Normal galaxies are, well, normal. They don’t have any of the extremely energetic things going on that Active Galaxies do. These are just your run of the mill galaxies.
Galaxies are large systems of stars and gas (and dark matter) that lie outside the Milky Way (the galaxy in which we reside). A typical large galaxy, like the Milky Way, is about 100 thousand light years across and contains 100 billion stars. Some galaxies are much smaller, less than 10% the size of the Milky Way, while others are more than ten times bigger. The nearest large galaxy to ours is the Andromeda galaxy (M31) which lies about 2.5 million light years away. It is part of our Local Group of galaxies, several dozen mostly small galaxies, along with Andromeda and the Milky Way, that comprises our local neighborhood. Most galaxies lie much farther away than Andromeda, and far beyond the bounds of the Local Group. The brighter ones in our night sky, which can be found in the Messier and New General catalogs, are typically tens of millions to hundreds of millions of light years distant. Much fainter galaxies lie beyond, stretching all the way back to the edge of the observable universe, with lookback times more than 12 billion years in the past. Use the links at left to explore some of the properties of galaxies and the observational characteristics we use to try to understand them.
The true nature of the galaxies was first convincingly demonstrated by Edwin Hubble when he showed that they are far outside the Milky Way. On the basis of photographic images Hubble suggested there are three major categories of galaxies. An example of each is shown below.
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elliptical (E)
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spiral (S)
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irregular (I)
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 M87 |
 M51 |
 Small Magellanic Cloud |
| All Images NOAO/AURA/NSF | ||
Hubble expanded this preliminary classification system into what has now become known as the “tuning fork” diagram.
Hubble Tuning Fork Diagram, Courtesy Space Telescope Science Institute.
In this picture the ellipticals are distributed along the “handle” of the tuning fork, with decreasing ellipticity to the left. The spirals are distributed along two sequences. The splitting was necessary because of the presence of two apparently similar sequences of spirals. One contains the “normal” spirals, in which the spiral arms appear to come from a central galactic bulge. On the other, the “barred” spirals have spiral arms that appear to come from the ends of a linear bar-like structure that passes through the bulge.
The spirals can be placed in a sequence along which the arms become more open and the bulge becomes less prominent. This is indicated by the designations Sa through Sc (and SBa to SBc for barred spirals). Hubble thought this ordering might have been an evolutionary sequence, with objects evolving from tightly wound to a more open state. Thus, the Sa galaxies are still referred to as “early” galaxies while the Sc galaxies are referred to as “late” galaxies.
Hubble expected there should be an S0 class which formed an evolutionary bridge between the ellipticals and the spirals. Though he never detected such an object, the S0’s were added later by Hubble’s student, Allan Sandage. Sandage greatly expanded Hubble’s original system. Additional enhancements were also suggested by Gerard de Vaucouleurs, who added the class Sd as a class later than the Scs. An Sd galaxy has extremely open arms and a negligible or even non-existent nucleus.
We now know that the tuning fork diagram bears no relation to the evolution of galaxies, at least, not in the sense that galaxies evolve from “early” to “late” as envisioned by Hubble. However, we do see collisions in which two spirals can merge to form an elliptical, counter to the evolutionary development assumed by Hubble. This is only one way to form an elliptical though, and many galaxies likely form as ellipticals from the very start. Also, in the early universe we see that large ellipticals and spirals seem to be forming from small irregular galaxies; there is even strong evidence that this process continues to the modern day.
NGC 2787, a lenticular galaxy. Image: NASA/ESA Hubble Space Telescope
Spirals like the Milky Way and Andromeda have cannibalized small galaxies in the past, and they continue to do so (see the image of M51 at the top of this page). The Milky Way is currently in the process of devouring the Large Magellanic Cloud, one of its small companion galaxies. The well-known “globular cluster” Omega Centauri seems in fact to be the remnant nucleus of a small elliptical galaxy that has been consumed by the Milky Way. Furthermore, in several billion years observational evidence suggests that the Milky Way and Andromeda will collide and merge, eventually settling down to form a large elliptical galaxy. The image at right shows two galaxies engaged in a similar encounter.
Evidence now suggests that the process of galaxy evolution is one of titanic mergers and cannibalism, in striking contrast to the slow migration envisioned by Edwin Hubble early in the Twentieth Century. So even though his tuning fork diagram is not at all related to the evolution of galaxies, the classification scheme originated by Hubble is still a useful way to describe the current appearance of a galaxy, if not where it came from or where it might be headed.
Image: Hubble Heritage Site, NASA/ESA Hubble Space Telescope
| Luminosity | Luminosities or absolute magnitudes may be determined for a galaxy by measuring an apparent magnitude and determining the distance. Magnitude determinations are difficult for galaxies since it is hard to define the precise location of the “edge” for a nebulous object. In addition, corrections must be applied because of the attenuation by dust in our own galaxy, the attenuation due to dust in the external galaxy, the orientation of the galaxy in space (edge-on or face-on), and the K-correction due to the redshift of the galaxy.
The following results are believed to be typical absolute magnitudes.
Notice that the absolute magnitude for a dE galaxy is only one magnitude brighter than the absolute magnitude for the brightest single supergiant stars in our own galaxy and in the LMC. Just for reference, the absolute magnitude for the sun (a G2 main sequence star) is about +5, while the absolute magnitude for a star like Sirius (an A0 main sequence star) is about +1. A typical red giant star might have an absolute magnitude of about 0 (zero). |
| We customarily classify galaxies and attempt to describe their morphological characteristics primarily on the basis of visual light images. This is in part a historical accident in that we first examined galaxies (spiral nebulae or elliptical nebulae) 200 years ago using visual inspection of telescopic images. Beginning 100 years ago photographic emulsions began to be used to record astronomical images. While the spectral sensitivity of photographic emulsions is slightly different from the spectral sensitivity of the eye, the two are close enough so that they are both commonly referred to as “visual light” images. Of course, it turns out that the visual-light region is the most advantageous one for studying galaxies in general since this is the region where stars (and hence galaxies) usually emit the most radiation.
However, as technological advances have made it possible to image astronomical sources outside the visible band, it has become apparent that some morphological structures are more pronounced in the IR, radio or even x-ray than they are in the visible. Indeed, some relatively featureless galaxies have revealed vast amounts of structure in non visible bands. For example, in spiral galaxies the dust scatters and obscures visible light while allowing IR, radio and x-ray photons to pass through relatively unimpeded. Furthermore, the dust becomes a stronger emitter of IR photons at the temperatures of the interstellar medium. In addition, neutral atomic hydrogen can be observed in the radio (1.414 MHz/21 cm) and yields views of galaxy interactions that are not at all obvious in other bands. As this technology has improved, we have come to recognize that a far clearer understanding of galaxies is possible when we observe both visible and non-visible photons. |
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Color
When we refer to galaxy color we are generally using it as a proxy for the stellar population.
Early type galaxies like ellipticals do not contain cool gas or dust. As a result they do not
form stars and are dominated by an older, redder stellar population. Spirals, on the other hand,
do form stars and therefore have a younger, bluer stellar population mixed in with their
underlying old disk population. This effect is not a large one, however, and most spirals are
not appreciably bluer than elliptical galaxies.
Besides stellar content there are other properties that affect the color of a galaxy. One is the
presence of dust. Dust will scatter short wavelength light while letting the longer red
wavelengths through. Therefore, extremely dusty galaxies will appear red regardless of their
underlying stellar population (this is the same process that causes the Sun to be redder at
sunrise and sunset than at midday). Indeed, such dusty galaxies will often be undergoing
extremely vigorous star formation, but their thick dust content blocks a direct view of their
young, bright stars. Instead we may see that they have inordinate emission in the infrared, the
result of the massive stars warming the dust and causing it to glow in the IR.
Sometimes a galaxy can be bluer than we expect given its morphology or stellar content. This
will be the case for galaxies with extremely old stellar populations. Generally we think of old
stars as being red, but if the stars are extremely old they would have formed before the galaxy
had produced many metals (in astronomy, “metals” are all elements heavier than hydrogen and
helium, and all those heavier than boron are formed inside stars). They will therefore be
metal-deficient. The preponderance of atomic transitions for many of these elements is in the UV
part of the spectrum. In stars with many metals the absorption in this region somewhat blocks
emission there; much of the flux that would normally be emitted in the UV must come out at
longer wavelengths, making these stars redder than they would otherwise be. On the other hand,
low-metal stars are able to emit more light in the blue and UV parts of the spectrum, which
somewhat diminishes their emission at longer wavelengths. While this is a small effect, it can
subtly change the color of the stars in a galaxy, making it slightly bluer would be expected
(actually, slightly less red is a better way to think about it).
So color is not always a simple thing. Generally a red galaxy will contain older, cooler stars.
But in some cases there is more to it, either because of the presence of dust in the galaxy or
because of low metal content.
Comparison of galaxies of different ages.
Size
Sizes of galaxies are determined by measuring their angular extent on the sky and determining
their distance.
Here s is the linear size, ø is the angular size in arc seconds, and d is the distance to the object. The units of s will be the same as the units of d. Of course, there are two difficulties with the practical application of this equation: (1) the precise determination of the angular size of a “fuzzy” object like a galaxy is difficult to make, and (2) the precise determination of the distance is also difficult.
The following results are typical measured values for galaxy diameters.
| dE dwarf ellipticals | 3 kpc |
| S spirals | 15 kpc – 20 kpc |
| E ellipticals | 60 kpc/td> |
| cD giant ellipticals | 2 mpc |
Luminosity
Luminosities or absolute magnitudes can be determined for a galaxy by measuring its apparent
magnitude and combining that with its distance. Here is it assumed that the flux of the galaxy
diminishes as the inverse of the square of its distance from us (strictly, this is true only for
point sources, but the methods used to measure galactic distances generally employ such
sources). Just as for size, apparent magnitude determinations are difficult for galaxies since
it is difficult to define the precise location of the “edge” for a nebulous object. In addition,
corrections must be applied because of the attenuation by dust in our own galaxy, the
attenuation due to dust in the external galaxy, the orientation of the galaxy in space (edge-on
or face-on), and the K-correction due to the redshift of the galaxy.
The following results are believed to be typical absolute magnitudes.
| dE | -8 |
| S | -21 |
| E | -22 |
| cD | -25 |
Notice that the absolute magnitude for a dE galaxy is only one magnitude brighter than the absolute magnitude for the brightest single supergiant stars in our own galaxy and in the LMC. Just for reference, the absolute magnitude for the sun (a G2 main sequence star) is about +5, while the absolute magnitude for a star like Sirius (an A0 main sequence star) is about +1. A typical red giant star might have an absolute magnitude of about 0 (zero).
Antennae Galaxies seen by NASA Hubble
Elliptical Galaxies
Elliptical galaxies appear in the sky as luminous elliptical disks. Elliptical galaxies have more stars concentrated at their center than at their outer edges, but otherwise have little or no structure. The light distribution is smooth with the surface brightness decreasing smoothly outward from the center.
M87
Image Provided By Anglo-Australian Observatory
The actual light distribution of ellipticals is well represented as
Where C0 is the central brightness, and I(r) is the brightness at radius r from the center. These galaxies are classified by means of the elongation of the projected image. That is, if a and b are the major and minor axes of the projected ellipse, then the ellipticity class, Ec,
is a measure of the ellipticity. This quantity is observed to vary fairly smoothly from E0 to E7. Statistical studies indicate that the true shapes of the galaxies are uniformly distributed from E0 to E7. There is no indication of the presence of any dust in these galaxies.
cD Galaxies
This type of galaxy appears similar to the ellipticals, and they are sometimes referred to as
giant ellipticals. The type was introduced by W. W. Morgan. The “c” refers to an old
spectroscopic designation for supergiants, while the “D” simply stands for diffuse. The objects
appear to have greatly extended envelopes and frequently exhibit structure and multiple nuclei
near their centers. They may have sizes which are ten times larger than a normal
elliptical.
ABELL 779
Image Provided by John Gretchen
cD galaxies are probably the result of two or more objects merging together, or of a larger object “swallowing” or cannibalizing smaller galaxies.
Irregular Galaxies
Irregular galaxies show no symmetrical or regular structure. Galaxies of type Irr I have resolved OB stars and HII regions. Irr II do not resolve into stars and are generally amorphous. Irregulars seem to contain a higher than usual concentration of dust and gas. Because of the dependence on resolution, it is impossible to distinguish between Irr I and Irr II for more distant objects.
NGC 55
Image Provided By Anglo-Australian Observatory
Hubble described this class as lacking both dominating nuclei and rotational symmetry.
In general, this class displays a lack of any organized structure.
The suffix p, for peculiar, should generally be reserved for galaxies with obvious structure
that has been distorted by the tidal interaction with close companions.
Lenticular Galaxies
Lenticular galaxies (Type S0) are intermediate between the E7 ellipticals and the Sa spirals.
They are flatter than E7s and have a thin disk as well as a spheroidal nuclear bulge. When they
are seen edge on they sometimes have the shape of a convex lens, so they are also called
lenticulars. The disk components have a light distribution that falls off more slowly than in
the ellipticals.
An S0 galaxy seen edge-on is very difficult to distinguish from an Sa seen edge-on. An S0 seen face-on is very difficult to distinguish from and E0. High quality images are necessary in order to discern the faint disk. Thus at increasing distances the identification of the S0s becomes increasingly more difficult.
NGC 936
Image Provided By Sloan Digital Sky Survey
S0 galaxies are disk galaxies, like spirals, but they have far less dust and gas than a normal spiral. It is possible that S0s are early type spirals that have lost their dust and gas as a result of tidal interactions with other galaxies. It is even possible that S0s are early type spirals which have suffered collisions. One clue to the nature of lenticular galaxies is that they are preferentially found near the centers of rich clusters of galaxies, where the intracluster gas would strip the interstellar dust and gas from a galaxy as it falls through the cluster.
Ring Galaxies
There are galaxies known to have very large ring-shaped structures beyond the nucleus. Some of these galaxies look almost like the planet Saturn, with their large prominent ring surrounding the nucleus. Sometimes the nucleus is nearly absent, so the only structure apparent is a large ring. In either case, these galaxies are termed “ring galaxies” and have been classified as R (for “Ring”). If an underlying spiral structure is discernible, the galaxy might be classified as S(R) or SB(R). Note that the upper case R is used for large ring structures beyond the nucleus, while the lower case r is used for ring structures features within or near the nucleus.
Hoag’s Object
This image was produced by the Hubble Heritage Team (STScI/AURA) using data collected by the Hubble Heritage team (STScI/AURA) and Ray Lucas (STScI).
It has been suggested by some that ring galaxies are produced as a consequence of a galaxy-galaxy collision.
Dwarf Galaxies
Most of the galaxies with which we are familiar are bright and easily detected. However, the vast majority of galaxies in the universe are actually small and faint. They tend not to fit into the classification scheme as we have outlined thus far. The most common form for these galaxies appears to be elliptical, and since they generally contain little or no gas, they are as a class referred to as dwarf ellipticals, dE. In addition to being smaller than the bright elliptical galaxies, the dEs do not exhibit a bright nuclear region. Another classification used for dwarf galaxies is dIrr (for dwarf irregular).
Leo I
Image Provided by Anglo-Australian Observatory
De Vaucleures has proposed the term “compact” to designate the dwarf galaxies. Thus, one can encounter the designations cE, for compact ellipticals, and cI, for compact irregulars.Since these objects are faint, they can only be detected if they are relatively nearby. This in itself can present a problem because some dwarf galaxies are so sparsely populated with stars that they are difficult to distinguish from Milky Way foreground stars. Nonetheless, given the large numbers of dwarf galaxies that we find in the local group and in other nearby groups, these are likely the most common types of galaxies in the universe.
Spiral Galaxies
Spiral galaxies are either ordinary (S or SA) or barred (SB). Both types have spiral shaped arms, with two arms generally placed symmetrically about the center of an axis of rotation. In the ordinary spirals the arms emerge from the nucleus, while in the barred spirals the arms emerge near the ends of a bar-like structure which passes through the center of the nucleus. Both types are classified according to how tightly the arms are wound, how patchy they are, and the relative size of the nucleus.
NGC 5236
Image Provided by Anglo-Australian Observatory
Ordinary spirals of type Sa have smooth ill-defined arms that are tightly wound around a large
prominent nucleus. The arms are wound so tightly they are nearly circular. The intermediate Sb
galaxies have more open arms which are often partly resolved into HII regions and stellar
associations. The nuclei of Sc galaxies are usually quite small and the arms are well extended
and resolved into HII regions and clumps of stars. An Sd galaxy displays virtually no nucleus at
all.
Hubble described this sequence in the following manner:
“The arms appear to build up at the expense of the nuclear regions and unwind as they grow; in
the end the arms are wide open and the nuclei inconspicuous. Early in the series the arms begin
to break up into condensations, the [breakup] commencing in the outer regions and working
inwards until in the final stages it reaches the nucleus itself. The “breakup” referred to is
the appearance of HII regions and bright blue supergiants.”