Science Helpdesk

The Universe is about 14 billion years old, and is formed by more than 100 billion galaxies. A galaxy is a huge system of stars, interstellar gas and dust. Typical galaxies contain from ten million to one trillion stars, all orbiting a common centre of gravity. Some galaxies are elliptical shape, some are spiral, others are irregular. Galaxies are usually separated from others by distances on the order of millions of light years. Groups of galaxies gravitationally attracted between themselves are called galactic clusters.

The Solar System is located in the Milky Way galaxy, a spiral galaxy with a diameter estimated at about 100,000 light years, containing approximately 200 billion stars. The Milky Way belongs in a cluster of over 30 galaxies known as the Local Group. The galaxy of Andromeda is the nearest to the Milky Way and the biggest one in the Local Group.

The Solar System resides in one of the Milky Way's spiral arms, known as the Orion Arm, at about 27,000 light years from the galactic centre. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years.

Stars are massive, glowing balls of hot gases, mostly hydrogen and helium. Some stars are alone in the sky (the Sun, the North Star), others have companions (Sirius, which is a binary star system, and Alpha Centauri, which is a ternary star system). The nearest star to the Sun is Alpha Centauri C or Proxima Centauri. The brightest star in the Northern hemisphere's night sky is Sirius.

Not all stars are the same: stars come in all sizes, brightnesses, temperatures and colours:

• The colour of a star is due to its temperature. A blue or white star is hotter than a yellow star, which is hotter than a red star.
• The brightness of a star depends on two factors: (a) its distance from us and (b) its luminosity: how much energy it puts out in a given time, which, in turn, depends on its size. Think on a street lamp, which puts out more light than a hand torch. That is, the street lamp is more luminous. But if that street lamp is 1 km away from you, it will not be as bright, because light intensity decreases with distance. In this case, a hand torch held 10 m away from you would be brighter than the street lamp. The same is true for stars.

Constellations are groups of stars visibly related to each other in a particular pattern. Some well known constellations contain familiar patterns of bright stars. Examples are Ursa Major (containing the Big Dipper or Plough), Orion (that resembles the figure of a hunter) and Cassiopeia (with the shape of a "W"). The stars of a constellation, although appearing to be very near, may be millions LY away one to another.

The Solar System is the stellar system formed by the Sun and the group of celestial objects gravitationally bound to it:

• the eight planets and their 162 known moons,
• five dwarf planets and their six known moons, and
• thousands of small solar system bodies (SSSB).

The Sun is the main component of the Solar System, a star that contains 99.9% of the Solar System's mass. The Sun releases enormous amounts of energy in the form of electromagnetic radiation, which includes visible radiation (light), ultraviolet radiation and infrared radiation.

The planets are the biggest objects orbiting the Sun. Their orbit is almost circular. In order of their distances from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small and rocky planets; the four outer planets are gaseous giant planets with a small rocky core. All planets but the two first are orbited by natural satellites (usually called "moons"). The planets, with the exception of Earth, are named after gods and goddesses from Greco-Roman mythology. The following table shows some major magnitudes measured relative to the Earth:

The dwarf planets are also rocky objects orbiting the Sun, smaller than the planets, but bigger than asteroids. There are currently (20-sep-2008) five dwarf planets in the Solar System; the two best known of which are:

• Ceres, formerly considered the Solar System's biggest asteroid;
• Pluto, a trans-neptunian object formerly considered the Solar System's ninth planet;

The SSSBs comprise several types of celestial bodies, the best known of which are:

• Asteroids = planetoids = minor planets. They're the smallest rocky bodies orbiting the Sun. Unlike planets and dwarf planets, they are not spherical, but irregularly shaped. Most of them occupy orbits between the ones of Mars and Jupiter, and make up the asteroid belt. The biggest asteroid is called Vesta.
• Comets, very small icy trans-neptunian objects that orbit the Sun in very eccentric orbits. When a comet approaches the Sun, its icy surface begins to boil away, creating two long tails, one of gas and another of dust, which are often visible with the naked eye. Two well known comets are Halley and Hale-Bopp.

The International System of Units (abbreviated SI from the French language name Système international d'unités) is the world's most widely used system of units. The SI was developed in 1960 and, with few exceptions, is used in every country in the world.

The SI base units for the seven primary quantities are:

Quantity	Name	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Electric current	ampere	A
Temperature	kelvin	K
Amount of substance	mole	mol
Luminous intensity	candela	cd

Symbols are written in lower case, except for symbols derived from the name of a person. For example, the unit of electric current is named after André-Marie Ampère, so its symbol is written "A", whereas the unit itself is written "ampere". The only exception is the litre, whose original symbol "l" is unsuitably similar to the numeral "1"; thus it is recommended that "L" be used instead.

Abbreviated symbols should not be pluralized: for example "25 kg", not "25 kgs".

Symbols do not have an appended period (.) unless at the end of a sentence.

A prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth. The SI main prefixes are as follows:

Quantities are the measurable properties of the physical bodies. Some of the most important ones are the following:

Mass	Is the amount of matter in a body. Its unit in the SI is the kg.
Volume	Tells how much space an object occupies. Its unit in the SI is the m3.
Capacity	Is the amount of space that can be contained by a body. It is measured in L.
Density	Expresses how concentrated is the matter in a body, this is, how much matter is there in a given unit of volume in a body. It is always the same for the same type of substance, this is, it doesn't depend on the size of the object. Its unit in the SI is the kg/m3.
Temperature	It is not a kind of energy but a measure of the amount of heat in a body. It depends on the movement of its particles: the quicker the movement, the higher the temperature. Its unit in the SI is the K, but is commonly expressed in °C or °F.

What is it?

The water cycle, also known as the hydrological cycle, is the circulation of water between the different compartments or reservoirs of the Earth's Hydrosphere, involving changes in the physical state of water between liquid, solid, and gaseous phases. The water cycle is powered by the Sun's energy and the Earth's gravity.

The Earth's water cycle involves the following main physical processes:

Evaporation	Is the transfer of water from bodies of surface water into the atmosphere. This transfer involves a change in the physical state of water from liquid to gaseous phases, powered mainly by the solar radiation. 90% of atmospheric water comes from evaporation.
Evapotranspiration	Is the transfer of water from living beings into the atmosphere. This transfer involves a change in the physical state of water from liquid to gaseous phases, powered mainly by the solar radiation and the heat released by the metabolism of the living beings. 10% of atmospheric water comes from evapotranspiration.
Condensation	It takes place when water vapour in the air accumulates to form liquid water droplets in clouds and fog.
Precipitation	Is atmospheric moisture that has previously condensed (or solidified), falling to the surface of the Earth. This happens mostly as rainfall, but also as snow, hail, or fog.
Surface runoff	Includes the variety of ways by which land surface water moves down slope to the oceans: snowmelt runoff to streams, streamflow, riverflow… Water flowing in streams and rivers may be delayed for a time in lakes. Much of the precipitated water evaporates before reaching the ocean or infiltrates into the soil.
Infiltration	Is the transition of land surface water into the ground. The infiltration rate depends on soil or rock permeability. Infiltrated water may become part of the soil moisture or accumulate in aquifers: in this case it is called groundwater.
Groundwater flow	Includes the movement of groundwater in aquifers. Aquifers tend to move slowly, so the water may return as surface water (into rivers, lagoons, oceans or through springs) after thousands of years in some cases. Water returns to the land surface at lower elevation than where it infiltrated.
Absorption or drinking	Are the ways in which soil moisture or surface water is taken in by living beings.

Volume of water stored in the water cycle's reservoirs:

	Volume (106 km3)	Percent of total
Seas and oceans	1370	97.25
Ice caps, glaciers and snow covers	29	2
Groundwater	9.5	0.7
Lakes	0.125	0.01
Soil moisture	0.065	0.005
Atmosphere	0.013	0.001
Streams and rivers	0.0017	0.0001
Living beings	0.0006	0.00004

Average reservoir residence times:

Groundwater: deep	10,000 years
Seas and oceans	3,200 years
Groundwater: shallow	100 to 200 years
Lakes	50 to 100 years
Ice caps and glaciers	20 to 100 years
Streams and rivers	2 to 6 months
Seasonal snow covers	2 to 6 months
Soil moisture	1 to 2 months
Atmosphere	9 days

Sedimentary rocks can be made up of clasts or detritus (rock grains) weathered from other rocks by river waters, wind, coastal sea waters, glaciers, living beings, etc. This kind of sedimentary rocks are called detrital rocks.

They're classified upon the grains that they are made up of. First up, grains may be all the same size, uniform, as in a sandstone, or different sizes, as in a conglomerate. Secondly, grains may be bigger or smaller. The following table shows roughly how rock grains are classified upon their size:

	Diameter
Clay	< 0.004 mm
Silt	< 0.06 mm
Sand	< 2 mm
Gravel	< 6 cm
Cobble	< 25 cm
Boulders	> 25 cm

The term lustre refers to the appearance of a mineral surface in reflected light. The main types of mineral lustre are the following:

Lustre	Opacity	Examples
Adamantine	Transparent	Diamond
Vitreous	Translucent	Quartz, Halite, Olivine
Wet	Translucent	Fluorite
Resinous	Translucent	Yellow and red sphalerite varieties
Wax-like	Translucent	Talc
Greasy	Variable	Milky quartz
Silky	Variable	Fibrous gypsum
Pearly	Poorly translucent	Orthoclase, Mica aggregates
Dull metallic	Opaque	Cinnabar aggregates
Metallic	Opaque	Pyrite, Chalcopyrite, Magnetite

This is not the most modern way of classification of living beings, but still proves to be quite useful to understand and organise the huge diversity of the living beings: