Topic Option: Astrophysics Flashcards
Solar System
The Sun is orbited by planets, moons, asteroids and comets.
Elliptical orbit
Non-circular path which has the star at one focus and is called eccentric. A body in an elliptical orbit changes speed as it travels.
Planets and Moons
Planets orbit their star with a slightly elliptical orbit. There are 8 planets around the Sun and many dwarf planets. Most planets have moons; Jupiter has more than 60.
Asteroids and Comets
The Asteroids Belt is billions of rocky objects which orbit the Sun between the inner and outer planets. Comets are made of ice and rock and orbit the Sun with highly eccentric orbits.
Stellar clustar
a group of stars (with gas and dust) held together by gravity/forming a globular/open arrangement;
Constellation
A pattern of stars in the sky as viewed from Earth. They are probably not near each other nor gravitationally bound.
Astronomical unit
The mean distance from the centre of the Earth to the centre of the Sun.
1 AU = 1.5 x 10^8 m
Light year
The distance that light travels in one year. 1 ly = 9.46 x 1015 m
(1 ly = 63 000 AU)
Galaxy
a large-scale collection/large number of stars/star clusters, gas, and dust. Typically contains 10^11 stars and is 10^5 ly across. Distances between stars approx. 1 ly. Distance between galaxies 106 ly. Shapes can be spiral, globular or irregular.
Apparent motion of stars
In 24 hours, the stars appear to rotate about the extension of the Earth’s axis. Each day they move slightly forward at the same time of night. In one year, some stars move above and below the horizon due to the Earth’s tilt.
Nuclear fusion
Inside a star, protons fuse to make helium in a complex reaction also producing positrons, neutrinos and gamma rays.
Stable star
There is a balance between collapse due to gravitational force and expansion due to KE of particles (radiation pressure). The star’s source of energy keeps it stable.
Luminosity
The total amount of energy emitted by the star per second. Unit: watt. Depends on the star’s temperature and its size.
Apparent brightness
The amount of power received per unit area. W/m^2
Black body radiation
A star is approximately a perfect emitter of a continuous radiation spectrum which changes depending on the temperature of the body. It is usually plotted as wavelength vs intensity.
Stefan Boltzmann Law
For a black body, the total power per unit area (intensity) is proportional to the fourth power of the absolute temperature.
Wein’s Law
For a black body spectrum, the most intense wavelength is inversely proportional to the absolute temperature.
Absorption spectra
The continuous spectrum from a star includes dark absorption lines corresponding to elements in the star’s outer layers. This can be used to identify the elements. The relative strengths of the spectral lines can accurately predict the temperature of the star.
Spectral classification
The Harvard system from hottest (60 000K) to coolest (2 000K) is O-B-A-F-G-K-M
Binary stars
Two stars which orbit a common centre of mass.
Red and blue shift
The wavelength of the light from stars which are moving towards or away from the viewer is shifted slightly in the blue (shorter wavelength) or red (longer) directions respectively.
Hertzsprung Russell Diagram
A graph which shows stars classified by colour and luminosity. The regions of the H-R diagram are Main Sequence, Giants, Supergiants, White Dwarfs.
Cepheid variable stars
The apparent magnitude of the star and therefore its luminosity vary periodically. It’s predictability makes it useful for estimating distances.
Stellar parallax
Viewed from different positions of the Earth’s orbit, stars appear to change their position in the sky. This can be observed for stars up to a distance of 100 pc.
Parsec
The distance to a point whose maximum parallax viewed from the Earth is one second of arc. 1 pc = 3.26 ly = 2.1 x 10^5 AU = 3.1 X 10^16 m
Apparent magnitude
A measure of how bright a star appears from Earth. The fixed points on the scale are +1 (brighter) and +6 (dimmer). +1 is 100 times brighter than +6
Absolute magnitude
The magnitude of a star viewed from a distance of 10 pc.
Spectroscopic parallax
Method of measuring distance to stars which are too far away for stellar parallax. It combines Wien’s Law, the HR diagram and apparent brightness. Limit is 10Mpc.
Newton’s model of universe
This assumes that the universe is infinitely old, infinitely large, static and uniform.
Olber’s paradox
According to the Newtonian model, there should be stars in every direction one looks, so night sky should be bright.
Expansion of universe
The light from galaxies is red-shifted, and even more so the further away they are. This implies that the universe is expanding.
Big Bang model
From the speed of the receding galaxies, we can deduce that the universe originated from a single point 13.8 billion years ago. At the moment of the Big Bang, space and time came into existence.
Cosmic microwave background radiation
Electromagnetic (Black body) radiation in the microwave region that fills the universe equally. It is received from all directions in the universe.
Critical density
Density at which the expansion of the universe tends to zero at infinite time.
International astrophysics research
There are many projects which involve cooperating nations, such as the International Space Station and the Hubble Space Telescope.
Protostar
Collapsed dust cloud which is contracting and very hot but does not yet shine nor does fusion occur yet.
Pre main sequence star
Dust cloud has blown off protostar, thus shining. Contracting and still increasing in temperature, but still no fusion occurring. When fusion initiates, joins main sequence.
Giant star
When all hydrogen in the core fused to helium, the core collapses, heats up and initiates fusion in surrounding gas which expands. Cooler and more luminous than main sequence star.
Red giant star
The core of a star of similar mass (up to 4 solar masses) to the Sun collapses once the hydrogen is fused to helium and no more fusion occurs.
Electron degeneracy pressure
The Pauli Exclusion Principle sets a limit to how much the core of a red giant can contract as electrons in the core cannot occupy the same space.
White dwarf star
The core of a red giant loses its outer layers and cools down.
Red supergiant star
Stars of more than four solar masses get hot enough for helium to fuse to heavier elements starting with carbon and continuing to iron.
Chandrasekhar limit
Sets upper limit on mass of white dwarf. If the core of a red supergiant is larger than 1.4 solar masses, its contraction can overcome electron degeneracy pressure and protons and electrons combine to form neutrons. This star does not form a white dwarf.
Neutron degeneracy pressure
When electron degeneracy is overcome in a core above the Chandrasekhar limit, the core contracts further until the neutrons oppose the contraction when they are as close as in the nucleus
Supernova
When the core has contracted to neutrons, the outer layers fly off in a huge explosion.
Neutron star
The core of a red supergiant after a supernova. It consists of neutrons and is as dense as a nucleus.
Oppenheimer-Volkoff limit
Sets upper limit on mass of neutron star. A neutron star with a mass greater than 2-3 solar masses will overcome neutron degeneracy pressure and will continue to contract.
Black hole
After a supernova, a core larger than the O-V limit overcomes neutron degeneracy and collapses to a singularity. Space and time are so distorted by gravity that even light does not escape.
Visual binary
Distinguished as 2 stars with telescope.
Spectroscopic binary
Distinguish 2 stars with periodic shift in wavelength (analysis of spectrum of light). Condition: periodic splitting in spectrum due to Doppler effect.
Eclipsing binary
Distinguish 2 stars from analysis of brightness of light from star. Conditions: the Earth/observer must be on the plane of the orbit;
alignment of stars is such that they can block the light from the other star as seen by the observer
Hubble’s Law
Recessional speed of a galaxy is directly proportional to distance from Earth
Galactic cluster
Hundreds or thousands of galaxies bound together by gravity
Galactic supercluster
Groups of galactic clusters
Dark Matter
Does not interact with light, accounts for 96% of mass of universe.
Dark energy
Fills all space and causes outward pressure counteracting inward gravity force
Expansion of the universe
Distant galaxies are all moving away from each other/Earth
