D. Astrophysics SL 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
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
Planet = a spherical body moving in an elliptical orbit round a star.
Conditions 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
Star = a luminous sphere of plasma held together by its own gravity.
Binary stars
o Binary stars are two stars orbiting around the centre of mass of both stars.
Planetary system
Planetary system = a group of gravitationally bound non-stellar objects in orbit around either a star or a star system.
Constellation
Constellation = A group of stars forming a recognisable pattern in the sky often named after the mythological character they represent.
Stellar cluster
Stellar cluster = A group of stars that are close together and are formed from the same gas cloud.
Open cluster
o 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
o 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
Nebula = a region of dust and gas where stars are born.
Galaxy
Galaxy = a large collection of stars (hundreds of billions) held together by their mutual gravity.
Spiral galaxy
o 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
o 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.
