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Q: What is a galaxy? Q: Describe the naked eye appearance of the Milky Way.
Q: Describe the appearance of the Milky Way through binoculars or a small telescope.
Q: What are we looking at when we observe the ‘Milky Way’?
Q: What would we observe from a viewpoint ‘above’ the plane of the Milky Way?
Q: What are the three main components of the structure of our Galaxy?
Q: Describe the disc of the Milky Way.
Q: Describe the bulge of the Milky Way.
A: The Milky Way (Latin: Via Lactea) is named after its appearance as a faint band of light (with some darker patches in it) stretching across the sky and passing through constellations such as Cassiopeia, Auriga and those whose brightest stars make up the Summer Triangle.
A: Huge, gravitationally-bound collections of stars.
A: We are looking into the plane of our Galaxy where most stars are located.
A: It is possible to resolve thousands of individual stars and view the Milky Way in all its splendour.
A: The disc, the bulge and the halo.
A: A giant disc-shaped Catherine Wheel rotating about a central bulge (much like Andromeda, 700 kpc away).
A: Surrounds the nucleus, thought to contain a massive black hole at the centre.
A: Diameter roughly 30 kpc and thickness 1-2 kpc), containing young stars, gas, dust and sites of star formation in four main spiral arms; the Solar System is in one such arm, about 10 kpc from the centre.
Q: Describe the halo of the Milky Way.
Q: What is found in the spiral arms of the Milky Way, aside from stars?
Q: Explain the radio emission of hydrogen atoms in the spiral arms of the Galaxy.
Q: How can astronomers use 21 cm radio waves to determine the rotation of the Galaxy?
Q: Who proposed the most commonly used galaxy classification system in the 1920s?
Q: What are the four main types of galaxy?
Q: How can we sub-classify S and SB galaxies?
Q: How can we sub-classify E galaxies?
A: Huge clouds of gas and dust (perhaps in the process of forming stars) that are opaque to visible night. This light-absorbing material prevents astronomers from ‘seeing’ the spiral arms and from being able to map the structure of our Galaxy and in particular its spiral arms.
A: Contains mainly globular clusters – dense groups of relatively old stars, in a spherical distribution about the galactic centre.
A: EM radiation at this wavelength can penetrate dust (and the Earth’s atmosphere), and so the neutral hydrogen that pervades the spiral arms of the Galaxy can be detected on Earth by radio telescopes ‘tuned’ to this frequency.As the Galaxy rotates, the wavelengths of the 21cm radio waves become slightly Doppler-shifted to shorter and longer wavelengths because of the relative motion of the hydrogen with respect to us (in the same way that we hear the pitch of ambulance sirens slightly different as they approach us or move away).Radio astronomers use these changes in wavelength to determine the relative velocities of the (gas in the) spiral arms. This then allows them to determine how various parts of the Galaxy rotate.
A: A hydrogen atom consists of a single electron in motion around a single proton. We can picture the electron as spinning in one of two ways. When the electron reverses its spin, it emits EM radiation of frequency 1420 Hz; this corresponds to a wavelength of 21 cm (in the microwave/radio region of the spectrum).
A: Spiral, S (flattened, disc-shaped systems), barred spiral, SB (similar, but with a bright ‘bar’ running through the nucleus from which spiral arms emerge), elliptical, E (relatively smooth spheroidal distributions of stars, showing a gradual decrease in brightness from the centre outward), and irregular, Irr (very little structure or symmetry).
A: Edwin Hubble.
A: By assigning a number: 0-7 depending on their shape. E0 is almost circular, E7 is highly elongated.
A: By assigning a letter: S and SB galaxies can be given letters a, b or c, depending on the strength of the central bulge and ‘openness’ of the spiral arms (Sa galaxies have a predominant nucleus and tightly wound arms; Sc galaxies have a relatively small nucleus and open arms).
Q: How did Hubble originally present his galaxy classification scheme?
Q: How can you contribute to the classification of galaxies?
Q: What type of galaxy is the Milky Way? Q: What is the Local Group?
Q: Name some of the other galaxies in the Local Group.
Q: How are galaxies organised on a larger scale?
Q: Which cluster of galaxies do we belong to?
Q: Name two nearby clusters to the Virgo Cluster.
A: In July 2007, Galaxy Zoo was launched. This web-based project has invited members of the public to access images from the Sloan Digital Sky Survey telescope in New Mexico in order to classify galaxies (www.galaxyzoo.org).
A: The Tuning Fork Diagram (so-called because of the shape’s resemblance to that of a tuning fork). It was first published in 1936 in his book The Realm of the Nebulae. At first Hubble believed that his classification scheme represented an evolutionary sequence (from left to right), but astronomers now know this to be incorrect.
A: In a similar way that stars are found in large collections, the galaxies themselves are grouped on both large and small scales. Our Galaxy is a member of the Local Group comprising about 50 galaxies; the group is about 3 Mpc across.
A: Traditionally, the Milky Way has been classified as an Sb type, but there is an increasing amount of evidence to suggest that it contains a well-defined bar, thus re-classifying our Galaxy type SBb.
A: Into gravitational agglomerations called rich clusters that often contain a giant elliptical galaxy in their central region.
A: Andromeda Galaxy (M31; giant spiral, and largest in the group), Large and Small Magellanic Clouds (two irregular companion galaxies to ours), Triangulum Galaxy (M33, spiral galaxy), several satellite and dwarf galaxies such as Pisces Dwarf and Aquarius Dwarf.
A: The Fornax Cluster and Coma Cluster (so-called because of the constellations in which they lie).
A: The Virgo Cluster, containing about 2000 galaxies (its centre lies 15-20 Mpc away).
Q: On a larger scale, how are clusters of galaxies organised? Q: What is an active galaxy?
Q: What is an active galactic nucleus (AGN)? Q: What is a radio galaxy?
Q: What is a Seyfert galaxy?Q: What is a quasar (quasi-stellar radio source, or quasi-stellar object)?
Q: What is a blazar?Q: What powers an active galactic nucleus?
A: Galaxies that emit vast amounts of energy at wavelengths other than those of visible light, particularly in the radio and X-ray regions of the spectrum.
A: Clusters collect together in superclusters. They are also held together by gravity. Our Local Group of galaxies and the clusters of galaxies such as Fornax and Coma are members of the Virgo Supercluster.
A: Discovered in the 1950s, these (mainly) elliptical galaxies are such intense sources of radio waves that they emit more than a million times more energy at radio wavelengths than normal galaxies; the radio emission often originates from symmetrical ‘lobes’ that lie on opposite sides of the galaxy.
A: The source of energy of an active galaxy, thought to be an extremely small nucleus.
A: Discovered by radio astronomers in 1963, their compact appearance makes them look like stars, but they emit huge quantities of energy in all regions of the spectrum from X-rays to (sometimes, but not always) radio waves; their high redshifts reveal that quasars are some of the most distant objects in the Universe.
A: Discovered by US astrophysicist Carl Seyfert in 1943, these spiral galaxies have extremely compact but bright nuclei, strong emission lines in their spectra and show strong and often variable emission of radiation at IR and X-ray wavelengths.
A: Matter falling into a super-massive black hole at the centre of the galaxy.
A: Blazars (of which BL Lacerta, or BL Lac objects are a subset) were discovered in the 1970s. They appear like star-like objects, but lie much closer than quasars, emit strongly in all regions of the spectrum from radio to gamma-rays and can vary dramatically in optical brightness on a timescale of one or two days.
Q: Describe how astronomers using different parts of the EM spectrum discovered quasars.
Q: What is cosmology?
Q: What is the Doppler Principle?Q: What did Edwin Hubble and Milton Humason discover in the 1920s?
Q: What is redshift? Q: What is the implication of the Hubble / Humason data?
Q: What is radial velocity (v)?
Q: Mathematically, what is the relationship between the change in wavelength of light and the radial velocity?
A: Cosmology deals with the large-scale structure of the Universe, its past and its future.
A: In the 1950s, many strong radio sources had been matched with stars, but it was not until 1962 that that the radio source 3C 273 had its position determined precisely by radio astronomers Cyril Hazard and John Bolton at the Parkes Radio Observatory in Australia (thanks to some fortunate occulatations by the Moon).The exact location of the optical counterpart of 3C 273 allowed astronomer Maarten Schmidt to obtain its spectrum using the 200-inch Hale telescope on Mt. Palomar in California. Schmidt noticed strange emission lines that he soon realised were hydrogen-redshifted lines of hydrogen, and he then calculated the corresponding recession velocity of quasar 3C 273 as 47,000 km/s.
A: They studied the spectra of almost 50 galaxies using the 100-inch Hooker Telescope at the Mt. Wilson observatory in California. They discovered that the wavelengths of well-known absorption lines in the galaxies’ spectra were all longer than the corresponding wavelengths when measured in the laboratory.
A: The effect, discovered by Austrian mathematician and physicist Christian Doppler in 1842, arises from the relative motion between a source of waves and an observer. This relative motion affects the wavelength or frequency of any wave, including sound and light. The shifts in frequency cause ambulance sirens, racing cars/bikes and train whistles to sound higher in pitch as they approach us and lower as they move away.
A: The fact that all Hubble’s sample of galaxies showed redshift in their spectra implies that they are all moving away from us.
A: The apparent increase in wavelength of the light of a body that is receding from the observer.
A: (λ – λo) / λ = v / c
where λ is the observed wavelength of the spectral line, λ0 is the true (rest) wavelength measured in the lab and c is the speed of light (taken to be 300,000 km/s).
A: The towards-or-away movement of a celestial body; positive if the object is receding, negative if it is approaching.
Q: What is the implication of a galaxy with a larger redshift?
Q: Which galaxies display a small blueshift?
Q: What is the implication of the discovery that all the distant galaxies are moving away from us?
Q: Describe Hubble’s Law.
Q: State Hubble’s Law mathematically.
Q: What is the value of Hubble’s constant?
Q: What is the Steady State theory of the Universe, and who proposed it?
Q: What is the Big Bang theory of the Universe, and who proposed it?
A: A few galaxies in our Local Group display small blueshifts in their spectra, meaning they are moving towards us. Our close neighbour the Andromeda Galaxy is a good example, and astronomers predict that it will ‘collide’ with our own Galaxy in about 10 billion years’ time.
A: The larger the redshift, the faster the galaxy is receding from us.
A: Hubble found that the fainter (and therefore more distant) galaxies showed greater redshifts in their spectra, i.e. the recession velocity of a galaxy (v) was proportional to its distance (d) from us.
A: The Universe is expanding.
A: The value of H has been difficult to determine exactly, but most cosmologists agree on 77 km/s/Mpc with an uncertainty of 15%.
A: v = Hd
where v is the recession velocity of a galaxy (in km/s), d is its distance (in Mpc) and H is Hubble’s constant (or parameter).
A: The main contributions to this theory cam from George Gamow in 1948. According to this model, the Universe: is expanding; is finite and ever-changing; was created some ~14 billion years ago from an event known as the Big Bang; may have an end depending on its density.
A: Proposed by Fred Hoyle, Hermann Bondi and Thomas Gold in 1946. According to the model, the Universe: is expanding; has unchanging density; spontaneously created matter, especially hydrogen, from empty space to keep the density the same; had no beginning and will have no end.
Q: How can we work out the age of the Universe?
Q: What is the Cyclic Universe theory?
Q: What is the major evidence in favour of the Big Bang model of the Universe?
Q: How was CMB radiation discovered?
Q: What was the significance of the COsmic Background Explorer (COBE) satellite?
Q: What is the significance of the Wilkinson Microwave Anisotropy Probe (WMAP)?
Q: What is dark matter?Q: Name some dark matter candidates.
A: There have been a series of Big Bang / Big Crunch scenarios and these will continue in the future. It suffers from a lack of hard evidence.
A: The Hubble constant can be expressed in units of ‘per second’ (the two distances – in km and Mpc – can be converted into the same distance unit and so cancel each other out). When this conversion of units is completed, inverting H gives us a value in seconds that cosmologists interpret as the age of the Universe. The value is 4.35 x 1017
seconds, or 13.8 billion years.
A: Discovered in 1965 by Arno Penzias and Robert Wilson. They were attempting to detect faint reflections of radio waves from Echo balloon satellites at the Bell Laboratories in Holmdel, New Jersey using a giant horn radio antenna. Having eliminated all sources of radio interference from their data, they discovered a steady uniform ‘noise’ from all parts of the sky and at all times of day and night.This steady noise was several times stronger than expected and through chance conversations Penzias and Wilson became aware that they had discovered what a team of astrophysicists led by Robert Dicke at nearby Princeton University had been searching for.
A: It explains the abundance of light elements, especially the 25% helium in the Universe; and the cosmic microwave background (CMB) radiation that pervades the Universe and perfectly matches the radiation emitted by a ‘black body’ with a temperature of 2.7 K.
A: Launched by NASA in 2001 in order to study the CMB in more detail and help cosmologists to refine their models of the composition, evolution and age of the Universe, its scientific instruments also provided data that enabled cosmologists to estimate the contribution of dark matter in the Universe and to confirm the existence of dark energy.
A: It was equipped with very sensitive detectors to study the CMB. The data confirmed the uniform nature (isotropy) of the CMB on a large scale, but also detected minute fluctuations or ‘ripples’ in the radiation. This data allowed cosmologists to study how matter (that would eventually develop into galaxies) and energy were distributed in the early Universe.
A: Black holes; brown dwarfs; unknown atomic particles (cold dark matter); a sea of fast-moving neutrinos (hot dark matter – though models show it cannot be much of this); exotic particles known as WIMPs (Weakly Interacting Massive Particles).
A: Matter that is invisible to astronomers because it does not emit or reflect radiation, but has detectable gravitational effects.
Q: Who proposed the existence of dark matter and why? Q: What is gravitational lensing?
Q: What proportion of the matter in the Universe is thought to be dark matter?
Q: What are the implications of the total mass of dark matter in the Universe?
Q: Is the expansion of the Universe speeding up or slowing down?
Q: What is the term given to the mysterious force of negative gravity?
Q: What is Olbers’ Paradox?Q: Describe the events shortly after the Big Bang.
A: Observed multiple and distorted images of the same galaxy whose radiated light has passed close to, and been gravitationally ‘bent’ by, a huge amount of dark matter (in a similar way that a convex lens refracts and focuses light).
A; Swiss astrophysicist Fritz Zwicky, in order to explain the unusual rotation of spiral galaxies. Using a simple gravitational model that stars revolve in times that depend on their distance from, and mass of, the central nucleus, he was unable to explain why stars at large distances from the nucleus were observed to be travelling much faster than expected. Zwicky suggested that the galaxy would need to contain a great deal of invisible ‘missing mass’ to explain the discrepancy.
A: It has implications for the ultimate fate of our expanding Universe, since the mass of the Universe determines the density. If the density of the Universe exceeds a ‘critical density’ then instead of expanding forever, gravitational forces will eventually halt its expansion and the Universe will subsequently contract, ultimately experiencing a Big Crunch.
A: Most cosmologists agree that dark matter contributes over 90% of the total mass of the Universe.
A: Dark energy (a term coined by cosmologist Michael Turner).
A: Until about 10 years ago, there was a general consensus among cosmologists that the expansion of the Universe was slowing down. However, in 1998, two teams of astronomers used observations of distant supernovae to suggest that the expansion was actually accelerating under the influence of mysterious repulsive force of negative gravity.
A: The Big Bang was very hot and extremely dense. In the first few seconds, matter in the form of hydrogen and helium nuclei was produced in abundance. About 25% of the matter in the Universe is helium. During the expansion that followed the Big Bang, the Universe cooled. After billions of years, atoms, then stars and then galaxies were formed.
A: Stated by the astronomer Heinrich Olbers, if the Universe is infinite then, no matter in which direction we choose to look, we would eventually see a galaxy. Therefore, the night sky should be bright – which it is most definitely not!
