A brief summary of some objects in the universe

The IB says you need to know about: planets, comets, stars (single and binary), planetary systems, constellations, stellar clusters (open and globular), nebulae, galaxies, clusters of galaxies and superclusters of galaxies. Essential descriptions are below, titles and other coloured text show links - click for pictures and much more information.

PLANETS: The three criteria for major planets in our solar system is that they orbit the Sun, they are big enough to be at least approximately spherical and they are the dominant object in their orbit. You should know the names (and order in terms of average distance) of the major planets from the Sun (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune). Dwarf planets, minor planets and asteroids may be big enough to be spherical, and orbit the Sun, but they do not dominate their orbits, they share their orbits with other, similar objects. Planets can be mostly rock, mostly ice or mostly gas, but they do not generate heat and light by nuclear fusion. That's what stars do. Moons orbit planets. Some moons are big enough to be circular, and some are as big as planets, but as they orbit planets they are not planets.

COMETS: Comets are minor solar system bodies, mostly made from ice. Mostly originally from the outer solar system, they sometimes fall in towards the sun and are occasionally trapped into elliptical orbits around the sun). As they get close to the sun they warm up and gas and dust is discharged from the comets and form large atmospheres and huge tails.

STARS: Stars are large spherical objects, mostly made from hydrogen and helium, that generate heat and light by nuclear fusion. Many stars are SINGLE, with no other star within light years, but most stars are members of BINARY or multiple systems with two or more stars. In a BINARY system the stars orbit their common centre of mass / centre of gravity (this is important, it is wrong to say they orbit each other, or orbit the biggest star - this was an exam question recently). The size of a star is determined by its mass and the balance between the radiation pressure pushing outwards and the force of gravity pulling in. This is an example of HYDROSTATIC EQUILIBRIUM. Much of the rest of this unit is about how stars are born, live, work and die.

PLANETARY SYSTEMS: This is a star (or binary / multiple stars) with surrounding planets and, presumably, minor planets, comets, etc. All of it gravitationally bound together.

CONSTELLATIONS: A constellation is a culturally based group of stars in the same-ish area of the sky as seen from Earth. This a line of sight effect (some stars are fainter but close, some are much brighter but further away). There are 88 modern constellations recognised by the IAU (the International Astronomical Union) and they are of some use in identifying where in the sky something is, but they aren't of much interest to professional astronomers.

STELLAR CLUSTERS: These are real, gravitationally-bound, groups of stars (unlike constellations). There are two types. GLOBULAR CLUSTERS are normally very large, very dense and old clusters that orbit a galaxy. OPEN CLUSTERS are much smaller and younger clusters of stars. They are the result of a burst of star formation for a type of nebula. They tend to be young as they 'evaporate' over time as stars are dispersed from the cluster.

NEBULAE: A nebula (plural: nebulae - latin for cloud(s)) is an interstellar cloud of dust and gas, with masses normally much larger than the largest stars. Dense only in comparison to empty space there are several types. The most important to us are those that act as Stellar Nurseries, where stars are born. From Earth they can appear dark (where they block the light of more distant stars) or glowing (when heated by nearby newborn stars).

GALAXIES: A galaxy is a large gravitationally-bound collection of stars, nebulae, clusters, etc. They have at least 100 million stars and can be as large as 100 trillion stars! Our own galaxy, the Milky Way, is a reasonably large galaxy with a spiral shape, a diameter of around 200 000 light years and has as much as 400 billion stars. Galaxies are not spread out evenly across the universe. They tend to be found in CLUSTERS and clusters of galaxies tend to form SUPERCLUSTERS.

D.1a Objects in the universe presentation