and lower concentrations among spirals, but the evidence wasn't all that persuasive. This figure comes from an article in 1973.
More recent work (published in 2001) based on digital images shows that the correlation between the concentration of light and morphology really is pretty good. Alas, this particular paper -- and much of the recent work -- adopts a new definition of "concentration index" which is roughly the inverse of the previous one, so in the figure below, small values of C mean the light is highly concentrated.
Figure 10 from
Shimasaku et al., AJ 122, 1238 (2001)
You've seen the examples, so why not give it a try yourself? I've set up a "virtual classroom" in the Galaxy Zoo project for use in this class. Please
Today in class, we'll go through just one section of this exercise; we'll save the rest for later. Your job in class is to classify images of 22 galaxies, and to record your results for later use.
When you're ready to begin,
and then work your way through the set of 22 galaxies. For each one, please
When you are finished,
Copyright © Michael Richmond.
This work is licensed under a Creative Commons License.
