Section 8 - Astrophysics - Paper 1

Describe the different orbit types in our solar system

Planets - almost circular/slightly elliptical

Moons - almost circular/slightly elliptical

Artificial satellites - circular

Comets - highly elliptical

What happens to the acceleration to an object travelling in a circle?

The object is constantly changing direction and so it is constantly accelerating. The resultant force causing this acceleration is a centripetal force, acing towards the centre of the circle.

Discuss centripetal force

Centripetal force would cause an object to just fall towards whatever it is orbiting, but because the object is already moving, it just causes it to change direction.

Gravitational attraction provides the centripetal force needed to keep planets in orbit.

What keeps an object in an orbit travelling in a circle?

The instantaneous velocity (which is perpendicular to the acceleration).

What happens to the instantaneous velocity the stronger the force?

The larger the instantaneous velocity needed to balance it. So the close to a star/planet you get, the faster you need to go to remain in orbit.

What happens to a comet as it nears the Sun?

It travels much faster, due to the increased pull of gravity from the Sun’s gravitational field.

Give two examples of natural satellites

Moons and asteroids.

Describe a polar orbit

* Takes the satellite over the Earth’s poles
* Travels close to the Earth
* Travels at very high speeds (nearly 8000m/s)
* Used for Earth mapping and some weather satellites

Describe geostationary orbits

* Take 24 hours to orbit the earth, so they appear to remain in the same place when viewed from the ground (because they move with Earth’s rotation)
* Higher above the ground than polar orbits
* Travel more slowly
* Used for communication and broadcast satellites

What temperature is a star if it emits higher frequencies of light?

Very hot - the star will appear blue.

What is a nebula?

A cloud of dust and gas - often remnants from a supernova

Describe a main sequence star

* A long stable period, during which the outward pressure (caused by thermal expansion from nuclear fusion) balances the inward force of gravity.
* The period lasts several billion years.
* The heavier the star is, the shorter its main sequence is.

Describe how a star is born

* Form from a nebula - a cloud of gas and dust
* Gravity pulls the dust and gas together, forming a protostar.
* The star gets **denser** and **more particles collide** with each other, causing the protostar’s **temperature to rise.**
* When the temperature gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei, giving out huge amounts of energy, which keeps the core of the star hot.
* A star is born.

What happens to stars after the hydrogen in the core begins to run out?

* The force due to gravity is larger than the outward pressure of thermal expansion.
* This causes the star to compress, until it is dense and hot enough so that the energy created makes the outer layers of the star expand.
* The star becomes a red giant (small-medium size) or a red supergiant (large size).
* The stars become red because their surface cools.

Describe a planetary nebula

* A small-medium sized star (e.g. the Sun) then becomes unstable and ejects its outer layer of gas and dust.
* What is left behind is a hot, dense, solid core - a white dwarf star.

What happens to larger stars after they become red supergiants?

* They undergo more nuclear fusion to make heavier elements.
* They expand and contract many times, as the balance shifts between thermal expansion and gravity.
* They continue fusing heavier elements until iron is formed. Iron releases no energy when fused.
* This leads to the star becoming unstable as the force of gravity is much greater than the outward pressure.
* The star suddenly explodes in a supernova.

Describe a supernova

* Outer layers of gas and dust are thrown into space.
* The star releases large amounts of energy, which allows elements heavier than iron (e.g. uranium, gold) to be produced. These are expelled out into space.
* A very dense core is left behind - a neutron star.
* If the star is massive enough, it will collapse and become a black hole - a super dense points in space that not even light can escape from.