2023 Fall semester Astronomy 101 with Prof. Welsh

ASTR101 EXAM #2 INFORMATION and STUDY GUIDE

** Exam #2 MONDAY Oct 16, 2023 **
 (minor update made on Oct 11th)

This is a 50-question multiple-choice exam. Print out and bring the Gradescope bubblesheet scantron form (available in .pdf format on Canvas). Make sure it is clean, unfolded, and uncrumpled. If the bubblesheet cannot be automatically read by the scanner, it won't be scored and the student will earn zero points for the exam.
No calculators/iphones/smartphones/etc are allowed.

Before the exam starts, PRINT your name (first name then last) on the Gradescope bubble form. Please write very neatly and carefully. Gradescope uses an artificial intelligence algorithm to read your name. If it can't read your name, it may result in a delay in scoring your exam (and possibly a loss of points if you didn't follow instructions). Also carefully write your RedID number. Leave the Version bubble blank. Write "ASTR-101" in the "Section" box, and "Exam 2" in the "Other" box.

** The class lectures are by far the most imporant source of information for this exam. The lectures contain much more information than the class notes, and the notes are more relevant for the exam than the textbook.
The exam is based mostly on the class lectures and covers everything we discussed in class since the previous exam. The chapters in the textbook to study are:
+ Chapter 3 Radiation: Information from the Cosmos
+ Chapter 4 Spectroscopy: The Inner Workings of Atoms
+ Chapter 5 Telescopes: The Tools of Astronomy
 Many concepts and definitions from the first part of the course do carry over, but there will not be any questions about the ancient Greek astronomers or Copernicus for example. There will be no phases of the moon questions on this exam. But there will be at least one question about the seasons, a question about solar eclipses (especially an annular eclipse), a question about gravitational waves, and possibly a question or two that uses Kepler'r 3rd law since it is so important.

Don't forget the definitions of terms such as perihelion, speed of light, temperature, AU, eclipse, ellipse, ecliptic, eccentric, etc., and Occam's Razor. And especially don't forget the Kepler's 3rd law, Newton's law of gravity, and why we have seasons.

* Please be sure to understand the interplay between light and matter, such as:
+ the relation between frequency and wavelength
+ the relation between a photon's energy and its wavelength
+ the relation between electron energy levels and photon energy
+ the relation between spectral lines and electron energy levels
+ Wien's law
+ Stefan-Boltzmann law (luminosity of a blackbody)
+ the Doppler effect law for radial velocity.

* Be sure to know the definitions of atom/element, atomic number, isotope, ion, etc, and understand
- The energy of a photon, given its wavelength or frequency;
- How electrons can jump up/down energy levels in an atom and how this will absorb or emit a photon;
- Kirchhoff's laws;
- Planck's law of thermal radiation;
- How to calculate things using Wien's law;
- How to calculate things using the Doppler effect;
- How to calculate the luminosity of a star.
- Why we need telescopes for astronomy.
- Why we put telescope in space, even though it is extremely expensive.
- The different types of telescopes and their advantages
- The different types of instruments used on telescopes
- The reason we need BIG telescopes
- The diffraction limit and how the Earth's atmosphere affects resolution

Remember that the study guide is a guide and helps you focus your studying. You should not "study" the study guide - that makes no sense. You should study your lecture notes, the class notes, and the textbook. Then use the guide to check that you understand the material.
Specifically for Exam #2, here are examples of the kinds questions you should be able to answer. This is not meant to be a comprehensive or complete list.

- Which star is hotter, the blue-white star Rigel or the red star Antares? How can you tell?
- What causes the seasons on Mars? (hint: it is the same thing that causes seasons on Earth)
- What design is used for the biggest telescopes?
- Who invented the reflector type telescope?
- What is the difference between a convex and a concave lens?
- What modern techniques are used to make huge mirrors?
- How many nm (nanometers) in a meter? How many nm in a micrometer?
- How many microns in a meter?
- How fast is the speed of light in meters per second? In km/s?
- If a planet has an orbital period 3x longer than another planet, how do their semimajor axes compare?
- If a planet in a highly elliptical orbit revolves about the Sun in 182 days, on average, how close is it to the Sun compared to the Earth?
- If the mass of an object is reduced to 1/3 its original value, how will this affect the gravitational force?
- If the separation between two object is made five times larger, how does the force of gravity change?
- What is a ion? What is an isotope?
- What is photoionization?
- What is the electromagnetic spectrum?
- How are frequency and wavelength related in a wave of light?
- How are energy and wavelength related in a wave of light?
- How much energy does a wave of light contain?
- How does the energy in light depend on the frequency of the light?
- What are Kirchhoff's laws?
- What is a blackbody? Why is the concept important?
- What is Wien's law? What does it tell us?
- How do you tell the temperature of a thermal spectrum?
- If a spectral line is observed to be at a larger wavelength, is the object redshifted or blueshifted?
- What does the Doppler effect tell us? How?

- If a star has a surface temperature of T=2900 K, at what wavelength is the star the brightest?
- If the H-beta line from a galaxy is shifted to the red by 10 nm, what is the radial velocity of this galaxy? [the rest wavelength of H beta is 486.1 nm]
- How much brighter does a star become if you triple its radius and but keep the same temperature?
- How much brighter does a star become if its size is constant but its temperature is tripled?

- At what wavelength is the maximum intensity of the light emitted from a blackbody of temperature 5500 K?
- What is the radial speed of a star with respect to the Earth if it's H alpha absorption line is measured to be at 657.0 nm, when normally the line is at 656.3 nm?
- How much brighter does a star become if its size is constant but its temperature is tripled?
- If a star is moving toward the Earth at 73 km/s, at what wavelength will it's Halpha line be located? (note: this problem is harder than usual)

- How do we know what stars are made of?
- How do we know the temperatures of stars?
- How do we know how fast stars are moving?

- Why don't stars twinkle in space? What causes twinkling?
- What is the main function of an astronomical telescope?
- Why do we use telescope that can detect IR or UV or X-rays?
- Why are telescopes often put on the tops of mountains?
- Why are telescopes sometimes put into space?
- Why are modern large telescopes made using segmented mirrors?
- What two things make the Hubble Space Telescope so useful?
- What is a Newtonian reflector?
- What is adaptive optics?
- What is interferometry?
- What is a CCD?
- What is "photometry"?
* What is a gravitational wave?
* What is the difference between a total and an annular solar eclipse?
* If the Earth's equator were in the plane of the ecliptic, would we have seasons? In other words, if the Earth's equator and the Earth's orbit were exactly in the same plane, would there be seasons?

A FEW PRACTICE QUESTIONS FROM OLD EXAMS:

1. Which of the following is approximately the speed of light in a vacuum?
A) 186,000
B) 300,000
C) 300,000,000
D) 300,000 km/s
E) none of the above

2. The planet Mercury orbits the Sun in approximately 88 days. How many AU from the Sun is Mercury?
A) (1/88) squared
B) square root of (88/365)
C) cube root of (88/365)
D) cube root of {(88/365)*(88/365)}

3. The star Antares has a surface temperature of about 3500 K. If its temperature was 35,000 K, how much brighter (more luminous) would it be?
Assume the star's size remains the same.
A) 10 times
B) 100 times
C) 1000 times
D) 10,000 times
E) 100,000 times

4. Suppose an Astronomy 101 student discovers a planet that orbits the Sun at a distance of 10 AU. How long would it take for this planet to revolve around the Sun?
A) 10 years
B) 100 light years
C) 1000 years
D) 31.6 (=square root of 1000) years
E) 4.6 (= cubed root of 100) years

5. Suppose "The Daily Aztec" newspaper reports the discovery of a new planet. According to the news story, the temperature of the surface of the planet is minus 800 K. What conclusion can you draw about this report?
A) The planet must be many hundreds of AU away from the Sun.
B) The planet has no atmosphere.
C) The planet must be at the aphelion of its orbit.
D) The planet must be a Kuiper Belt Object.
E) The report is erroneous since it is impossible to have a negative absolute temperature.

6. What does the astronomical term "seeing" mean?
A) the combined effects of flaws in the telescope optics and CCD
B) the twinkling and blurring of an image due to turbulent air currents in the Earth's atmosphere
C) the amount of haze or thin cloud in the atmosphere that affects the brightness of an image seen through a telescope
D) the effect of Cassegrain nebulosity, that can cause a shepherding equinox effect in the Cassini division
E) light pollution glare that blinds the prime focus of a telescope

7. Approximately at what wavelength is the Sun the brightest?
A) 5 nm
B) 500 nm
C) 2,000,000 nm
D) 70 nm
{To do the above problem, you need to know Wien's law and the temperature of the Sun's photosphere. And don't forget to use units of nanometers, not meters! Also, for the semester that this question was asked, students could use a calculator.}

8. Consider a star with a "surface" (photospheric) temperature of 58,000 K. At what wavelength is the intensity of its thermal radiation the greatest?
A) 5.0 meters
B) 58 m
C) 50 nm
D) 500 nm
E) 50,000 nm
{Note: For the semester that this question was asked on the exam, students could use a calculator.}

9. Which of the following is a speed? In other words, which of these has units of speed or velocity?
A) (0.10 nm / 656.3 nm) x 300,000
B) 656.4 nm - 656.3 nm
C) (-0.1 nm) x 300,000 km/s
D) (-0.1 nm / 656.3 nm)
E) (0.1 nm / 656.3 nm) x 300,000 km/s

10. The semimajor axis "a" of an ellipse is
A) the distance from the center to one focus of the ellipse.
B) half the distance between the foci of the ellipse.
C) the distance from the center of the ellipse to one end, along the largest diameter of the ellipse.
D) the distance from one focus to any point on the circumference of the ellipse.

11. Launching a telescope into space can be exceedingly expensive and risky. Yet there are several observatories in space. What is the MAIN (most important) reason that telescopes are put into space?
A) to be closer to the stars and planets
B) to get above the blurring effects of the Earth's atmosphere
C) since there is no gravity in space, the telescope is much easier to operate
D) since it is always dark in space the telescope can be used anytime, we don't have to wait until night
E) to be able to detect light that cannot penetrate through the Earth's atmosphere

[hint: The answer questions #11 is not choice (B).]

A student observes the spectral lines from a distant star for part of her senior thesis project. She sees both the H beta line (level 4 to level 2 transition in hydrogen) and the H alpha line (level 3 to 2). The H beta line is measured to be at 486.5 nm. In the lab, the rest wavelength is 486.1 nm.
- Is the spectrum redshifted or blueshifted (or not shifted at all)?
- How fast is the star moving, and it is heading towards us or away?
- Does the H beta photon contain more or less energy than the H alpha photon?
- Does the H beta photon have a longer or shorter wavelength than the H alpha photon?
- Does the H beta photon have a lower or higher frequency than the H alpha photon?

General hints for doing well in this course:
- Ask questions in class if you are unsure of something.
- The class lectures are the most important source of information.
- Read the textbook before the material is discussed in class.
- Go to the Astronomy Help Room to practice and get help
- Read the "Chapter Review Summary" section at the end of each chapter several times.
- Answer the questions in the textbook, especially the multiple choice questions
- Attend Zoom office hours if you have any questions.
- Read the additional material from on-line sources (those links in Canvas) to help reinforce ideas and concepts
- Go to the Astronomy Help Room - go! Let the TA's help you earn an A grade!