Unit 9 - Astrophysics

Question 1

Newtons Universal Law of Gravitation

Answer 1

The gravitational force of attraction between 2 point masses is directly proportional to the product of their masses and inversly proportional to the sqaure of the distance between them. The force acts along the line joining the 2 point masses

Question 2

Newtons Universal Law of Gravitation Formula

Answer 2

F = G * (m1 * m2)/r^2

Question 3

Universal Gravitation Constant

Answer 3

Rep using G
Value = 6.67 * 10^-11
SI Unit = Nm^2 Kg^-2

Question 4

Gravity vs Gravitational Force

Answer 4

Gravitational force is the force of attraction between 2 point masses

Gravity is the gravitational force between a planet/celestial body and any other object

Question 5

Centre of Gravity

Answer 5

The point where we assume all the gravitational force to be concentrated in

Question 6

Centre of Mass

Answer 6

The point where all the mass in the body is concentrated and allows us to idetnfity motion of an object and predict direction of object

Question 7

Is value of g contstant on earth and why?

Answer 7

No its not constant. It more towards the pole because earth isnt a perfect sphere but rather an elipse

Question 8

Gravitational Feild Strength, formula, si unit, and what is it also equal to

Answer 8

Can be defined as the force per unit mass expirienced by a small mass
Formula = Force/Mass
Si unit = Nkg^-1

Gravitational feidl strength = accelration due to gravity

Question 9

Kepliers Laws - 1st Law and its implications

Answer 9

Law of Orbits - All planets revolve around a sun in eliptical shaped orbits with the sun being present in any one of the 2 focal points

Implications - Distance between sun and the planets is never constant

Question 10

Kepliers Laws - 2nd Law and its implications

Answer 10

Law of Areas - the radius vector sweeps out equal area in equal time intervals

Implications - The distance it travels wont be equal since the orbit is an elipse. The closer a planet it to the sun, the more distance it will cover. Hence we can say that if a planet is closer to the sun, its velocity will also be higher

Question 11

Kepliers Laws - 3rd Law and its formula

Answer 11

Law of Periods - The square of the Time period of a planet to orbit its sun is directly proportional to the cube of the semi major axis

Formula = T^2 ∝ R^3

Question 12

Major and Semi major axis

Answer 12

Major axis - Line passing through the 2 focal point and extending to the periphery

Semi major axis - Half of the major axis

Question 13

Circular Motion

Answer 13

If a body travels in a closed strcuture, changing its direction of motion at each and every point while maintain a constant dustance from a reference point, then the body is said to be in circular motion

Question 14

A body is said to be in constant circular motion and travelling with constant speed, however its still classified as accerlated motion, Why?

Answer 14

Because at each and every point, the body is changing direction hence magnitude of velocity is constant, but direction isnt. If velocity isnt constant then acceleration isnt constant as well

Question 15

Centripetal force with example

Answer 15

A force which is centre seeking and which pulls a body back to its centre.

Eg - Tension in a string or Gravity with planets

Question 16

Centripetal accelreation

Answer 16

Accleration of a body moving in a circular motion where the direction of accelration is towards the centre

Question 17

formula for centripetal force and centripetal accleration

Answer 17

Accleration:
a = v^2/r

Force:
F = (m * v^2)/r

Question 18

Moons defination

Answer 18

Natural satellites which orbit around a planet

Question 19

Planets defination

Answer 19

Celestial bodies with a gravitational force which orbit around a star in fixed elliptical orbits

Question 20

Stars defination

Answer 20

Celestial bodies which are hot balls of gas which do fusion do create heat and light at the cost of mass

Question 21

Moons vs Planets vs Stars

Answer 21

• Moons orbit planets, planets orbit stars, and stars are the centre of the solar system
• A star creates it own light by fusion reaction, and the planet and moon reflect this light
• Moons have very weak gravity, Planets have medium gravity and a Start has a very high gravitational force
• Moons are made of rock/ice, planets are made of rock or gas or maybe both, but stars are purely made of gases such as Helium and Hydrogen

Question 22

Asteroid Belt and theories of its creation

Answer 22

Torus shaped region located between Mars and Jupiter, containing rocks and millions of asteroids

Theories:
1. Two planets existed before our current system but colloided at the position of our current asteroid belt
2. Some mass of Mars and Jupiter, due to Jupiters high gravitational force couldnt combine and was left as debris

Question 23

Planetisimals

Answer 23

Stuff from which planets are made of

Question 24

Kupier belt and how did it form?

Answer 24

Disc shaped region after Neptune which has comets, asteroids, icy objects, space debris and even has some dwarf planets.

It was formed after the collisions of planets and inability of some planets to form due to gravity after our solar system was made

Question 25

Oort Cloud

Answer 25

Sphere shaped shell of icy objects located after the Kuiper Belt and Pluto, which covers our entire solar system Most distance part of our solar system Due to gravity, some planetesimals became scattered across our solar system and eventually settled down to form the oort cloud

Question 26

Life cycle of a star

Answer 26

All stars start of a setllar nebula. Then if it meets the Rayleigh's criteria, it forms into a protostar which then becomes a main sequence star. Then it becomes a Red Giant. After that, if the mass of the start meets the Chandreshekar limit then it turns into a supernova explosion, where the outer layer becomes a setllar nebula. For the inner core, if it meets the volkoff - oppenheimer limit, then it becomes a black hole, if not then it becomes a neutron star If the original, chandreshekar limit isnt met, then the red giant's outer layer becomes planetary nebula and the inner layer becomes a white drawf, before evetnaully turning into a black dwarf indiciating the death of a star

Question 27

Stellar Nebula

Answer 27

Region of dust and gas where new stars are born. Very high gravitational force

Question 28

Rayleigh's criteria

Answer 28

If the criteria is met during the setllar nebulla's collapse, then a protostar us formed, toerhwise the star isnt formed and is left as a Brown star

Question 29

Protostar

Answer 29

First phase of a star, where fusion prepares to start to take place. It tries to meet the sufficient density and temperature for fusion. There is a radiation pressure, which counteracts the inwards gravitational force. Once these two forces are equal and the start stops shrinking, and the Collapsing gas cloud collapses on itself (converting GPE to KE) , the core contracts and becomes hot enough to become a Mian sequence star where it can start fusion.

Question 30

Main sequence star

Answer 30

Gravitational force = Radiation pressure causing it to become stable for a long time But at this stage, the star starts to run out of hydrogen to do fusions. Since the fusions it can do goes down, the radiation pressure decreases and gravitational force increases, causing the start to shrink again. The moment 12% of the initial hydrogen is exhausted, the fusions now start to take place in a shell surrounding the core, which turns the start into a Red Giant

Question 31

Whats the longest phase in cycle of a star

Answer 31

Main sequence star

Question 32

What phase is our sun in?

Answer 32

Main sequence star

Question 33

Red Giant

Answer 33

Fusions continue to take place but at the cost of mass. If mass decreases, then GF decreases and hence the radiation pressure increases, causing the star too expand.

Question 34

Where do fusions happen in a red giant and what gives it its red appereance?

Answer 34

Fusions happen in a shell outside the inner core, expanding the star and giving it its red appereance

Question 35

Chandreshekar limit

Answer 35

Mass of star >= 1.4 * mass of the sun

Question 36

After expanding from Red giant, what happens if the chandreshekar limit isnt met?

Answer 36

The outer layers fade away as a plantery nebula. In the core, some fusions still take place emitting light and converting into a white dwarf, which a small and dense star. After all the fuel in the white drawf has ended, it becomes a black dwarf, where it can no longer emit heat and light. It becomes inert and just floats in space.

Question 37

White Dwarf and Black Dwarf

Answer 37

White dwarf: Small and dense star with very loss mass and generally the size of a planet. Fusions start to slow down, signaling the start of the death of the star Black dwarf: Once a white dwarf's fuel is all used up, it cools down and no longer emits heat and light, becoming into a black dwarf. This star is said to be inert and called dead. It just floats away in space but has a mass and a Gravitional field.

Question 38

After expanding from Red giant, what happens if the chandreshekar limit is met?

Answer 38

Fusions continue to take place, but there is still a massive pressure difference. The outer layer explodes in a supernova explosion, creating another stellar nebula For the inner core there are 2 cases: Case 1) Volkoff-Oppenheimer limit isnt met. Then inside the inner core, electrons are pushed into neutrons and the start becomes a neutron star, a star composed fully of neutrons Case 2) Volkoff-Oppenheimer is met. Then the core of the star collapses on itself, because its gravity is so strong, forming a black hole

Question 39

Black hole

Answer 39

An infinitly dense point which appears as an empty hole Very high gravity which even light cant escape

Question 40

Neutron star

Answer 40

A star composed fully of neutrons, due to gravitational force pushing electrons into protons. It spins around, obeying the principle of conservation of angular momentum, emitting colourful light and beams of radiation

Question 41

Volkoff-Oppenheimer limit

Answer 41

Mass of core >= 3 * mass of the sun

Question 42

Doppler's effect or Red Shift Blue shift

Answer 42

If an object is moving away from the observer, its frequency decreases and wavelegnth increases, appearing as red to the human eye If the object moves coser to the observer, its frequency increases and wavelength decreases, appearing blue to the human eye

Question 43

Big Bang Theory

Answer 43

The entire universe we know today is moving away or expanding. Therefore a few billion years ago, all the mass today could have been concentrated at a point which would have exploded with a big band

Question 44

scientific proof of the big bang

Answer 44

1. Dopplers effect - Red shift - galaxies appear more red now than before 2. Cosmic Microwave Backround Radiation (CMBR) - Faint microwave radiation filling all of space. But EM waves can be given out as an result of an explosion 3. The composition of gases in terms of hydrogen and helium match with theoretically predicted quantitues based on the big bang.

Question 45

Hubbles law

Answer 45

The recession velocity of a galaxy is directly proportional to the distance between the galaxies

Question 46

recession velocity

Answer 46

Speed at which galaxies are moving away from us due to the universe expanding

Question 47

Hubbles Law formula

Answer 47

v = H_0 * d H_0 = Hubble's constant d = distance to galaxy

Question 48

Hubbles constant vaue and si unit

Answer 48

Si unit = s^-1 Value = 2.3 * 10^-18

Question 49

What is 1/H0 represent in hubbles law

Answer 49

Approximate age of the universe

Question 50

Conclusions of Hubbles law

Answer 50

The farther away the galaxies are from us, the faster it was moving and the faster it will m0ove away, suggesting that the universe in expanding