IB Physics SL Option D: Astrophysics

Asteroid

Small rocky object orbiting the sun

Comet

Irregular object a few kilometers in diameter, made from frozen gas, rock and dust. They have very elliptical orbits around the sun with varying periods, when near the sun the gases vaporize making the recognizable "shooting star" tail

Meteor

An asteroid on a collision course with another planet

Order of Planets from the Sun

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

Dwarf planet

Small round bodies that orbit the sun but have not cleared the area around their orbits of other orbiting bodies

Constellation

A group of stars that forms a recognizable pattern as viewed from earth, stars can be very far apart in reality.

Stellar cluster

A group of stars that are close together in space, are held together by gravitational forces.

Astronomical unit

The average distance from the earth to the sun.

Parsec (pc)

A measure of distance equal to 3.26 ly. 1 pc is equal to the distance at which one astronomical unit subtends an angle of one arcsecond/ star with a parallax angle 1 arcsecond.

Lightyear (ly)

The distance light travels in one year.

Arcsecond

1/3600 of a degree.

Galaxy

A huge group of single stars, star systems, star clusters, dust, and gas bound together by gravity.

Universe

All of space and everything in it.

Luminosity

Total power radiated by a star.

Apparent brightness

Power received per unit area.

Red shift

When a star is receding from earth.

Blue shift

When a star is approaching earth.

Spectral classes

In decreasing temperature: OBAFGKM.

Red giant

Comparatively cool, large in size and red in color, one of the later possible stages for a star. Energy comes from fusion of elements other than hydrogen.

White dwarf

Small in size and white in color. Comparatively hot. One of the final stages for small mass stars. Fusion is no longer taking place, it is just a hot remnant that is cooling down. Eventually it will cease to give out light when it becomes sufficiently cold (becomes a Brown Dwarf)

Red supergiant

Even larger than red giants

Main sequence star

Fuse hydrogen to produce energy, main part of a stars lifetime, 90% of stars are in this phase. Luminosity is directly proportional to mass^3.5

Planetary nebula

Ring shaped nebula formed by an expanding shell of gas around an aging star

Supernova

A star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass

Neutron star

Post-supernova remnants of some larger mass stars. Gravitational pressure has forced a total collapse and the madd of a neutron star is not composed of atoms but neutrons. Density is enormous. Rotating ones are called pulsars.

Black hole

Post-supernova remnant of larger mass stars. Escape velocity of a black hole is faster than the speed of light.

Cepheid variables

Have a regular variation in brightness and luminosity due to an oscillation (contractions and compression) in the size of a star. There is a link between the period of the brightness variation and their average luminosity so they can be used to calculate the distance to some galaxies (act as "standard candles"

Binary stars

Two or more stars in orbit around their common centre of mass

Wein's law

Wien's displacement law states that the black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the temperature.

Hubble's law

The distant galaxies we see in all directions are moving away from the Earth, as evidenced by their red shifts. Hubble's law describes this expansion.

Cosmic microwave background

Isotopic microwave radiation from all directions in space corresponding with a temperature of 2.73K, which fits with the Big Bang models predictions of the universe expanding (wavelength increases) and cooling (temperature matched prediction).

Big Bang Theory

All matter originated from the same point 15 billion years ago, the Big Bang model predicts that since a certain moment when things were all together with very high pressure and temperature there was an initial rapid expansion, and then now a steady expansion of spacetime during which the Universe's temperature and density have been decreasing. Big Bang = creation of space and time.

Red shift equation

Evidence of universe expanding, galaxies moving away from us, light waves experience a red shift ( their velocity is small compared to the velocity of light) because the wavelength is altered by the Doppler effect as it is moving away.

Chandrasekhar limit

1.4 solar masses. Below this limit electron degeneracy pressure prevents further collapse, above this the gravitational pressure is too high, so it is the maximum mass for a white dwarf.

Oppenheimer-Volkoff limit

2-3 solar masses. This is the largest mass a neutron star can have, above which the neutron degeneracy pressure outwards is overcome by gravitational force inwards and it collapses into a black hole.

Visual binary

Can be distinguished as two separate stars using a telescope

Spectroscopic binary

Identified from the analysis of the spectrum of light from the 'star'. Over time the wavelengths show a periodic shift or splitting in frequency.

Eclipsing binary

Identified from the analysis of the brightness of the light from the 'star'. Over time the brightness shows a periodic variation.

Nebula

Interstellar clouds of dust, hydrogen and other ionized gases

Pulsar

Cosmic sources of weak radio wave energy that pulsate at a very rapid and precise frequency. Linked to rotating neutron stars.

Quasar

Point-like sources of light and radio waves that are very far away. (very large red shifts, limits of observable universe, so they must be emitting very much energy... theoretical models link them to energy being radiated as a result of whole stars falling into black holes)

Dark energy

Mechanisms that cause an accelerating universe are not fully understood, must involve an outward accelerating force to counteract inward gravitational, must be a source of energy and matter we can't see, this has been given the name "dark energy" and "dark matter"

Type Ia supernovae

Explosions involving white dwarf stars. The amount of energy released can be predicted accurately and so they can be used as standard candles. By comparing the luminosity of a type Ia supernova and its apparent brightness as observed in a given galaxy, the distance can be calculated.