Galaxies are massive systems, which contain stars, gas and dust. These systems come in a variety of sizes, ranging from several million to several trillion stars. They also come in a variety of shapes. These shapes can be classified using the Hubble Classification system. The four classes of galaxies are the spiral (S), the barred spirals (SB), ellipticals (E) and irregulars (I). In this report I will be discussing spiral and barred spiral Galaxies and the reason why they spiral.
Spiral Galaxies and barred spiral Galaxies have similar appearances as they all contain arced arms of stars, a central bulge and a spherical halo. The arms have stars continuously being born and dying. These areas are full of small, young, metal rich population I stars, HII regions or clouds of cold ionized hydrogen, as well as heavier elements, which are sent into space when stars die. The central bulge is usually about a few thousand light years across. This area is made up mainly of Population II stars. Population II stars are older, reddish, metal poor stars. There are very few of these stars being born. Finally the halo is composed of dark matter and older star, which holds the galaxy together by surrounding the disk. A difference between the spiral and barred spiral galaxy is the area of the galaxy where the arms extend from. The arms of the barred spiral Galaxy are extended out of the ends of the bar shaped area, which runs through the galaxies nucleus, instead of from the nucleus itself as the spiral galaxy does.

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Once we have the galaxies classified into spiral and barred spiral, we can further classify them into a, b, and c types. There are several differences between these types. First there is the difference in gas content, which increases from a to c, second there is the difference in the size of the nuclear bulge, which decreases from a to c, third the nature of the spiral arms differ with younger features such a HII regions in showing up more in type c than a. The pitch angle of the arms and total mass of the galaxy are the last two features. The pitch angle is tighter in a-type galaxies and more open in c type galaxies. Lastly type a galaxies are more massive than type c. Below are picture from left to right type a, type b, and type c.
Winding Dilemma Theory
One of the earliest theories, which tried to explain spiral Galaxies, was the winding dilemma theory. In this theory it was proposed that the material in the arms was simply in the spiral shape from the time it was formed and is fixed in the spiral position. However, this is only possible if the Galaxy spun like a spiral disk and we know that stars, dust and gas orbit the centre of the Galaxy at approximately the same speed in a state of differential rotation. This means that the angular velocity of rotation is decreasing as the radius from the centre of the Galaxy increases (Г╟=v/r). This causes a problem as stars closer to the centre have a shorter orbit than the stars farther away from
Pictures of Galaxies can be found at the following site:M65: www.seds.org/messier/more/m065.more.html; M77:www.seds.org/messier/m/m077.html; M99: www.seds.org/messier/m/m099.html; None of the photos said who they were taken by.
the centre, yet stars that are closer have a greater angular speed. Because of the difference in angular speed the material in the arms would be stretched as the end of the arm moves slower than the front. After one or two periods of revolution the galaxy would become a featureless disk of stars, instead of a spiral disk. This is shown in the illustrations below.
Density Theory
In the 1940’s another theory was proposed by a Swedish astronomer Bertil Lindblad to solve the spiral arm mystery. This theory suggested that the spiral arms were part of a wave pattern which moved through the galaxy much like ripples in a pond. This wave theory was further studied in the 1960’s by two American astronomer’s, Chia Chiao Li and Frank Shu. To simplify the explanation of this theory it is compared to the waves formed when a stone is tossed into a pond. The water molecules pile up at the crest then
Illustrations taken from Universe 6th Edition (see Bibliography) Page 577 spread out again. The density wave is predicted to move around the galaxy. At the crest of the wave stars, gas and dust pile up creating an area of high density. These crests are the areas where we see the arms. Just like in the model with the stone thrown into the pond were the water molecules nudge the molecules next to them and spread the wave throughout the whole pond, stars, gas and dust do the same. Even though stars, gas and dust are separated by large amounts of distance they still apply gravitational forces on each other. So they nudge each other with their gravitational force, this spreads the density wave throughout the entire galaxy. There are two cases that occur when a wave is moving around a disk. The first case is when the matter is moving faster than the density wave. When the matter catches up to the wave it must slow down, this is what causes the pile up of matter. The matter is stuck in this slow moving wave until it is able to work its way through the dense area of the wave and out the other side, it is then able to continue rotation around the central bulge at its normal speed. This is how the wave works when it in the portion of the arm closer to the central bulge. As we recall the matter in the arm further from the bulge has a slower angular rotation. This means that the process were the matter is caught in the wave is reversed and the wave gets caught in the matter. How this works is the waves catches matter as it moves around kind of like a net. Matter is pushed ahead at a speed faster than it usually moves by the wave till it is able to work its way out and slow down to its normal speed. The first case can be demonstrated as being much like a slow vehicle on a freeway. The slow vehicle causes the vehicles behind it to slow down momentarily until they are able to pass it. These pile ups of matter can cause changes in the gravitational forces in the galaxy. By piling up the stars, gas, and dust we are creating an area of higher density. This area then has more gravitational force than smaller objects outside of the wave. The greater gravitational force draws in the smaller objects which further increases the mass of the wave.
One of the results of increased gravitational for is found in the motion of the stars. As the stars and gas travel around the galactic centre astronomers have noticed a wobble in the orbit, called an epicycle. This is said to be due to the gravitational pull of massive gas clouds as and the arms themselves. A resonance occurs if the time it takes to complete one epicycle is equal to the time it takes a star to travel from one arm to the next. If a galaxy is symmetrical then a star will complete one epicycle for every arm in the galaxy, so if there were two arms then the star will complete two epicycles per orbit. Resonance is when the star absorbs the energy from a gravity wave. The stars epicycle will then become larger. As well if all the energy from a spiral density wave is absorbed then the wave can not move beyond that arm.
Star Formation
Star formation takes place mainly in the spiral arms, where a galaxy can produce a few new stars per year. When matter is piled up there is a large amount of mass being compressed into a small amount of space. This compressed gas and dust is called a nebula. If the compressed gas is dense enough, 100-10,000 particles/cm3, and has a low enough temperature, about 10K, then this area is called a dark nebula, which is the birth place of stars. An example of a dark nebula is the Horsehead nebula. The densest part of the dark nebula will collapse first this happens when the area is so dense it collapses under its own gravitational force. When these gas clouds collapse a protostar is formed.
These protostars start are cold blobs of matter that start contracting to support its gaseous structure. As it contracts it begins to heat up and give off light. The protostar then turns into a star. When these stars are formed it is more typical of several small star forming rather than one large star. Therefore most stars formed are small with low mass and appear orange or red. The newly formed stars take their place and orbit around the centre of the galaxy. Most new stars have a short life span of 3-15 million years. For this reason new stars usually die before they are able to move out of spiral arm they were born in. These stars are most commonly found in or near the spiral arm they are born in. However these are not the only stars in the galaxy. There are also stars with much longer lifetimes. These stars can be found anywhere in the disk, both in arms and the space between the arms.
Now that we have a basic understanding of density waves there is still the matter of what keeps them going. A wave of this magnitude would need an extremely large amount of energy to keep it moving and compressing the gas and dust. A density wave would be expected to slowly fade away without the input of the needed energy. This is a topic that has been studied by American astronomers Debra and Bruce Elmegreen. They have a theory that the gravitational pull from within the galaxy is the supplier of the necessary energy, their theory is still being investigated though and doesn’t have much information on it yet. Another theory is that the energy needed may be supplied by the gravitational interactions with other galaxies rather that from within its own galaxy.
The Infall Theory
A third theory I found suggests that the spiral disks are form slowly from infalling matter. This falling matter is created due to perturbations with large collapse times. Perturbations are when an objects orbit is disturbed due to another objects gravitational pull. These disturbed objects will fall slowly into the galaxy with their tails following. The central bulge of the galaxy is actually a separate system, which has an angular momentum of about a third of that of the disk. The angular momentum is gained through tidal torque and conserved as the disk is built up from gases of falling shells. Density is dispersed exponentially through out the disk, which grows from the inside out. Due to this infalling material the disk is usually unstable. There is some stability created however as the infalling gas is changed into stars. Galaxies with a high infall rate create a system containing mostly young blue stars as well the system arms of the system will be open making larger angles with the central bulge. Also if the galaxy has a lower infall rate then the galaxy will contain mostly old, red stars and the arms will be tighter. The infall of material sustains the spiral structure, which would otherwise disappear after a couple of orbits. The shape of the spiral structure is affected by how fast matter is falling. The rate at which material falls into a galaxy is determined by the strength of the gravitational pull of the galaxy and the distance the material is away. This is because galaxies compete for material. The one with the strongest gravitational influence will pull the object from its orbit and it will start falling into the galaxy.
Concluding Remarks
In the last 60 years our civilization has made great advancements in the study of galaxies. Starting with the classification of galaxies to the theories of why they behave the way they do. The three theories I have outlined here are all very good guesses at what is going on in the vast space of a galaxy. I found the winding dilemma theory to be a great idea for the explanation of the spiral arms. As this theory needed more explanation the density theory was born giving answers to questions within the winding dilemma. However bringing the density theory brought out new questions such as energy sources for theory. These questions are still being answer as I was unable to find a lot of information on the topic. The third theory I found called the infall theory, was only found in one book. This is also an interesting theory but doesn’t seem as likely as the density theory, however that does not exclude from having the answers to out questions of what is happening out there.
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