The Galaxies Within Our universe

The Visual Morphology of a Galaxy

The Hubble Sequence

A galaxy's morphological classification is a system used in astronomy to divide galaxies into similar groups based on their visual appearance. Though there existed a system for doing so prior to 1920, the most widely used scheme today is the famous Hubble tuning fork sequence, devised by Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan Sandage. This classification system derives it's name from the obvious layout of the visual chart as seen at right. Hubble was not the only astronomer to base galaxy classification on visual or empirical grounds. In 1966, Halton Arp compiled a catalog of peculilar galaxies and based the arrangement of over 300 galaxies on a system of visual composition. Generally speaking, the reason for such a less ridgid scientific standard was due mostly to the fact that in the 1920's galaxies were only just beginning to receive any degree of scientific analysis, a situation that existed through the 1960's until further surveys and newer technological instruments were able to penetrate the mysteries and composition of galaxies as well as resolving the visual structures of these distant objects. At left is a diagram of Hubbles "'Tuning Fork" Diagram for Galaxy Classification Credit: ESA/Hubble

So far so good...

But what about those galaxies we classify as "peculiar" or "polar ring" or "irregular"?

"Some galaxies fall outside the commonly recognized sequences. Many of these are "train wrecks", transient forms produced by the interaction or merger of galaxies. These forms include ring (not ringed) galaxies, polar rings, shells, and systems with tails, as well as double nuclei and highly asymmetric galaxies. Possibly related are what Hubble called Irr II and de Vaucouleurs calls I0 systems. The type example is M82; a galaxy with early-type stellar spectrum, but no particular spiral structure and amorphous appearance. These are almost always found in dense environments and appear to result from interaction-induced bursts of star formation." [1]   So, while they actually do have a place on the chart, it's been left up to us to find that place. You see, back in 1926, when Edwin Hubble first wrote about a classification scheme for galaxies he did so almost capriciously; while answering a bit of criticism regarding his classification outline he also clairified the system but only breifly expanded upon his earlier discussion regarding the workings of his scheme. As noted By Allan Sandage in EDWIN HUBBLE 1889-1953

"[t]he barest outline of a new galaxy classification scheme had been given by Hubble, almost as an aside, in his fundamental paper on the nature of diffuse galactic nebulae (Hubble 1922a)...note that the famous tuning fork diagram which summarizes the system at a glance did not appear in Hubble's 1926 technical paper in The Astrophysical Journal, but rather only in his popular book The Realm of the Nebulae."   [2]

As may be guessed, this early system, though not the earliest ( Wolf 1908 ), left a lot of room for revision, especially in light of the advancements made in ground-based telescopes over the last twenty or so years and especially since space observatories like Hubble, Spitzer and Chandra were launched. In the end, it seems fair to say that the original system, as described by Edwin Hubble was intended as a interim measure for organizing the data that had been accquired up to that point, not a definitive scientific tool. If we modernize the tuning fork diagram we might see something similar to the below

And the modern version?

The Hubble classification scheme or tuning fork diagram is the prime galaxy classification tool still in use today and at right is a more modern version of the diagram using images of the galaxy types themselves. Again, it continues to provide for the categorization of galaxies through their phyical appearence with changes and additions to the original scheme a result of the nature of the astrophysics inherent when classifying galaxies. In the end, the basic system follows a standard pattern for galaxy types:


^{Credit: Elliptical M 84 Aladin Spiral ESO 269-57 APOD Barred Spiral NGC 1365 Aladin Irregular Galaxy Sextans A Aladin Peculiar Galaxies Arp 295 APOD}

Of course, there is a lot of scientific ground that lies in between the above examples but we'll keep it simple for now and discuss the various sub-types as we progress through each category. For those wishing a more thorough review of galaxy classification there are links on the last page of this section that detail Hubble's system in much greater depth ( go to the "bibliography section ).

Spiral Arms, Irregular Shapes

A Quick Visual of Galaxy Types

When we think of galaxies we usually picture something similar to the above images of Spiral galaxies M74 and M81, also known as NGC 628 & NGC 3031 respectively ( Images: M74: The Perfect Spiral Gemini Observatory, GMOS Team and Galaxy Wars: M81 versus M82 M81 by Rainer Zmaritsch & Alexander Gross ). However, galaxies come in many different shapes and sizes, from Unbarred Spiral to Irregular, Dwarf galaxies that contain only several billion stars, our milkyway contains 200-400 billion, to Giant Galaxies like M87. As discussed earlier, when we use the term morphological we are talking about the visual shape of a galaxy. The images below are an example of the various morphological types.


Astronomical data on each of the above galaxy can be displayed below by clicking this link here.

^{Image: NPS/Dan Duriscoe Big Bend National Park, Night sky panorama from the summit of Emory Peak}
Galaxies from the Evolutionary Cosmos

Redshift and Blueshift, Colliding and Interacting Galaxies, Milkway and Andromeda, Hubble's Law and Computer Simulations, Compact Groups and Black Holes, Tidal Tails and Bridges, the Universe and the Rate of Expansion. Believe it or not, all these terms and names have one thing in common: they are related to the stellar matter within our universe for which science and technology work hand-in-hand exploring, defining, analysing and seeking to understand in order to form a basic picture of the creation of our universe. Current scientific thought on this subject is threefold: The Big Bang Theory, The Steady State Theory and The Pulsating Theory.

Though beyond the scope of this section, suffice it to say that when astronomers peer into and survey the universe, what they are doing is analyzing and exploring the workings of outer space, galaxies, stars, gases, matter, planets and even the life and material which form our world called Earth. Though it may never answer all the crucial unknowns that one ponders when questioning the origins of our cosmos [fn.1], the later developmental nature of our universe is a process that science has begun to understand and in turn, this knowledge has helped us to understand our beginnings. This has come about in the same large leaps and bounds that our technology has, especially in the last twenty years and though conversely it has produced as many questions and surprises, it has also produced some very amazing results and even more amazing images. But let us backup a bit and start at the beginning.

How did the first galaxies originally form?

Glad you asked! The current answer is we do not know. That said, "The most popular version of the dark matter theory says that galaxies began small and grew over time through collisions and slow accumulation of material from their surroundings." from Galaxy Hunters — The Search for Cosmic Dawn [ ©1996-2009 National Geographic Society, all rights reserved ]. When the universe was very young, after the big bang, the super heated particles of matter ( mostly in the form of atoms of gas ) were scattered in the extremely violent blast, travelling in all directions at once, at super high velocity and rolling along, at first, behind the shock wave produced as a result of the event. But while the light of the shockwave continued on the material from the blast slowed and began to cool. With this material still moving about the young universe the first processes of galaxy and stellar formation began: "Some astronomers believe the universe was built with small pieces, such as gas clouds and star clusters, that merged over time to form galaxies and clusters of galaxies. Others theorize that the early universe broke first into colossal clumps that contained enough building materials to make structures on the grandest scale — great walls and sheets of millions of galaxies — that fragmented into increasingly smaller gas and clouds, ultimately resulting in individual galaxies." [3]   Today the universe continues to expand and, as clearly shown in many images, the formation of new stellar populations is in large part, an ongoing process of merging and colliding galaxies.

What happens when galaxies collide?

When galaxies collide the general result is that they merge into one large galaxy like NGC 2207 and IC 2163, seen at left, or they "interact" with one another, continuing on their way, reshaping their physical geometry to one extent or another due to gravitional effects, perhaps meeting again billions of years later like NGC 4676, seen at right. Recent scientific research has discovered many important facts regarding the formation of galaxies and stellar populations. Astronomers studying the Hubble Deep Field and Hubble Ultra Deep Field images saw that early galaxies were more often than not involved in frequent collisions and interactions, noting the unusual and distored shapes of galaxies seen in the images.

"Earlier Hubble pictures show that nearly a third of very distant galaxies, which existed early in the history of the universe, appear to be interacting galaxies, like the Antennae. In particular, the Hubble Deep Field (a "long-exposure" image from Hubble looking at galaxies far back into time), uncovered a plethora of odd-shaped, disrupted-looking galaxies. They offer direct visual evidence that galaxy collisions were more the rule than the exception in the early days of the universe." [3]   "When galaxies collide, direct hits between stars are extremely rare, but collisions between huge gas clouds in the galaxies can trigger a stellar baby boom. The most massive of these stars race through their evolution in a few million years and explode as supernovas. Heavy elements manufactured inside these manufactured inside these stars are blown away by the explosions and enrich the surrounding gas for thousands of light years." [5]   » Clicking on the filmstrip at right will take you to the Chandra X-ray Observatory website where you can view an animation "depicting the collision of two large galaxies which now form The Antennae. Gas and stars from the galaxies are ejected into long arcs. The animation then shows how collisions between huge gas clouds in the central region of the merging system trigger a stellar baby boom."

On the Subject of Collisions

If you reviewed the astronomical data on the twelve galaxies previously imaged ( by clicking on the data box above ) you would have noticed that each galaxy has a Radial Velocity or Redshit value in the positive save one, the Andromeda Galaxy which has a negitive value of -301. Why is this important? Glad you asked! First of all, for the purposes of this section, that negitive number of -301 indicates a BLUESHIFT as opposed to a REDSHIFT of the measured light spectrum (doppler effect), meaning that the Andromeda and our Milkyway galaxies are moving towards each other! Secondly, scientists have recently revised their forcast models of when a collision with the Andromeda galaxy will occur and "just how much of the two galaxies will be involved. In both cases the revise prediction is a much sooner collision involving more of each galaxy's mass [when the two] collide with one another." [6]   But hold on and don't run for cover yet — the event is not due to occur for another 2 or 3 billion years. While we're waiting around for the collision, we can view the wonderful, high-resolution image at right by clicking on it. This image is a wide-angle view of our Milky Way looking towards its center. A key asterism seen in this image is the teapot of the constellation Sagittarius — "Although it is supposed to represent an archer, an asterism in the constellation looks distinctly like a teapot. Look for the handle to be on the east side and the spout to be on the west side. The brightest star in Sagittarius is the star marking the bottom right corner of the teapot, Kaus Australis. Also known as Epsilon Sagittari, this magnitude 1.79 star lies 144 light-years away." [7]   This image was taken at the Cerro Tololo Inter-American Observatory in Chili and was retrieved from the National Science Foundation website.

"The Hubble Law states that the recessional velocity of a distant galaxy is proportional to its distance from us. The recessional velocity of a galaxy is measured by examining the Doppler shift of lines in the spectrum of light from the galaxy. The distance to the galaxy is more difficult to measure, but can be estimated from its apparent angular size or by the brightness of objects in it." — The Expanding Universe educational project website from Montana State University. Seen in this image is the Andromeda, imaged from the Galaxy Evolution Explorer ( GALEX )


The Past — Hubble and the HUDF

If you took your camera into space, pointing it at the darkest most uninteresting area you could find and held the shutter open for an exposure time of over 270 hours, know what you would you get? Easy, the Hubble Ultra Deep Field image at right. "The snapshot includes galaxies of various ages, sizes, shapes, and colors. The smallest, reddest galaxies, about 100, may be among the most distant known, existing when the universe was just 800 million years old. The nearest galaxies - the larger, brighter, well-defined spirals and ellipticals - thrived about 1 billion years ago, when the cosmos was 13 billion years old...[i]n ground-based photographs, the patch of sky in which the galaxies reside (just one-tenth the diameter of the full Moon) is largely empty. Located in the constellation Fornax, the region is so empty that only a handful of stars within the Milky Way galaxy can be seen...[t]he image required 800 exposures taken over the course of 400 Hubble orbits around Earth. The total amount of exposure time was 11.3 days, taken between Sept. 24, 2003 and Jan. 16, 2004." [8]   For a zoomable version of this image click here. « Clicking the filmstrip at right will take you to the HubbleSite video page where you can view a variety of movies which pan selected areas of the HUDF. You can also watch interviews with the principal astronomers whom were responsible for the different phases of Hubble's ultra deep infrared survey. The HUDF project is the third relevent survey of this type conducted in the last 15 years, the others were the 1995 HDF-North and the 1998 HDF-South. The images that Hubble produced where and are a milestone in terms of evolutionary astronomy; the HUDF is the closest we have come to viewing what our universe looked liked when it was only 5 percent of its present age, shortly after the event known as the Big Bang.

In November of 2006 the findings of the HUDF scientific team were published in an issue of the Astronomical Journal ( Steven V. W. Beckwith et al 2006 ). The report affirmed earlier conclusions made after examination of the first set of images from the Hubble Space Telescope's HDF-North and South surveys, stating that "[v]isual inspection of the images shows few if any galaxies at redshifts greater than ~4 that resemble present-day spiral or elliptical galaxies. The image reinforces the conclusion from the original Hubble Deep Field that galaxies evolved strongly during the first few billion years in the infancy of the universe." [9]

"Although the luminosity density of galaxies at wavelengths of ~1400 Å decreases modestly from redshifts of a few out to redshifts greater than 6, vigorous star formation was already underway when the universe was less then 1 billion years old." [Ibid, pg.27]

The Future — Computers and Mice Model a Galaxy

^{3-d animation of the Mice Galaxies and possible future collision, Joshua E. Barnes et.al.}
The image at right is from Joshua E. Barnes & John E. Hibbard's site Sneaking Up On The Mice (NGC 4676) where "The numerical models presented here show how two ordinary galaxies can be transformed into a dramatic interacting system." Their goal was "...to find a plausible model matching the morphology and VLA observations..." Graduate student Lisa Chien and Joshua E. Barnes have created what looks to be "...the most realistic renditions of N-body simulations now in existence." The simulation models the collision of the Mice Galaxies along with the image at left, a simulation of what may eventually become of the two galaxies as they collide and merge. "Models including star formation can be combined with the output of a stellar population synthesis code to produce realistic images in standard photometric wavebands." Computer modelling has come a long way in recent years, benefitting greatly from a wave of new scientific data as well as the trend resulting in the increase of computer technologies. Modeling simulations of galaxies, their interactions and collisions have given astronomers a way of verifying their theories and predictions. But most important of all, a computer simulation can accomplish what we, as humans are unable to do — study the evolution of a single galaxy as it changes over billions of years.

"By combining test-particle and self-consistent techniques, we have developed a method to rapidly explore the parameter space of galactic encounters. Our method, implemented in an interactive graphics program. This software is available at http://www.ifa.hawaii.edu/faculty/barnes/research/identikit/, can be used to find the parameters required to reproduce the observed morphology and kinematics of interacting disk galaxies." ( Joshua E. Barnes et al 2009 ) [10] Image description: Identikit model (points) matching a self-consistent N-body simulation of a galactic collision (grey scale). Each panel shows a different projection: top left shows sky-plane (X,Y), top right shows velocity-position (V, Y); bottom left shows position-velocity (X, V), bottom right shows (X, Z). « Clicking the filmstrip at left will take you to Joshua E. Barnes & John E. Hibbard's website where you can view their computer simulations on the formation and interactions of NGC 4676 ( the Mice Galaxies ). Particularly noteworthy is the section entitled "Photorealistic Images" which simulates the Mice galaxy prior to, during and what may eventually be in store for these two as they collide.

NGC 4676: When Mice Collide
"These two mighty galaxies are pulling each other apart. Known as " The Mice" because they have such long tails, each spiral galaxy has likely already passed through the other. They will probably collide again and again until they coalesce. The long tails are created by the relative difference between gravitational pulls on the near and far parts of each galaxy. Because the distances are so large, the cosmic interaction takes place in slow motion -- over hundreds of millions of years. NGC 4676 lies about 300 million light-years away toward the constellation of Bernice's Hair (Coma Berenices) and are likely members of the Coma Cluster of Galaxies. The above picture was taken with the Hubble Space Telescope's Advanced Camera for Surveys which is more sensitive and images a larger field than previous Hubble cameras. The camera's increased sensitivity has imaged, serendipitously, galaxies far in the distance scattered about the frame." Credit: ACS Science & Engineering Team, Hubble Space Telescope, NASA   [11]

No matter what we may imagine right now, the last and greatest thing we ever imagine is the next thing that comes along which we never imagined would — anonymous

---

Footnotes
^{1 — The term "cosmos", as used herein, refers to our universe regarded as an orderly, harmonious whole.}

A Heartfelt Thank You
In addition to the numerous links that appear on the pages of this website, there is also a wealth of astronomical data from all psossible sources across the world wide web and this list of sources is quite extensive. As such, the bibliography appears at the end of each section. For those wishing a complete listing for the entire topic under discussion, such a file will be made available after the completion and posting of the entire section. Some of the websites that are regularly used herein I would like to recognize and thank, especially the individuals therein who make possible the material that is available to everyone, everwhere in the pursuit of knowledge: NASA/IPAC Extragalactic Database ( NED ) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA, the Simbad Data Query & Aladin Sky Atlas websites, the HEASARC Data Archive, the Astronomy Picture of the Day website, the ESO, the Spitzer Space Telescope website, the Chandra X-Ray Observatory website, the Sloan Digital Sky Survey website, the Anglo-Australian Observatory website, the European homepage for the NASA/ESA Hubble Space Telescope, the Pulkova Observatory Saint Petersburg Russia website, the Astronomical Journal website and the NASA/JPL website.