Year 10 AstronomyReference Guide

Celestial Observations.THE CELESTIAL SPHEREAlthough the celestial sphere is not a physical object and does not truly exist, it is a key reference point for astronomers. It functions by imagining an enormous sphere around the Earth, with all celestial objects able to be located somewhere on it. The horizon lines close off the sphere, turning it into a semi-sphere around the observer. An observer’s horizon line becomes the new celestial equator and from this, we are able to locate various celestial objects around them. For example, if an observer is located in the capital city of New Zealand (Wellington), they are at 41 degrees south (latitude). It is possible to recreate the celestial sphere as seen by them to identify celestial objects. It is important to remember that due to the Earth’s rotation, it seems that all stars rotate about the poles (NCP or SCP). This means that observers at 90 degrees latitude will only see circumpolar stars while other observers will see different stars for different lengths of time and at different angles.OBJECTS ON THE CELESTIAL SPHEREzenith: the point directly above an observer on the celestial sphere (makes an angle of 90 degrees with the Earth)meridian: the circular line passing through the zenith from one celestial pole to the otherhorizon: the circle of visibility around an observer (equivalent to half of the celestial sphere)CIRCUMPOLAR STARSDue to our horizon line, some stars never come into our sky. A well-known example is Polaris which never crosses into the southern hemisphere. This is because all stars appear to rotate around the celestial poles due to the Earth’s rotation on its own axis. Circumpolar stars are those that appear to always be in our sky. Depending on an observer’s latitude, the stars will be part of a different portion of the celestial sphere. An observer at 40 deg. S will see the SCP at 60 deg Lat and from 60 to its horizon, all stars will be circumpolar.CELESTIAL COORDINATESOn the celestial sphere, latitude is described using declination (dec.) and found in degrees. Longitude is measured by right ascension (RA) but is measured in basic time units – hours, minutes and seconds. One Earth hour is equal to 15 degrees on the Celestial Sphere since the stars appear to move at this speed (it takes about 24 hours for stars to move 360 degrees). The meridian of RA is the vernal equinox and faces Pisces (beginning of spring in northern hemisphere). The amount of time in which a star crosses the north-south meridian after the vernal equinox corresponds to its RA.FINDING FEATURES OF THE CELESTIAL SPHERE*FACT* Our Sun appears to move 15 degrees per hour and it rises in the east and sets in the west due to the Earth’s rotation around itself.latitude) To find the latitude of an observer from their sphere, find the angle from the horizon to the visible celestial pole. In the north, this can be depicted simply as the star ‘Polaris’ but in the south, it will be indicated in some way since no star is close enough to the SCP at this moment in Earth’s history.Polaris) The angle of Polaris in the sky is currently approximately equivalent to an observer’s latitude if they are located in the northern hemisphere. E.g. standing at 30 deg N, you will see the North Star at an altitude of approximately 30 deg. Sun’s path) Each day, the same basic shape of the Sun’s path is the same. However, it translates horizontally in relation to the Earth’s orbit of the Sun. This means that its shape remains the same

(the one during the equinoxes that goes from the zenith on the ground to the latitude) but then shift it across the celestial sphere. On the summer solstice, the day will be at its longest and the Sun’s altitude will be 23.5 degrees higher than at the equinoxes due to the Earth’s tilt (the opposite goes for the Winter Solstice).solar noon) Solar noon is the time of day in which the Sun is at its highest altitude. To find the angle of elevation of solar noon, locate the highest point in its arc and then use the degrees of the celestial sphere to find its coordinates.shadows) The longest shadows occur when the altitude of the Sun is near its lowest. This means that to find the longest shadow, you must find the first point in the Sun’s arc that produces a visible shadow – the shadow cannot be extended past the horizon, but must be seen on the Earth.finding the time/date) The time can be approximately found with a celestial sphere by finding if it is morning, midday, afternoon, etc. Morning is any time before the Sun has reached its peak altitude for the day and the opposite is true for the afternoon. Midday can be difficult to find as it is not always solar noon but finding the Sun’s highest point in the sky is usually what you will be asked to find. To find the approximate date, you need to compare the Sun’s arc to what it looks like at the equinoxes. If the arc is higher in the sky, it is nearing summer – if it is 23.5 degrees higher, it is at the Summer Solstice. If the arc is lower, it is close to winter – 23.5 degrees lower means it is the Winter Solstice. Dates in between can be deduced from the number of degrees different to the equinoxes.CONJUNCTION OF PLANETSConjunction, or planetary alignment, is the when any two astronomical objects appear to be close together in the sky, as observed from Earth. They may not be physically close together but to observers on Earth they do.

The Seasons.REASONSWhen a hemisphere is leaning slightly towards the Sun, light hits the Earth at a more direct angle, making it more intense. Due to this higher intensity, it is also more effective in heating the surface of our planet, resulting in a warmer season. Furthermore, duration of insolation is higher when the Earth is leaning towards the Sun due to the tilt of the Earth.*FUN FACT* The Arctic Circle (Land of the Midnight Sun) begins at 67 degrees latitude since that is the area in which the Earth receives the most intense insolation at the Summer Solstice (therefore least intense at the Winter one). It is 90 degrees minus 23.5 degrees (Earth’s tilt), resulting in parallel rays from the Sun at perihelion and aphelion. (The opposite to the Arctic Circle is, as expected, the Antarctic Circle).POSITION OF EARTH AND INSOLATIONVernal Equinox – 12 hrs sunlightSummer Solstice – 18 hrs sunlightAutumnal Equinox – 12 hrs sunlightWinter Solstice – 6 hrs sunlightAPHELION = the point of the Earth’s orbit in which it is furthest from the SunPERIHELION = the point of the Earth’s orbit in which it is closest to the Sun

Our Moon.ORBITThe plane of the Moon’s orbit around the Earth is only 5 degrees different to the ecliptic which the Earth follows to orbit the Sun. It takes the Moon 27 days to travel the whole ecliptic but as the Earth orbits the Sun itself, it must travel an extra two and a half Earth days to complete its cycle of phases (as seen from the Earth). This is because the Sun’s rays on the Moon will differ by around 30 degrees. This Synodic Month relates to about 1/12th of the Earth’s orbit and presents a new moon as the Earth and Sun align with the Moon in the middle to make it seem dark. A Sidereal Month is the time it takes for the Moon to revolve around the Earth once. So, to recapitulate, a Sidereal Month is the Moon’s trip around the celestial sphere while a Synodic Month is the time between new moons and is longer than the former due to the Earth’s movement along the ecliptic.ORDER OF ITS PHASESAs the moon becomes fuller, it grows from the left side to the right side. As it is increasing in visibility, it is called ‘waxing’, it goes from a ‘waxing crescent’ to a ‘waxing half’ and then to a ‘waxing gibbous’. The next stage is the full moon and then as it decreases, it moves towards the right side. This is the ‘waning’ section of the phases of the moon, beginning with ‘waning gibbous’ and moving to ‘waning half’ and finally ‘waning crescent’. As you may be able to tell, this is the same stages as the ‘waxing’ phase but in reverse order.Due to the specific observation positions in which people must be to view the different phases of the Moon, there are very particular times associated to the highest altitude of each phase. For example,

a new moon will always rise with the Sun and set at sunset. One quarter of the way through its cycle, (at waxing half), the moon will rise at midday and set at midnight and so on, with the full moon rising at sunset and setting at sunrise.NEW MOON: occurs when the moon is between the Earth and the Sun but does not usually cause an eclipse since it won’t always intersect the Earth’s orbit and therefore block out the Sun.

By the waxing half, the moon will have completed ¼ of its orbit.*REMEMBER* The phases increase in a counter-clockwise manner.ECLIPSESLunar Eclipses) This can only occur when the Moon is on a node. A node is a point where the Moon’s orbit intersects the plane of the Earth’s ecliptic. This intersection occurs about twice a year but most eclipses are not total. A lunar eclipse itself is when the moon is situated in the Earth’s shadow and less light reaches it. This phenomenon can only happen during a full moon.Solar Eclipses) This occurs when the Earth moves behind the moon and falls into its shadow causing the moon to block out the Sun’s light. For this to occur, the moon must be in the first phase of its cycle; New Moon.ROTATIONAn interesting fact to consider is that of Synchronous Rotation. This refers to the fact that we only ever see the same side of the moon. This is because its period of rotation is the same as its period of revolution; 27.3 days. Also, the Moon appears to move 12 degrees in our sky each day and has the

fastest apparent motion of all celestial objects.

The Planets.To be considered a planet, a celestial object must fulfil the following three requirements.Firstly, it must be in orbit around a star.It must have enough mass to tighten itself into a circular shape (hydrostatic equilibrium).Thirdly, it must be the dominant gravitational body in its orbit (meaning it has cleared it of other objects since its mass is greater than any other object near its orbit) and also be pulled into a stable orbit by its star.All planets in our solar system have an elliptical orbit around the Sun but their rotation speed and orbital periods are very different depending on their size and distance to the Sun, as dictated by Kepler’s Laws. Between the terrestrial planets (the warmer, smaller ones) and the gas giants (colder, larger ones) there is an asteroid belt that helpfully separates the planets in our solar system based on their composition.

Our Sun.DURATION OF INSOLATIONThroughout the year, the duration of insolation of places at different latitudes changes dramatically due to the position of the Earth in its orbit around the Sun. When the North Pole is entirely facing the Sun, at the Winter Solstice (for the Northern Hemisphere), it will receive a full 24 hours of insolation and the opposite evidently occurs at the Summer Solstice. In between, there are the two equinoxes; the autumnal and the vernal equinox. The autumnal equinox occurs between summer and winter and results in the entire Earth experiencing precisely 12 hours of insolation. During the vernal equinox (between winter and summer), the exact same thing happens as the Earth has crossed a specific point in its ecliptic in which it is in line with the Sun at 23.5 degrees.LATITUDEDifferent latitudes experience different durations of insolation throughout the year. At zero degrees latitude (the equator), 12 hours of insolation are experienced year-round.THE SUN’S PATHEach day, the Sun moves approximately 1 degree to the East (relative to the stars) due to the Earth’s rotation of itself. This also means that the Sun rises 4 minutes later than the previous day.THE SUN’S LAYERSThe Sun consists of three inner layers and three outer layers.- INNER LAYERSCore: where thermonuclear fusion consumes hydrogen to form helium during nuclear fusion.Radiative Zone: energy generated by nuclear fusion moves outwards as electromagnetic radiation. In this way, energy is conveyed by photons. Convection Zone: on its surface, photons are created. In this region, rising currents of hot gas carry energy toward the Sun’s surface. The atmosphere of the Sun becomes less dense and hotter at this point.- OUTER LAYERSPhotosphere: this section of the Sun is visible and emits energy detected by us as sunlight.Chromosphere: this part of the atmosphere is not seen except in total solar eclipses as the photosphere is much brighter than it. It actually emits a reddish glow due to the super-heated hydrogen it is burning.

Corona: free electrons in the corona move in magnetic fields. Solar winds transport particles of energy (of various wavelengths) within the corona and through space.KEY WORDSSolar Wind – a stream of charged particles continuously released by the SunSolar Flare – sudden bursts of electromagnetic radiation (highly energetic eruptions)Coronal Mass Ejections (CME) – a large cloud of energetic and highly magnetised gas is emitted from the solar coronaSunspots – like hurricanes of the photosphere made of intense magnetic activity. They’re dark because they’re cooler than the surrounding area

The Stars. NUCLEAR FUSIONNuclear fusion occurs when positively-charged nuclei repel each other in a high energy collision. As gravitational forces cause a star’s giant molecular cloud to contract, the pressure grows along with the temperature. For stars with a mass lower than or equal to 1, nuclear fusion follows the PP chain (proton-proton). In this chain, protons collide with each other to create helium. Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons, primarily protons and neutrons. Stars with masses greater than 1.3 follow the CNO cycle (Carbon-Nitrogen-Oxygen). Although Helium is created nevertheless, more energy is needed to overcome the larger gravity of those stars. In both of the processes, mass is lost as light energy. Thermonuclear fusion releases enormous amounts of energy including radiation, electricity, solar wind, light and heat. Once fusion has begun, stars enter the main sequence as they have enough outward energy from fusion to counteract the massive gravity pushing in on them.SPECTROSCOPYThis is the investigation and measurement of spectra produced when matter interacts with or emits electromagnetic radiation. Black bodies are theoretical objects that absorb all electromagnetic radiation around them, therefore emitting thermal radiation in a continuous spectrum according to their chemical composition. The wavelength range is split into three categories, known as UBV (ultraviolet, blue and violet, and visible). Visible light produces a continuous rainbow spectrum. If it passes through gas, the latter will absorb some of the wavelengths, leaving dark bands in the spectrum called absorption lines. The electromagnetic radiation spectrum includes radio, infrared, UV and x-rays.EMISSION AND ABSORPTION SPECTRAStars can be classified by studying the spectrum they create when their light passes through a prism or a diffraction grating. Dark gaps will appear in the spectrum called absorption lines since those wavelengths have been absorbed by the star. Every element creates a unique spectrum of absorbed lines and they can therefore be used to identify the elements in a star and, subsequently, its temperature. Temperature can usually be deduced since the element types and quantities are known to result directly from a star’s temperature. However, the predicted elements are not always entirely accurate due to the fact that interstellar dust and gas can absorb blue light (in particular) from the spectra.SPECTRAL CLASSESOBAFGKM (0-9 each) is the order of spectral classes. It is crucial to remember that spectral classes do not directly relate to temperature but instead chemical composition. Nevertheless, chemical composition does result in specific temperatures (therefore temperature can be deduced).

HERTZSPRUNG-RUSSELL [H-R] DIAGRAM

(http://astronomy.swin.edu.au/cosmos/H/Hertzsprung-Russell+Diagram)

To interpret a H-R diagram, one must understand that it is a graph of brightness versus temperature. Brightness also takes into account size while temperature involves understanding a star’s colour, chemical composition and thus spectral class.Stars on the diagram are divided into three types:- GIANTS very large very bright very cool found in the top right of the diagram supergiants are extremely rare- DWARVES very small very dim hot stars found in the bottom left- MAIN SEQUENCE found in the middle of the chart, running diagonally from the bottom right to the top left

most stars are found here red, low-mass stars are the most common bright, hot, blue main sequence stars are very rareTHE LIFE CYCLE OF STARS- LOW MASS STARSAny star begins as a cloud of gas and dust (at least a few light years across). Gravity causes the gas cloud to contract until nuclear fusion begins but this is where the similarities end. Brown dwarves are created before nuclear fusion begins as they do not have enough gravity to create this immense heat. Instead, the materials in this celestial object reach a stable state.Brown Dwarf Definition: A substellar object that emits mainly infrared radiation. It is heavier than the largest gas giants but still lighter than the smallest solar masses (in density).As soon as nuclear fusion begins, a star is catalogued as a yellow or red main sequence star. In the process of their creation, fusion in stars’ cores continue until almost all hydrogen has become helium. At this dangerous point, the core shrinks, creating a hotter centre to allow the hydrogen to burn faster. Even more immense heat is created as the core continues to shrink and it reaches a stage called Helium Flash, the moment in which it can begin to fuse Helium in its core (in the triple alpha process). At this point, the star begins pulsating as it gets smaller, hotter and bluer in colour. Once the core is made up of mainly Carbon and Oxygen, it collapses and the star rapidly grows outwards to become a giant. The outer layer is then rejected and a much smaller core is left behind to slowly cool, contracting until a stable white dwarf is produced due to its inability to overcome electron degeneracy pressure. The material left behind is a planetary nebula and this can go on to form new stars.- HIGH MASS STARSCompared to the creation of low mass stars, the gas clouds contain much more mass and therefore the force pushing inwards is stronger. Due to this fact, faster fusion occurs due to the hotter temperatures reached and the stars become hotter, bluer and brighter. These stars have shorter life spans since they burn their fuel faster (caused by hotter temperatures). As the core contracts and heats (similarly to low-mass stars), the star will swell up, becoming a red giant or a supergiant. All stars reach a red giant phase in their life time but hotter, heavier stars will reach it earlier. The core within these stars gets so hot that it can fuse heavier elements up to and including iron which has 32 protons. (Fe) itself is so stable that the star collapses at this point, due to its loss of fuel – it is unable to fuse iron with other elements due to its stability. At this point, the star collapses and ejects most of its heavy nucleus into space in an event known as a supernova. The result of a supernova can be one of two things.If the star is from 1.4 to 3 solar masses, the core will collapse and all of its electrons will be squeezed into protons, creating neutrons. A neutron ball is formed, containing all of the mass originally in the core of the star.If the star is greater than 3 solar masses, neutron degeneracy pressure is not enough to stop gravity and neutrons are crushed together, producing a black hole.THE LIFE CYCLE OF STARS - SEQUENCE- stellar nebulae can become average or massive stars(average/low mass star) – planetary nebula – red giant – white dwarf(high mass star) – planetary nebula – supergiant – neutron star/black holeSTAR SYSTEMSBinary star systems contain two stars which orbit around a common barycentre. When systems are composed of two or more stars, they are named multiple star systems. Due to their huge distance from observers on Earth, they often appear to be a single point of light.PULSARS

These are celestial objects thought to be rapidly rotating neutron stars. They emit regular pulses of electromagnetic radiation at enormously fast speeds.

Luminosity.APPARENT MAGNITUDE (APPARENT BRIGHTNESS)The brightness of an object as seen from Earth. It is found on a scale that ranges from -30 to +30. This scale appears counter-intuitive as the brightest objects have the lowest numbers but it is because it began as a ranking system: the brightest stars in the sky were assigned with the highest rank (the lowest number). Examples of celestial bodies on this scale include: the Sun = -26.7 // Venus (when brightest) = -4.4 // binocular limit = +6Hipparchus invented this scale when attempting to order stars. 5 jumps on this scale (e.g. M1 to M6) correlate to 100 stars needed to equal the former in brightness (100 M6 stars are needed to recreate the luminosity of a single M1). This is a logarithmic scale where each value is approximately 2.5 times as bright as the previous one.(the fifth root of 100).ABSOLUTE MAGNITUDEAbsolute magnitude is the apparent magnitude of a star if it were 10 parsecs away and is calculated on the same scale as apparent magnitude. To transition between apparent and absolute magnitude, an observer must convert the distance to a star to equal 10pc using simple multiplication/division.LUMINOSITY (INTRINSIC BRIGHTNESS)Intrinsic brightness calculates the watts of total radiated power by a star, using its apparent brightness.L = b x 4 π R2

L = luminosity in wattsb = apparent brightness in watts per metre squaredR = distance to the star in m.APPARENT BRIGHTNESSThis is a measure of watts per received radiation. On Earth, it can be used to estimate the power that can be collected on solar panels. Apparent brightness is different to the intrinsic value because distance affects it through an inverse square law and other factors can impact it based on location.

Motion.APPARENT MOTIONApparent motion is a complex amalgamation of both an object’s actual motion (proper motion) and the Earth’s revolution around the Sun.PROPER MOTIONThis is calculated by the apparent motion of a star across the celestial sphere at right angles to the observer’s line of sight. It is defined as the angular velocity across the sky of a celestial object. It is typically expressed in milliarcseconds per year due to its seemingly minuscule value. In this course we are not required to be able to calculate proper motion, only understand what it means.PARALLAXStellar parallax is a method for measuring the distance of stars to Earth. Although parallax itself is the apparent change in position of an object caused by the motion of the observer. Stellar parallax is calculated with the unit of the parsec: the distance that shows a parallax angle of one arc second (an arcsecond is 1/3,600 of a degree into space). The formula for parallax is d = 1/p. aka distance = 1/parallax angle in arcsec

UNITS OF MEASUREMENTA.U. = Astronomical Unit = the radius of the Earth’s orbit (distance from Earth to Sun, a.k.a. semimajor axis)L.Y. = Light Year = the distance light can travel in 365 Earth days

distance in … m AU LY PC1 AU 1.496 x 1011 1 0.000016 0.0000051 LY 9.461 x 1015 63 240 1 0.30661 PC 3.086 x 1016 206 265 3.2616 1

Evidence of the Earth’s Motion.Let’s quickly recapitulate the key points we’ve discussed about the Earth’s motion and provide evidence for its rotation and revolution patterns:ROTATION* Earth rotates about 15 deg p/h*1. Focault’s PendulumThis French physicist knew that pendulums cannot change direction on their own and used this knowledge to prove the rotation of the Earth. He created a huge pendulum hanging from a cathedral roof and set it moving along the North-South line. After a few hours, it seemed to have moved and was now swinging East-West. As the pendulum could not have changed direction alone, he concluded that the Earth was rotating beneath it.2. The Coriolis EffectThis phenomenon can only occur on rotating bodies and affects fluids (like wind and water). It is their deflection to the right in the northern hemisphere and left in the southern one caused by the turning motion of the body.REVOLUTION1. The phases of VenusWhen Galileo observed that Venus appeared differently at different times of the year, he concluded that this could only happen if the planet had a smaller orbit around the Sun than the Earth. This was since the planet appeared differently when the Earth was facing the Sun directly compared to observations at sunrise, sunset, etc.2. Stellar Parallax and ConstellationsDue to the size of the Earth’s orbit, some nearby stars appear in different positions at different times during the year. This shows that the Earth is moving across the sky yet we always see the Sun. Therefore, astronomers concluded that the Earth must be revolving around the Sun. Additional observations show that different constellations are seen in the night sky throughout the year, reinforcing this idea as well as that of Earth’s own rotation.(constellation: a group/cluster of stars that forms an image in the sky - named after characters of Greek mythology)3. The Seasons.As the Earth moves around its orbit, the climate in different areas of the Earth changes. Due to the tilt of our planet, different latitudes receive different amounts of sunlight and therefore have more or less insolation and intensity.PRECESSION OF THE EQUINOXESCaused by gravity (gravity-induced), this is the continuous change in the orientation of a celestial body’s rotational axis. It is often very slow; on Earth, a full cycle is completed after 25,772 years.

Models of The Universe.GEOCENTRICEven in early times, the Earth was known to be round for these main reasons.1. The crow’s nest at the top of a ship’s mast could see further than the deck of the ship and vice versa (it could be seen for longer when sailing away). This could only be possible if the ship was sailing into the other half of a sphere.2. Travelling further north (northern hemisphere), the Sun appears lower in the sky while Polaris is closer to the zenith.3. During a lunar eclipse, the shadow of the Earth as seen on the Moon is curved.4. Sticks in different places would produce shadows of different lengths under the noon sun. This is a famous example undertaken by the Ancient Greeks. Eratosthenes was a Greek mathematician and was also the head of the library of Alexandria. He was able to prove that the Earth is round by measuring the shadows cast by the noon Sun in two different locations, Alexandria and Syene. Seeing that the shadows were of different sizes, he concluded that the ground was not facing the Sun with the same angle and so the Earth was round.The geocentric model had been present for many years when Ptolemy attempted to improve the ancient model. Aristotle, for example, taught that the Earth was at the centre of the Universe and was surrounded by a distant realm water, air and fire (the four elements). Beyond these, one could find the heavenly space of the Moon and Sun, along with the wandering “planets”. Even further out were the stars, alongside the homes of the Gods. Hipparchus (in 200 BC) suggested that the motion of the planets could be represented by a series of circles found in epicycles. The deferent (or largest circle) was the revolutionary path of the epicycle (smaller circle) and this model helpfully explained retrograde motion, the backwards motion of planets in the sky. Ptolemy later improved this model even though Aristarchus had already suggested a heliocentric universe in 300 BC.HELIOCENTRICDue to the importance of religion at the time, the heliocentric model was largely rejected and considered heretic. The Polish mathematician Copernicus was not afraid to challenge societal norms and he developed a heliocentric model of the universe. He was also able to find the orbital radii of the inferior planets by developing a formula for this from his model.Danish nobleman Tycho Brahe’s aim was to catalogue the stars and the motions of the planets and he was able to do this due to his wealth. When he noticed (what we now know to be) a supernova in the sky, he carefully observed it. This helped him to develop his theory that a nearby object must produce parallax if the Earth spins around the Sun. Due to the extreme distance of most stars, he was unable to find parallax over the course of his experiments and concluded that a geocentric model was still relevant. Just one year before his death, in 1600, Johannes Kepler came to work for Brahe as an assistant. Kepler went on to study Brahe’s data for most of his life, coming up with three laws relating to celestial motion which are discussed later in the document.EVIDENCE OF A HELIOCENTRIC UNIVERSE1. Mercury and Venus never appear in the midnight sky although others do. This is only possible if they have a smaller orbit since the Earth must be facing away from them (and the Sun) at that time2. A simpler explanation of retrograde motion in planets.3. Jupiter’s moons. In 1610, Galileo Galilei noticed four faint ‘stars’ orbiting the king of the planets -Jupiter. Realising that they were changing sides on it, switching from the left to the right, he decided that they were orbiting the planet. From this, he thought the planet was most likely orbiting the Sun and drew similarities with the Earth to hypothesis that it was doing the same thing.RETROGRADE MOTION

Retrograde motion is a result from the slower orbital periods of planets that are more distant from the Sun. For example, Mars appears to move backwards sometimes during the year as the Earth overtakes the other (slower) planet during its orbit of our star.

The galaxies.A galaxy is a large system comprised of stars, dust, gas (mainly H) and dark matter. Galaxies orbit a common centre and are bound together by gravity but there are enormous distances between different ones. At the centre of each galaxy, there is a central bulge where the nucleus can be found. Galaxies have spiral arms which extend out from their nucleus. These, along with the nucleus and central bulge, house the majority of a galaxy’s stars. Around the galactic disk, one can find globular clusters; groups of old stars. Surrounding the galaxy is a halo, a large dim region that consists of hot gas and is thought to house dark matter.FORMATION OF GALAXIESGalaxies are still forming to this day and are caused by the collapse of gas and dust clouds. Gravitational attraction increases as matter comes closer together. Within these enormous clumps of matter and gas, energy is released when H and He clash together. Throughout the universe (currently), galaxies are grouped in clusters, bound together by their respective gravities. This gravitational attraction also means that they have the ability to collide. Interactions between galaxies (such as collisions) can either cause the two to pass through each other or form new, larger galaxies after millions or billions of years. Superclusters are composed of many galaxies that are not gravitationally bound to each other.THE HUBBLE SEQUENCEThis sequence, created by Edwin Hubble, helps astronomers classify galaxies based on their shape by splitting them into 4 categories.An elliptical galaxy has an oval shape and no gas or dust is found within it. Within these galaxies, no visible bright stars or spiral patterns can be found and they do not have a galactic disk. They are classified with an E. E0 is the most circular shape and 7 the most elliptical. About 60% of the galaxies in the universe are elliptical but they are typically very small (nevertheless, some are huge).Spiral galaxies are the brightest of the galaxy types, consisting of about one fifth of the universe (20%). They can be further divided into two categories: ‘barred’ and ‘normal’. They both consist of a flattened disk, a central bulge (in which many stars are concentrated) and also stars forming a spiral structure out from the galaxy’s centre. Normal spirals have a disk shape with a bright centre and evident spiral arms. They are described using a capital S and then a lower-case letter (out of a, b and c) based on how tightly-wound their spiral arms are. To provide context, this means that Sc galaxies have very tightly wound spirals. Barred spirals have a more elongated bulge, making it seem like there is a bar inside them. Similarly to normal spirals, they are described by an SB with a following lower-case letter (a indicates tighter spirals).Lenticular galaxies house a bright central bulge and are almost elliptical in shape. They are classified with the combination S0 since they have no visible spirals and there is no evidence of recent star formation. With this in mind, they can be difficult to distinguish from E0 galaxies as they are quite round. Lenticular galaxies bridge the gap between spiral and elliptical galaxies and are found at the dividing point of the Hubble tuning fork.Irregular galaxies (a.k.a. peculiar) have no defined structure and are in this category due to the fact that they don’t match with any other Hubble Sequence galaxy type. In the past, they were referred to as Irr I and Irr II and include to this day the magellanic clouds (two irregular galaxies orbiting the Milky Way). Irregular galaxies are typically quite small.ACTIVE GALAXIES

Normal galaxies consist primarily of visible light distributed evenly throughout them. However, for some, immense amounts of light are ejected from their nucleus. They are also emitting energy in x-ray, UV, infrared and radio wavelengths from their centre. These active galaxies are only a small portion of the entire universe and a current theory states that they have a supermassive black hole at their centre. The four types of active galaxies are Seyfert galaxies, radio galaxies, quasars, and blazars:- Seyfert Galaxies: Spiral galaxies that change brightness every few weeks. They move very fast in the Universe, about 30 times faster than most.- Radio Galaxies: Elliptical galaxies emitting jets of high-velocity gas vertically from their nucleus. These streams of energy interact with magnetic fields and emit radio waves.- Quasars: Extremely bright galaxies that fluctuate in brightness daily.- Blazars: Similarly to Quasars, these galaxies emit jets of gas but these are pointed towards Earth rather than space in general and are therefore observed to be different to Quasars on our planet.NEBULAEA nebula is a cloud of gas and dust which is visible in the night sky. They may be seen as indistinct bright patches or dark silhouettes against other luminous celestial objects. Planetary nebulae result from the death of stars. They are ring-shaped nebulae formed by gas expanding around the dead stars. Gaseous nebulae themselves are interstellar clouds of dust and gases (both ionised and non-ionised).

The Big Bang Theory.EVIDENCE OF THE BIG BANGThe Big Bang Theory is currently the accepted hypothesis on the creation of the Universe due to evidence we can see surrounding its circumstances.1. It would have had to have been extremely hot; heat radiation would have been emitted as the universe expands, wavelengths did too (cosmic {microwave} background radiation). Wavelengths of radiation expand over time so the radio waves would still be present.2. If the quick inflation had not occurred at all, all the mass contained in the singularity would have collapsed into a black hole. However, if the expansion had taken any longer, atoms would not have formed due to insufficient pressure, energy and heat.3. The Universe is still expanding. This can be proven by the current separation of galaxies due to this expansion as can be seen with telescopes. This phenomenon is easily represented by blowing a balloon with coloured dots on it. The dots move away not because of their own motion but because the space on which they are found expands.REDSHIFTWhen light approaches an observer, it is compressed and appears bluer, shifting into the UV end of the spectrum rather than the infrared end. Vice versa is the case for the distancing of light – it becomes redder. Redshift in our current Universe shows that it is expanding since light emitted from galaxies appears more red than what it truly is.CEPHEID VARIABLESThese stars vary in brightness at a periodic rate. The longer the period, the brighter the star so they’re used as distance markers by comparing their period to their intrinsic and apparent brightness in order to calculate their distance from observers.QUASARSQuasi Stellar Radio Sources are the brightest, furthest, oldest objects in the visible Universe that emit radio waves.HUBBLE’S CONSTANT

In the 1920s, Edwin Hubble discovered that galaxies were red-shifted by looking at their continuous light spectrums. This light spectrum was moved towards the red end as wavelengths were extended due to the recession of galaxies that is noticed by astronomers on Earth. This phenomenon was labelled the ‘Doppler Effect’ and describes the velocity of galaxies as relative to distance.

Forces.GRAVITYGravitational force depends on distance and size of objects. As objects move further apart, gravitational attraction decreases. As object mass decreases, gravity also decreases.Force = Gravitational Constant x {(mass1 x mass2) / radius squared}F = G (m1*m2/r^2)ECCENTRICITYeccentricity = distance between foci / length of major axisE = d (f) /d (m.a.)APPARENT DIAMETER OF SUNAs the Earth moves towards the Sun, the apparent diameter of the Sun increases and vice versa when it moves away. Gravitational Attraction and Orbital Velocity also decrease when the Earth is further from the Sun as per Kepler’s Laws.

Kepler’s Laws.Johannes Kepler inherited all of Tycho Brahe’s notebooks which consisted of 20 years of data describing the motion of the planets. With this information, he was able to create three laws of planetary motion which are still relevant today.LAW ONEThe planets revolve around the Sun in an elliptical orbit.- since ellipses can come in various shapes, the best way to describe them is based on their eccentricity (see Forces section)- this means that the Sun is one of two foci of a planet’s orbit. When the Earth is closer to the Sun, it’s at PERIhelion and when it’s furthest, that point is called APhelion.LAW TWOAll planets cover equal areas of orbit in equal times.- The elliptical shape also impacts a planet’s orbital velocity, making it faster when closest to the Sun and slower when further out. This is due to eccentricity and gravitational attraction which we will talk about later on.LAW THREEThe further a planet is from the Sun, the longer its orbital period. This is because…1. Further planets have slower orbital velocity (due to gravitational attraction)2. Further planets must cover a larger distance in their orbitThis is defined by the rule r^3/t^2 where r is the radius of the planet’s orbit and t is the square of the period of the planet (the time it takes to revolve around the Sun, squared).

Geometrical Optics.The laws of geometrical optics describe light propagation in terms of rays. The rays used are only abstractions but are useful for approximating the path of light in different circumstances. Rays

themselves state the direction of wave propagation which is the transfer of energy in an electromagnetic wave from one point in space to another. The rays utilised are vectors found perpendicular to the wave fronts. Wave fronts themselves are surfaces passing through points of a wave that have the same phase and amplitude.REFLECTIONWhen light travels between/through mediums, some of it is reflected and some is transmitted at the boundary between the two media. Reflection can be specular or diffused. When light reflects from a smooth surface, it undergoes specular reflection and parallel rays will be reflected all in the same direction (at the same angle). Contrarily, when light reflects from a rough surface, diffuse reflection will occur and parallel rays will be reflected in various different directions. The law of specular reflection states that the incident angle is equal to the reflected angle relative to the normal. For reference, the normal is the imaginary line perpendicular to the reflected surface.PLANE MIRRORSPlane mirrors are flat mirrors that reflect light. They have the following properties:* image distance = object distance* image is not magnified* image is virtual* image is not inverted* left and right are reversedTherefore, in these mirrors, d(o) = d(i) and h(o) = h(i). This formula means that the distance from the object to the mirror is equal to the distance from the image to the mirror. This is the same for the height of an object as this measurement is equal to the height of the image. Images are formed at one of the following locations of intersection; (1) where the rays of light leaving an object intersect or (2) where the rays of light from an object appear to originate. A real image is found at the former (light leaving the object that intersects) and a virtual one at the latter description (ways appear to originate) The magnification of an image is defined by the image height over the object height. If the magnification is negative, the image is inverted. { FORMULA: m = h(i) / h(o) }SPHERICAL MIRRORSSpherical mirrors can be concave or convex. The principal axis of this type of mirror is the straight line between the centre of curvature (C) and the midpoint of the mirror When parallel rays coincide with a spherical mirror, the reflected rays intercept at point F, the focal point. For concave lenses, this is in front of the mirror. We will be focussing exclusively on concave lenses. Their focal length (f) is the distance from the surface of the mirror to its focal point. It is equivalent to half the radius of curvature so f = ½R. RAY TRACINGTo determine where an image will be located, one can use a P ray, F ray and C ray.P Ray = the parallel ray, the one that reflects through the focal pointF Ray = the focal ray, the one that reflects parallel to the principal axisC Ray = the centre of curvature ray, the one that reflects back along the same line as its incoming pathTHE MIRROR EQUATIONRay tracing indicates where an image will be located. It’s distance from the mirror { d(i) } can be found with the mirror equation 1/d(o) + 1/d(i) = 1/f. That is, the number one divided by the focal length is equal to 1 divided by the distance from the object to the mirror plus 1 divided by the image to the mirror. There are some important sign conventions to follow with this equation that you can find in this table. ( https://slideplayer.com/slide/4901095/ )REFRACTIONThe speed of light changes when travelling through different materials. The index of refraction (n) is

the ratio of the speed of light in vacuum (c) over speed of light in the material (v). Although the velocity and wavelengths of the speed of light change in different materials, its frequency is constant. Astronomical refraction is present on Earth and consists of the displacement of celestial objects towards the zenith due to atmospheric refraction.LENSESA lens is an object that uses refraction to bend light in order to form images. Light is reflected from a

mirror but it is refracted through a lens.

Astrology.Astrology uses the relative positions of stars and planets to predict the future. The zodiac is used in astrology; it consists of 12 major constellations found along the ecliptic. These constellations are patterns of stars in the night sky.