History of Astronomy: Second Term Exam Review
**Exam Review questions and video notes
Chapter 8 Review Questions
1. What was William Herschel looking for when he discovered Uranus? What made Herschel
particularly suited for discovering planets? In what 2 ways did Uranus reveal itself to
Herschel as a planet? What is it about Uranus' orbit that caused it to be considered a planet
instead of a comet? What is the origin of Uranus' name?
He was looking for stellar parallax when he discovered Uranus. He used to build his own telescopes
and built one of the largest telescopes of the time in his backyard to observe the skies. He realized it
was a planet because it moved relative to the stars rather than with the stars.Also, when he
increased magnification of the body, it appeared to become larger. This is different than stars
because stars remain point-like in shape when magnified. Uranus was seen to have a near circular
orbit around the Earth which deemed it a planet. Uranus was the father of Saturn and grandfather of
Jupiter in Greek mythology.
2. What did Kepler believe existed between the large space between Mars and Jupiter?
Explain why Bode's Law supported this idea.
Kepler believed there was an undiscovered planet in the large space. Bode’s law was a numerical
trend that he applied to the orbits of the planets; however there was one value in between Mars and
Jupiter which did not apply to a planet. He believed this meant there was an undiscovered planet for
3. What is the name of the object found in 1801 between Mars and Jupiter? What was this
object finally classified as, and why? What is this object a part of?
Ceres was the name given to the large body found in this area. It was later classified as an asteroid
because it was small (smaller than our moon). It is a part of the asteroid belt in between Jupiter and
4. What caused Adams and Leverrier to suspect the existence of a planet beyond Uranus?
When this planet was found, what was it named, and why?
Uranus’orbit was seen to deviate from its usual nearly circular path so they believed that there must
be a planet beyond Uranus that is causing the deviation due to the planet’s gravitational pull. This
planet was found in 1846 and was named Neptune (greek god of the sea) because of its blue colour.
5. How were the masses of Uranus and Neptune 1st measured? How do their masses compare
with the other planets? Based on the masses and sizes, what do we know about their densities
By observing each planet’s moons they could use Kepler’s laws to determine the mass of the
planets. They are more massive than the other planets but when comparing the mass to their size
they are not very dense. This suggests they are made up of a lot of light-density matter such as gas
so they are deemed “gas giants”. 6. What is today's theory for why all the planets in our solar system orbit the Sun in the same
direction and in roughly the same plane? What is the observational evidence for this theory?
The theory is that stars are born of nebulas that spin fast enough to form a star at the centre with
interstellar dust orbiting around it. This interstellar dust spinning around the star is called the
circumstellar disk and the debris in it eventually clumps together as gravitational forces pull in
more debris to form masses of debris. These masses of debris eventually become large enough to
resemble planets which explains why they will all be spinning in the same direction and same plane
as the circumstellar disk. It was observed that nebulas have rings around them either made of gas
(bright rings), rocky debris (dark rings) or a combination of both which are believed to result in
solar systems after many millions of years.
7. What is today's theory for the asteroid belt? What about planetary rings?
Asteroid belts are what happens when there are other planets that exhibit enough gravitational force
on the space debris to prevent it from clumping together into planets and instead stay as debris.
Planetary rings are the same except moons prevent the formation of other bodies and they occur in
the orbits around planets instead of the orbit around the sun.
8. What 3 attributes are unique to the 4 planets beyond Mars? What is the current
explanation for this?
The outer planets beyond Mars are all gaseous, ringed and have numerous moons (massive). The
reason for this is that the outer planets had more matter available to collect in the circumstellar disk
and as their mass grew their larger gravitational attraction could collect more debris and more gas
making them massive and gaseous. The fact that they were more massive allowed them a larger
gravitational pull to collect debris to orbit them to create moons.
9. Which planets are the terrestrial planets? What is the current explanation for their lack of
Venus, Mercury, Earth and Mars are all terrestrial planets. Since they are inner planets they had less
debris available to them so as they formed, their hard core formed first and never had a chance to
grow large enough to form a large gas layer like the outer planets.
10. What is the current theory for the origin of Mars' two moons? What is the evidence to
support this theory?
It is believed that Mars’moons were large asteroids from the asteroid belt that were knocked out of
the belt and fell into Mars’gravitational pull. Mars’moons are not spherical and represent asteroids
more than planets.Also, they move retrograde to the planets (clockwise rather than counter
clockwise) which contradicts that they were part of the original formation of the solar system.
11. Why is it unusual for Earth to have such a large moon? What is the current theory for the
origin of Earth's moon?
Usually the inner planets are believed to have been too small to have a moon form around them. It
is believed that Earth’s moon was the product of a large collision of a Mars-sized body with Earth.
This ejected a lot of debris into space which was caught in Earth’s gravitational pull which
eventually formed our moon. 12. What belief about Mars was popularized by Percival Lowell, and why? Was he right?
He popularized the belief of an intelligent Martian civilization on Mars. He believed that there were
intricate trenches built on Mars which could only be the product of an intelligent civilization. He
was wrong; they were just optical illusions of shadows cast by Mars’natural canyons.
13. Why was there suspected to be a planet beyond Neptune?
Neptune’s mass had been recalculated and they saw that the new mass did not account for the
deviation of Uranus’orbit alone. They believed there had to be a planet beyond Neptune which
helped in the deviation of Uranus’orbit.
14. When Lowell's predicted planet was finally detected, what was it named, who named it, and
why did she choose this name?
It was named Pluto for the god of the underworld since it was dark and gloomy. It was suggested by
a little girl in England at the time.
15. Why did the discovery of Pluto turn out to be a coincidence?
Because when Neptune’s mass was recalculated again, it turned out that the new mass indeed
accounted for the deviation of Uranus’orbit alone and Pluto was too small to have played a role in
16. The discovery of what kind of objects motivated the IAU to define the conditions for being a
planet? Where are these objects? What was the significance of the object now known as Eris?
The discovery of KBOs had forced the IAU to redefine the definition of the term planet. These
objects are all beyond Neptune and are pieces of ice debris. Objects like Eris (which was larger than
Pluto) made IAU question what to define a planet because if Pluto was one, there would be many
more just like it or bigger.
17. What are the IAU's planet conditions? Which condition(s) does Pluto fail? What
condition(s) do comets fail? What condition(s) do asteroids fail?
Aplanet must orbit the sun, be a sphere by its own gravity and it must have cleared the debris
around its orbit. Pluto fails the third condition because it is part of the ice belt beyond Neptune.
Comets and some asteroids are excluded due to the second condition. Kuiper Belt Objects are
excluded because of the third condition.
18. What is Pluto now considered? What are the condition(s) for this type of object? Is this type
of object expected to be common in our solar system?
Pluto is now considered a Dwarf Planet because it only fails the third criteria of a planet.ADwarf
Planet is anything that satisfies the first two criteria of a planet. This object is expected to be very
common in our solar system but cannot be confirmed until we have methods of telling whether
distant planets are spherical or not. Chapter 9 Review Questions
1. Why did all attempts to detect stellar parallax fail until the 1830s?
All scientists were trying to detect parallax on stars that were bright because they thought that
meant they were the closest. This was false because some stars were brighter than others and they
were too far to detect parallax.
2. What did Halley announce about stars in the early 1700s? How did this discovery aid the
detection of stellar parallax?
Halley compared ancient star catalogues with current star positions and found moving stars, not
moving within constellations but independently, thus it couldn’t be due to earth’s precession or due
to parallax (all stars would be effected), suggested that stars have unique motion across the sky. He
suggested using old star charts to determine which stars moved the most as this would help display
3. What is our nearest star, and how far away is it (approximately) in light years?
Alpha Centauri is the nearest star, it is appx. 4.4 light years away
4. What is the definition of a light year? What is the definition of a parsec? (NOTE: the
specific values in km aren't necessary here). Which unit is bigger, and by approximately how
Alight year is the distance light travels in a year and a parsec is the inverse of parallax measured in
arcseconds. The parsec is larger; one parsec is equal to about 3 light years.
5. According to the wave theory of light, what are all light waves composed of?
All light waves are composed of a magnetic wave and an electric wave that propagate
perpendicular to each other.
6. Describe how a wave from a bright blue light source is different from a wave from a faint
red light source.
Abright blue source wave will have a large amplitude, low wavelength (and so high frequency). A
dim red light will have a low amplitude, long wavelength (and so a low frequency).
7. What colour of visible light is shortest in wavelength? What colour of visible light is
longest in wavelength? What colour of visible light has the lowest frequency? What colour of
visible light has the most energy?
Blue is the shortest wavelength and highest frequency (highest energy) and red is the longest
wavelength with the lowest frequency (lowest energy). 8. In the full spectrum of light, what kind of light is shortest in wavelength? What kind of
light is longest in wavelength? What kind of light has the most energy?
Radio waves are the longest and lowest-energy waves, while gamma waves are the shortest and
highest energy waves
9. What is a spectroscope?
Aspectroscope an apparatus that produces a light spectrum that can be observed through a
10. What is Fraunhofer’s Spectrum a spectrum of? What does it contain (ie., continuum
emission, absorption lines, and/or emission lines)?
He found black lines on his sun spectrum every time and the lines always appeared at the same
colors on the spectrum. He mapped the lines and labeled them with a letter. He never explored what
those lines meant but they were absorption lines
11. Describe (briefly) how the chemical composition of a star can be determined.
Through spectroscopy the chemical composition can be measured through a spectra of light where
different elements emit different amounts of light of different colors on the spectra. Each element
emits light at certain wavelengths and thus would have its own unique identifiable “fingerprint”
12. What did Bunsen and Kirchhoff observe in the spectrum of sunlight combined with a gas
flame? What was their correct explanation for this, and what did it cause them to predict
about the Sun? How was their prediction proven in the 1860s?
They noticed that the absorption lines got thicker when combined with the gas flame. They
hypothesized that the element was absorbing its unique wavelength of light so the white light from
the sun was seen in every wavelength except those that were absorbed by the element in the flame.
This was used to hypothesize that the sun had an atmosphere of elements that would absorb certain
wavelengths of light. In the 1860s the sun was observed during a solar eclipse so that only the
outer atmosphere of the sun would be observed and emission lines were observed at the
wavelengths where the absorption lines were previously observed proving that there was an
atmosphere emitting those wavelengths of light.
13. How did the element Helium get its name?
It was discovered as a new emission line in the sun’s atmosphere and was named after Helios, the
Greek God of the sun
14. In a graphical spectrum, what do emission lines appear as? What do absorption lines
On a graphical spectrum peaks are emission lines and dips are absorption lines
15. What does the spectrum of a star contain (ie., continuum emission, absorption lines, and/or
emission lines)? How about the spectra of a gas cloud, a star cluster, and a gas/star system? Stars contain continuum spectra with absorption lines, gas clouds are just emission lines, star
clusters are continuum and absorption spectra and gas/star systems are continuum, absorption and
16. Why was the element Oxygen originally named "Nebulium"?
The emission lines of a spectrum of a planetary nebula were originally unidentified until matched
17. What does the spectrum of a planetary nebula contain (ie., continuum emission, absorption
lines, and/or emission lines)? When Huggins discovered this, what did it prove about
planetary nebulae (ie., what are they made of)?
He saw that it contained only emission spectra so the nebula had to be made of gas.
18. What is radial motion? How does it effect light waves? What is the name of this effect?
the approaching or receding motion; only detectable from star’s light spectrum via the “Doppler
effect” (the stretching or compressing of a wave due to the approaching or receding motion of the
source of the wave)
19. How does a light source's spectral lines reveal that the spectrum is redshifted? How
aboutblueshifted? If a spectrum is redshifted, what does this tell us about the motion of the
light source? How about if the spectrum is blueshifted?
Stretched waves are redshifted, compressed waves are blueshifted. If a spectrum is blueshifted then
the source is approaching, if it is redshifted then it is receding
20. What is astrophotography? Why does it allow us to see deeper into space than with the eye
and telescope alone? Who was one of the pioneers of this? What sort of survey did he
Henry Draper was a Pioneer of astrophotography (the production of long-exposure sky photos
using a camera attached to a telescope) Cameras can use long exposure to capture more light than
our eye would be able to. Sky photos allow us to see deeper into space and can be stored on film
for later analysis. He photographed the 1st stellar spectrum and began a photographic spectroscopic
survey of all visible stars so they could be classified through chemical composition
21. What were the members of "Pickering's Harem" hired to do?
They were hired to classify the stars according to their chemical compositions revealed by their
22. What property of stars was used to place the spectral types in their original alphabetical
order? What property of stars was used to re-arrange their order into their current order (O-
They were initially instructed to classify stellar spectra into alphabetical spectral types by the
strength of their Hydrogen lines. Strong hydrogen lines would be anAstar and weak ones, a Q star.
The property used to re-arrange them into their current order was their color, (hottest to coldest) 23. What do we know about the chemical composition of the reddest stars, due to their
numerous spectral lines?
The reddest stars have more metals than other types
24. If a star has a spectral type of B8, which type of star is it more similar to, O orA?
It would be more similar toA
25. What is a photon?
Aphoton is a particle of light
26. According to the Bohr model of the atom, what are the 3 components of an atom? What
determines an atom's chemical element?
It consists of a central nucleus of protons and neutrons surrounded by a shell of orbiting electrons.
The # of protons determines the chemical element of an atom
27. Explain (briefly) what causes atoms to produce absorption lines. What about emission lines?
When an electron absorbs a photon with the exact energy it needs to get excited, the electron jumps
to a higher shell causing an absorption line.An emission line is formed when an excited electron
drops down a shell, it emits a photon with the exact energy to return to ground state.
28. What unusual discovery did Cecilia Payne make about the chemical composition of stars?
She discovered that all stars are predominately composed of H (and He)
29. Describe how Eddington proved one of the predictions of Einstein's General Relativity
During a total solar eclipse, he proved general relativity by observing that when light rays pass
near the sun, their path is bent by the curved space around the sun.
30. What was Eddington and Bethe's correct explanation for the energy source of stars? Why
does this process release energy? How does it explain why stars produce so much energy?
How does it explain the existence of Helium in stars? How does it explain the full spectrum of
light that we receive from stars? How does it explain the long lifetimes of stars?
He proposed that the energy from stars is produced from the fusion of Hydrogen nuclei into Helium
at high temperatures. Nuclear fusion explains why stars produce enormous energy (Hydrogen atoms
are plentiful in stars). Helium is a product of hydrogen fusion and thus is found in large quantities
in stars. Stars emit a full spectrum of light (the gamma-rays downgrade to other wavelengths as
they interact with atoms in the star). Stars last for billions of years, the initial fusion reaction of
Hydrogen takes a billion years
31. At what rate does light dim with distance? Light dims with distance according to the inverse square law (the amount of light we receive from a
source is 1/d^2 of its actual light production)
32. What is the difference between the apparent brightness and the intrinsic brightness of a
Apparent brightness is the brightness of a light source as it appears from earth, intrinsic brightness
is the total amount of light that a source produces.
33. Which appears brighter, a star with an apparent magnitude of 2 or a star with an apparent
magnitude of -2?
Astar with a magnitude of - 2 would be brighter
34. For a group of stars in the same star cluster (i.e. at the same distance from Earth), how can
we tell which stars are the most luminous?
We have to take into account which star has the lowest absolute magnitude, not their apparent
magnitude. The brightest one would be most luminous.
35. What relationship among Main Sequence stars was found by Hertzsprung & Russell in the
1910s? What two kinds of stars were found to not follow this relationship?
Hertzsprung and Russel found that sprectral type and luminosity. Red giants do not follow this
trend. White dwarfs also do not follow this relationship.
36. What is the spectral type of the Sun?
The sun is a yellow dwarf
37. Using the HR diagram, compare the luminosity, temperature, colour, and size of Red Giants
to the Sun.
Red giants are more luminous than the sun, colder than the sun, more red than the sun and much
larger than the sun (1000x bigger)
38. Describe the mass and size of a White Dwarf. Using the HR diagram, compare their
luminosity, temperature, and colour to the Sun.
White dwarfs are smaller, hotter and bluer than the sun
39. What relationship among Main Sequence stars was found by Eddington in the 1920s? What
2 things did this discovery tell us about the Sun?
He found that there was a link between mass and luminosity; the more massive a star was at birth,
the brighter it was. This told us that our sun is a low mass star that is not very bright which is the
most common type of star on the main sequence.
40. Compare the luminosity, temperature, colour, size and mass of a Main Sequence O star to a
Main Sequence M star. Amain sequence O star is brighter, bluer and larger than a main sequence M star
41. What determines a star's spectral type on the Main Sequence? How do we know that a
star's Main Sequence spectral type can't change?
It is determined by luminosity and mass, this can’t change because it would violate conservation of
mass and energy
42. What was Hoyle's correct explanation for the existence of elements heavier than Helium in
stars? How do these elements get on to planets like ours?
Hoyle’s explanation used nucleosynthesis (the creation of heavier elements from existing atoms) to
explain the existence of elements heavier than helium in stars. When stars die, they eject their heavy
elements into the interstellar medium. Gas clouds in the chemically enriched medium eventually
collapse into new stars and planets
43. How do we know that our solar system must have come from the ejecta of other stars?
Our solar system must have come from the eject a of other stars since it contains Carbon since
when the universe was first born, the only available elements were Hydrogen and Helium
44. When a star is on the Main Sequence, what allows its properties to remain stable?
Astable star remains stable when its forces are equal (gravitational pressure = radiation pressure)
45. What causes stars to leave the Main Sequence?
When a star’s gravitational pressure is not equal to its radiation pressure. When hydrogen fusion
runs out it leaves the main sequence
46. Why do we find more low-mass Main Sequence stars than high-mass ones?
The H-fusion phase is shorter in high mass stars, as fusion is faster at high temperatures. This
leaves more low-mass stars on the main sequence
47. Describe what will happen to our Sun from the time it leaves the Main Sequence to its
ultimate corpse. Include how its luminosity, temperature, colour and size will change as it
evolves, and how these changes are due to the shifting in the balance between gravity and
The sun will run out of Hydrogen and fuse He which will cause it to swell into a red giant. It is now
much larger, much redder, brighter and cooler than it was. This is because the energy output is so
large from fusing heavier atoms and so the radiation pressure will be much larger than the gravity
pressure. After this the star will not be hot enough to fuse He products so it will run out of He to
fuse and the gravitational pressure will be much larger than the radiation pressure and the star will
shrink down to a white dwarf. This is smaller, blue-white, hotter and dim. The outer layers of the
star are ejected as a planetary nebula.
48. What kinds of stars are able to fuse elements heavier than Helium, and why?
High mass stars because they have the mass and thus the temperatures to fuse the heavier products. 49. Describe what will happen to an O-star from the time it leaves the Main Sequence to its
ultimate corpse. Include how its luminosity, temperature, colour and size will change as it
evolves, and how these changes are due to the shifting in the balance between gravity and
It will start consuming He and become a red giant (more red, larger, brighter, cooler due to larger
radiation pressure than gravitational pull). Once He is all used up it will start consuming the
products of the He fusion and become a red supergiant (radiation pressure is larger than
gravitational pull) and becomes larger, cooler, redder and brighter. When all the products are used
up and heavy materials remain, the gravitational pull will become much larger than the radiation
pressure and collapse into itself (super nova). The core of the star will fuse together and if the mass
is larger than 2 sun masses it will become a blac hole, if not it will become a neutron star.
Chapter 10 Review Questions
1. What is the "Island Universe" theory?
It was proposed by Kant that elliptical nebulae are groups of stars shaped like a disk (Island
Universes) and that we live on one such system
2. Why did theAncient Greeks use the word "galaxy" to describe the bright strip of light
across the sky? What do we call this strip of light today? What part of our galaxy does this
The bright band of light seen across the night sky was named “galaxius” which means milky in
greek. Today we call it the milky way. Kant correctly explained: if we live in a disk shaped star
system then the Milky Way our view through the disk’s length and we are within the disk.
3. What did William, Caroline and John Herschel produce together? What was Herschel
attempting to prove from this research? Why was he not able to prove this?
They produced a catalogue of over 5000 nebulae. They attempted to identify whether nebulae are
distant star systems (like Kant said) or nearby glowing gas clouds. If they were gas clouds it would
prove that we are the only galaxy in the universe. Since there are a large variety of nebulae, (some
with just gas and no stars) they couldn’t come to a general conclusion about their composition. M33
was a mix, M17 was just gas and M9 was a cluster of dense stars.
4. What did Herschel's map of the visible stars reveal about the shape of our galaxy?
It revealed that our star system is disk-shaped even though he judged the distances based on
5. What did Lord Rosse discover when he looked at the elliptical-shaped nebulae through his
Leviathan telescope? He discovered that many elliptical nebulae are spiral nebulae (a bright core surrounded by a fuzzy
sort of ring).
6. What is a variable star? What characterizes the Cepheid variables? Explain briefly
why Cepheids pulsate.
Variable stars are stars that change in brightness at regular time intervals. Cepheids are bright, high
mass stars which pulsate rapidly and intensely (at intervals of days to weeks). They are physically
pulsating, they grow and shrink in size. It has a lot of twice ionized He atoms which can be tightly
packed which prevent light from escaping (make it opaque). It’s photons push outward (radiation
pressure) and then the star will push outward, the light escapes (brighter) and then the gravitational
pressure builds and it collapses back again.
7. What are the SMC and LMC? What did Henrietta Leavitt discover when she
observed Cepheids in the SMC? Why was this discovery so valuable? Who used her discovery
to determine the boundaries of our galaxy?
Large Magellanic Cloud and Small Magellanic Cloud. By assuming the SMC Cepheid's are equally
distant from Earth, Leavitt found: the brighter the Cepheid, the longer its period (P). So you could
use its period to get its luminosity (or magnitude), which the luminosity could be used to find
distances. Then you could find distances to other star systems. She used apparent magnitude
because she did not have the distances she could not find luminosity in her original findings.
Harlow Shapley used her discovery; he converted the apparent mags to absolute mags (and then
luminosity) using cephieds with known distances and then used these relationships to find the
boundaries of our galaxy.
8. What are the 3 components of our galaxy, according to Shapley's model? Where is the Sun
located in this model? Where is the Galaxy's centre? In what 2 ways is this model incorrect?
The disk is 300 000 ly in diameter, which implied all nebulae are systems within our galaxy, our
sun resides in the outskirts of its disk (the galaxy is not heliocentric), the Milky Way centre is in the
direction of Sagittarius. He was wrong because our actual diameter of our galaxy is 100,000 ly and
there are many systems beyond our galaxy; the milky way is not the only galaxy (he rejected the
9. Why was Shapley convinced that the spiral nebulae are within our galaxy's boundaries?
He believed that if they weren’t contained in our galaxy, then we would be able to see them all
around the sky (we couldn’t see them near the disk of our galaxy). If they were beyond our
boundaries, why would they be affected by where the Milky Way disk is?
10. What is the Zone ofAvoidance? What was Shapley's explanation for it? What was Curtis'?
Who was correct?
The Zone of avoidance is the middle band where our galaxy is contained and there are no spiral
nebulae, Shapely believed that they were pushed from our disk by radiation pressure from its high
density of stars. Curtis was convinced that our galaxy was a spiral galaxy and the other spirals were
extra-galactic (much like the island galaxy theory). Curtis was correct. Curtis correctly proposed: If
the Mily Way’s disk contains a band of dust, then if spirals are outside our Galaxy, they aren’t seen
in the Zone of Avoidance because they are obscured by our dust. 11. What observation led Curtis to suspect that the disks of galaxies are filled with interstellar
dust? Where does this dust come from?
Curtis completed a high resolution survey of spiral nebulae, revealing obscuring matter (metal dust)
in their disks. This dust comes from the ejecta of dying stars, the metal dust absorbs starlight (it
glows in IR).
12. What did Vesto Slipher observe about the spectra of spirals?
He observed large redshifts in the absorption lines of spirals, suggesting recessional speeds as high
as 1000s of km/sec
13. In the Shapley-Curtis Debate, what did Shapley argue? What did Curtis argue? What 4
pieces of evidence did Curtis present?
Shapley argued that our galaxy is so enormous that it must be the entire universe and spiral nebulae
are contained within it. Curtis argued that the spiral nebulae were island universes like our own
galaxies and our galaxy is one of the many spirals. His four pieces of evidence were:
1. Spiral radial velocities are much larger than star velocities,
2. Spiral spectra are indicative of star systems (they have absorption lines like galaxies),
3. The faintness of what appear to be novae in the spirals suggest that they are distant star systems,
4. Spirals show evidence of obscuring matter in their disks; if the Milky Way is such a spiral, this
explains the Zone ofAvoidance
14. Who finally resolved the Shapley-Curtis debate? Explain how he did this. What was
NASA's tribute to this discovery?
Edwin Hubble ended the debate. He found a Cepheid in M31 (Andromeda spiral nebula), he plotted
the light curve and then could measure the period and find the luminosity and he found a distance to
M31 of ~1 million ly, this proved that spirals are galaxies beyond the Milky Way even with
Shapley’s estimate of our galaxy being 300,000 ly large. NASAhonoured his achievement by
launching the Hubble space telescope to explore space.
15. Describe (briefly) the 4 types of galaxies in Hubble's Classification system? Which are the
most numerous? Which are the least numerous? What are the two types of spiral galaxies?
What type of galaxy is the Milky Way?
1. Spiral (S) : ~30 of known galaxies, 60% of which are barred (SB; eg. The Milky Way).A
flattened disk containing spiral structure and possibly a centre bulged. Barred spiral (SB- spiral
with central bar. Sub classified as
Sa – tightly wound with bright bulge or
Sc – loosely wound with faint bulge. Theory that they come from the merging of two galaxies
2. Lenticulars (S0): ~ 15% of known galaxies – a flattened disk and possibly a central bulge, but
no spiral structure (They can have bars too [SB0])
3. Ellipticals (E): ~5% of known galaxies, they have no disk and no spiral. They are an elongated
spheroid, designated a number, the higher the number the more elongated the elliptical. Believed
to be formed when two merging galaxies are equal in mass
4. Irregulars (I): ~50% of known galaxies. They have no uniform structure. The small dwarf
irregulars (dIs) are by far the most numerous type of galaxy in the Universe 16. What is the Local Group? How does the size of the Milky Way and M31 compare to other
galaxies in the Local Group? How did the Milky Way and M31 get this way?
The local group is a cluster of galaxies that contains about 30 galaxies with 2 of them being spirals
(milky way andAndromeda). They are much larger than the other galaxies in the local group. They
grew to this size through galactic cannibalism (consuming dwarf galaxies).
17. What is the Local Supercluster?
Galaxy groups cluster into superclusters containing thousands of galaxies. The local group is part of
the local supercluster, known as the Virgo supercluster
18. What is Hubble's Law? What does the slope of Hubble's Law tell us? What does the inverse
of the slope tell us?
The larger a galaxy’s redshift (or recessional velocity), the further its distance. The rate at which the
universe is spreading out is given by the slope (acceleration) of Hubble’s law. This is called
Hubble’s Constant (H ).0The inverse slope (1/ H ) 0ells us the age of the universe
19. Prior to the discovery of Hubble's Law, who had found theoretically that the Universe is
Einstein theoretically found that it was expanding. He did not believe this was possible for the
universe to grow or shrink so he added a cosmological constant to “fix” this equation.
20. Describe what Einstein referred to as the "biggest blunder" of his career.
When Hubble showed the Universe is expanding, Einstein called his constant the biggest blunder of
21. Describe the 4 possible fates of the Universe according to Einstein's cosmological equation,
including the density and Cosmological Constant associated with each fate.
1. If density of the universe (p) = critical density required for gravity to halt the initial expansion
momentum (Pcrit) and cosmological constant=0 then the universe’s expansion will halt (big
freeze) [gravity of universe = force of bang]
2. If density of the universe > critical density required for gravity to halt the initial expansion
momentum and cosmological constant = 0, the universe’s expansion will halt and contract (Big
crunch) [gravity of universe is greater than force of bang]
3. If density of the universe < critical density required for gravity to halt the initial expansion
momentum and cosmological constant= 0, universe will expand forever (big chill) [gravity of
universe is smaller than force of bang]
4. If density of universe is < or = to critical density required for gravity to halt the initial expansion
momentum and cosmological constant >0, the universe will expand at an accelerating rate forever
22. What is the Big Bang theory? What is the Steady State theory? What is the Steady State
explanation for Hubble's Law?
The big bang theory claims that the Universe began from a single point and is expanding with time,
decreasing in density (spreading out). Steady State: the density of the Universe is constant: as galaxies recede from each other, new
galaxies take their place. The steady state explanation for Hubble’s Law proposed that just because
galaxies are receding from each other it does not mean that density is decreasing, the new galaxies
would preserve the density.
23. Describe the 2 pieces of observational evidence for the Big Bang.
1. Comparing of distant galaxies with nearby galaxies, showed that spacing of distant galaxies was
more dense (so it can be implied that the past was more dense) . Radio surveys of the most distant
galaxies revealed that they are closer together than the nearby galaxies. This showed that the
universe is expanding.
2. Remnants of big bang were actually seen (big bang radiation has expanded to microwaves).
24. Describe the observational evidence that the expansion of the Universe is accelerating. What
does this tell us about Hubble’s Constant?
The Supernova cosmology Project : 1990s-2010s: a survey of extragalactic supernovae was used to
measure distances to the most distant galaxies in order to determine the rate of the Universe’s
expansion in the past
The results: Hubble’s Constant (H0) was lower in the past and therefore the universe is expanding
at an accelerating rate toward a Big Chill or Big Rip (so it’s not really a constant if it’s changing).
25. What does a map of the CMB (Cosmic Microwave Background) give us a picture of, and
It can give us the initial density of matter and energy in the universe. It gives us the locations where
structures of matter were forming just after the Big Bang, it shows us galaxy formation.
26. What did WMAPtell us about the composition of the Universe? What effect is "dark
energy" believed to be having on the Universe? How is dark energy now accounted for in
Einstein's cosmology equation?
Normal matter comprises 27% of the Universe (only 4% we can see), the other 73% is dark energy,
which may be responsible for the Universe’s accelerated expansion. It is denoted by Einstein’s
cosmological constant which quantifies the dark energy in the universe.
27. Describe the ways that dark matter has been detected in galaxies and in intracluster space.
Outer stars in galaxies rotate faster than predicted by a galaxy's visible mass, suggesting the
presence of intergalactic DM.
Gravitational lensing of background galaxies suggests the presence of dark matter in galaxies halos
as well as in intracluster dark matter (i.e., in the space between the galaxies in clusters).
Chapter 11 Review Questions
1. What are the advantages of radio astronomy compared to visible-light astronomy? Make sure
that you understand *why* radio waves have each of these advantages.
- They can be detected on Earth both day (not overwhelmed by sunlight) and night (24h a day), as
well as through clouds. - The sun does not emit many radio waves so radio waves from space can be detected above the
sun’s emitted radiowaves.
- They are easily detected with ground level telescopes, they completely penetrate earth’s
atmosphere (the atmosphere can only absorb low wavelengths well).
- They reflect off of metallic most metallic surfaces, unlike visible light.
- Not absorbed by interstellar dust because they are so big.
- Strongly emitted by distant (young) galaxies, allowing us to observe galaxies as they appeared in
the distant past
2. Who was the 1st person to detect radio emission from space? Where was this emission
coming from? How did he know this?
Jansky was the first to detect radio emission from our galaxy. He noticed a radio hiss coming from
all around the sky with a daily cycle, and the coordinates of the strongest intensity were in the
Sagittarius constellation (the centre of the galaxy) thus suggesting the hiss must come from our
galaxy and is due to the fact that the centre of the milky way rises and sets every day due to the
earth’s daily motion. It is loud when the galaxy rises, it dims when the galaxy sets.
3. When Grote Reber mapped out the radio emission from the Milky Way, what did he find?
He found that there was a broad radio peak surrounded by fragile contours and to the left of that,
two smaller peeks which indicate other sources of radio emission, they were coming from discrete
sources. It was later found that Hydrogen atoms naturally emit a 21 cm radio wave when their
electron flips over
4. Describe the process that is producing the 21-cm radio emission in our galaxy.
Hydrogen atoms naturally emit a 21 cm radio wave when their electron flips over (from the aligned
state to opposite directions).
5. What did maps of the 21-cm radio emission in our galaxy reveal about the structure of our
galaxy? Where is the Sun located in this structure?
It revealed that our galaxy has a spiral structure, the sun resides in the orion arc (the sun points
towards the orion constellation.
6. What did Stanley Hey identify as the source of the radio emission that was interfering with
British radar during WWII?
He discovered that solar flares from sun spots were the source of high radio emissions that would
interfere with equipment.
7. What are solar flares?
Solar flares are eruptions of radiation which trace out magnetic field lines from sunspots
8. Describe how the Sun spins. How has this effected the Sun's magnetic field,
and What happens to the Sun's magnetic field every 11 years as a result?
The Sun spins differentially and so it spins faster at the equator and slower near the poles [ie. It is a
fluid so different parts spin at different rates].As a result, its magnetic field is not a simple bar-
magnet, but a complex field which twists and untwists every 11 years. This causes the frequency of sunspots and flares to increase and reach a peak every 11 years. When the sun reaches its most
twisted up state every 11 years the field lines poke out of the surface which cause a pair of sunspots
where particles exit one and enter the other spot.
9. What causes magnetic storms on Earth?
Magnetic storms are the temporary scrambling of earth’s magnetic field. They are caused by winds
of charged particles produced by sunspots (more frequent during some parts of the sun’s 11 year
10. What causes aurora on Earth? Why are aurora most often seen at high latitudes?
They are caused by solar winds of charged particles produced by sunspots. When the particles reach
the earth they glow. They are most often seen at high latitudes because the sun’s magnetic field is
strongest at the poles
11. What happens to the number and intensity of sunspots, magnetic storms and aurora every
11 years, due to the Sun's magnetic cycle?
There is an increase.
12. What was developed in the 1960s to extend the limits of the observable Universe in radio
Radio arrays (many radio dishes) were used, that allow us to detect the most distant objects ever
observed. Bringing a bunch of dishes together they can combine the performance to act as a single
giant dish (one giant dish is a lot more expensive and cumbersome).
13. Describe the radio emission from a pulsar. What did Hewish initially suspect that the signal
was coming from? What is the true explanation for the pulses?Are all neutron stars
pulsars? Why/why not?
It is a pulsing radio signal. Hewish thought it was some form of alien communication at first (an
artificial source). The signal was actually coming from the cores of supernova remnants; spinning
charged particles produce magnetic fields, emit a beam of radio emission from the magnetic poles.
The pulse is analogous to a lighthouse; when it faces us, we see a pulse. Not all neutron stars are
pulsars, if their beams are not aimed toward earth then they are not seen as pulsars.
14. What is time dilation? How was it proven to exist using pulsars in binary systems?
Binary pulsars were found to tick slower when closest to their centre of mass, proving the
prediction of General Relativity that time slows down in a strong gravity field, this is an example of
time dilation.Abinary star system consists of a high mass star and a pulsar; they found the ticking
of the pulsar slowed down when it was closest to the gravity field of its sister star.
15. What is a radio galaxy?
Radio galaxies are typically giant ellipticals with jets of radio emission from the galaxy’s nucleus.
16. What does "quasar" stand for? What do quasars look like in visible light? What do they
look like in radio light? What revealed that quasars are distant galaxies?
Quasar stands for Quasi-Stellar Radio sources. They look as bright as nearby stars, they have an
enormous radio emission. Their spectra is indicative of galaxies with huge redshifts so they are
billions of light years away. 17. What is currently believed to be the cause of the radio emission produced by radio galaxies
and quasars? What is the observational proof in support of this?
the bright star-like appearance is a huge accretion disk (caused by the black hole sucking up matter
of a nearby star) of swirling matter around a supermassive black hole (up to billions of solar
masses) at the galaxy’s centre. The strong radio lobes are a result of the strong magnetic field
produced from the black hole’s spin. The evidence: high res images of radio galaxies reveal huge
disks of hot gas and dust around a central dark object
18. What is the current explanation for why quasars are only seen in the distant Universe?
Since only distant quasars are seen, they may be at an infant stage, when galaxy cores were denser
(more fuel for black hole), as they age they will consume all of the available matter and eventually
become quiet calm galaxies we see near us.
19. What is the current explanation for why radio galaxies aren't as radio-loud as quasars?
All galaxies likely have a central black hole, but most are presently quiet, having consumed all
20. What is suspected to be at the centre of the Milky Way? What is the observational evidence
in support of this?
Ablack hole. Strange ongoing at the centre of our galaxy – expanding gas rings, bursts of star
formation and a brilliant central radio source.
21. What kind of telescope is the Chandra Observatory, and why is it in space?
Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, enabled
by the high angular resolution of its mirrors. Since the Earth's atmosphere absorbs the vast majority
of X-rays, they are not detectable from Earth-based telescopes; therefore space-based telescopes are
required to make these observations. Chandra is an Earth satellite in a 64 hour orbit, and its mission
is ongoing as of 2013.
22. What kind of object is Cygnus X-1? What is believed to be the cause of its X-ray emission?
How was this proven in 1972? At what observatory was this discovery made, and in what
The strongest Galactic X-ray source was discovered (Cygnus X-1) and proposed to be from an
accretion disk around a stellar black hole. Proof of black holes was found in 1972, when optical
images of Cygnus X-1 revealed a massive star in orbit around an unseen companion. The optical
images were obtained by a UofT professor at the David Dunlop Observatory in Richmond Hill,
Chapter 12 Exam Review Questions
1. What is the van Karman line? What was the first man-made object to breach it?
Who designed this object, and where did he end up working?
It is a line 100km above sea level, the boundary at which Earth’s atmosphere is
too thin for air navigation. The V2 rocket, a German WWII missile, became the
1st man-made object to breach the Karman line. Werner von Braun (forced into Nazi party) designed the V2. He later became one of the architects of the American space
2. How were the 1st photographs of Earth taken from space?
V2 rockets were used by the US military to take the 1 photos from space, they attached
a camera to the rocket.
3. What kinds of animals were first launched into space by the U.S (after the fruit
flies)? What about the Soviet Union?
The U.S. sent monkeys into space, very few survived. The Soviet Union was sending
dogs into space, they had more success with the dogs than the US had with monkeys,
mostly due to less technical problems (no necessarily the result of different animals being
4. Which country launched the first satellite into Earth-orbit, and what was this
The soviet union launched the first satellite, it was called Sputnik 1
5. Which country launched the 1st human into space? Describe how this trip was
The Soviet Union launched the 1 human into space. They created a space pod with no
controls, not meant to be piloted.Asingle person would be placed in the pod, attached to
the nose of a rocket, launched into space, ejected and would land back on earth
6. Which country made the first unmanned landings and orbits of the Moon? What
did the Luna 3 take photographs of for the 1st time?
The Soviet Union landed 3 Luna crafts (Luna 1, 2 and 3). Luna 1 was made to orbit the
moon, Luna 2 was made to impact the moon and Luna 3 was made to fly by the moon. Luna 3
took photographs of the moon ’sfar side for the first time.
7. Why can't we see the far side of the Moon from Earth? Why is the far side more
cratered than the near side?
We can’t see the far side of the Moon from Earth due to the Moon`s synchronous orbit,
its rotation period is equal to its orbital period. The far side is more cratered because
craters on the near side have been filled with lava and solidified into maria (dark regions),
likely due to the Earth`s gravitational pull on the lava when it was still liquid. The Earth’s
gravity would have pulled the lava to the near side to fill in the craters to make the near
side look smoother.
8. Which country made the first manned landing on the Moon? What was this name
of the astronaut who first stepped on the Moon? The United States landedApollo 11 on the Moon, and astronaut NeilArmstrong becomes
the first human to walk on the Moon
9. What is an orbiter? What is a lander? What is a rover?
-An orbiter goes into orbit around a planet or moon, intended to study a large area of a
planet or moon ex. Its atmosphere
-a lander lands on planet at a specific location and stays in that location to study the area
around it, it is dropped onto the planet by an orbiter.
- Rovers are dropped by orbiters onto a planet, rovers move around on a planet`s surface
10. Which planets in our solar system have been landed on by human-made crafts?
Were these missions manned or unmanned?
Venus: unmanned, probe and landers(Venera program).
Mars: unmanned, rover and probes and landers. (Mariner program was the probe, Viking
program was the landers, Spirit and Opportunity were the two rovers)
11. What did NASA's Space Shuttle consist of? What was its purpose?
It consisted of a rocket, orbiter & re-entry spaceplane . It completed over 100 missions
such as satellite maintenance, space experiments and trips to the International Space
12. What is the International Space Station? What is its purpose?
Ahabitable satellite for conducting space experiments. It was launched into Earth-orbit
in a joint-effort by 5 space agencies. It has been continuously occupied for over 10 years.
The station is suited for the testing of spacecraft systems and equipment required for
missions to the Moon and Mars.
13. What is the Canadarm? Where are Canadarms permanently installed, and what is
It is a 15 metre robotic arm and was used in a Space shuttle mission. Canadarms have
now been used in over 50 shuttle missions and are currently installed on the International
Space Station and in the shuttles. It transfers cargo and equipment. On the International
Space Station they are used to hold on to astronauts.
14. What have our space missions left behind around the Earth?
The world`s satellites and space missions have created a shell of debris around the Earth
(space junk). Now estimated in the tens of millions, the debris includes defunct satellites,
equipment & collision fragments
15. What discovery was made when Pete Conrad returned from theApollo 12 mission
to the Moon? Acamera that had been left on the moon for 2 years contained bacteria from Earth
that had survived the Moon’s harsh conditions. This proved: organisms can
survive in space
16. What are extremophiles? What are some of the extreme environments on Earth
where extremophiles are found?
Life forms which can survive extreme conditions (conditions that can`t sustain human
life) Extremophiles are the most likely forms of life to be found on other planets or
moons in our solar system. TheAntarctic is one example of where extremophiles can be
found, salty and acidic composition..
17. What is the Habitable Zone? Which planets in our solar system are in this zone?
The distance from a star at which a p