In the universe there are thousands of galaxies exist in a variety of styles, shapes and sizes. In 1926 Edwin Hubble set out to classify the different types of galaxies that were being researched by the astronomers. the Hubble classification systems categorizes galaxies by their shapes as seen from the telescopes. As shown below, galaxies are divided into elliptical and spiral. Spiral galaxies are broken down into normal spiral and barred spiral.
Within the spiral classification of the galaxies there are two main types one is the regular spiral and the another one is the barred spiral.
They contain arms that spiral out from the center.
They have the appearance of a straight bar that spiraled arms extend out from.
Spiral galaxies are usually found in less galaxy dense regions of the universe. They consist of a flat rotating disk containing stars, gas and dust.
In the past 10 years, evidence has shown that black holes are found at the center of almost all spiral galaxies, including our own, the milky way. Surrounding the black hole is a central bulge of older stars, with dim stars creating a halo effect around the bulge. Younger stars in the galaxy are found towards the arms of the galaxy in the rotating disc matter.
It is believed that the galaxy we reside in, the milky way, is a spiral galaxy. Our sun is believed to be one of the arms of the milky way galaxy, meaning that at 4.5 billion years old, it is relatively old, it is relatively young.
Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long thin regions resemble a spiral and thus give spiral galaxies their name. naturally different classifications of spiral galaxies have distinct arm structure.
Some spiral galaxies have a bright line or bar running through them. these are called the barred spiral galaxies. Spiral galaxies are further classified by how tightly their spiral are wound. A galaxy with a very tight wound arms would be called as type A galaxy and the galaxy with a loose arm wound are called as the type B galaxy on the other hand the galaxies which have more loose arms than the type B galaxies, are known as the type C galaxy.
The arms of the galaxies have al lot of dust and gas, and they are often found in the areas where new stars are constantly forming. The bulge of the spiral galaxy is composed primarily of old red stars. very little star formation goes on in the bulge.
A bulge is a large tightly packed group of stars. The term refers to the central group of stars found in the most spiral galaxies, often defined as the excess of stellar light above the inward extrapolation of the outer disk light.
Using the Hubble classification, the bilge of SA galaxies is usually composed of the population II stars, which are old, red stars with low mental content. Further the bulge of SA and SB a galaxy tends to be large. In contrast the bulges of the SC and SBC galaxies are much smaller and consists of younger and blue population I stars. Some bulges have similar properties to these of elliptical galaxies others simply appear as higher density centers of disks, with properties similar to disk galaxies.
Many bulges are tough to host a supermassive black hole at their centers. In our own galaxy for instance the object called the Sagittarius A is believed to be a supermassive blackhole. There are many lines of evidence for the existence of black holes in the spiral galaxy center, including the presence of the active nuclei in some spiral galaxies, and dynamical measurement that find large compact central masses in galaxies such as NGC 4258.
Bar shaped elongations of stars are observed in roughly two thirds of all spiral galaxies. Their presence may be either strong or weak. In edge on spiral galaxies, the presence of the bar can sometimes be discerned by the out of plane X shaped or (peanut shell) shaped structure which typically have a maximum visibility at half the length of the in-plane bar.
The bulk of the stars in a spiral galaxy are located either close to a single plane in more or less conventional circular orbits around the center of the galaxy, or in a spheroidal galactic bulge around the galactic core.
However, some stars inhabit a spheroidal halo or galactic spheroid, a type of a galactic halo. The orbital behavior of these stars is disputed, but they may exhibit retrograde and or highly inclined orbits or not move in regular orbits at all.
The pioneer of studies of the rotation of the galaxy and the formation of the spiral arms was Bertel Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. since the angular speed of rotation of the galactic disk varies with distance from the center of the galaxy would quickly become curved as the galaxy rotates. The arm of the would after a few galactic rotations become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem.
Measurement in the late 1960 showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from the Newtonian dynamics but still cannot explain the stability of the spiral structure.
Since the 1970s there have been two leading hypothesis or models of the spiral structure of the galaxies:
Star formation caused by the density of the waves in the galactic disk of the galaxy.
The stochastic self propagating star formation model caused by the shock waves are caused by the stellar winds and supernovae from the recent previous star formation, leading to self propagating and self-sustaining star formation. Spiral structure then arises from differential rotation of the galaxy’s disk
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