A star in constellation Orion is labeled b-Orion. This indicates that A: The star is the second brightest in the constellation
How many sections or constellations is the sky divided into by astronomers? A: 88
Polaris seems not moving because A: It is at the extension of Earth rotation axis
Which statement correctly describes the sky over the Earth? A: The sky appears different in different months
Which statement is WRONG about celestial sphere? A: It has celestial coordinates latitude and longitude
How long is one sidereal day? A: 24 hours - 3.9 minutes
What is ecliptic? A: The plane defined by Earth orbit around the Sun
Earth has seasons because A: Earth's spin axis is tilted 23.5 degree with respect to its orbital plane
On the equinox day, which of the following statement is correct? A: Everyplace in the world has about 12 hours of daylight and 12 hours of night
What is the length of time needed for the Earth's rotation axis to complete one cycle of precession? A: 26,000 years
In waxing phases of the Moon, what is the correct sequence of appearance of lunar phases? A: new moon, crescent, quarter, gibbous, full
Which of the following is the correct position alignment when lunar eclipse occurs? A: Sun, Earth, Moon
Which is correct position alignment when solar eclipse occurs? A: Sun, Moon, Earth
Hardcore
You observe a particular star directly at the zenith at exactly 9:00 PM tonight. One month from now, on a clear night, at what time will that same star next be at the zenith? A: About 7:00 PM β sidereal day is ~4 minutes shorter than solar day.
Why do we always see the same face of the Moon from Earth? A: The Moon's rotation period equals its orbital period around Earth.
FRQs
Why does Polaris not appear to move while other stars trace circles? Model: Polaris sits almost exactly on the extension of Earth's rotation axis (the north celestial pole). As Earth spins, the celestial sphere appears to rotate around that axis, so Polaris stays nearly fixed while all other stars trace concentric circles around it. π **Textbook βΈ Ch 2.1:** The north celestial pole's altitude equals the observer's latitude; stars within that angle of the pole are *circumpolar* and never set. Polaris currently lies very close to the pole.
What are the celestial coordinates and how are they measured? Model: They are the sky analogue of latitude and longitude. **Declination** is the angle north (+) or south (β) of the celestial equator, measured 0Β° to Β±90Β° in degrees, arcminutes, and arcseconds. **Right ascension** is the angle around the celestial equator measured eastward from the vernal equinox, expressed in hours (0β24h, where 1h = 15Β° of arc). π **Textbook βΈ Ch 2.1:** RA's "Greenwich" is the *vernal equinox* β the point where the ecliptic crosses the celestial equator. The whole sphere appears to rotate once every 23 h 56 min (one sidereal day).
Is a solar day longer or shorter than a sidereal day, and why? Model: A solar day (24 h, noon to noon) is **longer** than a sidereal day (~23 h 56 min, one full 360Β° rotation relative to the stars). During one sidereal day Earth also moves ~1Β° along its orbit, so it must rotate ~4 extra minutes to bring the Sun back to the meridian. π **Textbook βΈ Ch 4.3:** "A solar day is slightly longer than a sidereal day because Earth not only turns but also moves along its path around the Sun in a day." Civil clocks use mean solar time.
How many constellations are there? Why are Betelgeuse and Rigel also called Ξ± and Ξ² Orionis? Model: The sky is divided into 88 official constellations. Within each, the brightest stars are labeled with Greek letters in order of decreasing brightness β Ξ± (alpha) for the brightest, Ξ² (beta) for the second, and so on. In Orion, Betelgeuse is Ξ± Orionis and Rigel is Ξ² Orionis. π **Textbook βΈ Ch 2.1:** "The celestial sphere is organized into 88 constellations, or sectors." The Greek-letter labeling is the Bayer system (Johann Bayer, 1603), which uses the constellation's Latin genitive (e.g., *Orionis*).
How long is one cycle of precession of Earth's rotation axis? Model: About **26,000 years** for one complete precession cycle. π **Textbook βΈ Ch 2.1:** Because Earth bulges at the equator, the Sun and Moon "cause it to wobble like a top." About 5,000 years ago the pole star was Thuban (in Draco); in ~12,000 years it will be Vega.
List all waxing and waning lunar phases. Model: **Waxing** (illuminated portion growing): waxing crescent β first quarter β waxing gibbous β full moon. **Waning** (illuminated portion shrinking): waning gibbous β third (last) quarter β waning crescent β new moon. The full cycle takes about 28 days. π **Textbook βΈ Ch 4.5:** The exact period from new moon to new moon (the *synodic month*) is **29.53 days**, slightly longer than the Moon's 27.3-day sidereal orbit because Earth itself has moved along its orbit in that time.
What causes Earth's seasons? What is an equinox? Model: Seasons arise from Earth's **23.5Β° axial tilt**, not its distance from the Sun. The hemisphere tilted toward the Sun gets more direct sunlight and longer days (summer). An **equinox** ("equal night") occurs around March 21 and September 21, when Earth's axis is perpendicular to the Sun line, giving roughly 12 hours of day and night everywhere. π **Textbook βΈ Ch 4.2:** OpenStax explicitly debunks the distance hypothesis: "Earth is actually closest to the Sun in January, when the Northern Hemisphere is in the middle of winter." The vernal and autumnal equinoxes are when the Sun crosses the celestial equator.
Positions of Earth, Sun, and Moon during lunar vs. solar eclipses? Model: **Lunar eclipse:** Sun β Earth β Moon are aligned in that order; Earth's shadow falls on the full Moon. **Solar eclipse:** Sun β Moon β Earth are aligned; the new Moon's shadow falls on Earth, blocking the Sun. π **Textbook βΈ Ch 4.7:** The shadow has two parts β the dark central **umbra** (total eclipse) and the diffuse **penumbra** (partial eclipse). Eclipses don't happen every month because the Moon's orbit is tilted ~5Β° from the ecliptic.
Write in scientific notation. Model: 1. 1,000,000 = **1 Γ 10βΆ**
2. 300,000 = **3 Γ 10β΅** km/s (speed of light)
3. 6,500 = **6.5 Γ 10Β³** km (radius of Earth) π **Textbook βΈ Ch 1.4:** OpenStax stresses scientific notation because astronomy spans 40+ orders of magnitude β from sub-nuclear distances (10β»ΒΉβ΅ m) to the observable universe (~10Β²βΆ m).
Module 2 β Models of the Universe β Kepler & Newton
Weekly
Who proposed geocentric model? A: Aristotle
Who proposed Heliocentric model? A: Aristarchus
Which statement is WRONG regarding retrograde motion? A: Earth has retrograde motion as seen from Moon
Which of the following was NOT used in Ptolemy's model of the universe? A: All planets orbit around the Sun
Kepler's first law states A: Planetary orbits are elliptical
Planet travels fastest nearest to the Sun. Which law is this conclusion based on? A: Kepler's 2nd law
Which of the following planets orbits the Sun at slowest average speed according to Kepler's 3rd law? A: Saturn
Galileo confirmed Heliocentric model with the observation of A: the phases of Venus
Which of the following statement is WRONG concerning Newton's universal law of gravitation? A: The larger the planet size, the larger the force
The orbits of planets are determined by A: gravitational force of the Sun
Hardcore
An asteroid orbits the Sun on an elliptical path with semi-major axis 4 AU. By Kepler's third law, its orbital period is: A: 8 years
A planet at perihelion is moving faster than at aphelion. The most rigorous physical reason is: A: Conservation of angular momentum requires v Γ r to remain constant, so v rises as r falls.
FRQs
Who proposed each model, and what's the difference? Model: The **geocentric model** was developed by Aristotle and refined by Ptolemy (~140 AD): Earth sits motionless at the center with the Sun, Moon, planets, and stars circling it on epicycles. The **heliocentric model** was proposed by Aristarchus (~250 BC) and rediscovered by Copernicus (1543): the Sun is at the center and Earth and the other planets orbit it. π **Textbook βΈ Ch 2.4:** Ptolemy's system used **epicycles** (small circles) riding on larger **deferents**, all offset slightly from Earth at a point called the **equant** β a baroque but predictively accurate scheme that ruled astronomy for ~1500 years.
What is retrograde motion of planets? Model: Retrograde motion is the temporary apparent **westward (east-to-west) drift** of a planet against the background stars, opposite its normal prograde motion. In the heliocentric model it occurs when faster-moving Earth overtakes a slower outer planet (like Mars), making the outer planet appear to drift backward from our perspective. π **Textbook βΈ Ch 2.4:** Heliocentric retrograde motion needs no epicycles β it's a pure parallax effect, like watching a slower car appear to "drift backward" as you pass it on the highway. This was Copernicus's most elegant simplification.
Main contributions of Copernicus, Kepler, and Galileo? Model: **Copernicus** revived the heliocentric model (Sun-centered, still with circular orbits). **Kepler** used Tycho Brahe's data to formulate three laws of planetary motion β orbits are ellipses, equal areas in equal times, and PΒ² = aΒ³. **Galileo** first used a telescope astronomically, discovering lunar mountains, sunspots, Jupiter's four large moons, and the phases of Venus β observational proof of heliocentrism. π **Textbook βΈ Ch 3.1:** Kepler's 3rd law in its simplest form is **PΒ² = aΒ³** when P is in Earth years and a in AU β so an orbit of a = 4 AU gives P = 8 years.
Phases of Venus predicted by Ptolemy vs. heliocentric model? Model: **Ptolemy** predicts only **crescent and new** phases, since Venus must always lie roughly between Earth and the Sun on its epicycle. The **heliocentric model** predicts all five phases β new, crescent, quarter, gibbous, and full β because Venus orbits the Sun. Galileo observed all five, confirming heliocentrism. π **Textbook βΈ Ch 2.4:** OpenStax notes Galileo also observed that Venus's *angular size* changes dramatically with phase β small when full (far side of Sun), large when crescent (near Earth) β exactly as Copernicus's geometry requires.
What force pulls planets to orbit the Sun and apples to the ground? State Newton's law of gravitation. Model: The same force β **gravity** β does both. Newton's universal law of gravitation: every pair of masses attracts each other with a force **F = GΒ·mβmβ / rΒ²**, proportional to the product of their masses and inversely proportional to the square of the distance between them (G = 6.67 Γ 10β»ΒΉΒΉ NΒ·mΒ²/kgΒ²). π **Textbook βΈ Ch 3.3:** Newton's insight (the "Great Synthesis") was that the **same** law governs the Moon's orbit and a falling apple β uniting "celestial" and "terrestrial" physics for the first time. The inverse-square dependence is what produces Kepler's elliptical orbits.
By Kepler's 2nd law, is a planet's orbital speed constant? Why? Model: **No.** Kepler's 2nd law says a planet sweeps out equal areas in equal times. Since its orbit is an ellipse, to keep that swept area constant the planet must move **faster when nearer the Sun (perihelion)** and **slower when farther (aphelion)**. π **Textbook βΈ Ch 3.1:** This is equivalent to conservation of **angular momentum** β Newton later proved Kepler's 2nd law follows directly from any central force, including gravity. The law applies to comets too: Halley moves fastest near perihelion and creeps for decades near aphelion.
Module 3 β Light, Waves & the Electromagnetic Spectrum
Weekly
A mechanic wave has wavelength of 5 meters and frequency of 300 Hz. What's the wave speed of this wave? A: 1500 m/s
What are propagating in electromagnetic waves? A: Electric field and magnetic field
Which statement is WRONG about electromagnetic waves? A: Different waves propagate in different speed
What's the speed of electromagnetic waves (light waves)? A: 300,000 km/s
Which of the following is NOT electromagnetic wave? A: Sound wave
Which of the following wavelength does NOT belong to visible light? A: 900 nm
Which of the following light waves can NOT penetrate the atmosphere and reach the ground? A: Ultraviolet wave
What's the freezing point of water in Kelvin temperature scale? A: 273 K
Which statement is correct about Wien's radiation law? A: The higher the temperature of astronomic object, the shorter the wavelength of its light emission
You observe a series of bright lines from some lighting gas on screen of spectrometer. Which spectrum is this? A: Emission spectrum
According to doppler effect, light emission from a star should shift its wavelength if it is receding from the Earth. How would its wavelength shift? A: The wavelength should shift to red wavelength that is so called red shift
Hardcore
A star's blackbody emission peaks at a wavelength of 580 nm (yellow). Using Wien's law, its approximate surface temperature is: A: ~5,000 K
A spectral line normally at 656.0 nm is observed at 660.0 nm in a distant galaxy. The galaxy is: A: Receding from us at ~1,800 km/s β redshift.
FRQs
Speed of EM waves? Six colors of visible light? Which EM waves reach the ground? Model: EM waves travel at the speed of light, **c β 3 Γ 10β΅ km/s** (3 Γ 10βΈ m/s). The six color components of visible light (~400β700 nm) are **red, orange, yellow, green, blue, violet**. Only **visible light, radio waves, and some infrared** can penetrate Earth's atmosphere; the rest are blocked. π **Textbook βΈ Ch 5.2:** OpenStax states explicitly: "Electromagnetic radiation with wavelengths between roughly 400 and 700 nm is called visible light." UV is mostly absorbed by ozone (Ch 8.3); X-rays and Ξ³-rays require space-based telescopes.
How do astronomers determine stellar temperature? Model: They use **Wien's law**: a blackbody's peak emission wavelength is inversely proportional to temperature (**Ξ»_max = 3 Γ 10βΆ / T**, Ξ» in nm, T in K). By measuring the wavelength at which a star is brightest (or comparing brightness at two wavelengths), they compute its surface temperature. Color reflects this directly: blue is hot, red is cool. π **Textbook βΈ Ch 5.2:** Wien's law in OpenStax form: **Ξ»_max(nm) = 2.9 Γ 10βΆ / T(K)**. Our Sun (5,800 K) peaks at ~500 nm (yellow-green), Betelgeuse (3,500 K) peaks in the infrared, and Rigel (~12,000 K) peaks in the ultraviolet.
What is Kirchhoff's 2nd law? How do astronomers identify elements in stars? Model: **Kirchhoff's 2nd law:** a hot, low-density (rarefied) gas emits light only at discrete wavelengths, producing a bright-line **emission spectrum**. Every element has a unique pattern of lines. Astronomers identify elements in stars by matching the dark **absorption lines** in stellar spectra to known laboratory line patterns of hydrogen, helium, calcium, iron, etc. π **Textbook βΈ Ch 5.5:** Each element has a unique spectral fingerprint set by its atomic energy levels (Bohr model). Helium itself was discovered in the **Sun's spectrum** (1868) before being identified on Earth β hence its name, from *helios*.
Principles of the three types of spectra? Model: **(1) Continuous spectrum** (Kirchhoff 1) β a hot dense solid, liquid, or dense gas emits a smooth blackbody curve. **(2) Emission-line spectrum** (Kirchhoff 2) β a hot rarefied gas emits bright lines at discrete wavelengths characteristic of its elements. **(3) Absorption-line spectrum** (Kirchhoff 3) β a cool gas in front of a continuous source absorbs at those same discrete wavelengths, producing dark lines on the continuum. π **Textbook βΈ Ch 5.3:** The Sun's spectrum shows hundreds of **Fraunhofer absorption lines** β produced as photospheric continuum light passes through cooler overlying photosphere/chromosphere gases.
Hydrogen-Ξ± rest 656.3 nm, observed 658 nm β red or blueshift? Receding or approaching? Model: The observed wavelength (658 nm) is **longer** than the rest wavelength, so this is a **redshift**. A redshift means the source is moving **away** from Earth, so the star is **receding**. π **Textbook βΈ Ch 5.6:** The radial-velocity formula is **ΞΞ»/Ξ» = v/c**. Here ΞΞ» = 1.7 nm and Ξ» = 656.3 nm, giving v β 0.0026 c β **780 km/s** away from us.
Module 4 β Telescopes
Weekly
Which of the following is NOT a type of optic telescopes? A: diffractor
Which of the following optic devices is used for refracting telescopes? A: lens
The image formed by prime focus reflecting telescope is A: inverted
Modern telescopes are all A: reflectors
Which of the following is NOT disadvantage of refractors comparing to reflectors? A: It is hard to make lens surface smooth
Which type of reflecting telescope has only one mirror? A: prime focus
Image acquisition of modern telescope is done by A: charge coupled devices
Which statement is WRONG regarding telescope size? A: The larger the size, the more objects being observed
Which of the following is NOT a solution for high resolution of telescope? A: Put more mirrors in telescope
Which statement is WRONG concerning radio telescope? A: It uses refractor
Which type of telescopes can observe 24 hours a day and not interfered by rain and snow? A: Radio telescope
Which of the following telescope is the best option if visible radiation is blocked? A: Infrared telescope
Hardcore
Telescope A has an aperture diameter twice that of Telescope B. Compared to B, telescope A collects how many photons per second from the same star? A: 4Γ as many β proportional to collecting area (ΟrΒ²).
FRQs
What are the two types of telescopes? Which is used in modern telescopes? Model: **Refracting telescopes** gather light with a **lens**; **reflecting telescopes** gather light with a **mirror**. All modern research telescopes are **reflectors** β large mirrors avoid chromatic aberration and light absorption by glass, only need one polished surface, and can be supported from behind (large lenses sag under their own weight). π **Textbook βΈ Ch 6.1:** The largest refractor ever built (Yerkes, 1897) has a **40-inch** lens β essentially the size limit before glass sags under its own weight. Modern reflectors like Keck I/II use **10 m segmented mirrors**.
Main advantages of large telescopes? Model: (1) **Greater light-gathering power** β collecting area scales with diameter squared, so a 4 m mirror gathers 16Γ as much light as a 1 m mirror, allowing fainter objects to be seen. (2) **Better angular resolution** β the diffraction limit improves linearly with aperture diameter, revealing finer detail. π **Textbook βΈ Ch 6.1:** Quoting OpenStax: a 4 m mirror collects **16Γ** the light of a 1 m mirror because area scales as ΟdΒ²/4. This is why ground-based observatories chase ever-larger primary mirrors (e.g., the 39 m ELT under construction).
What is resolution? How is it improved? Which is better, 2β³ or 5β³? Model: **Resolution** is the smallest angular detail a telescope can distinguish, measured in arcseconds. It improves with larger aperture, shorter wavelength, and reduced atmospheric blurring (mountaintop sites, space telescopes, adaptive optics). **Smaller numbers mean finer detail**, so a **2β³ resolution image is better** than a 5β³ image. π **Textbook βΈ Ch 6.2:** "One arcsecond is 1/3600 degree" β about the angular width of a US quarter at 5 km. Ground-based telescopes are typically limited to ~1β³ by atmospheric "seeing" unless they use adaptive optics.
Main advantages and disadvantage of radio astronomy? Model: **Advantages:** can observe 24 hours a day, unaffected by clouds, rain, or snow; sees through interstellar dust; reveals phenomena invisible at optical wavelengths. **Disadvantage:** long wavelengths give very poor angular resolution for a given dish diameter (only **interferometry** combining widely separated dishes can match optical resolution). π **Textbook βΈ Ch 6.4:** The world's largest single dish is China's **FAST** (500 m). The Very Large Array (VLA) and ALMA use **interferometry** to mimic a single dish kilometers across, matching optical resolution.
Other astronomies? Do different-wavelength telescopes see the same image? Model: Other branches: **infrared, ultraviolet, X-ray, and gamma-ray** astronomy. UV, X-ray, and gamma-ray observatories must be in space because the atmosphere absorbs those wavelengths. **No** β the same object looks very different at different wavelengths because different physical processes (cool dust, hot gas, high-energy plasma, etc.) dominate the emission in each band. π **Textbook βΈ Ch 6.5:** UV, X-ray, and Ξ³-ray observatories must operate in space because Earth's atmosphere absorbs those wavelengths. X-rays are focused via **grazing-incidence** mirrors (e.g., Chandra); gamma rays cannot be focused at all.
Module 5 β Solar System Overview β Asteroids, Comets, Formation
Weekly
What did early astronomers NOT know about the solar system? A: Jupiter has many moons
Astronomers know the distance of planets to the Sun by A: Kepler's law
Astronomers know masses of Astronomic objects by A: Newton's laws
The orbits of planets are all close to ecliptic except A: Mercury
Which of the following planet is NOT Jovian planet? A: Venus
Which statement is WRONG regarding differences between Jovian planets and Terrestrial planets? A: Terrestrial planets have lower density
Asteroid belt is located between A: Mars and Jupiter
Which fact is WRONG concerning asteroids and meteoroids? A: They are icy
Comets have the following components except A: Atmosphere
Which statement is WRONG concerning the two tails of a comet? A: They are perpendicular to each other
If a comet breaks up, it is likely becoming A: meteor shower
Crater is unlikely created by A: comet
Which of the following theory is NOT used to explain formation of the solar system? A: Mass-energy equation
Hardcore
According to the condensation theory, why are inner planets rocky and outer planets gaseous? A: Beyond the frost line, ices could condense, giving outer protoplanets enough mass to capture nebular hydrogen and helium.
FRQs
What are the components of our solar system? Model: The Sun (containing ~99.8% of the system's mass), the **eight planets** (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), several **dwarf planets** (Pluto, Ceres, Eris, Makemake, Haumea), over **180 moons**, and large populations of **asteroids, comets, meteoroids**, plus **interplanetary dust and gas** β all gravitationally bound to the Sun. π **Textbook βΈ Ch 7.1:** "The eight planets all revolve in the same direction around the Sun. They orbit in approximately the same plane, like cars traveling on a circular racetrack." Pluto was reclassified as a dwarf planet in 2006 (IAU).
Terrestrial vs. Jovian planets β main differences? Model: **Terrestrial** (Mercury, Venus, Earth, Mars): small, dense, rocky/metallic composition, close to the Sun, thin or no atmospheres, few or no moons, no rings. **Jovian** (Jupiter, Saturn, Uranus, Neptune): large, low-density, gaseous (mostly hydrogen and helium), far from the Sun, thick atmospheres, rapid rotation, many moons, and ring systems. π **Textbook βΈ Ch 7.2:** The compositional split traces back to the **frost (snow) line** in the solar nebula at ~5 AU β beyond it, ices could condense, allowing Jovian cores to grow large enough to capture H/He envelopes.
Components of a comet? Where do they come from? Model: A comet has four parts: an icy/rocky **nucleus**, a **coma** (gas and dust envelope around the nucleus), a vast **hydrogen envelope**, and two **tails** β a straight **ion (gas) tail** and a curved **dust tail**, both pointing away from the Sun. Comets originate in the **Kuiper Belt** (~30β50 AU, beyond Neptune) and the much more distant **Oort Cloud** (~2,000β200,000 AU). π **Textbook βΈ Ch 13.3:** Comet tails *always point away from the Sun* (regardless of motion direction) β pushed by the solar wind (ion tail) and radiation pressure (dust tail). This was noted as early as the 1500s.
Explain solar system formation via nebular contraction and condensation theory. Model: A giant interstellar cloud of gas and dust contracted under its own gravity. Conservation of angular momentum made it spin faster and flatten into a disk. Dust grains served as condensation nuclei: in the hot inner disk only rocks and metals could condense β terrestrial planets, while in the cold outer disk gases and ices also condensed β Jovian planets. The hot center became the Sun. π **Textbook βΈ Ch 14.3:** The **condensation sequence** is temperature-controlled: at ~1500 K only iron/silicates condense; below ~150 K, water ice; below ~50 K, ammonia and methane ices. This explains the radial composition gradient.
Module 6 β Terrestrial Planets
Weekly
Terrestrial planets are similar in the following properties except A: surface temperature
The largest terrestrial planet is A: Earth
Which planet has retrograde surface rotation? A: Venus
Which planet has no detectable atmosphere? A: Mercury
Caloris Basin belongs to A: Mercury
Which is the largest impact crater of Venus? A: Mead
Which is NOT the major feature of Mars? A: High density craters on Northern Hemisphere
Olympus Mons is the largest volcano in solar system. It has a base diameter of about A: 700 km
Venus has the highest surface temperature 730 K for the following reasons except A: It has violent volcano activities
Which fact is WRONG concerning green house effect of atmosphere of planets? A: It produces more Oxygen
Hardcore
Venus's greenhouse effect raises its surface to ~730 K β far hotter than Mercury, despite being farther from the Sun. The dominant reason is: A: Venus's atmosphere is ~90Γ denser than Earth's and almost pure COβ, trapping infrared radiation efficiently.
FRQs
What causes ocean tides? Explain spring and neap tides. Model: Tides are caused by the **differential gravitational pull** of the Moon (and to a lesser extent the Sun) on Earth β the near side is pulled more strongly than the far side. **Spring tides** (highest) occur at new and full moon, when Sun and Moon align and their tidal pulls add. **Neap tides** (lowest) occur at first/third quarter, when Sun and Moon pull at right angles and partially cancel. π **Textbook βΈ Ch 4.6:** Tidal force falls off as **1/rΒ³** (not 1/rΒ²) because it's a *differential* gravitational force. The Moon dominates Earth's tides despite the Sun's larger mass because the Moon is much closer.
Compare rotations of the four terrestrial planets. Which is retrograde? Model: **Mercury** rotates prograde and slowly (~59 days), locked in a 3:2 spinβorbit resonance with the Sun. **Venus** rotates **retrograde** β the only terrestrial that does β and extremely slowly (~243 days). **Earth** rotates prograde in ~24 hours. **Mars** rotates prograde in ~24.6 hours, nearly the same as Earth. So **Venus is the retrograde rotator**. π **Textbook βΈ Ch 10.1:** Mercury's 3:2 spinβorbit lock means it rotates exactly 3 times every 2 orbits β so one Mercury "solar day" lasts **176 Earth days**, longer than its 88-day year.
What is the greenhouse effect? Model: Sunlight passes through the atmosphere and warms Earth's surface, which re-radiates the energy as infrared. **Greenhouse gases** (COβ, HβO, CHβ) absorb this infrared and re-emit it back downward, trapping heat. This keeps Earth's surface significantly warmer than it would be without the effect β and enhanced greenhouse gas concentrations drive global warming. π **Textbook βΈ Ch 8.3:** Without the greenhouse effect, Earth's average surface temperature would be roughly **β18 Β°C** instead of the actual **+15 Β°C**. Venus (Ch 10.3) shows the runaway version: a 730 K surface from a thick COβ atmosphere.
Compare terrestrial atmospheres. Why is Venus so hot? Model: **Mercury**: essentially no atmosphere (too hot and low-gravity to retain one). **Venus**: extremely thick COβ atmosphere (~90 Γ Earth's pressure). **Earth**: moderate Nβ/Oβ atmosphere supporting life. **Mars**: thin COβ atmosphere (~1 % Earth's). Venus's surface reaches ~730 K because its thick COβ atmosphere traps re-radiated infrared in a **runaway greenhouse effect**, making it hotter than Mercury despite being farther from the Sun. π **Textbook βΈ Ch 10.3:** "The surface of Venus is also remarkably hot, with a temperature of 730 K (over 850 Β°F), hotter than the self-cleaning cycle of your oven." Surface pressure is ~92 bar (like 900 m underwater).
How do scientists study Earth's interior? Structure from inside out? Model: By analyzing **seismic waves** from earthquakes. **Pressure (P) waves** travel through both solids and liquids; **shear (S) waves** cannot propagate through liquids. The pattern of reflections and refractions reveals layer boundaries. From inside out: **solid inner core**, **liquid outer core** (both iron/nickel), **rocky mantle**, **crust**, hydrosphere, atmosphere, and magnetosphere. π **Textbook βΈ Ch 8.1:** The **mantle is the largest layer by volume**; the crust averages only ~30 km on continents and ~6 km under oceans. Earth's global magnetic field is generated in the convecting liquid outer core (geodynamo).
Open question: does Mars contain life? Model: No confirmed life has been found, but it remains a serious possibility. Evidence supports past **liquid water** (runoff channels, ancient deltas, polar ice caps), seasonal **methane** fluctuations, and confirmed **subsurface water ice**. Past microbial life or current life in subsurface aquifers is plausible but unproven. Missions like Curiosity and Perseverance are actively searching for biosignatures in Martian rocks. π **Textbook βΈ Ch 10.5:** OpenStax devotes an entire section to "Water and Life on Mars," highlighting evidence of an ancient northern ocean, polar water-ice caps, and the still-unexplained seasonal methane fluctuations detected by Curiosity.
Layers of the atmosphere? Which layer hosts weather? Model: From bottom to top: **troposphere, stratosphere, mesosphere, thermosphere, exosphere** (the ozone layer lies in the lower stratosphere; the ionosphere overlaps thermosphere and exosphere). **Weather takes place in the troposphere**, the lowest ~12 km, where convection driven by solar heating of the ground produces clouds, winds, and storms. π **Textbook βΈ Ch 8.3:** Earth's atmosphere is ~78% **Nβ** and 21% **Oβ** at the surface, with surface pressure ~1 bar. The stratosphere (12β50 km) hosts the protective ozone layer.
Compare the interiors of the four terrestrial planets. Model: All four share a layered structure β metallic iron/nickel **core**, rocky **mantle**, thin **crust**. **Mercury** has an unusually large iron core (~75 % of its radius). **Venus** is Earth-like in size and bulk structure but with a likely non-convecting outer core (hence no global magnetic field). **Earth** has a convecting liquid outer + solid inner core that generates its magnetic field. **Mars** has a small, partly liquid core. π **Textbook βΈ Ch 10.6:** All terrestrials underwent **differentiation** while molten, sorting dense iron to the core and lighter silicates to the mantle/crust. Only Earth currently sustains plate tectonics; Mars and Venus show only relict tectonic features.
Explain the origin of the Moon. Model: The leading explanation is the **giant impact hypothesis**: a Mars-sized body struck the still-molten young Earth in a glancing blow. The impact ejected huge amounts of material β mostly from Earth's mantle β into orbit, where it gradually coalesced under gravity to form the Moon. This explains the Moon's low iron content and its isotopic similarity to Earth's mantle. π **Textbook βΈ Ch 9.4:** The impact occurred about **4.5 billion years ago**, shortly after Earth itself formed. The Moon's bulk density (3.3 g/cmΒ³) matches Earth's mantle, not its iron core β strong evidence for the impact hypothesis.
Module 7 β Jovian Planets
Weekly
Which statement is WRONG about Jovian planets? A: Jovian planets have smaller mass comparing to Terrestrial planets
Which planet has extreme seasonal variation? A: Uranus
Which of the following is NOT major visible feature of Jupiter? A: Vocalos
Brown oval in Jupiter is A: a large gap in clouds
Which fact is WRONG concerning the atmosphere of Saturn? A: It has more cloud layers than Jupiter's atmosphere
Which fact is WRONG about Saturn? A: It has red spots like Jupiter
Which fact is WRONG about Uranus? A: It has solid surface
The three cloud layers of Jupiter's atmosphere are A: water ice, ammonium hydrosulfide ice, ammonia ice
The three layers of Jupiter's interior are A: rocky core, metallic hydrogen, molecular hydrogen
Hardcore
Jupiter radiates roughly 1.7Γ as much energy as it receives from the Sun. The dominant source of this excess internal heat is: A: Slow gravitational contraction releasing thermal energy (Kelvin-Helmholtz mechanism).
FRQs
Which Jovian planet has extreme seasonal variations, and why? Model: **Uranus.** Its rotation axis is tipped about **98Β°** β essentially lying in its orbital plane. As Uranus orbits the Sun (84 Earth years), each pole experiences roughly **42 years of continuous sunlight followed by 42 years of darkness**, producing the most extreme seasons in the solar system. π **Textbook βΈ Ch 11.2:** OpenStax: "The strangest seasons of all are on Uranus, which has a spin axis tilted by 98Β°β¦ Practically speaking, we can say that Uranus orbits on its side." Its rings and moons all orbit in this tilted equatorial plane.
Main characteristics of Jovian atmospheres? Visible features of Jupiter's? Model: Jovian atmospheres are dominated by **hydrogen and helium** with traces of methane, ammonia, and water; they show **banded cloud structure** (light "zones" and dark "belts"), strong **differential rotation**, high wind speeds, and persistent storms. Jupiter's visible features include alternating **zones and belts**, the centuries-old **Great Red Spot**, smaller white/brown ovals, and lightning storms. π **Textbook βΈ Ch 11.3:** OpenStax classifies Jovian "gases" (H, He) vs. "ices" (compounds like HβO, NHβ, CHβ that *can* freeze but may exist as liquid/gas at depth) vs. "rocks." The Great Red Spot has persisted for at least 350 years.
Interior structure of Jupiter and Saturn? Model: Both have a small **rocky/icy core** (~10β15 Earth masses), surrounded by a thick layer of **liquid metallic hydrogen** (where extreme pressure ionizes hydrogen, generating their powerful magnetic fields), then a layer of **liquid molecular hydrogen**, topped by the gaseous Hβ + He atmosphere. Saturn's structure mirrors Jupiter's but is thinner overall because of Saturn's smaller mass and weaker gravitational compression. π **Textbook βΈ Ch 11.2:** OpenStax: "On Jupiter, the greater part of the interior is liquid metallic hydrogen. Because Saturn is less massive, it has only a small volume of metallic hydrogen." Both planets radiate more heat than they receive from the Sun.
What is a ring system? Which planet has the most moons? What are Jupiter's four largest moons? Model: A **ring system** is a disk of countless small particles of ice, dust, and rock orbiting a planet, typically within the **Roche limit**. All four Jovians have rings; Saturn's are most spectacular. **Saturn currently has the most known moons** (80+). Jupiter's four large **Galilean moons** β discovered by Galileo in 1610 β are **Io, Europa, Ganymede, and Callisto**. π **Textbook βΈ Ch 12.1β12.2:** **Ganymede** is the largest moon in the solar system β bigger than Mercury. **Io** is the most volcanically active body known. **Europa** likely hosts a subsurface liquid water ocean beneath its ice crust.
Module 8 β The Sun
Weekly
The rotation period of the Sun is about A: 30 days
The surface temperature of the Sun is about A: 6000 K
The visible layer of the Sun is A: Photosphere
Nuclear fusion in the Sun takes place in A: Core
The total energy radiated in one second by the Sun is called A: Luminosity
In the mathematical model of the interior structure of the Sun, the inward pressure is given by A: gravity
Which statement is correct regarding the density of the Sun? A: The core has the highest density
Through which paths does the Sun transport its energy from inside to outside? A: Radiation and convection
The outermost layer of solar atmosphere is A: Corona
Sunspot appears dark because A: it is slightly cooler than its surroundings
Sunspots are linked by A: magnetic field lines
Sunspots rise and fall in numbers, leading a sunspot cycle of A: 11 years
Solar prominence is A: a large sheet of gas ejected in photosphere
Which statement is WRONG concerning solar flare? A: It has much larger energy output than solar prominence
Nuclear fusion in the core of the Sun requires temperature as high as A: 10 million K
A corona mass ejection emits A: charged particles
Hardcore
A photon produced by fusion in the Sun's core takes roughly how long to reach the photosphere, due to random-walk diffusion through the radiative zone? A: About 170,000 years
Sunspot numbers rise and fall on an 11-year cycle, but the Sun's full magnetic polarity reversal completes only every: A: 22 years β the Hale cycle.
FRQs
Layers of the Sun's atmosphere? Which is the visible "surface"? Model: From inside out: **photosphere**, **chromosphere**, and **corona**. The **photosphere** (~5,800 K, ~500 km thick) is the visible "surface" β the layer from which essentially all the Sun's visible light escapes. Above it, the much hotter but tenuous chromosphere and corona are normally invisible except during a solar eclipse. π **Textbook βΈ Ch 15.1:** Below the photosphere lie the **convection zone**, **radiative zone**, and **core**. The corona is paradoxically hotter (~1β3 million K) than the photosphere (~5,800 K) β a long-standing puzzle, likely heated by magnetic reconnection.
How do scientists study the solar interior? Model: Through three main methods: (1) **mathematical/computer models** built on hydrostatic equilibrium (gas pressure balancing gravity); (2) **helioseismology** β Doppler shifts of surface oscillations reveal interior structure, analogous to using seismic waves to probe Earth's interior; and (3) **neutrino detection**, since neutrinos produced by core fusion stream out of the Sun directly. π **Textbook βΈ Ch 16.4:** Helioseismology measures p-mode oscillations on the Sun's surface; their patterns probe interior structure like seismic waves on Earth. Neutrino observations resolved the long-standing **solar neutrino problem** by confirming neutrino oscillation.
Properties of sunspots: why dark, cycle length, what links them? Model: Sunspots are regions of intense magnetic field where the field suppresses convection; they appear **dark** because they are ~1,500 K **cooler** than the surrounding photosphere and so emit less light. Their number rises and falls in an **~11-year cycle** (full magnetic cycle is 22 years). Sunspots typically come in pairs **linked by loops of magnetic field lines** of opposite polarity. π **Textbook βΈ Ch 15.2:** Each sunspot has an inner dark **umbra** and a lighter outer **penumbra**. The full magnetic cycle is **22 years**, since polarities flip between hemispheres every 11 years (Hale's law).
What is a solar flare? What is a solar prominence? Model: A **solar prominence** is a large, bright, gaseous loop or arch of plasma extending outward from the photosphere into the corona, supported by magnetic fields, persisting for days to weeks. A **solar flare** is a sudden, violent explosion in an active region near sunspots that releases a comparable amount of energy in just seconds to minutes, often as X-rays and energetic particles. π **Textbook βΈ Ch 15.3:** OpenStax: "A typical flare lasts for 5 to 10 minutes and releases a total amount of energy equivalent to that of perhaps a million hydrogen bombs." Large flares can disrupt satellites and power grids on Earth.
What produces solar energy? What happens in the Sun's core? Model: Solar energy is produced by **nuclear fusion** in the core. Specifically, the **proton-proton (p-p) chain** fuses four hydrogen nuclei (protons) into one helium-4 nucleus. The helium nucleus is slightly less massive than the four protons; the missing mass is converted into energy via **E = mcΒ²**. This requires temperatures above 10 million K, met only in the core (~15 million K). π **Textbook βΈ Ch 16.2:** Each p-p cycle converts ~0.7% of the mass involved into energy. The Sun fuses ~**600 million tons of hydrogen per second**, losing ~4 million tons of mass per second β yet it has fuel for another ~5 billion years.
One parsec is A: distance from the Sun a star must lie in order for its observed parallax angle to be exactly 1 second degree
A star is observed to have 0.5 arcsecond parallax, its distance to the Sun is A: 2 pc
The nearest star to the Sun is A: Proxima Centauri
Which statement is WRONG concerning luminosity? A: It is apparent brightness a star appears in night sky
Apparent brightness follows inverse square law with distance from a star. If distance is doubled, apparent brightness would become A: 1/4 of original brightness
Two stars appears equally bright in sky. This means A: they might be a dimmer but closer one, and a brighter but farther one
Apparent luminosity is measured using logarithmic magnitude scale. If star A has apparent magnitude 5 and star B has apparent magnitude 10, which one appears brighter in sky? A: A
Red stars are relatively A: cooler
According to spectral classification, among B, G and M stars, which one has highest temperature? A: B
H-R diagram plots A: stellar luminosity against surface temperature
Following the main sequence on H-R diagram from lower right to top left, the surface temperature of stars A: increases
Following the main sequence on H-R diagram from lower right to top left, the mass of a star on the main sequence A: increases
Hardcore
Star A has a measured parallax of 0.04 arcseconds. Star B has a parallax of 0.01 arcseconds. The ratio of B's distance to A's distance is: A: 4:1 β B is four times farther than A.
A star sits in the upper-right region of the H-R diagram β high luminosity and low surface temperature. It is most likely: A: A red giant
FRQs
A star has parallax 0.2β³ β what is its distance in pc and km? Model: Distance d (pc) = 1 / parallax (arcsec) = 1 / 0.2 = **5 pc**.
In kilometers: 5 pc Γ 3.1 Γ 10ΒΉΒ³ km/pc β **1.55 Γ 10ΒΉβ΄ km**. π **Textbook βΈ Ch 19.1:** OpenStax: "1 pc equals 3.09 Γ 10ΒΉΒ³ km and it also equals 3.26 light-years." The nearest star (Proxima Centauri) has parallax 0.77β³ β d β 1.3 pc β 4.2 light-years.
Difference between luminosity and apparent brightness? Do two equally bright stars have the same distance? Model: **Luminosity** is the total intrinsic power a star emits β a property of the star itself. **Apparent brightness** is how bright the star looks from Earth, which depends on both luminosity and distance (inverse-square law). **No** β two stars that appear equally bright can be at very different distances: a nearby dim star can match a distant luminous one. π **Textbook βΈ Ch 17.1:** Apparent brightness obeys an **inverse-square law**: doubling distance reduces brightness by a factor of 4. Astronomers use the magnitude scale (logarithmic; a 5-magnitude step = Γ100 in brightness, lower number = brighter).
How do astronomers determine stellar temperature? Seven spectral classes hottest to coolest? Model: Temperatures are determined from a star's **color** (using Wien's law on its blackbody spectrum) or, more precisely, from the **absorption lines** present in its spectrum. The seven spectral classes from hottest to coolest are **O, B, A, F, G, K, M** β mnemonic "Oh, Be A Fine Girl/Guy, Kiss Me!" O stars are hot blue; M stars are cool red. π **Textbook βΈ Ch 17.3:** OpenStax adds three modern cool classes: **L, T, Y** for brown dwarfs and very cool stars. O stars exceed 30,000 K; M stars are below 3,500 K. Our Sun is type **G2V**.
What is the H-R diagram and its main sequence? Main determinant of position on the Main Sequence? Model: The **Hertzsprung-Russell diagram** plots stellar luminosity (vertical) against surface temperature (horizontal, hot on the left). Most stars (~90 %) lie on a diagonal band from upper-left (hot, luminous) to lower-right (cool, dim) called the **Main Sequence**, where they fuse hydrogen in their cores. A star's position on the Main Sequence is determined primarily by its **mass** β more massive stars sit higher and to the left. π **Textbook βΈ Ch 18.4:** ~**90 %** of stars lie on the Main Sequence (fusing HβHe). Stellar mass spans ~0.08 Mβ (brown-dwarf limit) to ~100 Mβ; high-mass stars burn out fast (millions of years), low-mass stars live trillions.
Module 10 β Stellar Evolution
Weekly
A stable star is in equilibrium between A: pressure out from gas expansion and pressure in from contraction of gravity
A star begins to leave main sequence as A: hydrogen in the core is consumed
For a star on main sequence, the composition of its core A: is changing
What would happen to the core when the fuel of the core, hydrogen, is used up? A: The core contracts
Which fact is WRONG regarding red giant stage of a star? A: It becomes hotter
In which stage would a star has the largest size? A: Red giant
In which stage would Helium flash occur? A: Horizontal branch
In which stage would a low mass star become two part? A: Planetary nebula
Which is NOT property of nova? A: It happens to high mass star
Which statement is WRONG regarding supernova? A: It can repeat
If a star cluster shows many white dwarfs in its H-R diagram, it indicates A: The star cluster is very old
Hardcore
For a 1 Mβ star like the Sun, the final remnant after all stellar evolution is complete is: A: A white dwarf, which slowly cools into a black dwarf over many trillions of years
The Chandrasekhar limit (~1.4 Mβ) is the maximum mass at which: A: A white dwarf can remain stable against gravitational collapse from electron degeneracy pressure.
FRQs
From stage 7, list all stages of a sun-like star's life cycle. Model: **(7)** Main-sequence star β **(8)** Subgiant (H exhausted in core, H-shell burning begins) β **(9)** Red giant (envelope expands, cools, brightens) β **(10)** Horizontal branch (helium flash ignites core He β C) β **(11)** Asymptotic giant branch (second red giant; H- and He-burning shells) β **(12)** Planetary nebula (outer layers ejected) β **(13)** White dwarf β **(14)** Black dwarf (cooled, non-luminous remnant). π **Textbook βΈ Ch 22.1 & 22.4:** OpenStax notes that the **black dwarf** stage is theoretical β the universe is not yet old enough for any white dwarf to have fully cooled. Planetary nebulae like the Ring Nebula (M57) display this stage in progress.
Nova vs. supernova? Two types of supernova? Model: A **nova** is a sudden brightening (~1,000Γ) caused by hydrogen accreted from a companion star igniting in runaway fusion on a white dwarf's surface; the white dwarf survives and the process can repeat. A **supernova** is a one-time, catastrophic explosion that destroys the star, ~10βΆ times brighter than a nova. **Two types: Type Ia** (white dwarf exceeds 1.4 Mβ from accretion and carbon-detonates) and **Type II** (core collapse of a high-mass star). π **Textbook βΈ Ch 23.2:** The **1.4 Mβ** threshold is the **Chandrasekhar limit** β the maximum mass electron-degeneracy pressure can support. Type Ia supernovae all reach a similar peak brightness, making them excellent **standard candles** for cosmic distance measurement.
Evolution of stars of the same age in a cluster on the H-R diagram? Model: In a cluster, all stars formed simultaneously but with different masses. The most massive (O, B) leave the Main Sequence first to become red giants/supergiants and explode; lower-mass stars follow over time. On the cluster's H-R diagram, the **main-sequence turnoff point** moves progressively to lower-mass (cooler) stars as the cluster ages, so the turnoff position serves as a reliable age indicator. π **Textbook βΈ Ch 22.2:** Globular clusters have turnoffs near solar-mass stars β ages **~12β13 billion years** (among the oldest objects known). Young open clusters like the Pleiades still have O/B stars on the Main Sequence β ages only ~100 Myr.
Module 11 β Variable Stars & The Milky Way
Weekly
Which of the following is NOT intrinsic variable star? A: nova
Cepheid variable has a period of A: 1-100 days
Which best describes the luminosity-period relationship of a Cepheid star? A: Luminosity increases linearly with period
From luminosity-period relationship, we can find out luminosity of variable stars if knowing oscillation period of its brightness. If we can observe apparent brightness from Earth, we can calculate A: its distance from us
Which of the following distance method can measure the furthest distance? A: Variable star
We know rotation of Milky Galaxy by observing A: Doppler shift
The diameter of our Milky Way is about A: 30 kpc
Which part are the youngest stars staying in our Milky Way? A: Galactic disk
Infrared view of our galaxy shows much more detail of the galactic center than the visible-light view does because A: infrared is not as much absorbed by gas and dust
Milky Way's spiral arms are formed by A: spiral density waves
The center of Milky Way is A: a black hole with several million solar masses
Hardcore
The Sun's position within the Milky Way is best described as: A: About 8 kpc from the galactic center, embedded in the disk near a spiral arm.
FRQs
Two types of intrinsic variable stars? How is distance found for each? Model: **RR Lyrae stars** (periods 0.5β1 day, all with roughly the same luminosity) and **Cepheid variables** (periods 1β100 days). For **RR Lyraes**, the known luminosity plus measured apparent brightness gives distance via the inverse-square law. For **Cepheids**, the **periodβluminosity relation** converts the observed pulsation period into luminosity, after which the inverse-square law gives distance (good to ~25 Mpc). π **Textbook βΈ Ch 19.3:** **Henrietta Leavitt** discovered the Cepheid periodβluminosity relation in 1908 from observations of variable stars in the Small Magellanic Cloud. This calibration enabled Hubble's later discovery that galaxies are external to the Milky Way.
Cosmic distance ladder β main methods. Model: 1. **Radar ranging** β within the Solar System
2. **Stellar parallax** β out to a few hundred parsecs
3. **Spectroscopic parallax** β out to several kpc
4. **Variable stars** (RR Lyrae & Cepheids) β to ~25 Mpc
5. **Tully-Fisher relation** (spiral galaxies) β to ~200 Mpc
6. **Type Ia supernovae** as standard candles β to ~1 Gpc
7. **Hubble's law** β out to the edge of the observable universe
Each rung is calibrated by the rungs below it. π **Textbook βΈ Ch 19 (intro):** OpenStax: "Each technique builds on at least one other method, forming what many call the cosmic distance ladder. Parallaxes are the foundation of all stellar distance estimates." Errors compound at each rung.
Main components of the Milky Way? Size in parsecs? Model: **Galactic disk** (with spiral arms, gas, dust, young stars), **galactic bulge** (older stars surrounding the center), **galactic nucleus** (a supermassive black hole), **galactic halo** (sparse old stars + dark matter), and **globular clusters** (~150 dense old-star clusters in the halo). The disk is roughly **30 kpc** (~100,000 light-years) in diameter, and the Sun sits ~8 kpc from the center. π **Textbook βΈ Ch 25.1:** Modern measurements give a disk diameter of **~100,000 light-years (~30 kpc)** with a thin (~1,000 ly) and thick (~3,500 ly) component. The Sun orbits the center at ~220 km/s and takes ~225 Myr per galactic year.
Spiral arms β density waves or differential rotation? Model: **Spiral density waves.** If the arms were physical structures rotating rigidly with the disk, differential rotation would wind them up tightly within just a few rotations. Instead they are wave patterns of compressed gas through which stars and gas clouds move β like cars in and out of a traffic jam. The compression triggers star formation, and bright young blue stars make the wave visible. π **Textbook βΈ Ch 25.3:** The Milky Way is a **barred spiral** (type SBb/SBc). Its major arms β Scutum-Centaurus and Perseus β emerge from the bar's ends. The Sun lies in a smaller spur called the **Orion Arm**.
What's at the galactic center? What's its mass? Model: A **supermassive black hole** named **Sagittarius A*** (Sgr A*), inferred from the rapid Keplerian orbits of stars whirling around an otherwise invisible compact mass. Its mass is approximately **3β4 million solar masses** (the lecture cites ~3.7 Γ 10βΆ Mβ). π **Textbook βΈ Ch 25.4:** "The evidence for a supermassive black hole at the center of the Galaxy is convincing indeed." In 2022, the Event Horizon Telescope imaged Sgr A* directly β confirming a ~4.1 Γ 10βΆ Mβ black hole.
Module 12 β Galaxies
Weekly
Which of the following is NOT the type of normal galaxies? A: Radio galaxies
Spiral galaxies are classified according to A: the size of their central bulge
Among spiral galaxies Sa, Sb and Sc, which type has the largest central bulge? A: Sa
The components of a spiral galaxy include disk, core, bulge, spiral arms and A: halo
The galaxy type SBc means a galaxy with A: spiral arms and elongated bar
Which of the following fact is WRONG regarding elliptical galaxies? A: They have an elliptical disk
Among E0, E3 and E7, which galaxy is most elongated? A: E7
Among the three cosmic distance methods Tulley fisher, Standard candles and Variable stars, which one can measure farthest distance? A: Standard candles
Our local group has a number of galaxies of A: 45
According to Hubble's law, if a galaxy is further away from us, its receding speed must be A: higher
Active galaxies do not fit into Hubble's scheme because A: they are too luminous
Which of the following is NOT the property of active galactic nuclei? A: stable energy output
Hardcore
A galaxy 100 Mpc away recedes from us at 7,000 km/s. From Hubble's law (v = Hβd), the implied Hubble constant is: A: 70 km/s/Mpc
FRQs
List all galaxy classifications and their characteristics. Model: **Spirals (S)** β disk, bulge, spiral arms; classified Sa, Sb, Sc by decreasing bulge size and progressively looser arms. **Barred spirals (SB)** β like spirals but with a central bar (SBa, SBb, SBc). **Ellipticals (E0βE7)** β no disk or arms, smooth shape, little gas/dust, old stars; E0 is nearly spherical, E7 highly elongated. **Lenticulars (S0, SB0)** β disk + bulge but no spiral arms and little gas. **Irregulars** β no symmetry, often gas-rich, frequently distorted by interactions. π **Textbook βΈ Ch 26.1β26.2:** Hubble's original "tuning fork" diagram placed ellipticals on the handle and the two spiral branches (S and SB) as forks. Despite Hubble's belief, this is **not** an evolutionary sequence β ellipticals are typically *older* than spirals.
Cosmic distance ladder β 7 methods. Model: Same seven rungs: (1) **Radar ranging**, (2) **Stellar parallax**, (3) **Spectroscopic parallax**, (4) **Variable stars** (RR Lyrae & Cepheids), (5) **Tully-Fisher relation**, (6) **Type Ia supernovae** as standard candles, (7) **Hubble's law**. Each rung is calibrated using rungs below it and extends measurement farther into the universe. π **Textbook βΈ Ch 28.1:** Beyond Hubble's law, modern cosmology also uses **baryon acoustic oscillations** (BAO) and **gravitational lensing** as distance probes, especially for the most distant galaxies (z > 1).
What is Hubble's law? How do you find galaxy distances from it? Model: **Hubble's law** states that galaxies recede from us with velocity proportional to their distance: **v = Hβ Γ d**, where Hβ β 70 km/s/Mpc. To find a galaxy's distance, measure its **redshift** to get recession velocity v (from the Doppler shift of its spectral lines), then compute **d = v / Hβ**. π **Textbook βΈ Ch 28.3:** A galaxy with v = 14,000 km/s and Hβ = 70 km/s/Mpc gives d = 200 Mpc β 650 Mly. Hubble's law also implies the universe has a finite **Hubble time** of ~14 Gyr (1/Hβ).
Classifications of active galaxies and their characteristics? Model: Three main types. **Seyfert galaxies** β outwardly normal spirals with extremely luminous, compact, variable cores. **Radio galaxies** β emit very strongly in radio, often with huge double **lobes** extending well beyond the visible galaxy (lobe-dominated vs. core-dominated views may be the same object seen at different angles). **Quasars** ("quasi-stellar objects") β point-like, enormously redshifted, the most luminous AGN, typically very distant. π **Textbook βΈ Ch 27.1:** This is the **unified model** of AGN: a Seyfert, radio galaxy, blazar, or quasar may all be the same kind of object β a supermassive black hole + accretion disk + jets β seen from different viewing angles relative to the obscuring dust torus.
Properties of active galactic nuclei? Model: AGN exhibit: (1) extremely **high luminosity** (up to thousands of times a normal galaxy); (2) **nonstellar/nonthermal radiation** spanning radio to gamma rays; (3) **rapid variability**, implying a very compact emitting region; (4) **jets** of relativistic particles and other signs of explosive activity; (5) **broad emission lines** indicating rapidly rotating gas. The central engine is believed to be a **supermassive black hole** with an accretion disk. π **Textbook βΈ Ch 27.2:** Variability over **hours/days** implies emitting regions only light-hours across β smaller than our solar system. A 10βΉ Mβ black hole has a Schwarzschild radius ~20 AU, fitting this constraint.
Module 13 β Cosmology
Weekly
The largest structure sloan great wall has a size of about A: 300 Mpc
The cosmological principle includes the assumptions of A: isotropy and homogeneity
Homogeneity means that the universe is A: same everywhere
The age of the universe is calculated from Hubble's constant H0 by A: 1/H0
The age of the universe is about A: 14 billion years
If the density of the universe is low, it will A: expand forever
According to current observation, the expansion of the universe A: is accelerating
What is the possible cause of the expanding of the universe? A: dark energy
The 3 K cosmic microwave background is the evidence of A: big bang
Which statement is WRONG concerning dark matter? A: It exists only in centers of galaxies
Hardcore
The cosmic microwave background has a near-perfect blackbody spectrum at 2.725 K, uniform in every direction to 1 part in 100,000. This is strong evidence that: A: The universe was once extremely hot and dense, and has since expanded and cooled β supporting the Big Bang.
A galaxy is observed at redshift z = 2. The scale factor of the universe when its light was emitted was: A: 1/3 the current size
Two different surveys give different values for Hubble's constant: 67 km/s/Mpc (from the CMB) and 73 km/s/Mpc (from local Cepheids/Type Ia SNe). If the *true* value were the higher one, the inferred age of the universe (assuming constant expansion, age β 1/Hβ) would be: A: Smaller than the CMB-based estimate
FRQs
What is the cosmological principle? Model: The **cosmological principle** assumes that on large enough scales (greater than ~300 Mpc) the universe is both **homogeneous** (looks the same from every location) and **isotropic** (looks the same in every direction). It implies there is no special place or direction, so the laws of physics work identically everywhere. π **Textbook βΈ Ch 29.2:** OpenStax: "on the large scale, the universe at any given time is the same everywhere (homogeneous and isotropic). As a result, the expansion rate must be the same everywhere during any epoch of cosmic time."
How is the age of the universe found? Using Hβ = 70 km/s/Mpc, what is it? Model: The age is estimated by extrapolating Hubble's expansion backward to a single starting point: **age β 1 / Hβ** (the "Hubble time"). Converting Hβ = 70 km/s/Mpc into consistent units (1 Mpc = 3.086 Γ 10ΒΉβΉ km) gives 1/Hβ β 4.4 Γ 10ΒΉβ· s β **14 billion years**, matching modern estimates of the universe's age. π **Textbook βΈ Ch 29.1:** The precise modern age is **~13.8 billion years** (from CMB + supernova + BAO data). The simple Hubble time slightly overestimates the age because expansion was decelerating early on, then began accelerating ~5 Gyr ago.
Is the expansion accelerating or decelerating? What drives it? Model: The expansion is **accelerating** β distant Type Ia supernovae appear fainter (farther) than a decelerating model predicts, showing galaxies receding faster than expected. The driving force is attributed to **dark energy** (also called the cosmological constant or vacuum pressure), a mysterious component making up ~68 % of the universe's energy density that exerts a repulsive effect on cosmic scales. π **Textbook βΈ Ch 29.4:** The 2011 Nobel Prize honored the discovery of acceleration (Perlmutter, Schmidt, Riess) via Type Ia supernovae. Energy budget today: ~68% dark energy, ~27% dark matter, ~5% normal (baryonic) matter.
What is the cosmic microwave background? Why is it evidence of the Big Bang? What causes the ripples? Model: The **CMB** is the faint, nearly uniform microwave radiation reaching Earth from every direction, with a blackbody spectrum at ~2.7 K. It is the redshifted remnant of photons released ~380,000 years after the Big Bang, when the universe cooled enough for electrons and protons to combine. The tiny temperature **ripples** trace small density fluctuations in the early universe β seeded by dark-matter clumping β that later grew into galaxies and large-scale structure. π **Textbook βΈ Ch 29.4:** The CMB was emitted **~380,000 years** after the Big Bang, when the universe cooled to ~3,000 K and electrons combined with protons to form neutral hydrogen ("recombination"). The discovery (Penzias & Wilson, 1965) won the 1978 Nobel Prize.