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.
