Topic 8 - Space Physics Flashcards
The Life Cycle of a Star
- Stars initially form from a cloud of dust and gas called a nebula. The force of gravity pulls the dust and gas together to form a protostar.
- As the star gets denser, the temp rises and more particles collide with each other. When the temp gets high enough, hydrogen nuclei undergo nuclear fusion to form helium nuclei. This gives out huge amounts of energy, which keeps the core of the star hot. A star is born.
- The star enters a long stable period where the outward pressure caused by the nuclear fusion that tries to expand the star balances the force of gravity pulling everything inwards. In this stable period, it’s called a main sequence start and it typically lasts several billion years. The Sun is in the middle of this stable period.
- Eventually, the hydrogen begins to run out. The star then swells into a red giant (if it’s a small star) or a red super giant (if it’s a larger star). It becomes red because the surface cools. Fusions of helium (and other elements) occurs. Heavier elements (up to iron) are created in the core of the star.
- A small-to-medium-sized star like the Sun then becomes unstable and ejects its outer layer of dust and gas. This leaves behind a hot, dense, solid core - a white dwarf.
- As a white dwarf cools down, it emits less and less energy. When it no longer emits a significant amount, it is called a black dwarf.
- Big stars however start to glow brightly again as they undergo more fusion and expand and contract several times, forming elements as heavy as iron in various nuclear reactions. Eventually, they’ll explode in a supernova, forming elements heavier than iron and ejecting them into the universe to form new planets and stars. Stars and their life cycles produce and distribute all naturally occurring elements.
- The exploding supernova throws the outer layers of dust and gas into space, leaving a very dense core called a neutron star. If the star is massive enough it will become a black hole.
Black hole- definition
> A super dense point in space that not even light can escape from.
Our solar system
> The solar system is the sun and all the stuff that orbits around it. This includes things like:
-planets
-dwarf planets
-moons
-artificial satellites.
Our solar system is a tiny part of the Milky Way galaxy.
The Sun
> The Sun is the centre of our solar system and is orbited by 8 planets, along with other objects.
Planets
> Theses are large objects that orbit a star.
There are 8 in our solar system -mercury, venus, Earth, mars, jupiter, saturn, uranaus, neptune.
They also have to be large enough to have ‘cleared their neighbourhoods’.
This means that their gravity is strong enough to have pulled in any nearby objects apart from their satellites.
Dwarf planets
> Like Pluto.
>These are planet-like objects that orbit stars, but don’t meet all of the rules for being a planet.
Moons
> These orbit planets.
>They’re a types of natural satellite.
Artificial satellites
> Satellites that humans have built.
>They generally orbit the Earth.
Satellite - definition
> An object that orbits a second, more massive object.
Milky Way galaxy
> A massive collection of billions of stars that are all held together by gravity.
Orbits
> The structure of the solar system is determined by orbits - the paths that objects take as they move around each other in space.
The planets move around the Sun in almost circular orbits.
Orbits - gravity
> If an object is travelling in a circle it is constantly changing direction, which means it’s constantly accelerating.
For an object to accelerate, there must be a force acting on it. This force is directed towards the centre of the circle.
This force would cause the object to just fall towards whatever it was orbiting, but as the object is already moving, it just causes it to change direction.
The object keep accelerating towards what it’s orbiting but the instantaneous velocity (which is at a right angle to the acceleration) keeps it travelling in a circle.
The force that makes this happen is provided by the gravitational force (gravity) between the planet and the Sun (or between the planet and its satellites).
Size of orbits
> The closer you get to a star or planet, the stronger the gravitational force is.
The stronger the force, the faster the orbiting object needs to travel to remain in orbit (to not crash into the object it’s orbiting).
For an object in a stable orbit, if the speed of the object changes, the size/radius of the orbit must do so too.
Faster moving objects will move in a stable orbit with a smaller radius than slower moving ones.
Expansion of the universe
> As big as the universe already is, it looks like it;s getting even bigger. All its galaxies seem to be moving away from each other.
Evidence for expansion of the universe
> When we look at light from most distant galaxies, we find that the wavelength has increased.
The wavelengths are all longer than they should be - they’re shifted towards the red end of the spectrum. This is called red-shift.
This suggests the source of light is moving away from us. Measurements of the red-shift indicate that these distant galaxies are moving away from us (receding) very quickly - and it’s the same result in whichever direction you look in.
More distant galaxies have greater red-shifts than nearer ones. This means that more distant galaxies are moving away faster than nearer ones - and so all galaxies are moving away from every other galaxy, not just ours.
The inescapable conclusion appears to be that the whole universe is expanding.
