Topic 1 Astronomy notes GCSE

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1.1: Planet Earth

There are three key features that determine our planet from all the others:

Life: in all diverse forms.

Atmosphere: 78% nitrogen and 21% Oxygen.

Water: Essential to life on Earth. Covers 70% of the surface.

Why is the Sky blue: If the Earth had no atmosphere then the sun would appear white on a black background, like the sun does from the moon. The atmosphere scatters light. Light interacts with the particles of the atmosphere and they are moved off course. Blue light has the shortest wavelength and is scattered much more than red which has the longest wavelength (white light made up of all colours). Green light is scattered in the middle. Blue light is scattered the most which is why the sky appears blue.

Earth's average diameter is 13000 km and it is flattened slightly at the poles (by 42 km). Therefore it is the shape of an oblate spheroid.

There is evidence that the Earth is spherical:

Arcs of air crafts: They travel in arcs as it is shorter therefore saying that the Earth is a sphere.

Ships: They disappear over the horizon.

Satellites: Wouldn't orbit Earth if it were flat.

Space images: To show that the Earth is a sphere.

Curvature of Earth's shadow: during a lunar eclipse.

The Equator is an imaginary line that splits the Earth into two halves: The Northern and southern hemispheres. In order to identify parts of the Earth, the surface contains a system or reference lines of latitude and longitude. These are shown as a series of horizontal and vertical lines. But on a globe they converge towards the poles.

The Angle between a point on the Earth's surface, the center of the Earth and a point on the equator is the latitude of the point. It is expressed in an angle of degrees North or South of the Equator. They parallel with the equator. E.g. the latitude of Edinburgh is 56 degrees north and Buenos Aires is 35 degrees south. The poles have angles of 90 degrees north and south.

Earth's polar axis tilted by 66.5 degrees to the plane on which it orbits the sun. Therefore the Sun lies directly above the equator on only two days a year-Vernal and autumnal equinoxes (21st March and 22nd/23rd September). Between these dates, the sun resides over different latitudes up to a maximum of 23.5 degrees. When the Sun is at these extreme latitudes there are imaginary circles called the Tropic of cancer (north) and the tropic of Capricorn (south). They take place on 21st June and 21st December. They are called the Summer and Winter Solstices.

The poles are located at the intersection of the Earth's axis of rotation and the Earth's surface. They are the Northern and Southern ends of the Earth's axis.

The Longitude of a point is the angular displacement East or West of the Observer's meridian from the prime meridian (0 degrees longitude) that passes through Greenwich, London. It is expressed as an angle East or West of Greenwich. E.g. Sydney. Australia has a longitude of 151 degrees east. Kansas in the USA has a longitude of 94.5 degrees west. They are not parallel as they intersect at the Earth's poles.

Zenith: The point in the night sky directly above the observer on the celestial sphere. 90 degrees above the observer.

Horizon: Where the Earth's surface and the sky appear to meet at the observer's point of view.

Light pollution:

Observations of the night sky are often hindered by skyglow-a form of light pollution. Lots of unwanted and wasteful light shines into the skies of urban areas at night which causes an orange background haze that prevents faint stars, nebulae and the milky way from being observed. The countryside is the best place to observe stars.

Main sources of terrestrial light pollution are:

Commercial and sports floodlights.

Urban streetlamps and motorway lights.

Domestic and industrial security lamps.

Lights above car parks and shopping centers.

The moon also effects the viewing but it is not artificial, it is natural.

There are two types of light pollution: Indirect and direct. Indirect happens because the light from these light sources reflect from the ground up to the sky. Direct happens when the light goes straight into the sky.

CfDS is the Campaign for Dark Skies in Britain. It was set up in 1989 for both Astronomers and non astronomers who appreciate the night sky. They agree that lighting is essential and that poorly designed light is directed upwards or unnecessarily bright. They campaign against excessive, irresponsible and inefficient lighting and work to persuade organisations about well directed lighting.

Eratosthenes and the Earth's circumference:

Eratosthenes made the first accurate calculation of the circumference of the Earth in the 3rd Century BC. he was a Greek mathematician and Geographer. He was employed as the chief librarian of a library in Alexandria in Egypt. He read an account that at noon on the 21st June in Syene, on the Tropic of Cancer, the sun lay directly over a well. It did not cast any shadows at this precise moment in time. Measurements of the Sun's position at noon in Alexandria on the same date showed that is was just over 7 degrees from the Zenith (equivalent to 1/50th of a circle). He learnt that Syene was 790 km further south than Alexandria and so he used geometry to deduce that the circumference of the Earth was 50 times that value i.e. 39500 km. He assumed that light from the sun is parallel and the used alternate angles. He could then calculate what fraction of the circumference the 790 km between the two cities was. He found that this angle was also 7 degrees.His calculation of the Earth's circumference was surprisingly accurate (within 5%) given the many uncertainties and that he was using sticks.

The Earth's atmosphere:

The atmosphere consists of 78% Nitrogen, 21% Oxygen, 1% argon, average 1% water vapour, 0.04% Carbon Dioxide and the rest include small traces of neon, methane and helium. With increasing elevation, the atmosphere gets thinner, gradually merging with outer space when it reaches an altitude of about 10,000 km. There is no actual height that separates the atmosphere from space.

The atmosphere provides us with many benefits:

Gamma rays: Absorbs harmful energetic x rays and gamma rays and stops them from reaching the ground.

Oxygen: Provides us with oxygen to breathe.

Meteoroids: The majority of meteoroids burn up as they pass through the atmosphere before reaching the ground and many are visible (shooting stars).

Ultra violet rays: Absorbs harmful UV radiation that can cause accelerated skin aging and skin cancer. Ozone layer absorbs this.

Temperature: It regulates the temperature, allowing water to exist in liquid form (temp on Earth's surface is 15 degrees Celsius). Temperature usually between -90 and +60 degrees Celsius.

Drawbacks of the atmosphere:

Refraction of light as it passes through the turbulent atmosphere. This causes stars to twinkle and restricting the resolution of an image.

Selective scattering of shorter wavelengths (bluer) of sunlight by gas molecules in the atmosphere. This makes the sky appear blue and observations cannot be done during the day.

Absorption/reflection of most electromagnetic radiation from space. Therefore they cannot reach sea level and these must be studied using Earth Orbiting satellites.

Most EM radiation from space fails to pass through the atmosphere.

The atmosphere has different effects on radiation:

Longest radio waves are reflected back into space by electrons in the ionosphere.

Shorter wavelength microwaves are absorbed by water vapour and oxygen.

Most infrared radiation is absorbed by water vapour, carbon dioxide and methane. There is a small window that is let through.

UV radiation is absorbed by ozone and at shorter wavelengths, oxygen.

X-rays and gamma rays are absorbed by oxygen and nitrogen. The radiation loses energy and fails to penetrate most of the atmosphere.

Telescopes and observatories

Now, good astronomers rarely look through telescopes, however the instrument remains one of their most useful tools. A telescope collects as much light as possible and produces a focused image that can be viewed, studied, photographed or analysed.

There are two types of telescope:

Refractor: A glass convex lens collects the light and brings it to focus.

Reflector: a curved mirror collects the light.

The lens or mirror is called the objective and the size of a telescope refers to its diameter.

Large telescopes have two advantages over small ones:

The collection of more light.

Higher resolution in proportion to diameter.

A 10m telescope collects 25x more light than a two metre telescope and has 5x higher resolution. More light entering a telescope means that observing times can be shorter and more can be observed.

Large mirrors can be made to a much higher precision than lenses therefore most professional telescopes are reflectors. Big lenses are hard to make. In practice, imperfections in mirrors and absorption by other optical components will result in some loss of light and atmospheric turbulence will effect resolution.

Ideal observing sites consider:

Atmospheric properties: cloud cover, air turbulence, sky brightness and water vapour content.

Geographic location: access, utilities, likelihood of earthquakes, ground stability.

Most optical and infra red observatories are located on the tops of high mountains such as Mauna Kea on Hawaii.

There are more advantages to using orbiting telescopes rather than on ground ones:

No atmosphere to blur images-increase resolution.

No light pollution.

No adverse weather problems.

Longer observing periods.

Ability to detect other wavelengths e.g. gamma rays, x rays and IR.

There are also drawbacks:

reduced lifetime

difficult or impossible to repair.

More expensive to build.

The temperature of space telescopes must be kept to 0 in order to prevent their own IR emission from interfering with objects from space. This achieved using cryogenic cooling systems.

Van Allen Belts

The Van Allen belts (radiation belts) are two donut shaped rings of spiraling high energy particles from solar wind held in place by the earth's magnetic field. They are named after the American physicist James Van Allen who led the university of lowa team that was responsible for their discovery.

The inner belt is compact, and consists of mainly high energy protons, formed by collisions between cosmic rays and atoms in the atmosphere. Its altitude is 600-10,000 km (0.2-2 earth radii).

The diffuse outer belt is more dynamic and consists mainly of electrons and other charged particles emitted by the sun due to increased solar activity. Its altitude is 15,000-65,000 km (3 and 10 earth radii=3-10 x earth's radius).

The Discoveries began in October 1957 when Sputnik 1 satellite was released by the Russians. The inner belt was discovered in January 1958 using a Geiger counter (measures radiation) on board the US satellite, Explorer 1. This was originally sent to detect and study the intensity of cosmic rays from further away. The existence of the inner belt was confirmed by Explorer 3 and Sputnik 3 later in the same year.

The outer belt was discovered in December 1958 by US space probe Pioneer 3. This was originally meant to be a lunar flyby but due to a fuel cut out, the spacecraft only reached 100,000 km. It then fell back to Earth but before burning up, its Geiger counters provided data that allowed Van Allen's team to detect the existence of the second. This was confirmed by pioneer 4.

1.2: The Moon

The Moon is the Earth's only natural satellite. Its diameter is just 3,500 km (1/4 of Earth's). It orbits at a mean distance of 380,000 km. Due to its relatively small mass, the gravity on the moon is only about 1/6 of that on Earth. The most likely reason is the lack of any significant atmosphere.

this is due to its low gravity. The escape velocity of the moon is significantly lower. It is easy for fast moving particles to escape the moon's influence.

One hemisphere of the moon always faces us (the near side). This is because the Moon's rotation period is the same as its orbital period (27.3 days). Therefore the far side of the moon cannot been seen when on Earth. The far side was first observed by Soviet probe Luna 3 in October 1959. 17 images were successfully transmitted back to Earth. This revealed that the terrain was very different to that on the near side. The first humans to view the far side were the Apollo 8 astronauts in December 1968.

Lunar Features

People who see the moon from far notice two contrasting features: Large, dark grey, smooth lunar seas (maria), and lighter grey, mountainous, highly cratered highlands (terrae). Lunar craters are formed by meteors and comets. Lunar seas are called seas as astronomers once thought that they were filled with water. Seas are formed by lave rising from craters and cooling. They are younger than highlands.

Lunar seas were not victims of heavy bombardment therefore they are younger and have few craters.

Through binoculars or a small telescope, a few distinct mountain ranges and craters are also clearly visible.

In contrast, the moon's far side is almost devoid of seas and is made up of mainly mountains and there are many craters. There are only few craters in the lunar seas compared to highland regions therefore implying that the former are much younger. The moon has been struck by solid bodies such as meteoroids from elsewhere. These collisions form craters on the lunar surface, more prominent ones being Tycho and Copernicus being caused by the impacts of asteroids. The far side of the moon can be seen by space probes, lunar satellites, Apollo missions.

Rilles are narrow channel like depressions in the lunar seas that can be straight, smoothly curved or sinuous. Most are believed to be caused by collapsed lava flows.

Wrinkle ridges are believed to be caused by the buckling of lunar surface as a result of compressive forces within the cooling, contracting lava, forming ridges that are very long.

Some lunar seas include the sea of tranquility (Mare Tranquillitatus) and the Ocean of storms (Oceanus Procellarum).

Manned Exploration

Objectives included:

Collection of Lunar soil and rock for analysis.

To deploy scientific experiments on the lunar surface.

To win the race to the moon against the Soviet Union and restoring national prestige.

To send humans to the moon and back safely.

The goal of sending man to the moon was finally achieved in July 1969 when Apollo 11 astronauts Armstrong and Aldrin stepped onto the surface of the moon. They spent 2.5 hours exploring the LM in the sea of Tranquility (where they landed). That is where they planted the US flag. Between the years of 1969 to 1972, there have been 5 more successful missions following Apollo 11. Altogether ,12 people have walked on the moon, and to this day, they are still the only people.

Although each astronaut was there for a few hours, they installed and activated scientific instruments that would provide experimental results. ALSEP (Apollo Lunar Surface Experiments Package) was what was left behind to collect data. The scientific instruments were designed to perform long term studies of the local environment and internal structure of the moon. They are powered by a Radiosotope Thermoelectric Generator (RTG).

Each instrument was deployed to measure, analyse and monitor:

Dust

Interior of the moon's structure

Wind intensity and direction

Atmosphere composition and pressure

Gravity changes

Thermal and electrical properties of the lunar sub surface.

Also, several ALSEPs included a Laser ranging Retroreflector which was used to determine accurately the Earth-Moon distance using radar technique.

Formation of the moon

There are various hypotheses about how the Moon was formed about 4.5 billion years ago. This include the:

Condensation or Co-formation Hypothesis: The Earth and moon formed at the same time and place from material within the solar nebula.

Capture hypothesis: The Moon was formed far from Earth and was captured by the Earth's gravitational pull during a close encounter.

Fission hypothesis: A rapidly spinning Earth caused a large part to spin off to form the moon.

But, evidence gathered during Apollo missions said that these hypotheses contained severe flaws. E.g. magnetometers unable to detect presence of any significant lunar magnetic field and analyses of rocks revealed that the average density of the Moon was different to that on Earth. the moon lacks an iron core and implies that the Earth and Moon were not formed together.

Most now accept the theory involving a big off centre collision between two bodies, one being Earth. This is called the Giant impact hypothesis in which Earth was struck by a mars-size object known as Theia that is believed to have shared the same orbit. Theia and part of Earth's outer layers melted and merged, and also lots of debris was thrown into Earth-orbit. This material came together to form the moon.

Scientific evidence includes:

Water: Lack of water in the lunar rocks suggests that there was a collision involving so much energy that water was vaporised during the Moon's formation.

Elements: Discovery of KREEP-rich rocks (K-potassium, Rare Earth Elements and Phosphorus) in the Ocean of Storms and sea of showers regions of the moon. The hypothesis predicts that an ocean of magma would have surrounded the moo and KREEP rich magma would have crystallised in a region below lunar crust.

Oxygen: Oxygen isotopes deficiencies in Moon rocks which weren't in Earth suggests that they were formed at the same distance away from the sun.

In 1984, a conference was held in Hawaii to reach an agreement of the origin of the moon.

1.3: The Sun

The sun has a diameter of 1.4 million km. This is about 100x bigger than the Earth's diameter. Because the Sun is at a distance of 150 million km, (1 au) the sun appears only half a degree in size. The Sun provides heat, light and energy requirements for life on Earth.

One astronomical unit is the mean distance from the Earth to the sun.

It is a rotating sphere of gas, made up of 75% hydrogen, 25% helium and small amounts of other elements. There are three main parts of the sun:

Photosphere: The visible surface of the sun which has a temperature of 5800 Kelvin.

Chromosphere: Above the photosphere and is called the sphere of colour. it makes up the Sun's atmosphere with the corona. It is not usually visible from Earth but can be observed just before totality of a solar eclipse. Temperature varies between 4,500 K and 20,000 K.

Corona: makes up the Sun's atmosphere and is visible during a solar eclipse. It is a glowing region of ionised gas and has a temperature of 2 million K. This is hot enough to emit x-rays.

The Sun's energy source

Nuclear fusion reactions occur and these are the source of the Sun's energy. The temperature is so hot (15 million K) that hydrogen nuclei (protons) are able to fuse together to form helium nuclei in a series called the proton proton chain. this happens in the core.

At each stage in these reactions, matter (m) is lost and converted into an equivalent amount of energy (E) according to Einstein's E=m c^2 where c is the speed of light. The sun loses over 4 million tonnes of matter each second. Some of the energy allows nuclear fusion reactions to continue and maintains its temperature. The remaining energy is radiated and then for the outer 20,000 km, convected to the Sun's photosphere from where it is radiated into space.

Safely observing the Sun

H-Alpha: This is a special filter that is put on the edge of a telescope. This is not recommended for beginners.

Indirect Projection: This is using a pinhole camera, pair of binoculars or a telescope which focuses on an enlarged image of the sun onto a screen, reducing the brightness to a safe level.

To improve the image's contrast, a card or wooden baffle should be used to create a dark shadow on the projection screen. The objective lens should be partly covered with black card to reduce the amount of light entering the telescope.

Sunspots

The projection method is the best way to observe sunspots. They appear as tiny dark patches on the photosphere but are actually the size of Earth. Sunspots are cooler areas (release less light) of the photosphere that correspond to strong localised magnetic fields. They are low temperatures as a result of the upward motion of convecting solar material and prevent it from reaching the top of the photosphere. They usually appear in pairs or groups of opposite magnetic polarity.

The typical structure of an individual sunspot consists of:

an Umbra: Central darker region-2,000 K cooler than photosphere.

a penumbra: A lighter, less dark surrounding area-200 K cooler than photosphere.

The Sun does not rotate as a solid body, Its rotation period varies from 25 days at its equator to 36 days at the poles. The relative number of sunspots follows a well known regular 11 year pattern called the solar cycle. The number of sunspots increases to a maximum before falling again.

As the sun rotates, any object o the surface rotates at the same speed. 360 degrees iss the degrees of a circle. If a sunspot moves 60 degrees in 5 days. 60 degrees is a1/6 of 360 degrees. It would therefore take 30 days for the sunspot to rotate around the sun once as 5 x 6 is 30. This is how astronomers determine the Sun's rotational period.

It is best to study the positions of sunspots by plotting their latitude against time on a Butterfly diagram. The cycle begins with relatively small sunspots at latitudes between 35-40 degrees north and south of the solar equator. As time continues, they tend to drift towards the solar equator, and terminate between latitudes of 5-10 degrees. There are more sunspots at the start.

The solar maximum is the peak of the solar cycle-when solar activity is at its highest and Sunspots are most frequent.

The Solar wind

This is a steady stream of charged particles (protons and electrons, sometimes ions of helium and other elements) flowing outwards in all directions from the Sun's corona at a speed of 400 km/s. The high temperature of the corona partly explains how these particles gain kinetic energy to escape the Sun's gravity. Other mechanisms are also responsible. In coronal holes, particles escape more easily and therefore solar wind is faster.

The Appearance of the Sun at different wavelengths

Astronomers can build up more comprehensive pictures by studying the sun with different wavelengths of the EM spectrum. Many space probes and solar observatories have been launched from Earth, designed to study the sun at different wavelengths.

When a telescope is fitted with an H-alpha filter, it allows a narrow range of wavelengths either side of 656 nm to pass through and blocks the remaining light. The increased contrast allows solar features such as:

Prominences (Huge clouds of cooler gas in the Sun's atmosphere)

Filaments (The same, but appear as dark silhouettes against the brighter photosphere)

Solar flares ( sudden release of energy)

Sunspots and the chromosphere to be observed safely.

Exceptionally hot regions of the Sun emit x-rays. A picture of the sun using an x-ray filter would show the corona as bright regions at the edge of the solar disc, and bright regions on the solar disc are associated with active region loops where ionised hydrogen flows along curved magnetic field lines. Astronomers would have to fit a special filter or use an x-ray telescope.

1.4: Earth-Sun-Moon Interactions

Lunar phases:

The moon's lunar cycle lasts for 29.5 days (one lunar month). The cycle starts with a new moon (when the moon aligns with the sun), then a waxing crescent, first quarter, waxing gibbous, full moon. The visible part is on the right. Then it goes from full moon to waning gibbous, last quarter, waning crescent and then back to a new moon. The full moon is when the moon appears directly opposite the sun. The moon reflects sunlight, therefore is visible. The visible parts depend on where it is when orbiting Earth in relation to the sun. The lunar month is 2.2 days longer than its orbital period. This is because the Earth is orbiting the sun the same way the moon orbits the Earth. Therefore the sun appears to move with respect to other stars and it takes the moon 2.2 days longer to return to the apparent position of the sun.

Eclipses

The sun and moon appear to be the same size in the sky. This is because the Sun is 400 times further from Earth than the moon but is also 400 times bigger. Therefore the angular sizes of the sun and the moon are exactly the same when observed form Earth.

The moon passes directly in front of the sun at times and this obscures light from the Sun's photosphere and produces a total solar eclipse for observers in the Dark shadow (umbra) on Earth. When there is a total solar eclipse, bright solar prominences can also be seen.

Three things happen during totality:

The sky darkens so some of the brighter stars are seen.

The temperature drops.

The Sun's corona is visible.

Bailey beads are small bright spots of sunlight caused by the Sun's rays shining through valleys on the moon. At the beginning and end of totality, when just one bead is visible, this is known as the Diamond ring effect.

Solar eclipses only occur at a new moon and last for only a few minutes. They do not occur at every new moon because the plane of the Moon's orbit is tilted by 5 degrees to the plane of Earth's orbit around the Sun. At most new moons, the Moon is either below or above the Sun in the sky.

If the moon obscures only part of the Sun's light (penumbra), then the observer will see a partial solar eclipse. The Sun appears to have a circular bite due to the silhouetted moon.

When the Moon is full, it is possible to observe a lunar eclipse. The Earth casts a shadow and prevents sunlight from reaching the moon. This does not happen every full moon but as the Earth is bigger, the chances of the Moon falling into the Earth's shadow is more probable.

Total lunar eclipses are visible from half the hemisphere at night (facing the moon) and is more common than solar eclipses from a given location. Totality lasts between 30 mins and 1 hour-whole eclipse can last several hours.

During totality, the moon appears as a red/copper coloured full moon. This is because light from the sun passes around the edge of the Earth and blue light is scattered to leave mainly red light. Red light is refracted by Earth's atmosphere (converging lens) and illuminates the moon.

A Partial lunar eclipse is when Earth prevents only some of the light from falling onto the moon, showing a curved shadow of Earth.

Time and its determination

The Earth spins from West to East (anticlockwise) in a time of 23 hours, 56 minutes and 4.1 seconds. This is called a sidereal day and is known as the time taken for successive crossings of a given star over the observer's meridian. A sidereal day measures the time interval of Earth's rotation using the stars for reference. It is the time for Earth to complete a whole turn in space.

A Solar day measures the time interval of Earth's rotation using the sun for reference. The solar day is about 4 minutes longer than a sidereal day as the Earth is also orbiting the sun and must complete slightly more than one full turn in space before the sun is on your meridian again.

4 extra minutes needed is similar to the moon requiring 2.2 extra days to bring it in line with the sun again. During one complete Earth rotation, the planet has moved slightly in it's orbit around the sun, and must rotate for an extra 4 minutes to allow the sun to return to the same position in the sky.

The apparent motion of the sun across the sky can be studied using a shadow stick consisting of a pencil placed vertically in a container (plant pot with sand). A flat, horizontal piece of card should be used to mark the shadow every 5 minutes or a bit longer. If this is carried out one hour before and after noon, the time of local noon can be determined.

Times should be recorded as GMT and care must be taken to subtract 1 hour in summer. If a graph of shadow length against time (GMT) is plotted, a smooth U-shaped curve can be drawn through the points and the time corresponding to the shortest shadow can be determined. This corresponds to the time at which the sun was highest in the sky and due south (local noon).

Sundials have been used to indicate the passage of time. it has a gnomon whose leading edge casts a shadow on a graduated scale. A sundial uses light from the sun, therefore determining apparent solar time. A watch or clock reads mean solar time, which in the UK's time zone is called Greenwich mean time.

The sun does not move across the sky in a uniform manner every day:

Earth orbits sun at elliptical orbit.

Earth's polar axis is tilted to the plane of its orbit, causing the sun to be higher in the sky in summer compared to winter.

We use a 'mean sun' that does move across the sky uniformly. We set our clocks by this pretend sun.

Difference between apparent solar time and mean solar time is called the equation of time (EOT):

EOT=apparent solar time-mean solar time (GMT)

On four days each year, the EOT is zero. On other dates, its value can be anything up to plus or minus 16 minutes.

Since the Earth completes one rotation in 24 hours, it follows that it rotated by 1 degree every 4 minutes. For every 4 minutes time difference later that 12:00, the observer is 1 degree west of Greenwich. For every 4 minutes earlier than 12:00 the observer is one degree east of Greenwich.

Aurorae

Aurorae look like green, yellow and red curtains or streamers of light in the sky. They are known as the Northern and southern lights (Aurora borealis and Aurora Australis). They are only visible from high latitudes but can be observed from lower latitudes when solar activity is very high.

They are caused by electrons from solar wind, that have been accelerated to high speeds in the Earth's magnetic field, exciting atoms and molecules of oxygen and nitrogen in the upper atmosphere (100-1000 km). As atoms or molecules de-excite, they emit light at certain wavelengths that are characteristic of the element concerned. Green and red are associated with oxygen and purple and red are associated with nitrogen.