ASTR1003 Stars Practice Exam Questions

Q1: How does the fact that objects can look the same size in the sky even if they're different distances away make it tricky for astronomers to figure out how big things in space really are?

• Objects can appear the same angular size despite vast differences in distance

• Example: Sun and Moon look similar in size but Sun is much larger

• Astronomers must use tools like triangulation and telescopic data to determine actual sizes

Q2: How does the parallax method work for measuring distances in space, and what challenges arise when trying to apply it to celestial objects with small parallax angles?

• Uses observations from two positions to form a triangle and calculate distance

• Parallax angles are extremely small for distant stars

• Challenges: atmospheric distortion, Earth’s curvature, need for precise timing

Q3: How did Aristarchus and Hipparchos use a solar eclipse to successfully estimate the distance to the Moon?

• Observed solar eclipse coverage from two locations

• Noted partial vs total eclipse at different sites

• Used geometry and timing to calculate parallax angle and Moon’s distance

Q4: How did astronomers in the 17th century attempt to measure the distance to the Sun, and what key realizations and technological advancements led to a more precise determination of the solar distance in the 18th century, particularly through the observation of Venus transits?

• Early methods used Moon phases at sunset

• 18th-century breakthroughs: heliocentric model, parallax, telescopes with micrometers

• Venus transits (1761 & 1769) enabled precise global measurements (~3% accuracy)

Q5: What significant characteristics of the Sun did astronomers discover, and how do these revelations reshape our understanding of the Sun's place in the solar system?

• Sun is far away, enormous, and energetically powerful

• Central to solar system both in mass and influence

• These insights changed the perception of Earth’s cosmic position

Q6: How did Isaac Newton's concept of universal gravity revolutionize our understanding of forces and what practical applications have emerged from this understanding?

• Gravity acts on all objects, not just on Earth

• Follows inverse-square law based on mass and distance

• Used in archaeology, hidden structure detection, GRACE satellites measure Earth’s gravity changes

Q7: How did Henry Cavendish's experiment contribute to our understanding of the Earth and Sun, and what insights did it provide about their compositions?

• Measured gravitational force to determine Earth’s mass and density

• Found Earth’s core likely contains heavy elements (iron/nickel)

• Sun’s low density suggests a different, hotter composition

Q8: What is a spectrum and how can you measure it?

• Spectrum = range of electromagnetic wavelengths

• Measured using prisms/diffraction gratings and spectrometers

• Provides info on composition, energy, and temperature

Q9: How does the overlapping sensitivity of the medium and long-wavelength receptors in human eyes contribute to colour perception, and how can colour vision be approximated using specific wavelength ranges?

• Medium & long receptors overlap in yellow-green-orange range

• Enhances sensitivity to slight colour differences

• Colour ranges: Blue (400–480 nm), Green (480–580 nm), Red (580–700 nm)

Q10: How do digital colour cameras capture images in astronomy, and how does the use of filters in cameras like the MastCam on the Mars Curiosity Rover impact the representation of colours in images?

• Use Bayer masks and filters for RGB channels

• Images often combined from separate filtered exposures

• Special filters capture non-visible wavelengths (e.g., infrared), often shown in false colour

Q11: Explain how the spectrum of a person, standing near you, is composed, considering both the reflected light from an incandescent light bulb and the emission from the person.

• Reflected visible light from bulb

• Thermal emission in mid-infrared

• Spectrum shows visible peak (reflected) + infrared peak (emission)

Q12: What causes the type of spectrum known as a Black Body Spectrum, and how can you use one to determine an object's temperature?

• Result of thermal equilibrium in dense, opaque object

• Smooth continuum spectrum with peak wavelength shifting with temperature

• Peak wavelength → object’s temperature

Q13: Why do hot gases exhibit emission lines in their spectra, and what information do these emission lines provide about the gas?

• Emission lines occur at specific wavelengths unique to elements

• Arise from electron transitions between energy levels

• Reveal gas composition and conditions (e.g., temperature)

Q14: Why do gases exhibit emission lines in their spectra, and how is this related to the energy levels of electrons in atoms?

• Electrons are limited to specific energy levels

• Transitions emit photons with energies matching level differences

• Lines provide data on element type, ionization, temperature

Q15: What are absorption lines in a spectrum, how are they formed, and what information can be obtained from them?

• Dark lines in spectrum caused by gas absorbing specific wavelengths

• Result from light passing through a cooler, transparent gas

• Identify elements and ionization states of the gas

Q16: What did Meghnad Saha and Cecilia Payne-GaposchkinWhat contribution did Meghnad Saha and Cecilia Payne-Gaposchkin make to understanding the composition of the Sun, and what surprising discovery did Cecilia Payne-Gaposchkin make about the Sun's elemental composition? contribute to our understanding of the Sun’s composition?

• Saha: linked ionization states to temperature using absorption lines

• Payne-Gaposchkin: used quantum mechanics to analyze the Sun’s spectrum

• Surprising discovery: Sun is ~80% hydrogen and ~20% helium, unlike Earth

Q17: Why does the theory that the Sun is cooling face significant challenges and what evidence contradicts the idea that the Sun is simply cooling down over time?

• If true, the Earth would have experienced extreme heat in recent history

• No historical evidence of boiling oceans or widespread burning

• Solar irradiance measurements show no major decline

Q18: Why can't chemical reactions, particularly combining hydrogen with oxygen, serve as the primary energy source for the Sun's sustained brightness?

• Hydrogen + oxygen reactions produce too little energy

• Not enough oxygen in the Sun to sustain fusion via chemistry

• Sun’s energy comes from nuclear fusion, not chemical burning

Q19: Explain the fundamental principle behind potential energy and its connection to kinetic energy. Illustrate this concept using the example of a roller-coaster and its potential and kinetic energy throughout its motion.

• Law of Conservation of Energy: energy transforms but isn’t lost

• Potential energy = stored energy due to position (e.g. rollercoaster at top)

• As it falls, potential converts to kinetic (motion) energy

• Total energy remains constant, shown as a flat line on a graph

Q20: Explain Lord Kelvin's theory on how gravity could power the Sun. How did he propose the Sun's energy was generated?

• Proposed gravitational contraction: Sun shrinks and heats up

• Shrinking releases gravitational energy

• Could power the Sun for a few million years

Q21: Why did Kelvin's theory, which suggested the Sun's energy came from gravitational energy released during its shrinkage, face challenges in reconciling with the geologists' estimates of Earth's age based on sedimentary rock layers?

• Theory implied Sun was only millions of years old

• Geology showed Earth needed to be billions of years old

• Inconsistent with sedimentary rock timelines

Q22: How did Ernest Rutherford's experiments challenge the prevailing atomic model, and what was the key discovery?

• Alpha particles mostly passed through gold foil, but some bounced

• Discovery: atoms have a small, dense, positively charged nucleus

• Disproved plum pudding model of atoms

Q23: What does the chart of atomic nuclei reveal, and how does the Strong Force contribute to the stability of atomic nuclei?

• Shows stable/unstable proton-neutron combinations

• Strong Force binds protons & neutrons despite their charge repulsion

• Acts over short distances, mediated by mesons

Q24: Explain how the Strong Force, electromagnetic repulsion, and the Pauli Exclusion Principle influence the stability of atomic nuclei as they increase in size.

• Strong Force: attractive, short-range; stabilizes small nuclei

• Electromagnetic repulsion: destabilizes large nuclei (more protons = more repulsion)

• Pauli Exclusion: limits particle arrangements, requiring more neutrons in larger nuclei

Q25: How does nuclear fusion occur in the Sun, and what is the significance of the proton-proton (PP) chain in the fusion process?

• High temperature/pressure allow protons to overcome repulsion (via quantum tunneling)

• Proton-Proton chain: forms helium from hydrogen via several steps

• Final step: ³He + ³He → ⁴He + energy — main energy source of the Sun

Q26: What are the key features of fusion power on Earth, and why do some physicists believe fusion will become increasingly important as an energy source?

• Uses deuterium/tritium; produces neutrons that heat water for turbines

• No greenhouse gases; safer than fission

• Challenges: achieving net positive energy output

• ITER aims to demonstrate practical fusion in 20–30 years

Q27: How do astronomers estimate the internal conditions of the Sun, considering that they cannot directly observe its interior, and what are the key elements in constructing a model of the Sun's composition, energy sources, and heat flow?

• Build models using physics and observational data

• Use spectroscopy to find composition (~75% H, ~24% He)

• Simulate layers with fusion in the core and outward heat transfer

• Validate using surface temperature (5778 K) and energy output

Q28: How is pressure generated in the Sun, and what role does it play in maintaining the stability of different layers of gas within the Sun?

• Caused by fast-moving particles colliding (due to high temperature and density)

• Balances gravity pulling layers inward

• Pressure must increase deeper into the Sun to keep layers stable

Q29: What are the key features of the "standard solar model," and how does it explain the Sun's structure and energy production?

• Includes surface conditions, temperature/density profiles

• Explains fusion in the core, radiative/convective heat flow

• Predicts mass and energy distribution using known physical laws

Q30: How does the Sun's core differ from the outer layers in terms of temperature, density, and energy production?

• Core: 15 million °C, high density, site of fusion

• Radiative zone: energy moves slowly via radiation

• Convective zone: heat circulates by rising/falling gas

Q31: Why did the HomestakeWhy did the Homestake neutrino experiment, initially designed to detect solar neutrinos, observe only one-third of the expected neutrino count? neutrino experiment detect only 1/3 of expected neutrinos?

• Detected only electron neutrinos

• Neutrinos change type en route to Earth (neutrino oscillation)

• Later experiments (Sudbury, Super-Kamiokande) confirmed total count matched predictions

Q32: How do scientists study the vibrations in the Sun, and what method is used to measure these vibrations?

• Study surface oscillations using Doppler effect

• Blueshift = inward motion, Redshift = outward motion

• Use ground telescopes & space missions (e.g. SOHO)

• Frequencies give insight into Sun’s interior (helioseismology)

Q33: What is the photosphere, and why is it challenging to observe the outer layers of the Sun at optical wavelengths?

• Photosphere: Sun’s visible “surface,” where photons escape

• Low density (~0.2 g/m³), not a solid surface

• Intense light from photosphere obscures outer layers

• Eclipse helps observe corona and chromosphere

Q34: Explain why plasmas are strongly affected by magnetic fields, and how this interaction is relevant in the context of the Sun's corona.

• Plasma = charged particles → interact with magnetic fields

• Move in spirals along field lines, forming structures like flux tubes

• Explains corona’s shape and behavior (studied in magneto-hydrodynamics)

Q35: What causes sunspots, and how are they connected to the Sun's magnetic field?

• Cooler, dark regions (~4000 K) caused by magnetic field flux tubes

• Usually appear in pairs (field emerges and re-enters)

• Associated with plasma loops and X-ray activity

Q36: Describe the solar cycle and its impact on sunspot activity.

• ~11-year cycle with sunspot number variation

• Solar max = many sunspots, active corona

• Sunspots migrate from high latitudes toward equator

• Visualized using “butterfly plots”

Q37: Is there a significant link between solar activity, particularly sunspot cycles, and Earth's climate change? What is the observed impact of changes in solar radiation on Earth's temperature, and are there valid claims regarding indirect mechanisms connecting solar variations to climate changes?

• Solar activity has minor impact (~0.1 °C variation)

• Climate change mainly due to human factors

• Indirect mechanisms (UV, cosmic rays) lack strong evidence

Q38: What drives the solar dynamo, and how does differential rotation contribute to the Sun's magnetic field changes?

• Driven by differential rotation (equator rotates faster)

• Twists magnetic fields into rings → sunspots

• Babcock-Leighton model explains magnetic polarity reversals every ~22 years

Q39: What are the two possible mechanisms suggested to explain the unusually high temperature of the Sun's corona, and why are they considered in the quest to solve this mystery?

• Magnetic plasma waves: accelerate as they rise

• Magnetic reconnection: field lines snap and release energy

• Parker Solar Probe is investigating this mystery

Q40: What is the solar wind, and how does it differ in terms of speed and origin based on the Sun's geography? Briefly explain what Coronal Mass Ejections (CMEs) are and how they distinguish themselves from regular solar wind.

• Solar wind: continuous charged particle flow

  • Slow (~400 km/s) near equator; Fast (~750 km/s) near poles

• CMEs: explosive ejections of plasma, more energetic and impactful

Q41: How does the solar wind influence comets, and what happens when the solar wind interacts with the Earth's atmosphere? Additionally, what potential effects can a coronal mass ejection (CME) have on Earth, particularly in terms of the aurora and technological disruptions?

• Solar wind creates comet tails

• On Earth: causes aurorae near poles

• CMEs: trigger widespread aurorae, disrupt power grids, affect satellites

Q42: How did galaxies, including our own Milky Way, form after the Big Bang, and what role did gravity play in shaping the structure of the early universe? Additionally, how do stars contribute to the creation of heavy elements that enrich the interstellar gas within galaxies?

• Gravity collapsed early dense regions into galaxies

• First stars made heavier elements via fusion/supernovae

• Stellar deaths enrich interstellar gas → new star formation

Q43: What is the "Jeans Mass," and how does it influence the collapse of gas clouds to form stars?

• Minimum mass needed for a gas cloud to collapse under gravity

• If mass > Jeans Mass → collapse forms a star

• Explains why stars, not planets, form in large clouds

Q44: Why do simple models of star formation, which convert most of the gas in a giant molecular cloud into stars quickly, not align with observations, and what role does feedback, radiation, and magnetic fields play in shaping the efficiency of star formation?

• Real star formation is slower and less efficient than models suggest

• Feedback: radiation, magnetic fields, and jets slow the process

• Helps prevent entire clouds from collapsing too quickly

Q45: How do scientists determine the age of the solar system, and what role do radioactive isotopes, specifically Uranium isotopes, play in this dating process? Describe the lead-lead dating technique, focusing on the use of lead isotopes in meteorites, and explain how the relative proportions of different lead isotopes in crystals help calculate the time since the rocks solidified.

• Use radioactive dating (e.g. Uranium → Lead decay)

• Lead-lead dating in meteorites (e.g. chondrules, CAIs)

• Consistent results ~4.568 billion years

Q46: How are isotope ratios measured in laboratories, such as Trevor Ireland’s lab, and what types of samples are analyzed?

• Tiny samples prepped and mounted

• Use mass spectrometry to separate isotopes

• Oxygen plasma → ionizes atoms → analyzed by magnets/electric fields

• Lead isotopes need separate process with chemical concentration

Q47: What are the key phases in the future evolution of the Sun, and what happens during the "Helium Flash"?

• Red Giant → Helium Flash (invisible core explosion)

• Becomes Yellow Subgiant → second Red Giant

• Final stage: white dwarf surrounded by planetary nebula