# Range of Physics Week Day 3: Angular Momentum on an Astronomical Scale

Yestarday we saw that a cycle does not fall because of the rotation

of its wheels

Today we will see some places where MASSIVE objects/phenomenon is caused/affected by rotations and thus angular momentum .

a) Accretion Disks in Binary Systems : Our Sun is a single star . But most

of the stars in the Universe occur in Pairs . They form what is known as

Binary systems . Now lets suppose that there are 2 stars in a binary

pair : A HUGE star with a low atmosphere density . Lets call this dude S

and we have a compact star s which is the companion of S . If s and S are

close to each other then it might happen that s may begin to attract

the gases that form the atmosphere of S . so the gases of S start moving

towards s . Now as they move they loose energy because of collisions .

So to conserve the Angular Momentum the gases start whirling around

s and form a disk/ring around s . This is called the accretion disk .

The ring is flat and rotates very fast . The ring is perpendicular to

the axis of rotation of the star s . This is because the gases at this

place experience more centrifugal force and can resist the gravity of s .

This is again in accordance with the consevation of Angular Momentum .

For a beautiful picture of this Phenomenon see :

http://physics.angelo.edu/~msonntag/physics1301/binarymasstransfer.htm

b) Formation of Galaxies :

Imagine : A HUUUUUUUUUUGE cloud of dust and gas spinning . IT collapses

because of the gravity of its individual parts or some local irregularities.

Now as mass falls towards the inside it starts rotating very rapidly.

THis is to conserve angular momentum . Again it collects in a disk shape

perpendicular to the axis of rotation . Now places where there is more

gas/dust mass start falling withn themselves to form stars . Again these

starts are rotating . The total angular momentum of the galaxy is

the sum of the rotational motion of the galaxy and the sum of angular

momentum of its parts( starts , dust etc ) .

So we see that the same theory that explains tops also explains

things zillions of miles across .