Option D Flashcards
Comet
a celestial object consisting of a nucleus of ice and dust and, when near the sun, a ‘tail’ of gas and dust particles pointing away from the sun.
Asteroid
a small rocky body orbiting the sun. Large numbers of these, ranging enormously in size, are found between the orbits of Mars and Jupiter, though some have more eccentric orbits.
Planet
a spherical body moving in an elliptical orbit around a star.
Condition for being a planet
o Clear the area around it with its gravity
o Orbit a star with a low eccentricity
o Have sufficient mass to form a spherical shape
Star
a luminous sphere of plasma held together by its own gravity.
Binary Star
Binary stars are two stars orbiting around the centre of mass of both stars.
Planetary System
a group of gravitationally bound non-stellar objects in orbit around either a star or a star system.
Constellation
A group of stars forming a recognisable pattern in the sky often named after the mythological character they represent.
Stellar Cluster
A group of stars that are close together and are formed from the same gas cloud.
Open Cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age (<10 billion years old).
Globular Cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities towards their centres (>10 billion years old).
Nebula
a region of dust and gas where stars are born.
Galaxy
a large collection of stars (hundreds of billions) held together by their mutual gravity.
Spiral Galaxy
Spiral galaxies consist of a rotating disk of stars and interstellar medium with a central bulge of (generally) older stars. Extending outward from the bulge are relatively bright arms.
Elliptical Galaxy
Elliptical galaxies range in shape from nearly spherical to highly flattened ellipsoids and range in size from hundreds of millions to over one trillion stars. They could be formed from the collisions of spiral galaxies.
Irregular Galaxy
Irregular galaxies have no specific structure. This could be because other, more massive galaxies have drawn away their matter.
Cluster of Galaxies
a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity with typical masses ranging from 10^14–10^15 solar masses.
Super Cluster of galaxies
a large group of smaller galaxy clusters or galaxy groups, which is among the largest-known structures of the cosmos.
Luminosity
the total energy emitted by the star per second.
Apparent Brightness
the energy reaching a square metre on Earth per second.
Black body
an object that absorbs all radiation that falls on it and emits all radiation within it
Stephan-Boltzmann Law
the luminosity of a star depends on its surface area and absolute temperature according to L = sigmaAT^4
Hydrostatic equilibrium
Where the radiation pressure is balanced by the gravitational attraction.
Main sequence star
Stars that produce energy through the fusion of hydrogen into helium.
Cepheid Variable
Stars that have a luminosity that varies periodically
Big Bang model
Both space and time (the universe) were created at a single point and have been continuously expanding from its beginning, about 13.8 billion years ago.
CMBR
Em radiation left over from the early stages of the universe
Chandrasekhar limit
Minimum mass a star remnant must have to become a neutron star (1.4 solar masses)
Oppenheimer Volkoff limit
Minimum mass a neutron star must have to become a black hole (3 solar masses)
Jeans Criterion
For a gas cloud to collapse into a star the gravitational potential energy of the particles must be greater than the kinetic energy.
Jeans Mass
This mass a cloud must be to form a star under certain conditions.
Proton-Proton Chain (Hydrogen fustion)
The proton–proton chain reaction is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium
CNO Cycle (Massive Stars)
The CNO cycle is one of the two known sets of fusion reactions by which stars convert hydrogen to helium. The CNO cycle is a catalytic cycle. It is dominant in stars that are more than 1.3 times as massive as the Sun.
Neutron Capture
The process where a nucleus absorbs a neutron which, if there is sufficient time, can decay by beta minus into a proton, making a heavier element.
S-Process
Neutering capture with a low neutron flux (eg in star) so beta decay can happen and heavier elements can be made.
R-Process
Neutron capture with a high neutron flux so there is insufficient time for beta decay so heavier isotopes are made.
Type IA Supernova
When a dense dwarf star in a binary system gains enough mass from the other star to cross the Chandrasekhar limit, meaning it collapses.
Type II Supernova
When a massive star has fused all its silicon into iron so has no radiation pressure and collapses.
Difference between IA and II supernovae
Brightness of IA falls sharply whilst it plateaus for a few days for type II.
Cosmological principle
The universe is isotopic and homogenous
Isotropic
same in all directions
Homogenous
same in all locations
Dark matter
Matter that does not interact with light thought to make up around 25% of the energy density of the observable universe.
Dark energy
Energy causing the expansion of the universe to accelerate.
MACHO
Massive compact halo object - normal matter that we haven’t observed.
WIMP
Weakly interactive massive particle - subatomic particle not described by the standard model that interacts with gravity but not light.
Standard Candles
An astronomical object of known luminosity, which can be used to calculate distances (to distant galaxies etc.) through the relationship:
Cosmic scale factor
o The cosmic scale factor is used to compare the size of the universe and two different times.
o This is done by comparing the distance between two points in space now and at another time
o As distances increase because of the expansion of space, this increase in distance can be found from the cosmic redshift
Hubble’s Law
The relative recessional velocity between galaxies is proportional to the distance between them.
Age of Universe
Ho = 40-90 kms^-1 Mpc^-1
Critical density
The density at which the universe would stop expanding after an infinite amount of time (gives rise to a flat universe)
