A Level Physics OCR A Module 5 - Chapters 19 & 20 Stars & Cosmology

A Level Physics OCR A Module 5 - Chapters 19 & 20 Stars & Cosmology

What is a planet?

An extremely large, round mass of rock and metal or gas that moves in a circular or elliptical orbit around the sun or another star.

How are planets formed?

When interstellar dust is attracted into large clumps which join together.

What is a planetary satellite?

A smaller body in the orbit around a planet.

What is a comet?

A large rocky ice ball. It has a highly elliptical orbit.

What is a solar system?

The sun together with all the bodies that revolve around it. They are any similar systems surrounding another star.

What is a galaxy?

A cluster of stars and planets that are being held together by gravity. Stars rotate around the galaxy's centre of mass.

What is the universe?

Everything that exists. The diameter is estimated to be 93 billion light years.

What is an astronomical unit?

The mean distance between the earth and the sun.

What is a light-year?

The distance light travels through a vacuum in 1 year.

What is a parsec?

The distance that gives a parallax angle of 1 second of arc.

What is a stellar parallax?

Where the position of nearby stars appear to move relative to the distant stars, as the Earth orbits the sun.

What is the first stage in the formation of a star?

A cloud of dust and gas contracts and gets denser under the force of gravity, they then turn into protostars which continue to contract and heat up.

What is the second stage in the formation of a star?

When the temperature is hot enough, hydrogen nuclei start to fuse to form helium. It continues to contract as temperature and gas pressure increases until it reaches a main sequence star.

What is radiation pressure?

A pressure exerted by electromagnetic radiation on any surface it hits.

How are red giants formed?

When the hydrogen in the core of a main sequence star runs out the outward pressure stops so the core contracts and heats up more under the weight of the star. The outer layer expands and cools making the star a red giant.

What is core hydrogen burning?

Hydrogen fusion produces pressure to balance the gravitational force trying to compress the core.

What is shell hydrogen burning?

The material around the core contains a lot of hydrogen that is fused from the heat of the contracting core.

How are white dwarfs formed?

In stars with a mass of 1.4 times the mass of the sun or under, the carbon-oxygen core isn't hot enough to continue fusing so it contracts under its own weight until the electrons exert enough pressure to stop it collapsing anymore. Then a hot dense white dwarf is left over.

What is the Chandrasekhar limit?

The maximum mass for which electron degeneracy pressure can counteract the gravitational force.

Why do bigger stars die quicker?

They fuse elements much faster to keep the volume contracting too fast, but it burns up their fuel much faster than smaller stars .

What are neutron stars?

The collapsed core of a massive star, 1.4 to 3 times the mass of the Sun, after a supernova. They are extremely dense and small.

What is a pulsar?

A rapidly rotating neutron star which emit radio waves.

What is a black hole?

A region of space created after a star greater than 3 times the mass of the star collapses into an infinitely dense point called a singularity. No light can escape.

What is luminosity?

The measure of how bright an object is. It is the total energy emitted per second which depends on temperature and surface area.

What is a continuous spectra?

The spectrum of what light light. Hot objects emit a continuous spectrum in the visible light and infrared regions.

How do gases produce line emission spectra?

When the electrons return to their ground state after being heated they emit energy as photons. When using a diffraction grating the lines produced correspond to a particular wavelength of light emitted by the source.

How is an absorption spectrum produced?

When light is passed through a gas most of the electrons will be on their ground state so absorb photons to get excited, those that dont are missing from the absorption spectrum.

What is Stefan's law?

Luminosity is proportional to the fourth power of the stars temperature and directly proportional to the stars surface area. L=4(pi)r^2oT^4.

What is Wien's displacement law?

Lambda(max) is proportional to 1/T..

What is the cosmological principle?

On a large scale the universe is homogenous and isotropic and the laws of physics are universal.

What does homogenous mean?

The same throughout.

What does isotropic mean?

Looks the same in all directions.

What is Hubble's Law?

V=Ho*d, where v is the recessional velocity and d is distance in megaparsecs.

How does red shift prove that the universe is expanding?

The spectra from galaxies shows red shift so the wavelengths are getting longer so they are moving away from us.

What is the Big Bang Theory?

The universe started off very hot and very dense and has been expanding ever since.

How is cosmic microwave background radiation evidence for the Big Bang theory?

Lots of gamma radiation was produced in the very early universe. As the universe expands the radiation has been stretched and is now in the microwave region.

What is dark matter?

An unknown substance that gives mass to galaxies. It takes up 25% of the universe in total, there is 5 times as much dark matter as there is ordinary matter.

What are MACHOs?

Massive compact halo objects. These are objects made up of a very dense form of normal matter.

Why does the existence of MACHOs challenge the Big Bang Theory?

They would require more protons and neutrons to exist than is compatible with the current understanding of the Big Bang theory.

What are WIMPS?

Weakly interacting massive particles. They don't interact with electromagnetic force but with gravity. They are purely theoretical.

What is dark energy?

A type of energy that fills the whole of space. It causes the expansion of the universe to accelerate.