IB Physics/Astrophysics SL

Thanks to Tímea Garlati

F.1 Measuring stellar radiation

F.1.2

Binary Star : Two stars orbiting a common centere

Black Dawrf : The remnant of a white dawrf after it has cooled down. It has very low luminosity

Black Hole : a singularity in space-time: the end result in the evolution of a very massive star

Brown Dawrf : Gas and dust that did not reach high enough temperatures to initiate fusion. These objects continue to compact and cool down.

Cepheid Variable : A star of variable luminosity. The Luminosity increases sharply and falls off gently with a well-defined period. The period is related to the absolute lumonisity of the star and so can be used to estimate the distance to the star.

Clusters of Galaxies : two or more Galaxies that are close enough to each other that they affect each others through gravitation.

Comet : a small body composed of mainly ice and dust that orbits the sun in an eliptical orbit.

Constellation : A group of stars which are in a particular pattern or design.

Clusters : Gravitationally bound system of galaxies/stars.

Constellations : Group of galaxies/stars given a specific name. The 12 zodiacs are examples - Pisces, Aries, Taurus, Gemini, Cancer, and so on.

‘’’ Dark Matter’’’: matter in galaxies that is too cold to radiate. Its existence is inferred form techniques rather then direct visual contact.

Galaxies : Giant assemblies of stars, gas, and dust held together by the gravitational forces they have on each other. Our Galaxy is called the Milky Way.

‘’’Interstellar medium’’’: Gases and dust that is filling the space between stars. Interstellar mass’s density is very low with about one atom of gas for every cubic centimeter of space.

‘’’Main Sequence Star’’’ : A normal star that is undergoing nuclear fusion of hydrogen into helium.

Neutron stars : A very dense star, consisting only of uncharged neutrons. They are created when very massive stars explode, leaving this neutron 'ball' behind. A neutron star is smaller then a white dwarf and extremely dense. It is microscopic and is a prime example of microscopic quantum physics.

Nebulae : From the Latin for 'cloud'. Used to label all sorts of stuff in space, that now are known as star cluster or galaxies. It is sometimes still used for a concentration of gas and dust.

‘’’Nova’’’ : A sudden increase of luminosity of a white dwarf caused by material from a nearby star falling into the white dwarf.

Parallax : The apparent motion of a star against the background of a more distant star, due to the motion of the Earth around the Sun. The angle is measured at different times during the year. The distance of the Sun to the Earth is known. Distances are specified in parallax angle in seconds of arc (parsec). At large distances the uncertainty becomes too large and it can't be applied. Example : angle = (6_10^-5)° = (6_10^-5)° (3600) = 0.22´´ of arc (seconds of arc) in parsecs: 1/0.22´´ = 4.5 pc.

‘’’Planetary nebula’’’ : The ejected envelope of a red giant star.

Pulsar : Send out sharp, strong burst of radio waves at regular intervals ranging between milliseconds and 4 seconds. They appear to be rapidly rotating highly magnetic neutron stars. The pulses are very energetic charged particles. The rotation and pulse rates slow gradually down as energy is radiated away.

Quasars : Small, extraordinarily luminous extragalactic objects with high redshifts. They do not seem to conform to Hubble’s Law. They are as bright as nearby stars, but display very large redshifts. According to Hubble’s law the quasars must be either extremely distant and incredibly bright (thousands of times brighter than ordinary galaxies) or that they are closer than the redshift suggests. There is either an unresolved brightness problem of an unresolved redshift problem. One theory is that quasars could be powered by black holes.

‘’’Red Dwarf’’’ : A very small star with low temperature. It is relatively red in color.

Red Giant : Luminous stars with low surface temperature. These stars are produced when the hydrogen in the core of the star has fused into the more heavy helium. Gravity forces the star to contract, but at the same time it heats up. The hydrogen around the core now burns more fiercely and causes the outer envelope of the star to expand and thus cool. This low surface temperature produces light at a longer wavelength.

‘’’Stellar Cluster’’’: a group of stars that are physically near each other in space, created by a collapse of the same gas cloud.

Supernova : Are gigantic stellar explosions. These occur when very massive stars explode. Because of Einstein's law, E=mc², fusion can not take place after that iron is created. The star collapses since there is no force that can outweigh the gravitational force. When it is impossible to compress it further it explodes violently giving out extremely luminous light.

White Dwarfs : Are developed as all fuel in a star is exhausted. Gravity forces the star (with a mass lower than 1.4 solar masses) to contract, heat up, at a high pressure. The atoms loose some of their electrons. It becomes a small hot white dwarf. 

-Within our own universe, there exist many celestial bodies each with their own unique properties; the planets being no exception.

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Note: All planets beyond Mars are gas giants, i.e. Jupiter is a failed sun.

With these given properties and the right equation, you can find the volume and density of any of these planets.

Volume: V= 4/3(pi) r^2 r = radius of the planet or sphere

Density: D= M/V M=mass V=volume

IT & CB

F.1.3

Apparent magnitude is a measure of observed brightness of a star seen from Earth. Absolute magnitude is a measure of luminosity, how much light a star radiates into space. (Astronomers define absolute magnitude as the apparent magnitude a star would have 10 pc away from Earth). A star seeming just as bright as one close to us, but being further away has a greater absolute magnitude.

F.1.4

Light Years

A light year is a unit of measurement of Ultra-solar system distances. It's the distance traveled by light in one year. The speed of light is 186,287.5 miles per second. You can find out the number of seconds in a year by multiplying the number of seconds in a minute (60) by the number of minutes in an hour (60). Then multiply that by the number of hours in a day (24), and multiply that by the number of days in a year (approximately 365.25).One light year is equal to 9.46 x 10^15m, which is also equal to 0.3068 Parsecs(Pc).

Example: The distance to the nearest star (Proxima Centauri)from the Earth is 4.31 light years, which is equal to 1.3pc. This means that it would take 4.3 years to send a message to Proxima Centauri.

M.H./P.F/E.W/T.S/J.M

F.1.5

Relative Distances

The average distance between stars in a galaxy is approximately 1 pc, which is equal to 3.26 light years. The average distance between galaxies within the same cluster ranges from 100 kpc (kilo-parsecs) - several kpc (kilo-parsecs). Galaxies in different clusters can be up to a few Mpc (mega-parsecs) apart. 1 Mpc is equal to 1000 kpc.

/M.H.

F.1.6

The spectra of light emitted by a source can reveal the chemical composition of the source since the 'emission spectrum' is the material's characteristic since excited gases emit light of only certain wavelengths.

F.2.1 Energy Source

All stars follow a simple proton-proton cycle in order to maintain an equilibrium between gravity and pressure. When the star is expending fuel it rises in temperature and therefore rises in pressure, this is required in order to keep a balance between the great force of gravity that is trying to compress the star. At the beginning of a star's life cycle the star consists mainly of hydrogen, in fact they are made of 98% hydrogen. There are three basic stages of the Proton-Proton cycle.
1.) Two hydrogens fuse to form a deuterium, plus a positron and a neutrino. Each positron is annihilated

F.2.2

See notes in F.1.1 and F.2.1.

F.2.3

A variable star is any star whose luminosity is not constant with time. It is assumed that red giants turn into pulsating variable stars before they finally die.

Red variables ???

Novae/Supernovae ???

F.2.4

A binary star is formed when two stars revolve around a common centre of gravity. Masses can be determined by angular size, and period of orbits.

A visual binary can be seen through a telescope, where two separate stars can be seen.

A spectroscopic binary can not be resolved by a telescope, only by its spectrum. There's a varying Doppler shift in the spectral lines as it approaches and recedes from Earth.

An eclipsing binary is situated so that one star passes in front of its companion and cuts off light at regular intervals, changing in brightness regularly.

F.2.5

Black Body Radiation

F.3 The Expanding Universe

F.3.1

The formula is l' = l Ö ((1+v/c)/(1-v/c)) , where l' is the wavelength measured on Earth, l the wavelength in the source's reference frame (on star), v is the speed of the source and c the speed of light.

If the source moves towards us, the frequency is higher and wavelength longer, moving away the opposite.

F.3.2

Redshift occurs when the source is moving away from us, since the wavelength of the light emitted appears longer to us, shifting towards the red end of the visible spectrum.

F.3.3

Hubble's law: v = Hd, where v is the speed, H is the Hubble parameter, and d is the distance. It describes Hubble's observation, that most lines in the spectra of other galaxies were redshifted, and the amount of shift was approximately proportional to the distance of the galaxy from us. So the velocity is proportional to the distance. H is approximately 50km/s/Mpc. Note that this does not work well for nearby galaxies.

F.3.4

The fact that with increasing distance the velocity increases suggests that there was a huge explosion some (large) number of years ago that accelerated the particles, and that the universe is expanding.

F.3.5

The U.S. physicists A. Penzias and R. Wilson detected (in 1956) microwave radiation coming equally from all directions in the sky, day and night. This radiation is like the one radiated by a black body at a temperature of 3 Kelvin, therefore the name 3K radiation.

This discovery supports the theory of Big Bang, where strong shortwave radiation was supposed to be sent out. The radiation spread filling the expanding universe uniformly. With time it cooled, to the now observed temperature of 3K, and now strikes the Earth as microwaves.

F.4 The Big Bang model of the creation of the universe

F.4.1

Olber's paradox : If the universe is infinite, how come then that we observe a black night sky? Shouldn't such a universe provide an infinite number of stars so that wherever we looked it would be bright?

F.4.2

One solution could be that the universe is not infinitely old, so light from distant galaxies that are travelling very fast with respect to Earth, has yet not reached us. This contradicts Newton's assumption of a static infinite universe, since in this case the universe is evidently expanding. The faster stars travel the greater is the redshift, so we could perhaps not even see the electromagnetic waves the most distant stars emit, since they don't fall in the visible spectrum.

F.4.3

The matter and radiation of our present time was initially all packed together into an extremely hot and dense fireball, that exploded giving rise to the Big Bang. Within seconds, matter was accelerated through 3 dimensions, expanding and developing very rapidly. Time became a measure of the rate of that expansion, the necessary 4^th dimension.

F.4.4

Curvature of space (also called four-dimensional space-time) : According to Einstein's general theory of relativity light is also affected by gravity. This means that light can be bent 'around' planets, and follow a curved path. Light must travel through the shortest distance available between two points, meaning that the curved path is the shortest distance, hence space itself is curved.

Open universe : The universe will continue to expand forever because the curvature of the universe is negative.

Flat universe : The curvature is zero, the universe is infinite.

Closed universe : The curvature is positive, so the universe is finite.

This can all be showed through non-Euclidean mathematics, where the sum of angles in a triangle either subtends of exceeds 180°.

F.4.5

Antimatter is the opposite of matter, with nucleus of antiprotons and orbiting positrons around it.

The Inflationary Epoch is the time after the Big Bang when two of the four forces of nature (gravity and strong nuclear forces) subsequently singled out, and the universe expanded very rapidly due to energy release.

As the universe gradually cooled (still talking about fractions of seconds) there was an excess production of matter over antimatter as they annihilated, when temperature dropped to the point where they could no longer be created

This excess forms electrons, neutrinos and hadrons (protons and neutrons). 3 minutes after the Big Bang, the hadrons start to form atomic nuclei such as deuterium and helium.

After 10 000 years at a temperature of 10⁵ K, atoms are formed.

300 000 years after the explosion electrons began to bind to the nuclei, and the dense 'fog' of particles cleared. Gas clouds formed and due to the pull of gravity they contracted into stars. The contraction also provides energy for nuclear fusion, so that the stars 'burn'.

There two theories on how galaxies and clusters of galaxies formed, one stating that one was created before the other, the other the inverse, we can only say for sure that they exist.