Astrophysics
Motion in the universe
The universe is a large collection of billions of galaxies.
A galaxy is a large collection of billions of stars.
A solar system is a collection of planets orbiting a common star. Our solar system is in the
Milky Way galaxy.
Gravity in Space
The gravitational field strength is the force per unit mass on a body in a gravitational field and is
measured in Newtons per kilogram (N/kg). It varies with the mass and size of the body and is
therefore different on other planets and the moon compared to the Earth.
weight = mass × gravitational field strength W = mg
Gravitational force enables the various bodies to orbit around others. For example:
Moons orbit planets
o The orbits are slightly elliptical with near constant orbital speed
Planets and comets orbit the Sun
o The orbits of planets are slightly elliptical with near constant orbital speed. The
orbits of comets are highly elliptical.
Artificial satellites orbit the Earth
The greater the orbital radius or the smaller the time period, the greater the orbital speed
2× π × orbitalradius
orbital speed =
time period
Comets have a greater speed nearer to the star (when r is smaller) because the ice inside them
melts as they get closer (& warmer) - causing their mass to decrease.
Stellar evolution
A star begins as a cloud of dust & gas called a nebula. The particles experience a weak
attraction towards each other due to gravity and begin to clump together.
They continue to clump together until the pressure and temperature is great enough for
nuclear fusion to occur.
o Hydrogen nuclei fuse together to form helium nuclei which releases a large amount
of energy and causes a great outwards pressure
o This outwards pressure balances with the inwards pressure due to gravity and the
star is now stable and called a main sequence star
Eventually the hydrogen in the star is used up. There is no longer enough outward pressure
from nuclear fusion, and it collapses under its
own gravitational attraction, becoming
unstable.
If the star has a similar mass to the Sun, it
expands massively and becomes a red giant. It
then becomes a white dwarf (and finally cools
into a black dwarf).
If the star has a mass larger than the Sun, it
expands and becomes a red super giant, before
exploding in a supernova. What remains is
either a neutron star, or if it even bigger, a black
hole
Motion in the universe
The universe is a large collection of billions of galaxies.
A galaxy is a large collection of billions of stars.
A solar system is a collection of planets orbiting a common star. Our solar system is in the
Milky Way galaxy.
Gravity in Space
The gravitational field strength is the force per unit mass on a body in a gravitational field and is
measured in Newtons per kilogram (N/kg). It varies with the mass and size of the body and is
therefore different on other planets and the moon compared to the Earth.
weight = mass × gravitational field strength W = mg
Gravitational force enables the various bodies to orbit around others. For example:
Moons orbit planets
o The orbits are slightly elliptical with near constant orbital speed
Planets and comets orbit the Sun
o The orbits of planets are slightly elliptical with near constant orbital speed. The
orbits of comets are highly elliptical.
Artificial satellites orbit the Earth
The greater the orbital radius or the smaller the time period, the greater the orbital speed
2× π × orbitalradius
orbital speed =
time period
Comets have a greater speed nearer to the star (when r is smaller) because the ice inside them
melts as they get closer (& warmer) - causing their mass to decrease.
Stellar evolution
A star begins as a cloud of dust & gas called a nebula. The particles experience a weak
attraction towards each other due to gravity and begin to clump together.
They continue to clump together until the pressure and temperature is great enough for
nuclear fusion to occur.
o Hydrogen nuclei fuse together to form helium nuclei which releases a large amount
of energy and causes a great outwards pressure
o This outwards pressure balances with the inwards pressure due to gravity and the
star is now stable and called a main sequence star
Eventually the hydrogen in the star is used up. There is no longer enough outward pressure
from nuclear fusion, and it collapses under its
own gravitational attraction, becoming
unstable.
If the star has a similar mass to the Sun, it
expands massively and becomes a red giant. It
then becomes a white dwarf (and finally cools
into a black dwarf).
If the star has a mass larger than the Sun, it
expands and becomes a red super giant, before
exploding in a supernova. What remains is
either a neutron star, or if it even bigger, a black
hole
