Topic 4 Astrophysics

8.2 key concepts of astrophysics(universe, galaxy, solar system)

• The universe is a large collection of billions of galaxies.

• A galaxy is a large collection of billion of stars.

• A solar system is a collection of planets orbiting a common star. Our solar system is in the Milky Way galaxy.

8.3 Why does the gravitational field(g) strength on different planets vary?

g = the force(of gravity) per unit mass on a body

• it varies with the mass and size(radius) of the planet

• the greater the mass, the greater the g

• the greater the size(greater the radius), the smaller the g

8.4 Orbits and gravitational force

Since gravitational force pulls any two objects with mass together, it causes:

• the moon to orbit the earth

• causes the planets to orbit the sun

• causes artificial satellites to orbit the earth

• causes comets to orbit the sun

The greater the mass of the two objects, the greater the gravitational pull.

The closer the two objects are next to each other, the greater the gravitational pull.

8.5 Differences in the orbits of moons, planets and comets

• The orbits of moons are almost circular (slightly elliptical), with nearly a constant orbital speed

• The orbits of planets are almost circular (slightly elliptical), with nearly a constant orbital speed

• The orbits of comets are highly elliptical, with varying orbital speed.

8.6 Formula for orbital speed

Since speed = distance over time, orbital speed = circumference of orbit over time period

→ v = 2πr / T

8.7& 8.8 Classification of stars

Stars can be classified according to their colour:

Oh - blue

Be - blue-white

A - white

Fine - yellow-white

Guy - yellow

Kiss - orange

Me - red

The temperature of the surface of the star defines its colour. The hotter a star, the bluer it is. The cooler a star, the redder it is.

8.9 Evolution of stars with masses similar to the sun

1. Nebula

2. Main-sequence star

3. Red giant

4. White dwarf → Black dwarf

Evolution of star - 1st stage

• The star begins as a cloud of dust and gas called a nebula.

• The particles experience a weak attraction towards each other due to gravity and begin to clump together, forming a protostar (like a baby star) 🌟

Evolution of star - 2nd stage

• The particles continue to clump together until the temperature and pressure are great enough for nuclear fusion to occur.

• Hydrogen nuclei fuse together to form helium nuclei, releasing a large amount of energy and causing great outwards pressure.

• This outwards pressure balances with the inwards pressure caused by gravity.

• The star is now stable and called a main sequence star. It will burn for billions of years.

Evolution of star - 3rd stage

• Eventually the hydrogen in the star will be used up.

• No more nuclear fusion occurs, so the outwards pressure decreases and can no more balance the inwards pressure due to gravity.

• The star collapses(becomes unstable), expands and cools down, becoming a red giant.

Evolution of star - 4th stage

• Due to the increased radius, the outer layer of the star will be more weakly bound due to gravity to the core of the star, and will eventually be ejected.

• It leaves behind a hot, dense core, called a white dwarf

• The white dwarf eventually cools down and becomes a black dwarf.

8.10 evolution of a star with a mass greater than the sun

• After the star has used up all the hydrogen, it expands, cools down and becomes a red super giant.

• Elements from helium to iron are formed.

• The star explodes in a supernova, forming elements heavier than iron.

• What remains will either be a neutron star (incredibly dense object mainly composed of neutrons) or if the star was exceptionally massive a black hole, which has an immense gravity, attracting everything even light.