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Galaxy classification
Galaxy cluster
Galaxies are found in groups and clusters.
Poor or rich clusters
The Milky Way belongs to the Local Group.
The nearest rich cluster is Virgo cluster.
Super cluster contains nearly 12 clusters.
The Local Group
The Coma cluster
The Coma cluster
Hubble’s law
The more distant a galaxy, the grater is the
redshift and it is receding from us more rapidly
The cosmological redshift is caused by the expansion of the Universe. Some nearby galaxies, however,
such as M31, show a blue shift rather than a red shift, showing that they have a physical motion toward
us that is larger then the cosmological redshift.
Expanding universe emerged from
Big Bang
Hubble flow shows universe
expanding for billions of years.
has
been
Past matter was closer together and so denser
than it is today
Astronomers combine mathematical models
with observations to develop workable theories
of how the Universe came to be.
According to the Big Bang model the universe
expanded from extremely dense and hot state
and continues to expand.
cosmic microwave background (CMB)
CMB, is radiation that fills the universe and can be detected in every direction.
When the CMB was initially emitted it was not in the form of microwaves at all, but mostly visible and ultraviolet
light. Over the past few billion years, the expansion of the universe has redshifted this radiation toward longer and
longer wavelengths, until today it appears in the microwave band.
CMB represents the earliest radiation that can be detected.
CMB shows temperature fluctuations that correspond to regions of slightly different densities at very early times, representing the
seeds of all future structure: the stars and galaxies of today.
Galaxy formation
After the big bang, collapsing gas and dust
clouds might have lead to the formation of
galaxies.
Interactions between galaxies, specifically
collisions between galaxies, play an important
role in their evolution.
Gravitational Lensing
Gravitational lensing effect of Einstein's theory of general relativity – Here, mass bends light.
Gravitational lensing was actually discovered for quasar before it was found for galaxies. Two quasars only 6 seconds of
arc apart were discovered and they were found to have identical redshifts and spectra.
Black hole
An object from which neither matter nor EM-radiation can escape is called black hole.
Surrounding a black hole, where the escape speed from the black hole equals the speed
of light, is the event horizon.
Distance from the center of a non rotating black hole to its event horizon is
Schwarzschild radius.
The point in side black hole where the entire mass crushed to zero volume is known as
singularity.
An object from which neither matter nor EM-radiation can escape is called black hole.
Surrounding a black hole, where the escape speed from the black hole equals the speed
of light, is the event horizon.
Distance from the center of a non rotating black hole to its event horizon is
Schwarzschild radius.
The point in side black hole where the entire mass crushed to zero volume is known as
singularity.
According to theory, there are three types of black holes: stellar,
supermassive, and primodial black holes
Sometimes relativistic jets of hot plasma are formed by strong magnetic
fields in the material around a rotating black hole.
Nearly every nearby galaxy appears to have a super-massive black hole
(∼10^6 to 10^9 M⊙ ) at its center
Amongst the galaxies harboring a super-massive black hole, a small fraction
emit an extraordinary amount of radiation across the entire band of the
electromagnetic spectrum. These galaxies are known as ‘active galaxies’
and their highly luminous nuclei as ‘active galactic nuclei’ (AGN).
Radio galaxies
hotspot
jet
core
FRII
FRI
The key emission process of radio galaxy is synchrotron emission which implies the presence of
relativistic electrons and magnetic fields.
The spectrum results from summing the emission spectra of individual electrons. As the electron
spirals around the magnetic field, it emits radiation over a range of frequencies peaking at ν0 ,
the critical frequency.
The synchrotron emission spectrum follows a power-law decay, where flux steadily declines with
frequency according to the
relation: F ∝ ν −α