Galaxies and cosmology

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  • Created by: katie
  • Created on: 01-06-14 01:26

Milky Way the basics

Stars and therefore solar systems are not spread evenly across the universe instead they are grouped in huge gravitationally bound collections called galaxies.
We belong to the Milky Way, thus named because it looks like a pale'milky' strip accross the night sky.
The Milky Way has two small satellite galaxies called the Small Magellanic Cloud(SMC) and the Large Magellanic Cloud(LMC).
You can view the Milky Way unaided if there is minimal light pollution. We view it side on from our location lookking down on it it looks like a huge Catherine wheel with a central bulge.
One of our closest galaxies is the Andromeda Galaxy which is only 700kpc away.
Our galaxy is made of three main bits:
1.Disc-made of young stars, gas , dust and sites of star formation, 4 real spiral arms.
2.Bulge-around the nucleus, strong belief that the center is a black hole.
3.Halo-Globular Clusters, relativly old stars in groups, big cirlce around the nucleus spread out not in disc.
You can't see through the spiral arms to view the structure, but they can detect the electromagnetic raditation from the changing direction of the electrons in hydrogen, the doppler effect alters the wavelengths, the amount of cahnge shows the relative velocities of the gas therefore showing how parts of the galaxy rotate.

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Galaxies the basics

Edwin Hubble's scheme for classifying galaxies based on apperance:
1.Spiral-flat disc shape
2.Barred spiral-same as spirla but there is a bright 'bar' which goes through th nucleus with arms emerging from it.
3.Elliptical-smooth spherodical distrabution, brightness decrease as distance from the center increases
4.Irregular-little structure or symmetry
This scheme is often shown on a tunning fork diagram as there are sub catagories for all but irregualr galaxies.
Spiral and barred spiral galaxies are classified with either a,b or c, a has the largest bulge with the most closed arms, c has the smallest bulge with the most open arms.
Elliptical glaxies are classified with a number between 0-7, 0 is the most spherical and 7 is the most elongated.
S0 is a special sub catagory for galaxies which are both spiral and elliptical galaxies
The Milky Way has been classified S(spiral)b, but there is increasing evidence that it has a bar meaning it would need to be reclassified as a SBb.

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Local Group and Clusters & Superclusters

Local Group
Stars collect in galaxies, galaxies collect in small and large groups.
The Milky Way is part of the Local Group containning 50 glaxies it is 50 Mpc across.
Within the Local Group are: Andromeda, Large and Small Magellanic Clouds, Triangulum and several satellite & dwarf galaxies.

Clusters & Superclusers
Looking bigger again galaxies are found in gravitational agglomerations-rich clusters, the central region of these clusters often contain a giant elliptical galaxy.
Scaling up again clusters come together to make superclusters, superclusters are also held together by gravity.

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Active Galaxies

Also known as Active Galaxtic Nuclei(AGN), these are galaxies which emmit wavelengths other than visible light,-radio and x ray- they are the most powerful energy source in the universe, there energy is thought to come from a small central nucleus hence the name AGN.
There are four main types:
1.Radio galaxies-discovered in the 1950's, they emit massie amounts of radio waves, these waves usually come from a symetrical pair of 'lobes' which lie on opposite sides of the galaxy and they are generally elliptical galaxies.
2.Seyfert galaxies-discovered 1943 by Carl Seyfert, spiral galaxies, compact bright nucleus and emits strong varying emmisions of IR and x-ray.
3. Quasars(quasi-stellar radio source or quasi stellar object), dscovered in 1963, they are really compact so they look like just 1 star, they emit high amounts across the spectrum from x-ray to radio waves and the huge redshift shows they are some of the most distant things in our universe.
4.Blazars-discovered 1970's, like quazars they appear to look like stars but they are much closer, they emit in all regions of the spectrum from gamma rays to radio rays and they can vary dramatically in optical brightness as well as timescale.

Now believed all AGN's are acctually all one type with slight variation jut viewed from a differnt position. It is also now widley agreed that black holes are at the cneter of all AGN's.

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Cosmology the basics

Cosmology is the study of the universe as a whole, its structure, its past and its future.
Doppler effect:
A source of waves motion relative to an observer results in thw waves frequency and wavelength being 'shifted', as they are inversely proportional they both must change.
Redshift- when the source is moving away the wavelength increases and teh frequeny decrease the waves are in essence being stretched so they shift towards the red end of the spectrum for visible light.
Blue shift-when the source is moving towards the observer the frequency increase and the wavelength decreases, the waves are squished so shift towards the blue end of the spectrum for visible light.
All waves typs are effected and the faster a source is moving the more shifted it is. The equation showing the radial speed(v) and amount of red/blue shift is:

λ-λ°=                                        observed wavelength-true wavelegth = radial velocity
λ°        c                                                      true wavelength                         speed of light

This shows the universe is expanding supporting the Big Bang theory as it shows it started at one specific point in time and space.

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Hubble's law

Hubble also found the more distant galaxies were more redshifted showing they were moving the fastest in comparison to us.
This relationship can be shown as the equation: v=Hd, v=recession velocity of a galaxy,d=distance(Mpc), and H=Hubble's constant-this changes as more data is added so it becomes more accurate most cosmologist think it is 77km/s/Mpc but with an uncertainty of 15%

Expanding universe
Cosmologist agree the universe is expanding there is lots of evidence and nothing which dirsctly opposes this conclusion.
Looking back it suggest the Universe has always been expansing ever since the Big Bang.
When looking at Hubble's constant this can be expressed in just seconds, cosmologist believe this to be the age of the Universe 13.8 billion years.

There is belief in The Steady State model which agrees that the Universe is expanding but thinks new matter is onstantly being created so the Universes density stays the same.
There is also some belief in the Cyclic Universe theory, where the universe goes around in a cycle with a series of Big Bangs and Big Crunches.
Neither of these have any hard evidence to validate them.

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Cosmic Microwave Background radiation

Massive argument in favour of the Big Bang, CMB is present everywhere in the Universe as far as we know.
In the black empty parts of space it is still detectable, it also has a temperature of 2.7K just above absolute 0 which also supports the theory that it is from the very begining of the Universe as it's thought the Universe was hot and dense then cooled as it exanded to just above absolute 0.

COsmic Bakground Explorer (COBE) was sent to collect data on CMB to show that it is always the same on a larger scale but it also picked up minor fluctuations in the CMB, this means cosmologis can study how matter and energy was distributed in the beginning and therefore how the Universe evolved and came to its current state.

Wilkinson Microwave Anisotropy Probe (WMPA) was launched to study CMB in more detail. The data it collects will allow cosmologists to study the Universe's age, composition and evolution as well as the contribution of dark matter and to confirm the existence of dark energy.

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Dark matter and dark energy

Dark matter:

Dark matter doesn't emit or reflect radiation so is in essence invisible but the effects of it can be seen, it has detectable gravitational effects. The nature of dark matter is unknownbut it could explain: black holes, neutrinos and WIPMs(Weakly Interacting Massive Particles).

Origionaly it was suggested to explain spiral galaxies movement, it also explains why light from galaxies can appear bent at times when there is apparently no force acting upon it.

Cosmologist think that 90% of the Universe's mass is made of dark matter this means we don't know how much matter is in the Universe meaning we don't know its density. If this density is too high it will exceed critical density meaning the gravitational force will force it to bgin to contract heading towards the big crunch.

Dark energy:
Untill recently it was believed the Universe's expansion was slowing but recent discoveries show the oposite, for this to be true there must be another force acting upon the expansion-negative gravity.
The proposed name for this negative gravity is dark energy as like dark matter we can't see it and don't understand it.

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