Unit 9 - Astrophysics Flashcards
Newtons Universal Law of Gravitation
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
Newtons Universal Law of Gravitation Formula
F = G * (m1 * m2)/r^2
Universal Gravitation Constant
Rep using G
Value = 6.67 * 10^-11
SI Unit = Nm^2 Kg^-2
Gravity vs Gravitational Force
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
Centre of Gravity
The point where we assume all the gravitational force to be concentrated in
Centre of Mass
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
Is value of g contstant on earth and why?
No its not constant. It more towards the pole because earth isnt a perfect sphere but rather an elipse
Gravitational Feild Strength
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
Kepliers Laws - 1st Law and its implications
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
Kepliers Laws - 2nd Law and its implications
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
Kepliers Laws - 3rd Law and its formula
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
Major and Semi major axis
Major axis - Line passing through the 2 focal point and extending to the periphery
Semi major axis - Half of the major axis
Circular Motion
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
A body is said to be in constant circular motion and travelling with constant speed, however its still classified as accerlated motion, Why?
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
Centripetal force with example
A force which is centre seeking and which pulls a body back to its centre.
Eg - Tension in a string or Gravity with planets
Centripetal accelreation
Accleration of a body moving in a circular motion where the direction of accelration is towards the centre
Asteroid Belt and theories of its creation
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
Life cycle of a star
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
Stellar Nebula
Region of dust and gas where new stars are born. Very high gravitational force
Rayleigh’s criteria
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
Protostar
First phase of a star, where fusion prepare to start to take place. It tries to meet the sufficient density and temperature for fusion. There is a radiaiton pressure, which counteracts the inwards gravitational force. Once these two forces are equal and the start stops shrinking, it becomes a Mian sequence star where it can start fusions
Main sequence star
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 main sequence start turns into a Red Giant
Whats the longest phase in cycle of a star
Main sequence star
What phase is our sun in?
Main sequence star
