Main shapes of galaxies
Galaxies or milkyways are actually enormous swarms of suns.
Just like in our own solar system the individual stars with their planets form a solar system based upon gravity and motions.
These planetary systems are in balance with one another and so form the star system or galaxy. The galaxies vary drastically in size, they comprise between 10 billion and 1 trillion stars.
The galaxies in turn lump together into groups called clusters, these clusters vary from a few systems to several thousands. The best known group is our local cluster, this comprises about 30 members, amongst which our own Milkyway and the Andromeda nebula.
The galaxies that are far beyond the Milkyway are sometimes called the extragalactic galaxies. The number of galaxies in the universe is estimated to between 50 and 100 billion.
A galaxy is born.The Orion Nebula is the central part of a huge cloud of dust and gases, containing lots of ionized hydrogen gas, which generates active star formation.
The young bright stars heat up the surrounding dust and gas to temperatures over 10,000 K.
The most important stars started to radiate only a few thousand years ago.
Almost all galaxies have a nucleus, which is the bright central part of the galaxy. The galaxy nuclei are made of millions of stars and contain large quantities of mass. There is strong evidence that there might even be enormous black holes in the center of some galactic nuclei.
The galaxies are classified by their shapes, commonly they are divided in two main groups, the elliptical shaped, indicated by the symbol E and spiral galaxies, for which the symbol S is used, sometimes they symbolize a bar shapes spiral galaxy with S.B. The so called irregular shaped galaxies can be distinguished by the symbol Ir.
A spiral galaxy is in principle, a large flat disc with a bulging center, which is called the nucleus. In the nucleus, the largest quantity of stars are situated to the edge of the spiral-shape, and in the rest the amount of stars decreases. Within the disc the stars are situated on pinwheel-like structures, the spiral arms, that wind out in a spiral-shape from the nucleus to the edge. They are regions of dust, gas, and stars where star formation is occuring. Also in between the spiral arms reside stars, the stars in these areas give the galaxy its characteristic shape, because they are younger and much brighter.
Spiral milkyways are comprised mostly of gas and dust. The youngest and most active stars are situated in the nucleus and the spiral arms.
Elliptical milkyways are hardly comprised of gas and dust. The shape is smooth and without any structure, it looks like a rugby ball.
In galaxies of this group, the stars are regularly divided over the stars system. The number of stars gradually decreases from inside to the outside.
Since elliptical galaxies have little or no dust or gas, they do not have much of a star formation. In other words, elliptical galaxies have completed their star formation.
They are so far away that it is impossible, even with the strongest telescopes to observe the individual stars of such a constellation. You can only see the common light of all suns as a hazy spot.
Irregular bar spiral shaped.
Sometimes a galaxy is deformed by the influence of gravity from within the galaxy or from an other galaxy in the neighborhood.
Some spiral galaxies have a "bar" running through the galaxy nucleus. Usually the spiral arms "connect" to the end of this bar. The bars are active regions where star formation is occuring.
Peculiar shapes and deformations.
Besides the spiral and elliptical shape, sometimes a subdivision is made based upon the irregularity of the galaxies.
Irregular galaxies do not fit into either of the above two categories. Often these constellations are too small to form the structures seen in spiral or elliptical galaxies. Their stars and gas clouds are scattered in random patches.
Sometimes, galaxies crash into each other or due to gravity rip each other apart. In any event, peculiar galaxies are just different. Some known peculiar galaxies are going through a very active period of their history right now with rapid star formation and a lot of supernova explosions.
After a collision with one of the smaller galaxies this Galactic Ring was formed, in the ring are regions of dust, gas, and stars where star formation is occuring. A Sombrero galaxy is spiral shaped with a large nucleus, that is comprised of almost no gas or dust.
Galaxies rotate relatively quickly. From the radiation at 21 cm we see that the hydrogen in the galaxy approaches us (blue) or moves away from us (red), see also Doppler effect.
source: P.T. de Zeeuw, Sterrewacht Leiden
Galaxies far away and in motion.
The galaxies are so far away, that it is impossible to distinguish the individual stars. We can only see the common light of all stars as a hazy nebula spot. The most distant milkyways that we know are more than one hundred million lightyears away from Earth.
All star systems fly away from us and each other, the universe expands more and more. Until recently, scientists suggested that most of the stars are grouped in galaxies, but in the beginning of 1998 they start to suggest otherwise.
Galaxies display themselves in a variety of different colors, depending on their age, type, and quantity of its gases and dust.
By studying the light from a galaxy, astronomers can also get more information about its chemical composition, its distance from Earth, and the speed at which it is traveling away from us.