Astronomy 121 - Exam 1 Review

Questions:

1 Distance Scales & Scientific Notation

• Be able to express very large numbers into scientific notation e.g. 1, 232, 000, 000 = 1.232×109.

• Be able to express very smal numbers into scientific notation e.g. 0.00000453 = 4.53 × 10−6.

• Know the basic prefixes for shorthand expression of large and small numbers:

Table 1: Scientific Prefixes

Prefix Figures Scientific English

giga 1,000,000,000 109 1 billion

mega 1,000,000 106 1 million

kilo 1 000 103 1 thousand

centi 1/100 10−2 1 hundredth

milli 1/1 000 10−3 1 thousandth

micro 1/1 000 000 10−6 1 millionth

nano 1/1 000 000 000 10−9 1 billionth

• What is an astronomical unit (A.U.)?

• What is a light year?

• How is it we see history in astronomy?

2 The Scientific Method

• What are the steps of the scientific method?

• Be able to argue if a study follows the method or is rather pseudoscience.

• What is the difference between a theory and a hypothesis?

• What is the placebo effect? Is it real?

• What are some of the fallacious arguments used to strengthen pseudoscience arguments?

• What is a Fermi problem?

3 Motions in the Sky

• Define the following terms: angular size and angular separation.

• Define the following terms: constellation, asterism, north and south celestial poles, the celestial

sphere, celestial equator, right ascension, and declination.

• Define the following terms: altitude, horizon, meridian, and zenith.

• Which star is directly above the Earth’s north pole? How does it relate to your latitude?

What does circumpolar mean?

• Would I see circumpolar stars at the equator? The north pole? What is it like in between?

• Be able to determine the range of circumpolar, rise and set, and never rise given a particular

latitude.

• Do you see the full sky from all latitudes?

• What is the difference between sidereal and synodic?

• what is the celestial sphere? How does it move daily?

• A star sets at 10 pm. What time will it set approximately tomorrow?

• Define the following terms: ecliptic and zodiac.

4 Planetary Laws

• Why did Tycho Brahe and the Greeks conclude that the geocentric model was correct based

on parallax? Why didn’t they observe it?

• What useful work did Tycho Brahe do to aid Kepler?

• What are Kepler’s three laws of motion? What is the meaning of each? How did he formulate

them?

• What does Kepler’s third law look like in A.U. and years?

• Define ellipse, focus, eccentricity, aphelion, perihelion, semi-major axis, period.

• How are the average distance of a planet on its orbit and semi-major axis related?

• What does the second law really say about the speed of a planet on its orbit?

• Planet X is discovered! It is determined to be an average distance of 100 A.U. from the Sun.

What is its period?

• Planet Y is discovered! It is determined to have a period of 8 years. What is its average

distance from the Sun in A.U.?

• What is Newton’s modified version of Kepler’s third law?

• The Moon has a period of 27.5 days. It is a distance of 0.00257 A.U. from the Earth. What

is the Earth’s mass in terms of solar masses?

• What is meant by geosynchronous or geostationary?

5 Newton’s Laws & Gravity

• What is velocity? What is acceleration? What are their metric units?

• What is the difference between weight and mass? What are their metric units?

• Write down Newton’s three laws of motion. Give an example of the application of each.

• Typically in the terrestrial environment, if an object doesn’t move, what does that say about

the forces operating on it? Does a planet moving at a constant speed on a circular orbit

accelerate? Is there a force?

• What is Newton’s universal law of gravitation?

• How many times more or less is the gravitational force between the Earth and the Sun if I

triple the distance between them?

• How about if I instead triple the mass of the Earth?

• How many times more or less is the gravitational force between the Earth and the Sun if I

halve their distance?

• Why are astronauts weightless when they are really not that far from the surface of the Earth

where they should be feeling its gravity?

• What is meant by escape speed?

• How was Neptune discovered? What makes this a good test for Newton’s gravity hypothesis?

6 The Nature of Light

• How did Galileo attempt to measure the speed of light?

• Explain how Roemer measure the speed of light?

• Explain how the further out into space we look, the further back in time we are seeing.

• What is a spectrum?

• How was light demonstrated to be a wave?

• How did Newton show that a prism separates light?

• Who is James Clerk Maxwell? What did his theory predict?

• What is an electric field? A magnetic field?

• Sketch an electromagnetic wave and explain how it can propagate without a medium.

• A radio station broadcasts at 93.1 mHz (mHz = megahertz). What is the wavelength of the

radio waves broadcast?

• What is a photon?

• what is the electromagnetic spectrum? What are the major bands?

• Write down the main bands of the electromagnetic spectrum in order of largest to smallest

in terms of A) energy, B) wavelength, and C) frequency.

• What are the orders of the colors in the visual (optical) part of the E+M spectrum in order

of largest to smallest in terms of A) energy, B) wavelength, and C) frequency?

7 Generation of Light

• Briefly describe the structure of an atom.

• Explain how emission lines are formed in terms of the Bohr theory of the atom.

• Explain how absorption lines are formed in terms of the Bohr theory of the atom.

• Under what circumstances can an electron transition to a higher energy level?

• Under what circumstances can an electron transition to a lower energy level?

• What is meant by the ground state? What is an excited state? What state do most atoms

prefer to be in?

• How can we identify the chemical elements that might be present by looking at a spectrum?

• What is a black body spectrum? What circumstances lead to a blackbody spectrum?

• Emission line spectrum? Absorption line spectrum?

• What is Wein’s law? What is the Stefan-Boltmann law?

• A star has a temperature of 3000K. What is the wavelength of its peak emission?

• I observe two stars, A and B. Star A appears reddish. Star B appears blueish. Which one of

these stars is likely hotter? Explain your reasoning.

• One star is 3 times the temperature of another but exactly the same size. What are their

relative brightnesses?

• One star is the same temperature as another but 3 times its size. What are their relative

brightnesses?

• What is ionization? Does it exhibit specific emission (absorption) lines?

• Name the properties that observing the spectrum (emission or absorption line) can tell us

about the objects that we are viewing.

• Describe the Doppler effect and what we can learn from it.

• What is meant by the term blueshift? Redshift?

• How do we determine the rate at which a star rotates?

• A certain star emits an Hβ line at 486.112 nm. A laboratory measurement of this line shows

that its normal wavelength is 486.133 nm. Is this star moving towards or away from us? At

what speed (c = 300,000 km/s)?

• What is the inverse-square law?

• A 100W light bulb is viewed from 1 m and 3 meters. Is it brighter/dimmer at 3m compred

with 1m? By what factor?

8 Telescopes

• What are the three properties of a telescope and which is the most important?

• How does the amount of light collected scale with area?

• Suppose I triple the diameter of a telescope. How many times more light does it collect?

• What is a refracting telescope? What is a reflecting telescope? Why do modern astronomers

mostly use reflecting telescopes?

• what are the two main types of reflecting telescopes and how do they differ?

• Suppose your reflector telescope has an objective mirror 20 cm in diameter with a focal length

of 2 meters.

• What magnification do you get for a 2m reflector telescope with eyepieces whose focal lengths

are: A) 9 mm B) 20 mm C) 55 mm?

• What is the current state of the art telescope in the U.S.? How was it used to determine the

mass of the black hole in the center of our galaxy?

• what sizes roughly will be the next generation of large telescopes?

• How do we create mirrors for the largest telescopes?

• What is Hubble? What is JWST?

9 Astronomical Instrumentation

• What are the advantages of CCD cameras over photographic plates or your eye as radiation

collectors for telescopes?

• What is the function of a spectrograph?

• What are adaptive optics and what do they aid a telescope in doing?

• What are the problems posed using Earth-based telescopes? Where might we place a telescope

to best overcome this issues?

• What advantages exist for placing telescopes in space?

• If you are to build an observatory, describe what your telescope may be like and where you

would locate it in terms of terrain and location.

• How do astronomers typically create color images? Why do they use black and white CCDs?

• How do radio astronomers achieve comparable resolution to optical telescopes?

• What is very long baseline interferometry (VLBI)? What has it been used for in the center of

our galaxy and that of M87?

10 The Sun’s Atmosphere

• How does the size of the Sun compare to that of the Earth?

• What is the composition of the Sun roughly?

• Describe the following regions of the Sun’s atmosphere: photosphere, chromosphere, transition

region, corona, & the solar wind.

• Explain the origin of most of the activity associated with the Sun.

• What is a magnetic field line?

• What are sunspots? Why are they cooler than their surroundings? How was it determined

they are related to the Sun’s magnetic field?

• How can sunspots tell us the Sun’s rotation? How does the rotation differ along its equator

compared with the poles?

• What are spicules?

• what are plages? Filaments? Be able to identify them on the surface of the Sun.

• What is a prominence?

• What is a flare?

• What is a coronal mass ejection?

• How do these phenomena relate to space weather? How can they impact us on Earth?

• How does solar activity relate to the Sun’s magnetic field?

• What is the sunspot cycle? What is solar minimum and maximum?

• How does the cycle relate to the Earth’s climate?

• What happens at solar maximum to fix the Sun’s magnetic field?

• What protects us on Earth from the solar wind?

• How are aurorae formed?

11 The Sun’s Interior

• What is thermonuclear nuclear fusion?

• Why does nuclear fusion require high temperatures and pressures to operate?

• Explain the process of how energy is formed in the Sun including the various reactions that

take place. What is the name of this reaction chain?

• Describe how energy is transported from the core to the surface.

• What mechanisms were proposed for powering the Sun? Why did they mostly fail? How did

Einstein solve the mystery?

• How long does it take photons created in the Sun’s interior to reach the surface on average?

Why does it take so long?

• What are granules?

• Make a sketch, labeling the major regions, of the Sun’s interior.

• What are neutrinos? What is antimatter?

• What are the three main ways astronomers study the interior of the Sun?

• What is the solar neutrino problem? How was it resolved? How are neutrinos detected?

Answer Key:

1. Distance Scales & Scientific Notation

- Scientific notation is a way to express large and small numbers using powers of ten.
    - Astronomical Unit (A.U.) is the average distance from the Earth to the Sun (~149.6 million km).
    - A light-year is the distance light travels in one year (~9.46 trillion km).
    - We see history in astronomy because light from celestial objects takes time to reach us.

2. The Scientific Method

- Steps: Observation, Question, Hypothesis, Experiment, Analysis, Conclusion.
    - A theory is a well-tested explanation; a hypothesis is an untested educated guess.
    - The placebo effect is a psychological effect where people experience changes due to belief in treatment.
    - Common pseudoscience fallacies include anecdotal evidence, cherry-picking data, and lack of peer review.
    - A Fermi problem is a rough estimation problem using approximate values.

3. Motions in the Sky

- The celestial sphere is an imaginary sphere surrounding Earth where celestial objects appear.
    - Circumpolar stars never set and are visible all night, depending on latitude.
    - The North Star (Polaris) is nearly aligned with Earth’s rotational axis.
    - Sidereal time is based on the stars; synodic time is based on the Sun.
    - The celestial sphere moves due to Earth's rotation.
    - A star that sets at 10 PM will set 4 minutes earlier each night due to Earth's orbit.

4. Planetary Laws

- Tycho Brahe's precise observations helped Kepler develop his laws.
    - Kepler's Laws:
      1. Planets follow elliptical orbits with the Sun at one focus.
      2. Planets sweep equal areas in equal times (varying speeds).
      3. P² = a³ (Orbital period squared is proportional to semi-major axis cubed).
    - A planet at 100 A.U. has a period of ~1000 years.
    - Newton’s modification accounts for masses of objects.

5. Newton’s Laws & Gravity

- Newton’s Laws:
      1. Objects remain in motion unless acted upon.
      2. F = ma (Force equals mass times acceleration).
      3. For every action, there is an equal and opposite reaction.
    - Weight depends on gravity, mass does not.
    - Doubling distance reduces gravitational force by a factor of 4.
    - Astronauts appear weightless because they are in free fall.
    - Escape speed is the speed needed to break free of gravity.

6. The Nature of Light

- Light is a wave and a particle (photon).
    - The electromagnetic spectrum includes gamma rays, X-rays, UV, visible light, infrared, microwaves, and radio.
    - Blueshift indicates motion toward us; redshift indicates motion away.
    - Doppler effect helps measure celestial motion.

7. Generation of Light

- Emission and absorption spectra help identify elements.
    - Wein’s Law: λ_peak = (2.9 × 10^6) / T.
    - A hotter star emits more blue light; cooler stars emit red.
    - Stefan-Boltzmann Law: L = σT⁴.

8. Telescopes

- Modern telescopes are mostly reflecting.
    - Light-gathering ability increases with telescope area (D²).
    - Space telescopes avoid atmospheric distortion.
    - Hubble and JWST are major space telescopes.

9. Astronomical Instrumentation

- CCDs are better than photographic plates.
    - Spectrographs analyze starlight.
    - Adaptive optics correct for atmospheric distortion.
    - Large telescope arrays improve resolution (VLBI).

10. The Sun’s Atmosphere

- The Sun has layers: photosphere, chromosphere, corona.
    - Sunspots are caused by magnetic fields.
    - Solar activity affects space weather.
    - Auroras are caused by solar wind interactions with Earth's atmosphere.

11. The Sun’s Interior

- Energy is generated by nuclear fusion (proton-proton chain).
    - Energy takes thousands of years to reach the surface.
    - Neutrinos help us study the Sun’s core.