SPACE

PHYSICS
 THE EARTH
• Earth orbits around the sun.
• Time Period = 365 days (365 days 6 hours 9 mins approx.)
(Time for 1 orbit)
• Orbit of Earth is elliptical.
Equal
Distance
Radius Slightly elliptical hence
Further Closer
generally treated as a
circular orbit

Circular Orbit Elliptical Orbit

Orbital Speed:
The average speed with which the earth or any body orbits around the other body.

The word ‘average’ has been used because:

“There is a slight variation in orbital speed as the distance of Earth from sun change due
to its elliptical orbit.”
An elliptical orbit has two focuses that is why speed is
considered as average because distance keeps on changing
in an elliptical orbit since there are two focuses.
Since, V = d / t so, V = 2πr / T
Where, V = Orbital speed
2πr = Circumference of Circle (length of orbit)
T = Time taken for one orbit
Since Earth’s orbit is very slightly elliptical thus it is considered as circular so for the
distance, we consider the circumference of the circle.
S.I Unit is m/s.
The axis of Earth’s rotation is tilted at 23.5° from vertical.

Seasons are created because of this tilted rotation of Earth.

Day & Night of equal length

Facing towards Facing away Because of being

(summers) (winters)
tilted at 23.5°, the
part of the earth that
faces away from the
sun have winters &
when that part faces
Longest Day Longest Night
Shortest Night towards the sun
Shortest Day
during the orbital
Day & Night of equal length
rotation then it has
summers.
 Equinox:
❑ Equal day and night
North-east
time.
❑ sun rises from pure
……………
east and sets at pure
west.
South-west

Length of the arc helps to understand

the length of daytime in solstices and Winter Solstice: Summer Solstice:
equinoxes. ❑ Shorter days and ❑ Longer days and
longer nights. shorter nights.
❑ Sun rises less from the ❑ Sun rises more from
surface of earth. Its the surface of earth.
angle of inclination is Its angle of inclination
less. is more.
❑ Rises south-east and ❑ Rises north-east and
sets south-west. sets north-west.
• The Earth takes 24 hours to spin around its axis.
• Moon orbits around the earth in approximately 27 days (one month).
(Lunar calendars are not perfectly synced with normal calendars.)
• Moon revolves around its own axis at the same speed as rotation of
Earth. Therefore only one side of the moon is visible on Earth.
THE SOLAR SYSTEM
Planets:
❑ Spherical shape.
❑ Must have an
orbit around the
sun.
❑ No debris in its
atmosphere.
Dwarf planets:
They are even
smaller than the
moon. Eg: Pluto,
Ceres, Eris,
Makemake.
Asteroids: Comet:
Large rocks that have no atmosphere of ❑ Comet is made out of dust
their own and have an orbit if in the embedded in ice and
asteroid belt. methane.
❑ when comets pass closer
➢ Meteoroids are smaller sized rocks to the sun, radiations cause
broken out of asteroids. ice to melt.
➢ When the enter the earth's atmosphere ❑ Motion of comet causes
they become meteor. dust and gas to be blown
Star: away hence it forms a tail
A celestial body that produces energy of of its own.
its own using fusion reaction. It comprises
mainly of Hydrogen that fuses into
Helium.
DATA ABOUT PLANETS IN SOLAR SYSTEM:
❑ Planets having more mass in less volume have more gravity.
Since { g = GM / R2} where, G= Constant
g ∝ Mass of planet g ∝ 1 / (Radius of planet)2
Earth and Saturn have approx. same gravitational field strength.
Earth: Mass = 5.97 x 1024 Kg Dia.= 12.8 x 103 Km
Saturn: Mass = 568 x 1024 Kg Dia.= 120 x 103 Km

❑ Surface temperature of a Planet depends upon:

1. Distance from the Sun:

Greater distance Less intensity of sun’s Hence less

from sun radiation reaches the surface temperature
planet
2. Atmosphere of the planet:
Thin atmosphere does not trap heat from the inner core of planet and that
received from sun. Hence surface cools down.

3. Surface Area of the planet:

Larger surface area promotes more heat transfer via infrared radiations so it
cools.

• Mars is cooler than Earth as it is further away from sun and also has a
thinner atmosphere.
• Planets after the asteroid belt (Jupiter, Saturn, Uranus, Neptune) are very
cool on surface as they are very far away from sun. Very less heat reaches
there.
❑ The greater the radius of orbit, the larger is the orbital time period.

T2 = Constant x R2

(Time period (Radius of orbit

to orbit) around sun)
❑ Gravitational field strength of the sun is strongest at the surface of sun
and gets weaker as the distance from the sun increases.

Circular Motion:
Gravitational force from sun to the planet
acts as a centripetal force.

Distance of Grav. Field strength less field time of orbital

orbit of sun on planet strength orbit speed
increases decreases causes less increases decreases
acceleration
 THE STARS & THE
UNIVERSE
• A star is a cloud of gas mainly hydrogen
that converts into Helium by fusion
reaction.
• It is a self sustained entity that is energy
to maintain fusion reaction going on is
provided by fusion itself as heat in the
core.
• Radiations coming out of star comprises
mainly of visible light, infrared and
ultraviolet rays.
• Our sun is a main sequence star with surface temperature of around
5600 Kelvin. It is stable and medium sized.
White or blue stars are hotter and red or yellow stars are less hot.

• Stars are extremely far away hence distance to them are measured in
light years that is distance travelled by light in space in one year.
Closest star is alpha Centauri which is 4.31 light years away.

Since, V = d / t Thus, d = V x t
d = (3x10⁸)(365x24x3600)
d = 9.4608 x 1015 meters equivalent to 1 Light year
Galaxy:
A Galaxy is a large cloud of gas and dust which contains stars and planets
etc. It spirals and moves in a universe.
The name of our Galaxy is
‘Milky Way’. It is a spiral galaxy
and we exist near its edge.

Milky way is around 13.6 billion years old

and 100,000 light years in diameter.
FORMATION OF A STAR:
• A large cloud of gas and dust collapse under its own gravity.

• What's formed is called a protostar.

• Collapse continues and temperature of core rises until it is large enough

to cause fusion reaction.

• Hydrogen turns into helium.

• Once fusion begins, star stops collapsing and heat produced by a fusion
reaction generates enough pressure from inside to balance the
gravitational force from the outside.
LIFECYCLE OF A STAR:
• When star begins to run
out of its hydrogen fuel,
the outward expansion
force is insufficient to
counter gravitational force.
• Core of the star gets denser
and it starts to fuse Helium
into other heavier
elements.
• The heat produced now,
causes outer hydrogen to
fuse and consume and heat
causes the expansion to
accelerate.
• Main sequence star now turns into a Red giant or Red Super Giant.

mass of red giant < 8 x Mass of sun small mass: red giant
mass of red super giant > 8 x Mass of sun large mass: red super giant

DEATH OF A MEDIUM/SMALL MASS STAR:

• The hydrogen fuel in the core begins to run out.

• The heavier elements in the core begins to fuse and temperature of core
rises.

• This high temperature causes hydrogen on the surface of star to fuse as

well into Helium.
• Core becomes denser and its strong gravitational field keeps it collapsing.

• Surface gases expand and cool due to large expansion force and turn to Red
Giant.

• At a point when the temperature is extremely high in the core, an explosion

causes surfaces gases to expel in outer space thus a planetary nebula
remains.

• Its Centre has the core of star called a White dwarf. Its white due to very
high temperature.

• Eventually when fusion reaction come to an end, they turn into black
Dwarfs. These Dwarfs mainly contain heavier elements like iron, sodium
etc.
DEATH OF A HIGH MASS STAR:
• They have masses more than 8 times that of the sun.

• Fusion reaction in them go at a higher rate as soon as they start to run

out of hydrogen.

• When hydrogen decreases, Helium starts to fuse and form heavier

elements like carbon etc.

• Heat produced causes surface hydrogen to fuse as well and expand. The
core collapses further due to extra gravitational force.

• The star turns into a Red Supergiant. (larger size due to larger mass)
• Eventually when elements fuse and start producing iron, fusion reactions
end in the form of a huge explosion called a ‘Super Nova’.

• The energy and light produced is very high such that even iron fuse to
form heavier elements which expel in outer space as clouds/nebula.

❑ If red supergiant was small, Neutron star is formed that has no fusion
reaction going on inside. But a large amount of heat is being radiated as
it was the core of the star. It contains heavier elements and spins at
extremely high rate generating radio wave pulse which surround it. It
does not glow much.

❑ If red supergiant was large, then extremely dense core of it turns into a
Black hole. Not even light can escape out of it.
• The Universe is expanding!
• Redshift in color of stars prove that stars are moving away from one
another and Big Bang would have occurred.

Violet Red
(wavelength increases)

Actual wavelength = Observed wavelength

Star moving away

Observed wavelength > Actual wavelength

 If in water ripples are being produced of the same
wavelength by a source but if the source is moved away the
center is changed while the ripples are still being produced
of the same wavelength. But to the observer, the
wavelength would appear to have increased. The same
happens with stars.

• When the source of a wave moves away from observer the observer, the
observed wavelength is longer (observed frequency is shorter) than the
actual. This difference in wavelength (frequency) is called Doppler's
effect.

• In case of stars, a redshift in their color was seen when observing via
telescopes. This proved that all stars are moving away and are universe is
expanding.
• It also validates the idea that Big Bang would have occurred. Everything
was concentrated at point and that point exploded giving all in this
universe an outward velocity.

• If a star recedes away faster, it shows a greater redshift.