IB ASTROPHYSICS OPTION DEFINITIONS AND CONCEPTS
SOLAR SYSTEM: The Sun is orbited by planets, moons, asteroids and comets.
ELLIPTICAL ORBIT: Non-circular path that 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 that 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 CLUSTER: A group of stars that are close to each other due to the collapse of a gas cloud. They are
gravitationally bound.
CONSTELLATION: A pattern of stars in the sky as viewed from Earth. They are probably not near each other nor
gravitationally bound.
ASTRONOMICAL UNIT (AU): The mean distance from the centre of the Earth to the centre of the Sun.
1 AU = 1.5 x 108 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 group of stars held together by gravity. It typically contains 1011 stars and is 105 ly across. Distances between
stars are approximately 1 ly. The distance between galaxies are 106 ly. The shapes of the galaxies can be spiral, globular
or irregular.
APPARENT MOTION OF THE 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 (L): The total amount of energy emitted by the star per second. Unit: watt. Depends on the star’s
temperature and its size.
APPARENT BRIGHTNESS (b): The amount of energy received per unit area. Unit: Wm-2
BLACK BODY RADIATION: A star is approximately a perfect emitter of a continuous radiation spectrum that 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.
WIEN’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 that orbit their common centre of gravity.
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 HR diagram are Main Sequence, Giants, Supergiants, White Dwarfs.
CEPHEID VARIABLE STARS: The size 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 (pc): 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 105 AU = 3.1 X 1016 m
APPARENT MAGNITUDE (m): 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 (M): The magnitude of a star viewed from a distance of 10 pc.
SPECTROSCOPIC PARALLAX: Method of measuring distance to stars that are too far away for stellar parallax. It combines
Wien’s Law, the HR diagram and apparent brightness.
NEWTON’S MODEL OF THE 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 what is
the night sky not bright?
EXPANSION OF THE 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: The Big Bang model predicts that after 300 000 years at 4 000 K,
atoms formed and the universe became transparent for the first time. The radiation that could now travel through it is
still visible today as microwaves coming from all directions.
CRITICAL DENSITY: The density of the universe at which it will continue to expand forever at a steady rate.
INTERNATIONAL ASTROPHYSICS RESEARCH: There are many projects that involve cooperating nations, such as the
International Space Station and the Hubble Space Telescope.