Galaxies and nebulae
Licenciatura en Ciencias de la Tierra
Facultad de ciencias
How are galaxies and nebulae formed?
In this project I will talk about the formation and constitution of galaxies and nebulae and the processes it involves according to “the bottom up model”
Galaxies are vast groupings of billions of stars and other material gravitationally bound together. Theytend to be found in clusters of dozens to a few hundred members. Different types of galaxies include spiral galaxies such as our own Milky Way, rich in dust and gas with stars still forming in their arms; elliptical galaxies devoid of gas, lenticular galaxies and irregular galaxies. (CSIRO, 2005)
A Model of Galaxy Formation
We do not yet have a definitive model of how galaxies form. Indeedthis is a lively and active area of debate among astrophysicists. One of the many problems they face in trying to construct a useful model is the need to match observations of galaxies in the current or nearby Universe with those and those in the early Universe with theoretical simulations. Recent work on the role of dark matter in the early Universe has led to the hierarchical or bottom-up modelgaining wide acceptance.
In the bottom-up model, the Universe contains cold dark matter (CDM). The "cold" in this case refers to the average speed of the dark matter particles; they are relatively slow compared to the speed of light. As the Universe expands these slight irregularities and the resultant gravitational instabilities cause gas clouds to collapse, forming extremely high-mass stars.These are thought to be among the first structures to form after the decoupling era. Gravity therefore plays the dominant role in galaxy formation.
These massive stars in turn form clusters of stars and gas. These structures then interact and merge into larger structures now known as galaxies.
Where galaxy interactions are common successive mergers are thought to mop up gas not boundin stars and smooth the shape. Large elliptical galaxies are the result. Examples of these in the nearby Universe show little or no free gas and low rates or no star formation.
If a galaxy evolves in relative isolation with few interactions then a spiral galaxy is likely to be the result. These still have gas in their spiral arms. We can observe star formation still taking place in the armsof spirals.
Image 1.1 (Obtained de Brad Whitmore (STScI) and NASA)
Formation of the first galaxies
After the Big Bang, the universe, for a time, was homogeneous. There was little-to-no structure in the universe, and thus no galaxies.
As the universe cooled clumps of dark matter began to condense, and within them gas began to condense. The primordial fluctuationsgravitationally attracted gas and dark matter to the denser areas, and thus the seeds that would later become galaxies were formed. These structures constituted the first galaxies. At this point the universe was almost exclusively composed of hydrogen, helium, and dark matter. Soon after the first proto-galaxies formed, the hydrogen and helium gas within them began to condense and make the firststars. Thus the first galaxies were then formed. (Wikipedia, 2012)
There are different types of galaxies. We have “protogalaxies”. After recombination, density enhancements either grew or dispersed. According to our hybrid top-down/bottom-up scenario, an assortment of enhancements formed of various sizes. Small, dense ones collapsed first, large ones formed slower and fragmented as theycollapsed.
The first lumps that broke free of the Universe's expansion were mostly dark matter and some neutral hydrogen with a dash of helium. Once this object begins to collapse under its own gravity, it is called a protogalaxy. The first protogalaxies appeared about 14 billion years ago.
Image 1.2 (Obtained de S.G. Djorgovski et al. & Digital Media Center, Caltech)
As we can see,...
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