Astrophysics ALL Slides

Q: What are the two postulates of special relativity?

A:

1. The laws of physics are the same in all inertial frames.

2. The speed of light in a vacuum is the same for all observers, regardless of motion.

Q: What are some consequences of the postulates of special relativity?

A: Time dilation, length contraction, and mass increase at speeds close to c.

Q: What is general relativity?

A: Einstein’s theory that gravity is the result of mass curving spacetime, not a force acting at a distance.

Q: What is length contraction?

A: Objects moving close to the speed of light appear shorter in the direction of motion to a stationary observer

Q: What is time dilation?

A: A moving clock ticks more slowly compared to a stationary observer's clock.

Q: What is mass increase in special relativity?

A: An object’s relativistic mass increases as its speed approaches the speed of light.

Q: What is the equivalence principle?

A: There's no difference between acceleration and a gravitational field — locally, they're indistinguishable.

Q: What is spacetime?

A: A four-dimensional framework combining space and time into a single continuum affected by mass and energy.

Q: What are the rules of spherical geometry?

A:

• Parallel lines eventually meet.

• Triangle angles add to more than 180°.

• No truly straight lines — all great circles.

Q: What are the rules for saddle (hyperbolic) geometry?

A:

• Parallel lines diverge.

• Triangle angles add to less than 180°.

• Space is negatively curved.

Q: What is LIGO and what does it do?

A: LIGO is the Laser Interferometer Gravitational-Wave Observatory. It detects gravitational waves from events like black hole and neutron star mergers by measuring tiny spacetime distortions.

Q: What is the proton-proton chain?

A: The main nuclear fusion process in the Sun where hydrogen nuclei fuse to form helium, releasing energy.

Q: What is the difference between fission and fusion?

A:

• Fusion: Light nuclei combine into heavier ones (releases energy in stars).

• Fission: Heavy nuclei split into lighter ones (used in nuclear reactors).

Q: What are the products and byproducts of the proton-proton chain?

A: Helium-4 (main product), positrons, neutrinos, and gamma rays (byproducts). 0.7% of the original mass is lost as energy.

Q: How is the Sun’s lifetime estimated?

A: By dividing total nuclear energy available by the Sun’s current luminosity (~10 billion years).

Q: What was the gravitational contraction theory?

A: The old idea that the Sun shines by slowly contracting; this would power it for only ~25 million years.

Q: What is the solar thermostat?

A: A self-regulating feedback: if core temperature rises, fusion increases, pressure expands the core, cooling it back down.

Q: Why do photons take a long time to reach the photosphere?

A: They undergo countless scatterings in a process called random walk, which slows their outward journey.

Q: What is radiative diffusion?

A: The process by which photons slowly carry energy outward in the Sun’s radiative zone, by bouncing off particles.

Q: What is convection in the Sun?

A: Energy is transported by the movement of hot plasma rising and cooler plasma sinking, in the convection zone.

Q: What are the three types of neutrinos?

A: Electron, muon, and tau neutrinos.

Q: What is the photosphere?

A: The visible surface of the Sun, where most of the Sun’s visible light is emitted.

Q: What is the chromosphere?

A: The layer above the photosphere, emits ultraviolet radiation and appears pinkish during eclipses.

Q: What is the corona?

A: The Sun’s outer atmosphere, visible during eclipses, source of X-rays, and extremely hot.

Q: What are sunspots?

A: Cooler, darker regions on the Sun caused by intense magnetic activity.

Q: How are magnetic fields related to sunspots?

A: Sunspots form where magnetic field lines emerge or loop back into the Sun’s surface, inhibiting convection.

Q: What is the Zeeman effect?

A: The splitting of spectral lines in a magnetic field — used to measure the Sun’s magnetic field strength.

Q: What are solar storms?

A: Violent eruptions of charged particles and radiation caused by magnetic disturbances on the Sun.

Q: What are solar flares?

A: Sudden, intense bursts of radiation and particles from active sunspot regions.

Q: What is the sunspot cycle?

A: An ~11-year cycle in the number of sunspots due to changing solar magnetic activity.

Q: What is the 22-year magnetic cycle?

A: Every 11 years, the Sun's magnetic field flips — two cycles complete a full magnetic polarity cycle (22 years).

Q: What is the inverse square law in astronomy?

A: Apparent brightness = Luminosity ÷ (4π × distance²); brightness decreases with the square of distance.

Q: What is parallax?

A: The apparent shift of a star’s position due to Earth’s orbit, used to measure distances to nearby stars.

Q: What does the spectral sequence OBAFGKM(LT) represent?

A: A classification of stars from hottest (O) to coolest (M), with L and T for brown dwarfs.

Q: What is a visual binary?

A: A binary star system where both stars can be seen separately through a telescope.

Q: What is an eclipsing binary?

A: A binary system where stars pass in front of each other from our point of view, causing dips in brightness

Q: What is a spectroscopic binary?

A: A binary system detected through periodic Doppler shifts in the stars’ spectral lines.

Q: What is the Hertzsprung–Russell diagram?

A: A graph plotting stellar luminosity vs surface temperature (or spectral type).

Q: What is the main sequence on the H-R diagram?

A: The diagonal band where stars fuse hydrogen in their cores — most stars, including the Sun, lie here.

Q: Where are giants and supergiants found on the H-R diagram?

A: Upper right — they are cool but very luminous due to their large size.

Q: Where are white dwarfs on the H-R diagram?

A: Lower left — hot but dim, because they are small and no longer fusing.

Q: What are the stellar luminosity classes I through V?

A:

• I: Supergiant

• II: Bright giant

• III: Giant

• IV: Subgiant

• V: Main sequence

Q: What is a variable star?

A: A star whose brightness changes over time, often due to internal pulsations or eclipses in a binary.

Q: What is the instability strip?

A: A region on the H-R diagram where stars (like Cepheid variables) become unstable and pulsate.

Q: What are Cepheid variables?

A: Pulsating stars with a predictable relationship between their luminosity and period, used to measure distances.

Q: What are open clusters?

A: Loosely bound groups of young stars found in the disk of the galaxy.

Q: What are globular clusters?

A: Dense, spherical groups of old stars found in the galaxy’s halo.

Q: How can we determine the age of a star cluster?

A: By identifying the main-sequence turnoff point on the H-R diagram — more massive stars leave first.

Q: What determines the life expectancy of a main-sequence star?

A: Its mass — more massive stars burn through their fuel faster and have shorter lifespans.

Q: What is the interstellar medium (ISM) mostly made of?

A: ~70% hydrogen, ~28% helium, and 2% heavier elements (by mass).

Q: What are star-forming clouds?

A: Cold, dense regions of gas and dust (usually molecular clouds) where stars are born.

Q: What are molecular clouds?

A: The coldest, densest parts of the ISM, mainly composed of molecular hydrogen (H₂), and the birthplaces of stars.

Q: What is interstellar reddening?

A: The effect where dust in the ISM scatters blue light more than red, making distant stars appear redder.

Q: What is a protostar?

A: A collapsing cloud core that is heating up but not yet undergoing nuclear fusion in its core.

Q: What role does rotation play in star formation?

A: It causes the collapsing cloud to flatten into a disk and helps conserve angular momentum.

Q: What is flattening in star formation?

A: The process by which a spinning, collapsing gas cloud flattens into a disk due to conservation of angular momentum.

Q: What is an accretion disk?

A: A rotating disk of gas and dust surrounding a protostar, feeding material onto the growing star.

Q: How are jets formed during star birth?

A: Magnetic fields and rotation launch jets of material from the poles of the protostar.

Q: What is degeneracy pressure?

A: A quantum mechanical pressure that arises when particles are packed so tightly that their positions can't be more precisely defined — independent of temperature.

Q: What is a brown dwarf?

A: An object with mass between a planet and a star (less than 0.08 M☉) that never gets hot enough for hydrogen fusion.

Q: What are L & T dwarfs?

A: Spectral classes of very cool, low-mass stars and brown dwarfs — L dwarfs are warmer than T dwarfs.

Q: What is the minimum mass for a star to sustain hydrogen fusion?

A: About 0.08 solar masses (M☉).

Q: What is the maximum mass limit for a star?

A: About 150 solar masses (stars above this become unstable and lose mass quickly).

Q: What is a hydrogen-burning shell?

A: A shell of hydrogen around an inert helium core where fusion continues after the core stops fusing hydrogen.

Q: What is the triple-alpha process?

A: A nuclear fusion process where three helium nuclei (alpha particles) combine to form carbon.

Q: What is a helium flash?

A: A sudden, rapid onset of helium fusion in a low-mass star's degenerate core.

Q: What is the horizontal branch?

A: A stage in low-mass star evolution where the star is fusing helium in its core and hydrogen in a surrounding shell.

Q: What is double shell burning?

A: A phase where a star fuses helium in a shell around a carbon core and hydrogen in a shell above that.

Q: What is a carbon star?

A: A late-stage red giant with a carbon-rich atmosphere caused by helium fusion and dredge-up of carbon from the core.

Q: What is a planetary nebula?

A: The glowing shell of gas ejected from a low-mass star at the end of its life, surrounding a white dwarf core.

Q: What is the CNO cycle?

A: A hydrogen fusion process in high-mass stars using carbon, nitrogen, and oxygen as catalysts to fuse hydrogen into helium.

Q: What is advanced nuclear burning?

A: Fusion processes in high-mass stars after helium burning — elements fuse into heavier nuclei up to iron.

Q: What is helium capture?

A: A fusion process where helium nuclei are added to heavier elements, forming elements with even atomic numbers.

Q: What is multiple shell burning?

A: In late-stage high-mass stars, multiple layers of different elements undergo fusion in shells around an inert iron core.

Q: Why is iron the endpoint of stellar fusion?

A: Fusing or splitting iron doesn’t release energy — fusion beyond iron requires energy input, not release.

Q: Why are even-numbered elements more abundant in the universe?

A: Because of helium capture during nuclear fusion, which tends to create elements with even atomic numbers.

Q: What is a supernova?

A: A massive stellar explosion marking the death of a star, either via iron core collapse or white dwarf detonation.

Q: What is mass exchange in a binary system?

A: When one star expands and transfers material to its companion, potentially leading to novae, Type Ia supernovae, or accretion disks.

Q: What is a white dwarf?

A: The dense, Earth-sized core left behind after a low- to intermediate-mass star sheds its outer layers.

Q: What is the typical size of a white dwarf?

A: About the same size as Earth, but with a mass comparable to the Sun.

Q: How dense is a white dwarf?

A: Extremely dense — about 1 ton per cubic centimeter.

Q: What is the Chandrasekhar limit?

A: The maximum mass a white dwarf can have: ~1.4 solar masses. Beyond this, it cannot support itself with electron degeneracy pressure.

Q: What is an accretion disk in a binary system?

A: A rotating disk of matter formed by material transferred from a companion star onto a white dwarf, neutron star, or black hole.

Q: What is a dwarf nova?

A: A small, recurring outburst of brightness from a white dwarf in a binary system due to instability in the accretion disk.

Q: What is a nova?

A: A sudden, temporary increase in brightness caused by runaway hydrogen fusion on the surface of a white dwarf in a binary.

Q: What happens when a white dwarf exceeds 1.4 M☉?

A: It undergoes uncontrolled carbon fusion and explodes as a Type Ia supernova.

Q: What is a white dwarf supernova (Type Ia)?

A: A supernova triggered when a white dwarf accretes enough mass to exceed the Chandrasekhar limit.

Q: What are Type Ib and Ic supernovae?

A: Core-collapse supernovae from massive stars that have lost outer hydrogen layers (Ib has helium; Ic does not).

Q: What is a Type II supernova?

A: A core-collapse supernova from a massive star with hydrogen lines in its spectrum.

Q: What is a neutron star?

A: The dense remnant of a massive star’s core after a supernova, composed mostly of neutrons.

Q: What is a pulsar?

A: A rapidly rotating neutron star emitting beams of radiation from its magnetic poles.

Q: What is a glitch in a pulsar?

A: A sudden change in the rotation speed of a pulsar, likely due to shifts in its internal structure.

Q: What is an X-ray binary?

A: A system where a neutron star or black hole accretes matter from a companion, producing strong X-rays.

Q: What causes X-ray bursts?

A: Runaway nuclear fusion on the surface of a neutron star after matter builds up from accretion.

Q: What is a black hole?

A: An object with gravity so strong that nothing, not even light, can escape beyond its event horizon.

Q: What is the event horizon?

A: The boundary around a black hole where escape velocity equals the speed of light.

Q: What is the neutron star mass limit?

A: About 2–3 solar masses; beyond this, the core collapses into a black hole.

Q: What is a singularity?

A: A point at the center of a black hole where density and gravity become infinite.

Q: What is spaghettification?

A: The extreme stretching of objects in strong tidal gravitational fields near a black hole.

Q: What effect does rotation have on a black hole?

A: It can distort space-time and form an ergosphere — allowing some energy extraction.