A star is a luminous sphere of plasma, with the sun being the nearest to Earth. Stars vary significantly in size and color, determined by their temperature, composition, and distance, while their life cycles can lead to phenomena such as supernovae and the formation of black holes. The Milky Way galaxy, which contains the solar system, hosts billions of stars, and its dynamics, as well as the life cycles of stars within it, contribute to the cosmic landscape we observe today.

1. STAR
A star is a luminous sphere of plasma held together by its own
gravity. The nearest star to Earth is the Sun. Other stars are visible
from Earth during the night, appearing as a multitude of fixed
luminous points in the sky due to their immense distance from Earth.

3. Birth of a Star

4. How does a Star Shine?

5. Dwarfs and Supergiants
• There are many different types of star.
• Supergiant stars are the largest stars in the universe. They can be
thousands of times bigger than our Sun and have a mass up to 100 times
greater. The largest known supergiant star, VY Canis Majoris, is up to
2,100 times the size of the Sun (based on upper estimates).
• Red stars are the coolest, with surface temperatures of around
2500 degrees Celsius. Blue-white stars are the hottest, reaching a
sizzling 40000 degrees. Yellow stars like the Sun are in-between,
with surface temperatures of about 5500 degrees.

6. Why stars have different
colours?
The difference in colours actually depends on a lot of different
factors. The first is the composition of a star. While stars are all
basically composed of atoms some stars have other trace elements in
them that can alter the wavelengths of light that they emit. The next
factor is surface temperature. This is the most significant contributor
to a star colour. The change in temperature changes the wavelength
of light a star emits. The last factor is distance in relation to the
Doppler Effect.

7. Composition does play a role in the colour of light a star emits. This
is because not all stars are formed exactly the same way. The reason
is due to being formed in different nebulas. Nebulas in the interstellar
medium are largely composed of hydrogen the main fuel for star
creation. However they do carry other elements. The mix of these
other elements varies from nebula to nebula. The change in colour
these elements add to stars is not very obvious. However it is
important in the field of spectro-analysis. Depending on the kind
light an element burns scientist can analyse a star light and basically
determine its elemental composition.

8. The most important factor to a star colour has to be its surface
temperature. If you have ever seen an open flame you would
understand why. A blue flame is flame burning at very high
temperatures. A yellow flame has temperatures that are cooler than
blue flames and red flames are the coolest of all. The same thing
happens with stars. Very hot stars tend to be blue stars. Stars with
an average surface temperature become yellow stars like our sun.
The coolest stars are Red Dwarfs. We call these temperatures as
colour temperatures and this is due to the phenomenon called black
body radiation.

9. Black Body Radiation
• A black body (also, blackbody) is an idealized physical body that
absorbs all incident electromagnetic radiation, regardless of
frequency or angle of incidence. A white body is one with a "rough
surface [that] reflects all incident rays completely and uniformly in
all directions.
• Black-body radiation is the type of electromagnetic radiation
within or surrounding a body in thermodynamic equilibrium with
its environment, or emitted by a black body (an opaque and non-
reflective body) held at constant, uniform temperature. The
radiation has a specific spectrum and intensity that depends only
on the temperature of the body.

12. Doppler Effect
• Sometimes the same star seems to appear in a different colour this
is due to the star shift similar to that of doppler effect in sound
waves.
• As the stars move away, the wavelengths from the light they emit
stretch. They shift to the red end of the 8
• spectrum because that end has longer wavelengths. Cosmologists
call this phenomenon the redshift. A star's redshift is an
indication of how quickly it is moving away from Earth. The
further toward the red end of the spectrum the light shifts, the
faster the star is moving away.

14. Star Death
Most stars take millions of years to die. When the core runs out of
hydrogen fuel, it will contract under the weight of gravity. However,
some hydrogen fusion will occur in the upper layers. As the core
contracts, it heats up. This heats the upper layers, causing them to
expand. As the outer layers expand, the radius of the star will
increase and it will become a red giant.
The radius of the red giant sun will be just beyond the Earth's orbit.
At some point after this, the core will become hot enough to cause
the helium to fuse into carbon. When the helium fuel runs out, the
core will expand and cool. The upper layers will expand and eject
material that will collect around the dying star to form a planetary
nebula. Finally, the core will cool into a white dwarf and then
eventually into a black dwarf. This entire process will take a few
billion years.

15. Stars heavier than eight times the mass of the Sun end their lives
very suddenly. When they run out of fuel, they swell into red
supergiant. They try to keep alive by burning different fuels, but this
only works for a few million years. Then they blow themselves apart
in a huge supernova explosion.
For a week or so, the supernova outshines all of the other stars in its
galaxy. Then it quickly fades. All that is left is a tiny, dense object –
a neutron star or a black hole – surrounded by an expanding cloud of
very hot gas.
The elements made inside the supergiant (such as oxygen, carbon
and iron) are scattered through space. This stardust eventually
makes other stars and planets.

16. SUPERNOVA
• Every now and again our Milky Way galaxy is lit up by a huge explosion.
Known as a supernova, this violent event marks the death of a
supergiant – a heavyweight star which is many times bigger than the Sun.
One of the last supernovas in the Milky Way took place about 340 years
ago in the constellation of Cassiopeia, so it is known as Cassiopeia A
(Cas A). Cas A is located ten thousand light-years from Earth.
Observatories such as the NASA-ESA Hubble Space Telescope have
made detailed studies of the left-over cloud of glowing gas and dust.
• The images show a shredded ring of material that is moving rapidly away
from the site of the explosion. Some of the material is moving at about
50 million km per hour (fast enough to travel from Earth to the Moon in
30 seconds!). The huge swirls of debris are glowing because they have
been heated by the shock wave from the supernova as it passed by.

17. • There are several types of supernova explosions. Cas A blew up when a
small star, known as a white dwarf, pulled a lot of material from a nearby
star. As the gas built up, the white dwarf became so hot and active that
it exploded. Other supernovas occur when massive stars run out of
nuclear fuel in their cores. Unable to give out any more energy, the core
collapses, destroying the star.
• Supernovas are important because they spread star material across the
galaxy. Almost everything on Earth (including us!) is made of elements
(such as carbon and iron) that came from this stardust.

19. WHITE DWARF
A white dwarf, also called a degenerate dwarf, is a stellar remnant composed
mostly of electron-degenerate matter. They are very dense; a white dwarf's
mass is comparable to that of the Sun, and its volume is comparable to that
of the Earth. Its faint luminosity comes from the emission of stored thermal
energy.
White dwarfs are thought to be the final evolutionary state of all stars whose
mass is not high enough to become a neutron star—over 97% of the stars in
the Milky Way. After the hydrogen–fusing lifetime of a main-sequence star
of low or medium mass ends, it will expand to a red giant which fuses helium
to carbon and oxygen in its core by the triple-alpha process.

20. If a red giant has insufficient mass to generate the core temperatures
required to fuse carbon, around 1 billion K, an inert mass of carbon and
oxygen will build up at its center. After shedding its outer layers to form a
planetary nebula, it will leave behind this core, which forms the remnant
white dwarf. Usually, therefore, white dwarfs are composed of carbon and
oxygen. If the mass of the progenitor is between 8 and 10.5 solar masses,
the core temperature is sufficient to fuse carbon but not neon, in which case
an oxygen-neon–magnesium white dwarf may be formed. Also, some helium
white dwarfs appear to have been formed by mass loss in binary systems.

21. The material in a white dwarf no longer undergoes fusion reactions, so the star
has no source of energy, nor is it supported by the heat generated by fusion
against gravitational collapse. It is supported only by electron degeneracy
pressure, causing it to be extremely dense.
A white dwarf undergoes carbon detonation if it has a normal binary star
companion close enough for the dwarf star to draw sufficient amounts of matter
onto itself, the siphoned matter having been expelled during the process of the
companion's own late stage stellar evolution.
If the companion supplies enough matter to the dead star, the white dwarf's
internal pressure and temperature will rise high enough to fuse the previously
unfusable carbon in the white dwarf's core. Carbon detonation generally
occurs when the accreted matter pushes the white dwarf's mass close to the
Chandrasekhar limit of roughly 1.4 solar masses.

22. A white dwarf is very hot when it is formed, but since it has no source of
energy, it will gradually radiate away its energy and cool. This means that
its radiation, which initially has a high color temperature, will lessen and
redden with time. Over a very long time, a white dwarf will cool to
temperatures at which it will no longer emit significant heat or light, and it
will become a cold black dwarf. However, the length of time it takes for a
white dwarf to reach this state is calculated to be longer than the current
age of the universe (approximately 13.8 billion years), and since no white
dwarf can be older than the age of the universe, it is thought that no black
dwarfs yet exist. The oldest white dwarfs still radiate at temperatures of a
few thousand kelvins.

23. NEUTRON STAR
A neutron star is a type of stellar remnant that can result from the
gravitational collapse of a massive star after a supernova. Neutron stars are
the densest and smallest stars known to exist in the universe; with a radius
of only about 12–13 km (7 mi), they can have a mass of about two times
that of the Sun.

24. Galaxy
A galaxy is a gravitationally bound system of stars, stellar remnants,
interstellar gas and dust, and dark matter. Nearly all stars belong to
gigantic groups known as galaxies.
The Sun is one of at least 100 billion stars in our galaxy, the Milky
Way. And there are billions of galaxies in the Universe.
Everywhere we look in the sky there are galaxies of different shapes
and sizes. Some are spirals, with curved arms wrapped around a
bright central core.
Some have a bar of stars across the centre, with arms attached at
either end. Others have no recognisable shape at all. The largest
galaxies look like squashed balls. They contain up to 10 million
million stars, but they have very little gas or dust. Nearly all galaxies
have a supermassive black hole at the centre.

25. • Galaxies were born only a few hundred million years after the
Universe was created. At that time, about 13 billion years ago,
the galaxies were small and much closer together. Collisions were
common. As the galaxies smashed into each other they grew in
size and changed shape.
• Since then, the Universe has been expanding. Most galaxies are
moving apart at high speed, except in galaxy clusters where they
dance around each other.

26. The Milky Way Galaxy
• The Milky Way is a barred spiral galaxy some 100,000 – 120,000
light-years in diameter which contains 100–400 billion stars. It
may contain at least as many planets as well. The Solar System is
located within the disk, about 27,000 light-years from the
Galactic Center, on the inner edge of one of the spiral-shaped
concentrations of gas and dust called the Orion Arm.
• The Milky Way as a whole is moving at a velocity of approximately
600 km per second with respect to extragalactic frames of
reference. The oldest known star in the Milky Way is at least
13.82 billion years old and thus must have formed shortly after
the Big Bang.

27. • We live in one of the arms of a large spiral galaxy called the Milky
Way. The Sun and its planets (including Earth) lie in this quiet
part of the galaxy, about half way out from the centre.
• The Milky Way is shaped like a huge whirlpool that rotates once
every 200 million years. It is made up of at least 100 billion stars,
as well as dust and gas. It is so big that light takes100 000 years
to cross from one side to the other.
• The centre of the Galaxy is very hard to see because clouds of
gas and dust block our view. Scientists think that it contains a
supermassive black hole that swallows anything passing too close.

28. • Outside the main spiral are about 200 ball-shaped clusters of
stars. Each 'globular cluster' is very old and contains up to one
million stars. The Milky Way belongs to a cluster of at least 40
galaxies. The so-called Local Group has two large spiral galaxies
– the Milky Way and Andromeda.
• The others are much smaller. They include two galaxies that can
be seen with the naked eye from countries south of the equator.
The galaxies are called the Magellanic Clouds, after the
Portuguese explorer Ferdinand Magellan.

30. ANDROMEDA

32. BLACK HOLE
• A black hole is a geometrically defined region of spacetime exhibiting
such strong gravitational effects that nothing—including particles and
electromagnetic radiation such as light—can escape from inside it.
• The theory of general relativity predicts that a sufficiently compact mass
can deform spacetime to form a black hole.
• The boundary of the region from which no escape is possible is called the
event horizon. Although crossing the event horizon has enormous effect
on the fate of the object crossing it, it appears to have no locally
detectable features.

33. • In many ways a black hole acts like an ideal black body, as it reflects no
light.
• Quantum field theory in curved spacetime predicts that event horizons
emit Hawking radiation, with the same spectrum as a black body of a
temperature inversely proportional to its mass. This temperature is on
the order of billionths of a kelvin for black holes of stellar mass, making
it essentially impossible to observe.
• Hawking radiation is black body radiation that is predicted to be released
by black holes, due to quantum effects near the event horizon. It is
named after the physicist Stephen Hawking, who provided a theoretical
argument for its existence in 1974,[1] and sometimes also after Jacob
Bekenstein, who predicted that black holes should have a finite, non-
zero temperature and entropy.

34. By absorbing other
stars and merging
with other black
holes, supermassive
black holes of millions
of solar masses (M☉)
may form. There is
general consensus
that supermassive
black holes exist in
the centers of most
galaxies.
Black holes of stellar mass are expected to form when very
massive stars collapse at the end of their life cycle. After a black
hole has formed, it can continue to grow by absorbing mass from
its surroundings.

35. • Despite its invisible
interior, the presence of a
black hole can be inferred
through its interaction
with other matter and with
electromagnetic radiation
such as visible light.
Matter falling onto a black
hole can form an accretion
disk heated by friction,
forming some of the
brightest objects in the
universe.
If there are other stars orbiting a black hole, their orbit can be used to determine
its mass and location. In this way, astronomers have identified numerous stellar
black hole candidates in binary systems, and established that the radio source
known as Sagittarius A*, at the core of our own Milky Way galaxy, contains a
supermassive black hole of about 4.3 million solar masses.

36. BINARY STAR
A binary star is a star system consisting of two stars orbiting around
their common center of mass. Systems of two, three, four, or even
more stars are called multiple star systems. These systems, especially
when more distant, often appear to the unaided eye as a single point
of light, and are then revealed as double (or more) by other means.
Research over the last two centuries suggests that half or more of
visible stars are part of multiple star systems.