Now we get to the really big stuff -- Galaxies. Now pay attention, because we need your help here. I'll come back to this at the end.
First, you have to get used to a new term -- a Parsec. The parsec ("parallax of one arcsecond", symbol pc) is a unit of length, equal to just under 31 trillion kilometres (about 19 trillion miles), or about 3.26 light-years.
That's about how long a 12 year old thinks it'll be before he or she gets finished with a school day. We need to use this term as it makes us impossible to understand by mere mortals -- and also, the numbers (distances) are so vast that we need a numbering system that is usable at this scale.
In case you are a brainy show-off, the parsec is defined as the length of the adjacent side of an imaginary right triangle in space. The two dimensions that form this triangle are the parallax angle (defined as 1 arcsecond) and the opposite side (which is defined as 1 astronomical unit (AU), the distance from the Earth to the Sun). Given these two measurements, along with the rules of trigonometry, the length of the adjacent side (the parsec) can be found.
One of the oldest methods for astronomers to calculate the distance to a particular star was to record the difference in angle between two measurements of the position of the star in the sky. The first measurement was taken from the Earth on one side of the Sun, and the second was taken half a year later when the Earth was on the opposite side of the Sun.
Thus, the distance between the two measurements was known to be twice the distance between the Earth and the Sun. The distance to the star could be found using simple calculations of trigonometry. Since it is based on an angle and the distance between the Earth and the Sun, it is fundamentally derived from the degree and the AU. The length of a parsec is about 30.857 petametres, 3.26156 light-years or 1.9174×1013 miles.
The first documented use of the term parsec was in an astronomical publication in 1913, and attributed to Herbert Hall Turner who was shot for just mumbling it.
OK, back to galaxies and more gorgeous images. You know, all the math aside, the universe really is a magnificent creation. A galaxy is a system of stars, nebulae and interstellar gas that is held together by that binding force -- Duct Tape - er, I mean Gravity.
Imagine our galaxy is a bicycle wheel. If you hold it so the wheel end, the tire, faces you, that's what you're looking at when you see the Milky Way.
"Near" us are another couple of Galaxies, the Andromeda Galaxy, the Triangulum galaxy, the "dwarf" Sagittarius Galaxy and about 50 other minor galaxies. Andromeda is estimated to hold about the same number of stars as us, and Sagittarius, only a puny 40 billion.
Our "local group" of galaxies is about 10 million light-years across. Try to imagine that.
From what we can see today, with our most powerful telescopes, we estimate there are at least 125 billion galaxies in the known universe. The farthest are almost 13 billion light years away.
We know too that these galaxies are moving. We also know that only about 5% of galaxies exist by themselves. Most exist in groups.
The notion of galaxies moving in different directions was the crowning work of Edwin Hubble (right, he changed his name to fit the Space Telescope /joke). During the past 100 years, astronomers have discovered quasars, pulsars, black holes and planets orbiting distant suns. But all these pale next to the discoveries Edwin Hubble made in a few remarkable years in the 1920s. At the time, most of his colleagues believed the Milky Way galaxy, a swirling collection of stars a few hundred thousand light-years across, made up the entire cosmos. But peering deep into space from the chilly summit of Mount Wilson, in Southern California, Hubble realized that the Milky Way is just one of millions of galaxies that dot an incomparably larger setting.
Hubble went on to trump even that achievement by showing that this galaxy-studded cosmos is expanding — inflating majestically like an unimaginably gigantic balloon — a finding that prompted Albert Einstein to acknowledge and retract what he called "the greatest blunder of my life." Hubble did nothing less, in short, than invent the idea of the universe and then provide the first evidence for the Big Bang theory, which describes the birth and evolution of the universe. He discovered the cosmos, and in doing so founded the science of cosmology.
The nearest object to the Milky Way was discovered only in 1994. The Sagittarius galaxy is represented by a point, but this belies the nature of this mysterious object. You may have figured out that the object is called Sagittarius because it is seen in that constellation in the night sky. However, parts of this galaxy have been seen on both sides of the Milky Way Galactic disk. So representing it with one point is really not too accurate. Sagittarius is a "dwarf spheroidal" galaxy that has been stretched and warped by our Milky Way. These types of galaxies appear as smudges in the sky in even the largest telescopes. With a low star density, these galaxies often resemble star clusters rather than galaxies. The first -- "Sculptor", was discovered in 1938 by Harlow Shapley.
DIFFERENT SHAPES OF GALAXIES
Galaxies come in three main types: ellipticals, spirals, and irregulars.
The Hubble classification system rates elliptical galaxies on the basis of their ellipticity (naturally), ranging from E0, being nearly spherical, up to E7, which is highly elongated. You can see examples in the image on the left. These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently these galaxies also have a low portion of open clusters and a reduced rate of new star formation. Instead these galaxies are generally dominated by older, more evolved stars that are orbiting the common center of gravity in random directions. In this sense they have some similarity to the much smaller globular clusters.
The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction of galaxies, resulting in a collision and merger. They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy clusters. Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy.
SPIRALS (Barred and Unbarred)
The Sombrero Galaxy in this page's header image is an example of an unbarred spiral galaxy.
Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge. An Sa galaxy has tightly wound, poorly-defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region.
In spiral galaxies, the spiral arms do have the shape of approximate organized spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms also rotate around the center, but they do so with constant angular velocity. That means that stars pass in and out of spiral arms, with stars near the galactic core orbiting faster than the arms are moving while stars near the outer parts of the galaxy typically orbit more slowly than the arms. The spiral arms are thought to be areas of high density matter, or "density waves". As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.
NGC 1300 is an example of a barred spiral galaxy.
A majority of spiral galaxies have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure. In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter (a, b or c) that indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to a tidal interaction with another galaxy. Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.
Our own galaxy is a large disk-shaped barred-spiral galaxy about 1 light year in diameter and a kiloparsec in thickness. It contains about two hundred billion stars and has a total mass of about six hundred billion times the mass of the Sun.
OTHER TYPES OF GALAXIES (IRREGULAR)
Hoag's Object is an example of a ring galaxy.
Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies. An example of this is the ring galaxy, which possesses a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy. Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.
A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars. (Barred lenticular galaxies receive Hubble classification SB0.)
NGC 5866, an example of a lenticular galaxy. Credit: NASA/ESA.
In addition to the classifications mentioned above, there are a number of galaxies that cannot be readily classified into an elliptical or spiral morphology. These are categorized as irregular galaxies. An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme. Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted.
Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds. These tiny galaxies are about one hundredth the size of the Milky Way, containing only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.
Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered. Dwarf galaxies may also be classified as elliptical, spiral, or irregular. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead.
A study of 27 Milky Way neighbors found that dwarf galaxies were all approximately 10 million solar masses, regardless of whether they have thousands or millions of stars. This has led to the suggestion that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.
Our knowledge of the origins of galaxies, indeed the origin of the universe, is still theoretical. Here is a timeline of how we came to know what we do today.
Before I ask for your help, I would be remiss in mentioning the work being carried out at the Sloan Digital Sky Survey team. This group, based in New Mexico and at NYU, is working on a project to map the universe (I know, what do they do after that?). Here is an example of their work, and as you can see, a lot images need naming
So this is how do I need your help. With all the galaxies we are finding, not many have been classified. So naturally, there is a website GalaxyZoo, that will give you a little training, and you can spend an occasional hour, classifying galaxies and helping a global effort. You might even get one named after you. Or at least a star. The are, after all more stars than all the people that have ever lived.