Q: Describe the appearance of stars in the night sky.

Q: What is a double star? Provide an example of an optical double.

Q: What is an asterism? Provide an example. Q: What is a constellation?

Q: Describe the appearance of an open cluster, and state an example.

Q: Describe the appearance of nebulae.

Q: Describe the appearance of globular clusters.

Q: Explain the Beyer system of labelling stars in a constellation according to brightness.

A: Binary stars are a pair of stars that interact gravitationally because they are very close to each other, whereas two stars in a close line of sight are known as optical doubles. Alcor and Mizar in the Plough are an optical double (~ 3 l.y. apart).

A: Stars appear as points of light when seen in binoculars and telescopes.

A: An area of sky containing a pattern of stars.

A: Smaller groups of stars than constellations that form familiar shapes, such as the Plough (in Ursa Major), the Great Square of Pegasus, and the Summer Triangle.

A: Faint fuzzy patches of light (Latin for cloud, singular: nebula). An example would be the Great Nebula in Orion (M42 and M43).

A: Loose groups of fairly bright stars that can be seen well using binoculars. The stars are not tightly packed, but are an impressive sight. The most easily seen open cluster in the night sky is the Pleiades (M45, the Seven Sisters).

A: Stars in a constellation are labeled with Greek letters α, β, γ, δ, ε... according to how bright they appear; in this scheme α is the brightest, β the second and so on. To complete the labelling, the Greek letter is followed by a shortened three-letter version of the genitive case of the constellation’s name.

A: Globular clusters appear as fuzzy, circular patches of light in binoculars. In telescopes, more of the stars towards the centre of the cluster can be identified. An example would be the Great Globular Cluster in Hercules (M13).

Q: What was the earliest list of constellations published?

Q: What are the probable origins of the names of the original constellations?

Q: Where do the names of the constellations come from?

Q: How were southern constellations added to the catalogue?

Q: What was the contribution of the Dutch theologian and cartographer Petrus Plancius?

Q: What was the contribution of the 17th century Polish astronomer Johannes Hervelius?

Q: What was the contribution of the French astronomer Nicolas Louis de Lacaille?

Q: How did de Lacaille divide up the extremely large Greek constellation Argo (the Great Ship, sailed by Jason and the Argonauts in their search for the Golden Fleece)?

A: Probably devised at the dawn of written history (~ 2000 BCE) by early Babylonian and Sumerian shepherds, farmers, navigators or desert-travelers, many of whom used the stars as simple calendars or for navigation purposes on their journeys.

A: Published around the year 150 CE by the Egyptian astronomer and geographer Claudius Ptolemaeus (Ptolemy) in his compendium The Almagest. It is almost certain that the positions of many of the stars listed in The Almagest were based on earlier by the Greek astronomer and mathematician Hipparchus of Nicaea, but the origins of the constellation names are less certain.

A: Many were added to the original 48 in the late 16th century, notably by two Dutch navigators, Pieter Dirkzoon Keyser and Frederick de Houtman. They added 12 constellations and named them mainly after exotic birds and other animals.

A: They were named after heroes and heroines from legends and mythological stories that they had heard (for example Hercules, Perseus and Andromeda) and both real and fabled creatures (such as Scorpius, Taurus and Pegasus).

A: 11 more faint constellations were introduced in order to fill the gaps that still remained in the northern sky. They were first published in his atlas Firmamentum Sobiescanium in 1690. They included Lacerta (the Lizard), Sextans (the Sextant) and Vulpecula (the Fox).

A: He added 3 constellations of rather faint stars in the northern hemisphere: Columba (the Dove), Monoceros (the Unicorn), Camelopardelis (the Giraffe).

A: Divided it into 3 separate constellations: Carina (the Keel), Puppis (the Poop Deck) and Vela (the Sails).

A: Set up a small observatory under the Table Mountain at Cape Town, South Africa in 1750. He invented 14 new constellations and named them (with one exception, Mensa, the Table Mountain) after important artistic and scientific instruments of the time such as Caelum (the Graving Tool) and Circinus (the Pair of Compasses).

Q: How many official constellations do we currently acknowledge?

Q: Describe two examples of how ancient civilisations had different versions of the constellations.

Q: Describe how ancient Chinese astronomers used different patterns of stars.

Q: Describe two examples of Native American associations with constellations.

Q: Draw the Plough. Q: Draw Orion.

Q: Draw Cygnus.Q: Draw Cassiopeia.

A: The Aztecs recognised Capricornus (the Sea Goat) as a whale, whilst astronomers in India saw it as an antelope. The Lakota (a Native American tribe) associated the stars in and below Orion’s Belt with the Chief of the Tribe’s Hand, and in Navajo mythology, the stars of The Plough represented seven brothers of the Changing Bear Maiden.

A: 88. Established in 1922 by the IAU (astronomy’s governing body).

A: The Lakota associated the stars in and below Orion’s Belt with the Chief of the Tribe’s Hand, and in Navajo mythology, the stars in the Plough represented seven brothers of the Changing Bear Maiden.

A: They divided their sky into 31 regions comprising 3 Enclosures (close to north celestial pole) and 28 Mansions (covering the zodiacal region).

A:

Q: What is a pointer? Q: Describe how pointers are used to identify Arcturus and Polaris.

Q: Describe how pointers are used to identify Sirius, Aldebaran and the Pleiades.

Q: Describe how pointers are used to identify Fomalhaut and the Andromeda Galaxy.

Q: Name three constellations visible from the UK all year long.

Q: Name two constellations that are seasonal (only visible from a given location for a few months of the year).

Q: Why are some constellations seasonal?

Q: What is the celestial sphere?

A: Following a line up from the two stars furthest right in the Plough leads to Polaris (the Pole Star, α UMi), whilst ‘following the arc’ of the handle leads to Arcturus (α Boo).

A: Pointers are used by astronomers to guide people to new regions of the night sky.

A: Using the asterism the Great Square of Pegasus: follow the line of the top two stars of the squareleft to find M31; follow the two furthest right downwards to find Fomalhaut in the Southern Fish constellation.

A: Follow the line of Orion’s Belt – up to the right is found the bright red star Aldebaran; down to the left is Sirius, the brightest star in the night sky. The Pleiades, an open cluster is found beyond Aldebaran.

A: Orion, Gemini. A: Ursa Major, Ursa Minor, Cassiopeia.

A: An imaginary sphere surrounding the Earth, whose centre is the same as that of the Earth.

A: Because of the Earth’s orbital motion around the Sun, and the fact that stars cannot be seen during the day.

Q: What is a meant by the term declination (dec)?

Q: What is meant by the term right ascension (RA)?

Q: When is the Vernal (Spring) Equinox?

Q: What units is right ascension measured in?

Q: What are the celestial coordinates and dates for the times of the year when the Sun is at its maximum declination?

Q: What is the Zodiacal Band?

Q: What is the declination of Polaris (α UMi)?

Q: How can we use Polaris to determine latitude?

A: The angular distance of a star from the vernal equinox, measured westward. It is the interval between the transit of the vernal equinox and the transit of the body concerned.

A: The angular distance of a celestial body north or south of the celestial equator. Expressed as an angle, with + and – depicting north and south of the celestial equator (zero declination).

A: Units of time (hours and minutes, where 1 hour is equivalent to 15 degrees) eastward (to the left on a star chart).

A: Around March 21st every year.

A: It straddles the ecliptic by ~ 8 degrees on either side. This is the part of the sky that contains the constellations of the zodiac and is the region in which planets and the Moon are located.

A: Maximum declinations of +23.5o

and -23.5o at RAs of 6 h and 18 h (on or near June 21st and December 21st respectively, the summer and winter solstices).

A: The angular elevation of Polaris above the northern horizon is equal to the latitude from where it is observed.

A: For the purposes of GCSE Astronomy, we take it to be +90o (precise declination at present is +89o15’51’’).

Q: Looking in a northwards direction, which direction do the stars appear to rotate over the course of a night? Why is this?

Q: What is meant by a circumpolar star?

Q: How can we mathematically work out whether a given star is circumpolar?

Q: Practically, how do we determine the rotation speed of the Earth using circumpolar stars?

Q: Mathematically, how do we determine the rotation speed of the Earth using circumpolar stars?

Q: Why is it more difficult to determine the Earth’s rotation speed in the southern hemisphere?

Q: What is meant by the term culmination?

Q: State three sources of information you could use to plan a stellar observation session?

A: A star that never sets from the place of observation.

A: An anticlockwise sense, due to the Earth’s rotation from west to east.

A: Using long-exposure photography. From a location where there is very little skyglow, the camera should be pointed at Polaris for exposure times of more than 2 hours to give arcs large enough to be measured. The image will show a large number of arcs centred on a point very close to Polaris and the angular distance of a few selected clear arcs can be measured using a protractor.

A: A star will be circumpolar from a given latitude on the Earth provided: declination > 90 – latitude.

A: Observers in the southern hemisphere do not have a bright star equivalent to Polaris above the South Pole and so it will be more difficult to locate the centre of the arcs at which to measure the angles.

A: Using simple ratios: rotation period of Earth / exposure time = 360o / mean arc angle.

A: Planisphere, star chart or computer software.

A: The maximum altitude of a celestial body above the horizon.

Q: If you have a star of known RA, how do you work out another star’s RA?

Q: Why do stars culminate earlier every day when observed?

Q: What points do you need to consider when planning a suitable date for naked-eye observing?

Q: How do you find out about conditions in advance of commencing a viewing?

Q: What equipment would you use to undertake a naked-eye observation?

Q: What are the two types of light-sensitive cells present in the retina of the eye?

Q: Why must the eyes be dark-adapted before serious observing can take place? How can you aid this process?

Q: Describe a naked-eye observation technique that allows you to see particularly dim stars or nebulae.

A: It only takes only 23 h 56 min for a star to return to an observer’s meridian (not 24 h), which causes the stars to culminate (or rise, or set) 4 min every day (and approximately 2 h earlier every month).

A: Subtract the RA from the desired star’s RA and you have an angle in hours and minutes. The equivalent time difference will give the interval of time before the desired star crosses the meridian.

A: Computer software such as Stellarium, a planisphere or star charts such as those published month-by-month in Astronomy Now and Sky at Night magazines.

A: The phase of the Moon (ideally this should be new – not visible – or at best half-full); the weather forecast (clear nights are ideal – even patchy cloud clover will hinder observations significantly); the likelihood of meteor shower (perfect for naked-eye observing); the visibility of a comet, planets or interesting Messier objects).

A: Rods (not colour-sensitive) and cones (colour-sensitive).

A: Comfortable reclining chair with small table; head torch with red filter; clipboard, prepared star chart with pen/pencil and rubber; warm clothes and refreshments.

A: Observers should look slightly to the side of the object (using averted vision) so that the rods are stimulated, allowing the object to be seen.

A: 20-30 min of darkness allows the retina’s rods to become fully light-sensitive. The use of a red light to illuminate sketches and diagrams will not have an adverse effect on dark-adapted eyes.

Q: What is the Messier Catalogue?Q: Who compiled the Messier Catalogue, and what does it contain?

Q: Why did Messier create his Catalogue?

Q: Are stars in a constellation physically related? Why is this?

Q: What is the difference between an optical double and a binary star?

Q: What is the apparent magnitude scale of stars?

Q: How were stellar magnitudes assigned in ancient times?

Q: What does a magnitude difference of 1 correspond to in terms of brightness ratio?

A: French astronomer Charles Messier in 1781, and contains over 100 fine examples of star clusters, nebulae and galaxies.

A: It contains a list of extended ‘fuzzy’ objects, many of which are just visible with the naked eye; binoculars or a small telescope can be used to reveal their true splendour.

A: No, it is merely a line-of-sight effect (this does not apply to stars in a cluster, which are linked gravitationally).

A: To avoid confusion with comets for which Messier and others were searching (Messier discovered over a dozen comets in his lifetime and earned the nickname Furet des Cometes (‘Ferret of Comets’) from the French king Louis XV. He has both an asteroid and a crater on the Moon named in his honour.)

A: The observed brightness of a star is measured in magnitudes where a difference of exactly 5 magnitudes between two stars corresponds to one star appearing 100 times brighter than the other.

A: Optical doubles merely appear to be closely aligned (one star may be many times further away than the other), whereas stars in a binary system are the same distance from us and orbit their common centre of gravity (mass).

A: A brightness ratio of the fifth root of 100 (which is 2.512 – for the purposes of GCSE a value of 2.5 will suffice).

A: Placed into 6 different magnitude classes; those of 1st magnitude were the brightest since they were visible first after sunset. The next brightest were classed as 2nd magnitude and so on down to 6th magnitude (the faintest stars visible).

Q: Convert these magnitude differences to brightness ratios: 1, 2, 3, 4, 5, 6.

Q: State 4 factors that affect the apparent magnitude of a star (symbol m).

Q: What is the brightest star in the night sky?

Q: What is the inverse square law of light intensity?

Q: Define the absolute magnitude of a star (symbol: M).

Q: Mathematically, how are the apparent and absolute magnitudes of stars related?

Q: What is a variable star? Q: What are the two types of variable star?

A: The total energy radiated by the star (this depends on the star’s size and temperature), the distance of the star, the amount of interstellar gas and dust, the amount of light absorbed by the Earth’s atmosphere.

A: 2.5, 6.25, 16, 40, 100, 250.

A: If the distance of a star (hypothetically) doubles, the star will appear four (22) times dimmer; if the distance increases by a factor of 5, the star appears dimmer by a factor of 25 (52), and so on.

A: Sirius (α CMa), magnitude = -1.5, but only because it is one of our closest neighbours. Cf. Deneb (α Cyg) is almost 200 times more distant than Sirius but still appears bright (magnitude = 1.3) because it has an extremely high luminosity.

A: Using the distance modulus formula:

M = m + 5 – 5logdWhere d is the distance to the star in pc.

A: A measure of the true brightness of a star, defined as the apparent magnitude the star would have if it was observed from a standard distance of 10 parsecs.

A: Binary stars and Cepheid variables.

A: Careful observations show that the amounts of radiation emitted by many stars vary in intensity on a regular basis – these are variable stars.

Q: What is a Cepheid variable?

Q: What do we call a graph showing how light intensity from a variable star changes over time? How can we determine the period of variability?

Q: What do we call the two stars in a binary system?

Q: What do we call it if the stars orbit in a plane along our line of sight?

Q: Name an example of a well-known eclipsing binary.

Q: Why does the intensity of light from a binary star vary?

Q: Why does the intensity of light from a Cepheid variable vary?

Q: Why can astronomers not use radar to establish stellar distances?

A: Light curves. Period of variability determined by simply reading off the time axis.

A: Giant yellow stars that regularly expand and contract in size (named after the prototype δ Cep).

A: Because the two stars will eclipse one another, we call it an eclipsing binary.

A: The brighter star is called the primary star, the dimmer star is called the companion or secondary star.

A: Because of stellar motion (the stars are orbiting one another).

A: Algol (β Per). In Greek mythology, this was associated with the winking evil-eye of Medusa, the Gorgon, whose slain head was held aloft by the hero Perseus.

A: Stars are so far away that the time taken for the ‘return’ signal would be in years, even for the nearest stars; any such reflected signal would be so weak that it would be undetectable from background noise; stars are spheres of gas and do not have hard reflecting surfaces.

A: Because the star expands and contracts, hence the size and temperature of the star have a dramatic effect on its intensity.

Q: What is parallax?Q: Describe the method of heliocentric parallax to determine distances to nearby stars.

Q: How is the parallax angle (p) defined?

Q: What is the definition of a parsec?

Q: How have Cepheid variables been used to determine stellar distances?

Q: What is the first stage in star formation?

Q: Give a well-known example of a nebula visible with the naked-eye. What type of nebula is this?

Q: What happens as a nebula begins to collapse?

A: If the (very small) angular shift in position of a nearby star over a period of 6 months can be measured to determine its distance.

A: The apparent motion of near and more distant objects with respect to one another (caused by the motion of the observer).

A: One parsec is the distance at which a star would have a parallax angle of one arcsec. (NB. 1 degree is 60 minutes of arc, and 1 minute is 60 seconds of arc, so 1 arcsec in 1/3600 of a degree.)

A: Half of the angular shift in position, related to distance by the formula:

d = 1 / pwhere d is in parsec and p is in seconds of arc (parsec).

A: The gravitational collapse of huge clouds of gas and dust (nebulae) that are found mainly in the spiral arms of galaxies. These clouds are ~15 kpc across and may contain enough material to form several thousand stars.

A: In 1912, US astronomer Henrietta Leavitt observed a number of these stars in the Small Magellanic Cloud, and established a simple relationship between a star’s pulsation period and its mean absolute magnitude. This period-luminosity law allows astronomers to determine the star’s absolute magnitude which, when combined with its apparent magnitude, allows the distance to the star to be calculated using the distance modulus formula.

A: Once a gas cloud begins to collapse it breaks up into smaller collapsing knots called protostars. These rise in temperature as gravitational potential energy is converted to kinetic energy. Eventually, the central temperature of a protostar reaches 15 million K, hot enough for the nuclear fusion of hydrogen nuclei to helium to begin.

A: The Orion Nebula is perhaps the best-known example of a site of star formation. This emission nebula contains glowing gas that has been excited by newly-formed stars and emits light by fluorescence.

Q: What happens after a protostar begins nuclear fusion?

Q: How long does a star remain on the main sequence?

Q: What happens to a star as it leaves the main sequence?

Q: What is the fate of a red giant star?

Q: What is a planetary nebula? Q: What happens to the core of a red giant?

Q: What happens to the core of a red supergiant? Q: What is a neutron star?

A: The main sequence phase of a star is very stable, and stars can spend anything from 100 million years (for a large mass star such as Spica) to 1 million million years (for a low mass star such as Barnard’s Star) in this state. The Sun is about half way through its estimated main sequence lifetime of 10 million years.

A: The outward pressure from the radiation generated in the star’s core halts any further collapse and the star settles to a stable size. It then begins to radiate energy from its hot surface and becomes a main sequence star.

A: With further hydrogen depletion and contraction, the temperature at the centre of a red giant can rise to as hot as 100 million K; this is hot enough for helium nuclei to fuse to form carbon. When this helium is depleted the red giant loses its outer layers in an expanding shell of gas.

A: Eventually, the hydrogen that fuels the star’s core runs out. The outward radiation pressure is now no longer present, and the star collapses once more under its own gravity. This causes a further temperature rise within the star, until hydrogen can undergo fusion in a shell surrounding the core, which is now rich in helium. The new nuclear reactions that occur cause the outer layers of the star to expand and cool to form a red giant or supergiant.

A: The inner part of the star collapses to form a dense, hot white dwarf with a mass about the same as the Sun, but with the size of the Earth. Eventually, white dwarf stars cool to become red, brown and eventually black dwarfs.

A: So called because of the resemblance of the first few to be observed telescopically to planetary discs. The outer layers of a red giant being lost as an expanding shell of gas.

A: Neutron stars have masses slightly greater than that of the Sun, but all contained within a sphere with a diameter of just 20 km. The high densities of neutron stars cause such strong gravitational effects that they are able to rotate very rapidly on their axes. Rotating neutron stars emit intense radio waves from their polar regions as they spin, and these regular short ‘pulses’ of radio waves enable radio astronomers to detect neutron stars as pulsars.

A: In a larger-mass supergiant star, the temperature of the central core is hot enough for further fusion reactions involving nuclei of elements up to iron occur. Once these are depleted, a violent explosion (a supernova) occurs at the outer part of the core and the supergiant blows away its outer layers at speeds of up to 5000 km/s.

Q: What information can be derived from a stellar spectrum?

Q:What do spectral lines in stellar spectra correspond to?

Q: What is the most common classification system of stars using relative numbers and wavelengths of spectral lines?

Q: Which is hotter: an O class star or an M class star?

Q: What class of star is our Sun?Q: What is the name of the diagram used to represent stellar types? What are the axes?

Q: What are the four ‘groups’ found on an H-R diagram?

Q: Which types of star cannot be represented on an H-R diagram?

A: Known wavelengths of light caused by atoms, ions (and sometimes molecules) in the outer regions of a star absorbing light at well-defined wavelengths.

A: Chemical composition, temperature (which in turn determines its colour) and radial (in the line of sight) velocity. Of a star by using the Doppler effect

A; O-type stars are hotter than M-type stars.

A: The Harvard scheme of spectral type that uses the letters OBAFGKM (mnemonic: Oh Be A Fine Girl Kiss Me).

A: The Hertzsprung-Russell diagram (after Danish and US astronomers Ejnar Hertzsprung and Henry Russell). Temperature on the x-axis (temperature increasing right to left), luminosity on the y-axis.

A: A G2 star (the higher the number, the cooler the star).

A: Neutron stars or black holes, owing to their small or zero luminosities.

A: ‘Normal’ main sequence stars (diagonal band from bottom right to top left), red giants (top right), red supergiants (slightly to the left of the red giants), white dwarfs (bottom left).

Q: What is a nova? Q: What are the two classes of supervova?

Q: How is a continuous spectrum produced? Q: What are absorption lines?

Q: What are emission line spectra? Q: Provide an example of a probable black hole candidate.

Q: What is the observational evidence for the existence of black holes?

Q: Complete the nursery rhyme, “Twinkle, twinkle, little star...”

A: Type I (formed by the explosion of a white dwarf component of a binary star, when the mass of a white dwarf is over 1.4 solar masses); Type II ( caused by the explosion of stars with a mass of > 8 solar masses, after which the core collapses to forma neutron star or a black hole).

A: This is an existing star (usually quite faint) whose luminosity suddenly increases by 10 magnitudes or more, before returning to pre-nova state. It usually takes a few days to reach maximum brightness and then about 40 days to recede back to a dimmer star. Novae occur in binary systems where one component is a white dwarf and the other a giant.. Matter drains from the giant (weaker gravity) to the dwarf (stronger gravity). Nuclear reactions involving the inflowing material cause a violent explosion on the surface of the white dwarf. This is what causes the luminosity to increase.

A: They contain dark lines emitted at particular wavelengths, superimposed on a continuous EM spectrum, produced when radiation from a hot source (a star, which is emitting a continuous spectrum) passes through a layer of gaseous material. The absorption spectrum shows the types of atoms present.

A: Produced by any hot source. It is the pattern produced of colours produced when starlight or sunlight is dispersed.

A: Cygnus X-1.

A: A series of bright lines produced by very energetic sources. The hottest stars have emission spectra lines in their spectra. They are also typical of luminous nebulae, as they are produced by any highly energetic source. The emission lines show the types of atoms present. The hotter the source the more clearly the emission spectra show up.

A: Go and do some proper revision.

A: It is not possible to observe black holes directly, but their presence is detectable by the X-rays that are emitted from accreting matter from a nearby companion star in a binary system.