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Galaxies
Hubble’s Galaxy Classification
The American astronomer Edwin Hubble was the first to categorize galaxies in a
comprehensive way. Working with the then recently completed 2.5-m optical telescope on
Mount Wilson, California in 1924, he classified the galaxies he saw into four basic types (solely
on the basis of their appearance):
spirals,
barred spirals;
ellipticals,
irregulars
Many modifications and refinements have been incorporated over the years, but the basic
Hubble classification scheme is still widely used today.
SPIRALS
We saw several examples of spiral galaxies—for example, our own Milky Way Galaxy and our neighbor
Andromeda. All galaxies of this type contain a flattened galactic disk in which spiral arms are found, a central
galactic bulge, and an extended halo of faint, old stars. The stellar density is the number of stars per unit
volume. The stellar density is greatest in the galactic nucleus, at the center of the bulge.
Spiral Galaxy Shapes Variation in shape
among spiral galaxies. As we progress
from type Sa to Sb to Sc, the bulges
become smaller while the spiral arms tend
to become less tightly wound. (W. Keel;
NOAO; D. Malin/AAT)
In Hubble’s scheme, a spiral galaxy is denoted by the letter S and classified as type a, b, or c,
according to the size of its central bulge:
Type Sa galaxies having the largest bulges,
Type Sc the smallest.
The size of the bulge is quite well correlated with the tightness of the spiral pattern (although the
correspondence is not perfect).
Type Sa spiral galaxies, with large central bulges, tend to have tightly wrapped, almost circular,
spiral arms.
Type Sb galaxies, with smaller bulges, typically have more open spiral arms.
Type Sc spirals, with the smallest bulges, often have loose, poorly defined spiral structure. The
arms also tend to become more “knotty,” or clumped, in appearance as the spiral pattern
becomes more open.
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Much of our description of the large-scale structure of the Milky Way Galaxy applies to spiral
galaxies in general.
Like the disk of the Milky Way, the flat disks of typical spiral galaxies are rich in gas and dust. The 21cm radio radiation emitted by spirals shows the presence of interstellar gas, and detailed photographs clearly
show obscuring dust in many systems. Stars are forming within the spiral arms, where the interstellar
medium is densest.
Type Sc galaxies contain the most interstellar gas and dust,
Type Sa galaxies contain the the least interstellar gas and dust.
A variation of the spiral category in Hubble’s classification scheme is the barred-spiral galaxy.
The barred spirals differ from ordinary spirals mainly by the presence of an elongated
“bar” of stellar and interstellar matter passing through the center and extending beyond
the bulge, into the disk. The spiral arms project from near the ends of the bar rather than from
the bulge (as they do in normal spirals). Barred spirals are designated by the letters SB and
are subdivided, like the ordinary spirals, into categories SBa, SBb, and SBc, depending on
the size of the bulge.
Colliding Galaxies
No human will ever witness an entire galaxy collision, for it would last many millions of years. Galaxies in
clusters apparently collide quite often. In the smaller groups, the galaxies’ speeds are low enough that
interacting galaxies tend to “stick together,” and mergers are a common outcome. In larger groups, galaxies
move faster and may pass through one another without sticking .
Although a collision may change the large-scale structure of the galaxies involved, it has no effect on
the individual stars they contain. The stars within each galaxy just glide past one another. Although we
have plenty of photographic evidence for galaxy collisions, no one has ever witnessed or photographed
a collision between two stars. Stars do sometimes collide in other circumstances—in the dense central
cores of galactic nuclei and globular clusters, or as a result of stellar evolution in binary systems—but
stellar collisions are a very rare consequence of galaxy interactions.
To understand why individual stars do not collide when galaxies collide, recall that
The galaxies within a typical cluster are bunched together fairly tightly. The distance between adjacent
galaxies in a given cluster averages a few hundred thousand parsecs, which is only about 10 times
greater than the size of a typical galaxy. Galaxies simply do not have that much room to roam around
without bumping into one another.
By contrast, stars within a galaxy are spread out much more thinly. The average distance between stars
within a galaxy is millions of times greater than the size of a typical star. When two galaxies collide, the
star population merely doubles for a time, and the stars continue to have so much space that they do not
run into one another. The stellar and interstellar contents of each galaxy are certainly rearranged, and
the resultant burst of star formation may indeed be spectacular from afar, but for the stars, it’s clear
sailing.
The Distribution of Galaxies in Space
Standard candles are easily recognizable astronomical objects whose luminosities are confidently
known. The basic idea is very simple. Once an object is identified as a standard candle—by its
appearance or by the shape of its light curve, say—its luminosity can be estimated. Comparison of this
luminosity with the object’s apparent brightness then yields the object’s distance and hence the
distance to the galaxy in which it resides. Note that, apart from the way in which the luminosity is
determined, the Cepheid variable technique relies on identical reasoning. However, the term standard
candle tends to be applied only to very bright objects.
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Planetary nebulae and Type I supernovae have proved particularly reliable as standard candles.
The latter have remarkably consistent peak luminosities and are bright enough to be seen and
identified out to distances of many hundreds of megaparsecs. The small luminosity spread of Type I
supernovae is a direct consequence of the circumstances in which these violent events occur. An
accreting white dwarf explodes when it reaches the well-defined critical mass at which carbon fusion
begins. The magnitude of the explosion is relatively insensitive to the details of how the white dwarf
formed, or how it subsequently reached critical mass, with the result that all such supernovae have
quite similar properties.
An important alternative to standard candles was discovered in the 1970s, when astronomers found a close
correlation between the rotational speeds and the luminosities of spiral galaxies within a few tens of
megaparsecs of the Milky Way Galaxy. Rotation speed is a measure of a spiral galaxy’s total mass, so it is
perhaps not surprising that this property should be related to luminosity. What is surprising, though, is how
tight the correlation is. The Tully-Fisher relation, as it is now known (after its discoverers), allows us to
obtain a remarkably accurate estimate of a spiral galaxy’s luminosity simply by observing how fast it rotates.
As usual, comparing the galaxy’s (true) luminosity with its (observed) apparent brightness yields its
distance.The Tully-Fisher relation can be used to measure distances to spiral galaxies out to about 200 Mpc,
beyond which the line broadening becomes increasingly difficult to measure accurately.
Local Group
The Local Group is made up of some 36 galaxies within approximately 1 Mpc of our Milky Way Galaxy. Only
a few are spirals; most of the rest are dwarf -elliptical or irregular galaxies, only some of which are shown here. Spirals are
colored blue, ellipticals pink, and irregulars white.
The Local Group’s diameter is roughly 2 Mpc. The Milky Way Galaxy and Andromeda are by far
its largest members, and most of the smaller galaxies are gravitationally bound to one or the other of
them. The combined gravity of the galaxies in the Local Group binds them together, like stars in a star
cluster, but on a millionfold larger scale. More generally, a group of galaxies held together by their
mutual gravitational attraction is called a galaxy cluster.
Does the universe have even greater groupings of matter, or do galaxy clusters top the cosmic
hierarchy? Most astronomers believe that the galaxy clusters themselves are clustered, forming titanic
agglomerations of matter known as superclusters.
Together, the galaxies and clusters form the Local
Supercluster, also known as the Virgo Supercluster. Aside from
the Virgo Cluster itself, it contains the Local Group and
numerous other clusters lying within about 20–30 Mpc of Virgo.
Most of the galaxies shown in Figure 16 are fairly large spirals
and ellipticals; the fainter irregulars and dwarfs are not
included. Galaxies are false-colored according to the local
galaxy density—white and yellow indicate the most congested
regions, green less dense regions, and blue the least dense.
The white, yellow and green galaxies trace out approximatelly
the supercluster's extent. All told, the Local Supercluster is
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about 40–50 Mpc across and contains some 10 solar masses
of material. Very irregular in shape and significantly elongated
perpendicular to the line joining the Milky Way to Virgo, its
center lies near the Virgo Cluster.
Local Supercluster The locations of numerous galaxies and galaxy clusters in the vicinity of Virgo. More
than 4500 galaxies are represented here, and several prominent galaxy clusters are labeled. Part (a) shows
the Virgo Supercluster roughly as it appears from the direction of our own Galaxy, which is located about 20
Mpc (two grid squares) above the page (Data courtesy B. Tully, U. Hawaii; visualization by S. Levy, NCSA)
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SPIRALS
Barred-Spiral Galaxy Shapes Variation in shape among barred-spiral galaxies. The variation from SBa to
SBc is similar to that for the regular spirals, except that now the spiral arms begin at either end of a bar
through the Galactic center. In frame (c), the bright star is a foreground object in our own Galaxy, and the
object at top center is another galaxy that is probably interacting with NGC 6872. (NOAO; AAT; ESO)
ELLIPTICALS
Elliptical Galaxy Shapes Variation in shape among elliptical galaxies:
(a) The E1 galaxy M49 is nearly circular in appearance.
(b) M84 is a slightly more elongated elliptical galaxy. It is classified as E3. Both of these galaxies lack
spiral structure, and neither shows evidence of cool interstellar dust or gas, although each has an
extensive X-ray halo of hot gas that extends far beyond the visible portion of the galaxy.
(c) This false-color X-ray image of the giant elliptical galaxy 3C295 displays hot gas (red) well beyond
the galaxy itself (white) as well as throughout the cluster of galaxies surrounding 3C295. (AURA;
SAO)
The next major category in the Hubble scheme contains the elliptical galaxies. Denoted by the letter E,
these galaxies are subdivided according to how elliptical they are. The most circular are designated E0,
slightly flattened systems are labeled E1, and so on, all the way to the most elongated ellipticals, of type
E7.
There is a large range in both the size and the number of stars contained in elliptical galaxies.
The largest elliptical galaxies are much larger than our own Milky Way Galaxy. These giant
ellipticals can range up to a few megaparsecs across and contain trillions of stars.
At the other extreme, dwarf ellipticals may be as small as 1 kpc in diameter and contain only a
few million stars.
The dwarfs are by far the most common type of ellipticals, outnumbering their brighter
counterparts by about 10 to 1. However, most of the mass that exists in the form of elliptical
galaxies is contained in the larger systems.
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Intermediate between the E7 ellipticals and the Sa spirals in the Hubble classification is a class of
galaxies showing evidence of a thin disk and a flattened bulge, but containing no gas and no spiral
arms. These galaxies are known as
S0 galaxies if no bar is evident
SB0 galaxies if a bar is present.
They look a little like spirals whose dust and gas have been stripped away, leaving behind a stellar disk
and bulge.
Observations in recent years have shown that many normal elliptical galaxies have faint stellar disks
within them, like the S0 galaxies.
As with the S0s, the origin of these disks is uncertain, but some researchers suspect that the S0s and
ellipticals actually form a continuous sequence, along which the bulge-to-disk ratio varies smoothly.
IRREGULARS
The final galaxy class identified by Hubble—irregular galaxies—named in this way because their
visual appearance does not allow us to place them into any of the other categories just discussed.
Irregulars tend to be rich in interstellar matter and young, blue stars, but they lack any regular structure
such as well-defined spiral arms or central bulges. They are conventionally divided into two subclasses:
Irr I galaxies - The Irr I galaxies often look like misshapen spirals. They tend to be smaller than
spirals but somewhat larger than dwarf ellipticals. They typically contain between 108 and 10 10
stars. The smallest are called dwarf irregulars. As with elliptical galaxies, the dwarf type is the most
common. Dwarf ellipticals and dwarf irregulars occur in approximately equal numbers and together
make up the vast majority of galaxies in the universe. They are often found close to a larger
“parent” galaxy.
Irr II galaxies. - The much rarer Irr II galaxies, in addition to their irregular shape, have other
peculiarities, often exhibiting a distinctly explosive or filamentary appearance. Their appearance
once led astronomers to suspect that violent events had occurred within them. However, it now
seems more likely that in some (but perhaps not all) cases we are seeing the result of a close
encounter, or even a collision, between two previously “normal” systems.
Galactic “Tuning Fork” Hubble’s tuning fork diagram, showing his basic galaxy classification scheme.
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