Astrophysics & Cosmology

What is a planet?

A body with enough mass for spherical shape, no fusion, and has cleared its orbit.

What is a dwarf planet?

A spherical body that has not cleared its orbit.

What is a planetary satellite?

A body that orbits a planet.

What is an asteroid?

A small rocky body orbiting the Sun, irregular in shape.

What is a comet?

A small icy body with an eccentric elliptical orbit.

What is a solar system?

A star and the objects orbiting it.

What is a galaxy?

A collection of stars, gas, and dust.

Approximate number of stars in a galaxy

~100 billion.

What is a nebula?

A large cloud of gas and dust where stars form.

What causes nebulae to collapse?

Gravitational attraction.

What energy change occurs during collapse?

Gravitational potential → thermal energy.

What is a protostar?

A hot dense sphere of gas forming a star.

What is required for a star to form?

Temperature and pressure high enough for fusion.

What fusion begins star formation?

Hydrogen → helium.

What is the main sequence phase?

Stable fusion phase of a star.

What balances gravity in main sequence stars?

Radiation pressure and gas pressure.

How does mass affect main sequence lifetime?

More mass → shorter lifetime.

Define solar mass (M☉)

1.99 × 10³⁰ kg.

Mass range of low mass stars

Initial mass less than about 8 M☉; final core mass less than 1.4 M☉.

What happens when hydrogen runs low in low mass stars?

Core collapses.

What does a low mass star become next?

Red giant.

Where does fusion occur in a red giant?

In outer shell.

Final stage of low mass star evolution

White dwarf.

What is a planetary nebula?

Ejected outer layers of a red giant.

Do white dwarfs undergo fusion?

No.

What supports a white dwarf?

Electron degeneracy pressure.

What is the Chandrasekhar limit?

1.44 M☉.

Mass range of massive stars

A high-mass star has an initial mass greater than about 8 M☉; its core reaches about 1.4 M☉ and collapses.

What do massive stars become?

Red supergiants.

Why can massive stars fuse heavier elements?

Higher core temperature.

What forms at the core before supernova?

Iron.

Why can iron not fuse further?

Fusion cannot continue beyond iron because iron has the highest binding energy per nucleon. Fusing iron into heavier elements would require energy instead of releasing it, so no further energy can be gained from fusion.

What event follows iron core formation?

Type II supernova.

Where are elements heavier than iron formed?

Supernovae.

What forms if core >1.44 M☉ and less than 3.00M☉

Neutron star.

How do neutron stars form?

Protons + electrons → neutrons.

What forms if core >3 M☉?

Black hole.

Why can light not escape a black hole?

Escape velocity > speed of light.

What does the HR diagram plot?

Luminosity vs temperature.

What can HR diagram position show?

Stellar type and evolution stage.

What are electron energy levels?

Discrete allowed energies.

Can electrons exist between levels?

No.

What is excitation?

Electron gaining energy.

What is de-excitation?

Electron losing energy.

What is emitted during de-excitation?

A photon.

Why are energy levels negative?

Energy needed to remove electron.

Define emission line spectrum

Bright lines on dark background.

Define continuous spectrum

All wavelengths present.

Define absorption spectrum

Dark lines on continuous background.

What causes absorption lines?

Photons absorbed to excite electrons.

Photon energy equation

E = hc / λ.

What determines photon wavelength?

Energy level difference.

What is spectroscopy?

Identifying elements from spectra.

What is a diffraction grating?

Many closely spaced slits.

Diffraction grating equation

d sinθ = nλ.

What does d represent?

Slit spacing.

What does n represent?

Order number.

What is Wien’s displacement law?

λ_max T = constant.

Value of Wien’s constant

2.9 × 10⁻³ m K.

What does Wien’s law determine?

Surface temperature.

What colour are hotter stars?

Blue/shorter wavelength.

State Stefan–Boltzmann law

L = 4πR²σT⁴.

Value of Stefan’s constant

5.67 × 10⁻⁸ W m⁻² K⁻⁴.

What does luminosity depend on?

Radius and temperature.

Define astronomical unit (AU)

Average Earth–Sun distance.

1 AU in metres

1.5 × 10¹¹ m.

Define light year

Distance light travels in one year.

1 light year in metres

9.46 × 10¹⁵ m.

Define parsec

Distance where 1 AU subtends 1 arcsecond.

1 parsec in metres

3.1 × 10¹⁶ m.

What is stellar parallax?

Apparent shift of nearby stars.

Parallax distance equation

d = 1 / p.

Units for d and p in parallax

pc and arcseconds.

Range parallax is accurate to

~100 pc.

State the cosmological principle

Universe is isotropic and homogeneous.

What does isotropic mean?

Same in all directions.

What does homogeneous mean?

Uniform density on large scale.

Define the Doppler effect

Change in observed wavelength due to motion.

What happens to wavelength if source approaches?

Decreases (blueshift).

What happens if source recedes?

Increases (redshift).

Doppler shift equation

Δλ / λ = v / c.

What is Hubble’s law?

Recessional velocity ∝ distance.

Hubble’s law equation

v = H₀ d.

What does redshift show about the universe?

It is expanding.

How can universe age be estimated?

1 / H₀.

Approximate age of universe

~14 billion years.

What is the Big Bang theory?

Universe began from hot dense singularity.

Key evidence for Big Bang

Redshift and CMB radiation.

What is CMB radiation?

Microwave background from early universe.

What happened during inflation?

Rapid expansion.

When did first atoms form?

~380,000 years after the Big Bang

When did first stars form?

~30 million years after the Big Bang

What is dark energy?

Hypothetical energy causing accelerated expansion.

Estimated % of dark energy

~68%.

What is dark matter?

Unseen mass affecting galaxy rotation.

Estimated % of dark matter

~27%.

Possible universe futures

Open, closed, flat.

What determines universe fate?

Overall density.