Black hole is a region of space having a gravitational field so intense that no matter or radiation can escape.

1. What is a Black hole?
- a place in space where gravity pulls so much that even light cannot get out. The
gravity is so strong because matter has been squeezed into a tiny space. This can
happen when a star is dying.
- form after a massive star runs out of fuel, sometimes resulting from a supernova
and other times without a supernova, which is called the direct collapse scenario.
Once a star has no fuel left to burn, it can no longer support its mass and
collapses.
 There are two basic parts to a black hole: the singularity and the event
horizon.
- The event horizon is the "point of no return" around the black hole. It is not a
physical surface, but a sphere surrounding the black hole that marks where the
escape velocity is equal to the speed of light. Its radius is the Schwarzschild
radius mentioned earlier.
- One thing about the event horizon: once matter is inside it, that matter will fall
to the center. With such strong gravity, the matter squishes to just a point – a
tiny, tiny volume with a crazy-big density. That point is called singularity. It is
vanishingly small, so it has essentially an infinite density. It's likely that the laws
of physics break down at the singularity.
What is a Supermassive black hole?
- The largest black holes are called "supermassive."
- These black holes have masses that are more than 1 million suns together.
- every large galaxy contains a supermassive black hole at its center.
- The supermassive black hole at the center of the Milky Way galaxy is called
Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a
very large ball that could hold a few million Earths.
If light can't escape a black hole, how can we see black holes?
- Astronomers don't exactly see black holes directly. Instead, astronomers observe
the presence of a black hole by its effect on its surroundings. A black hole, by itself
out in the middle of our galaxy would be very difficult to detect.
- Because no light can get out, people can't see black holes. They are invisible.
Space telescopes with special tools can help find black holes. The special tools
can see how stars that are very close to black holes act differently than other
stars.

2. - NASA is using satellites and telescopes that are traveling in space to learn more
about black holes. The spacecraft help scientists answer questions about the
universe.
Types of Black Holes
Stellar-mass black holes are formed when a massive
star runs out of fuel and collapses. They are found
scattered throughout the galaxy, in the same places
where we find stars since they began their lives as stars.
Supermassive black holes are found at the center of
nearly every large galaxy.
Recent studies have shown that the size of the black
hole is correlated with the size of the galaxy so there
must be some connection between the formation of the
black hole and the galaxy.
Black Hole or Neutron Star?
When stars die, depending on their size, they lose mass and become denser until
they collapse in a supernova explosion. Some turn into endless black holes that devour
anything around them, while others leave behind a neutron star, which is a dense remnant
of a star too small to turn into a black hole.
Where Could Supermassive Black Holes Exist?
Astronomers believe that supermassive black holes lie at the center of virtually all
large galaxies, even our own Milky Way. Astronomers can detect them by watching for
their effects on nearby stars and gas.
One possible mechanism for the formation of supermassive black holes involves a
chain reaction of collisions of stars in compact star clusters that results in the buildup of
extremely massive stars, which then collapse to form intermediate-mass black holes. The
star clusters then sink to the center of the galaxy, where the intermediate-mass black
holes merge to form a supermassive black hole.

3. Active Galactic Nuclei
- Active galactic nucleus (AGN), a small region at the center of a galaxy that emits a
prodigious amount of energy in the form of radio, optical, X-ray, or gamma
radiation or high-speed particle jets.
- The observed energy is generated as matter accretes onto a supermassive black
hole with a mass of millions or even billions of times that of the Sun. The accreting
matter can outshine the rest of the galaxy as it is heated in very high-speed
collisions outside the black hole’s event horizon.
Theories about Supermassive Black Hole come about.
1. Lumps from the Early Universe
The lumps from the early universe in the “Big Bang” the whole universe was in a really
dense state. So much that perhaps lumps could have formed and of matter dense enough
that a black hole was formed.There was enough surrounding matter that galaxies formed
around the lumps. A black hole is an extremely dense astronomical object from which
nothing can escape, not even light. When a star explodes in a supernova, a black hole can
be left behind. Alternatively, a supermassive star can burn through its fuel quickly and

4. turn into a black hole, no explosion needed. Scientists say this is how many massive black
holes form in rapidly assembling proto-galaxies.
2. The Stellar Seed Model
Provided that the surrounding environment is sufficiently rich in matter, a giant black
hole could result in an initial “stellar seed” of 10 M sun produced during a supernova.
3. Collapse of a Whole Star Cluster
- If the stars of a tight-knit cluster of the moderately young Universe had all relatively
the same size stars (above the Chandrasekhar Limit), there would be quite a few
Black Holes forming simultaneously causing smaller stars to be absorbed, and
black holes to combine. When the core runs out of hydrogen, these stars fuse
helium into carbon just like the sun. However, after the helium is gone and the core
has turned to iron, it can burn no longer. The star collapses by its own gravity and
the iron core heats up. The core becomes so tightly packed and shrinks to a
neutron core with a radius of about 6 miles (10 kilometers). The outer layers of the
star fall inward on the neutron core, thereby crushing it further. The core heats to
billions of degrees and explodes making them release large amounts of energy
and material into space. The shock wave from the supernova can initiate star
formation in other interstellar clouds. The remains of the core can form a
neutron star, black hole, or supermassive black hole depending upon the
mass of the original star.
Characteristics of AGN
1. Optical Jets
The magnetic fields around a black hole that are thought to produce the spectacular
jets of high-energy particles rushing away from black holes come from the disk of hot gas
around the black hole, not the black hole itself. The jets are made by the Magnetic field
of the matter before it goes into the Black Hole.
Emit Synchrotron radio signals
2. Tidal forces stretch farther, but are weaker
The tidal force is proportional to the mass of the black hole. In other words, as the
object gets more massive, the force should get bigger too. But the force is also inversely

5. proportional to the cube of the object's radius. As the hole gets more massive, its size
increases, but because of the cube factor, the force decreases much faster than any
possible mass increase can account for. The result is that big black holes have weak tidal
forces, and small ones have strong tidal forces.
3. Cannibalism
- Galactic cannibalism refers to the process in which a large galaxy, through tidal
gravitational interactions with a companion, merges or eats another galaxy
resulting in a larger, often irregular galaxy. One galaxy may have eaten one of its
slightly smaller, more primitive neighbors, that then spilled all its stars into the
outskirts.
Theory How Supermassive black holes formed
Once these giants have formed, they gather mass from the dust and gas around them,
material that is plentiful in the center of galaxies, allowing them to grow to even more
enormous sizes. Once these giants have formed, they gather mass from the dust and
gas around them, material that is plentiful in the center of galaxies, allowing them to
grow to even more enormous sizes.

6. Galaxy Classification
History
- In the 1600s and 1700s, the first astronomers to use telescopes to study the
heavens noted the presence of fuzzy patches of brightness which they called
nebulae. In the 1920s, astronomers came to understand that some of these
nebulosities are actually galaxies – systems of tens, hundreds, or even thousands
of billions of stars (and interstellar gas and dust) organized by their common
gravitational field. After astronomers discovered what galaxies really were, they
began to sort them according to their appearance and this is where the galaxy
classification came into play.
What is a galaxy?
- originally called white nebulae is a collection of millions of stars, stellar remnants,
interstellar gases, dust, and dark matter that are held together by gravitational
forces and extend over many billions of light-years.
Since astronomers discovered a lot of galaxies, in 1924, astronomer Edwin Hubble
set up a system to classify these galaxies and that is what we called the Hubble Sequence.
Hubble Sequence
- divides regular galaxies into three broad classes based on their visual appearance
or the originally recorded on photographic plates. These are the ellipticals,
lenticulars, and spirals. A fourth class contains galaxies with an irregular
appearance.
- Spirals are divided into two groups, normal and barred. The elliptical galaxies, and
both normal and barred spiral galaxies, are subdivided further.
What is Hubble Tuning Fork?
This Figure is called the “Hubble Tuning Fork” and this
is a morphological Galaxy classification scheme is
shaped like a tuning fork.
What are the different galaxy classifications?
a. Spiral
b. Elliptical

7. c. Lenticular
d. Irregular
Classification of Galaxies
a. Spiral Galaxies
- twisted collections of stars and gas that often have beautiful shapes and are made
up of hot young stars (e.g. Milky Way).
- spiral arms contain a wealth of gas and dust which means spiral galaxies have an active
star formation. Dust lanes in the spiral arms are created by previous generations of stars
that have died and seeded the galaxy with stellar material.
In Hubble classification, spiral galaxies
which are denoted by ‘S’ are sub-divided into
two groups - normal spirals and barred spirals.
The most important difference between
these two groups is the bar of stars that runs
through the central bulge in barred spirals.
The spiral arms in barred spirals usually start
at the end of the bar instead of from the
bulge. They do not twist all the way into the
center. It is thought that barred spiral galaxies
have a mechanism that channels gas to the center of the galaxy, possibly to a
supermassive black hole. The spirals were assigned letters from "a" to "c," which
characterize the compactness of their spiral arms.
Structure of Spiral Galaxy

8. Spiral galaxies may consist of several distinct components:
 A flat, rotating disc of stars and interstellar matter of which spiral arms are
prominent components
 A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
 A bar-shaped distribution of stars
 A near-spherical halo of stars, including many in globular clusters
 A supermassive black hole at the very center of the central bulge
 A near-spherical dark matter halo
Parts of a Spiral Galaxy
Spiral Arms
Spiral arms are regions of stars that extend from the center of barred and
unbarred spiral galaxies. Either way, spiral arms contain many young, blue stars (due to
the high mass density and the high rate of star formation), which make the arms so
bright.
Bulge
A bulge is a large, tightly packed group of stars. The term refers to the central
group of stars found in most spiral galaxies, often defined as the excess of stellar light
above the inward extrapolation of the outer (exponential) disk light. Using the Hubble
classification, the bulge of Sa galaxies is usually composed of Population II stars, which
are old, red stars with low metal content. Many bulges are thought to host a
supermassive black hole at their centers. In our galaxy, for instance, the object called
Sagittarius A* is believed to be a supermassive black hole. There are many lines of
evidence for the existence of black holes in spiral galaxy centers, including the presence

9. of active nuclei in some spiral galaxies, and dynamical measurements that find large
compact central masses in galaxies such as Messier 106.
Bar
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 (and lenticular)
galaxies, the presence of the bar can sometimes be discerned by the out-of-plane X-
shaped or (peanut shell)-shaped structures which typically have maximum visibility at
half the length of the in-plane bar.
Spheroid
The bulk of the stars in a spiral galaxy are located either close to a single plane
(the galactic plane) in more or less conventional circular orbits around the center of the
galaxy (the Galactic Center), or a spheroidal galactic bulge around the galactic core.
Elliptical Galaxies
Elliptical galaxies make up roughly half of the galaxy population. They have a smooth
featureless light distribution and appear elliptically shaped in photographic images. There
is very little star formation going on within them. They are found near the center of rich
galaxy clusters. They are gas-poor and dust-poor so no new stars are formed yet they
contain more than ten trillion stars.
Elliptical galaxies have a broader range in size than other types of galaxies. The
smallest are dwarf elliptical galaxies, which can be less than 10 percent of the size of the
Milky Way. But ellipticals can also stretch to more than a million light-years.
b. Elliptical Galaxies
- While spiral galaxies are bright, elliptical galaxies are dim.
- Spiral galaxies are hotbeds of star formation, but elliptical contain less gas and
dust, which means fewer new (and brighter) stars are born.
- The existing stars inside an elliptical galaxy tend to be older, giving off more red
light than younger stars.
- Because elliptical galaxies contain older stars and less gas, scientists think that they
are nearing the end of the evolutionary line for galaxies.

10. - A supermassive black hole is thought to lie at the center of these ancient galaxies.
These gluttonous giants consume gas and dust and
may play a role in the slower growth of elliptical
galaxies.
- Elliptical galaxies lack the swirling arms of their more
well-known siblings, spiral galaxies. Instead, they bear
the rounded shape of an ellipse, a stretched-out
circle.
- M87, identified as one of the largest galaxies in the
universe, is classified as an E0 elliptical galaxy.
In the Hubble sequence, they are designated as type E, with a subclassification
based on a measure of the ellipticity of their appearance.
Ellipticity- the degree of deviation from circularity (or sphericity).
In Hubble’s classification scheme, Galaxies classified as E0 appear to be almost
perfect circles, while those listed as E7 seem elongated or much longer than they are
wide. Galaxies with higher ellipticities have higher numbers.
c. Lenticular Galaxies
At the center of the Hubble tuning fork,
where the two spiral-galaxy branches and the
elliptical branch join, lies an intermediate class of
galaxies known as lenticulars and given the symbol
S0. Much like spiral galaxies they are disc-shaped
with a large central bulge but have no discernable
spiral arms. They have less interstellar matter than
spiral galaxies and, like elliptical galaxies, have little
ongoing star formation and mostly consist of aging
stars.

11. d. Irregular Galaxy
- designated by Irr, is the catchall name given to any galaxy that does not neatly fit
into one of the categories of the Hubble classification scheme.
- They have no defined shape nor structure and may have formed from collisions,
close encounters with other galaxies, or violent internal activity.
- They are among the smallest galaxies and are full of gas and dust. Having a lot of
gas and dust means that these galaxies have a lot of star formation going on within
them. This can make them very bright.
- About 20% of all galaxies are irregulars.
Instruments Used
Almost all galaxies are so incredibly distant
that they are too faint to be seen with the naked
eye (the Large and Small Magallenic Clouds and
the Andromeda Galaxy are the only three
exceptions). Small telescopes allow humans to
directly view a few scores of galaxies. However,
to study the faint details of galaxies more closely
astronomers generally collect the light with
photographic film or digital image detectors.
Now, there are several schemes used in galaxy classification because Hubble’s
Sequence was expanded and revised by several astronomers like Gérard de
Vaucouleurs and Allan Sandage because they believed that the Hubble Sequence does
not correspond to an evolutionary sequence of a galaxy during different phases of its life.
However, the Hubble Sequence is the most commonly used system for classifying
galaxies, both in professional astronomical research and in amateur astronomy.

12. Cosmology
What is Cosmology?
- cosmology comes from the Greek word “kosmos” which means universe, and
“logy” which means the study of.
- a relatively new branch of physical science that is the study of the universe
including its origin and development.
History
- Most early cosmologies are based on some form of anthropomorphism or the
perception of a divine being or beings in human form or the recognition of
human qualities in these beings. Some involve the idea that the physical world is
animated by supernatural beings that can help or harm mankind.
The modern era of scientific cosmology began with Einstein’s general theory of relativity,
published in 1915.
What is the general theory of relativity?
- general theory of relativity is a theory of gravity.
- basic idea is that instead of being an invisible force
that attracts objects to one another, gravity is a
curving or warping of space. The more massive an
object, the more it warps the space around it.
- general theory of relativity talks about gravity as a
warping space, and the principle of time dilation.
- allows us to understand not the origin of the cosmos in space and time, but the
origin of space and time themselves.
- forms the basis of the modern Big Bang model, which has emerged as the best
available description of the expanding Universe. The Big Bang is an example of
what relativity theorists call a singularity, a region
of space where the curvature of spacetime
becomes infinite.

13. Now, Einstein’s special theory of relativity was published in 1905. It stands
as one of the greatest intellectual achievements in the history of
human thought.
What is Special relativity?
- an explanation of how speed affects mass, time, and space.
As an object approaches the speed of light, the object's
mass becomes infinite and so does the energy required to
move it. That means no matter can go faster than light travels.
- The main difference between general relativity and special relativity is
that general relativity is related to gravity and acceleration, whereas special
relativity is related to speed and time.
There is no doubt that the general and special theory furnishes an elegant conceptual
framework, however, is that it involves some of the most difficult mathematics ever
applied to a description of nature. To give some idea of the complexity involved, it is
useful to compare Einstein’s theory with the older Newtonian approach.
Newton’s gravity is hard enough to apply in realistic situations, but Einstein’s theory is an
absolute nightmare. For one thing, instead of Newton’s one equation, Einstein has no less
than ten, which must all be solved simultaneously.
So, with precious little observational evidence to go on, Einstein decided that he would
simplify the Universe he described by making it homogeneous (i.e. the same in every
place) and be isotropic (i.e. looking the same in every direction). These twin
assumptions together form the Cosmological Principle.
What is Copernican Principle?
- The principle that we do not live in a special place in the Universe.
- Observed isotropy, together with the Copernican Principle implies the
Cosmological Principle.
What is Cosmological Principle?
- the universe is about the same in all places when viewed on a large scale.
- The universe is isotropic then which means you will see no difference in the
structure of the Universe as you look in different directions.
- When viewed on the largest scale, the Universe looks the same to all observers,
and the Universe looks the same in all directions as viewed by a particular observer.

14. Cosmological Principle