IB physics Option D

Constellation

A group of stars distant in space that form a recognizable pattern viewed from Earth.

Stellar cluster

A globular group of stars (including gas and dust) held together by gravity that are close to each other in space

Binary stars

Consist of two stars that rotate around a common centre of mass.

Luminosity of a star

The total power radiated by the star.

Apparent brightness

The power per square meter received at the surface of Earth

Stellar parallax

Half the angle between the star and Earth measured six months apart

Standard candle

Star with a known luminosity

Galaxies

Stars, gas, and dust held together by gravitational forces.

Hydrostatic equilibria

The balance between outward radiation pressure from fusing nuclei, and inward gravitational pressure.

Wien’s (displacement) law

Explains why stars are different colours. Using the blackbody model, stars have a peak emission wavelength. Hot stars have a shorter peak wavelength while cooler stars have a longer one.

Cepheid variable

A star which has luminosity that varies periodically over time due to the outer layers undergoing periodic expansion and contraction causing variations in surface area. When the surface area is large, the temperature is low and vise versa. they’re ‘standard candles’ since their luminosity is known using the period-luminosity relation/ graph

Hubble’s law

The recessional velocity of a galaxy is proportional to its distance away from Earth.

Big bang evidence

1. The cosmic microwave background (CMB) is the thermal radiation left over from the time of recombination in Big Bang cosmology. The predicted wavelength at the big bang and the measured wavelength today match up after accounting for the expansion of the universe.

2. Universe is expanding because every distant galaxy is moving away from us. Redshift.

Evidence of black holes

• The X-rays emitted by matter spiralling towards the edge of a black hole

• Unimaginably strong gravitational fields have been seen to spiral stars; a black hole has been detected in the centre of the milky way.

Chandrasekhar Limit

The upper limit to the mass of a white dwarf, equal to 1.4 solar masses. Further collapse is restricted by electron degeneracy pressure.

Oppenheimer-Volkoff Limit

The upper limit on a neutron star for which neutron degeneracy is able to resist further collapse into a black hole. 2<M<3 solar masses

Critical density

Density at which the universe will expand forever but the rate of expansion will approach zero.

Open universe

A universe which continues to expand and gravity slows the rate of expansion but cannot stop it.

Density of universe < Critical Density

Closed universe

A universe which will eventually collapse on itself resulting in a big crunch, 

(the reverse of Big Bang)

Density of universe > Critical Density

Flat universe

In-between an open and closed universe, gravity keeps slowing down the expansion but theoretically it takes infinite time to come to rest. 

Density of universe = Critical Density

Evidence for accelerated expansion of the universe

By using Type Ia supernovae as standard candles to estimate galactic distances and measure their redshifts, evidence suggested the universe is undergoing an accelerated expansion.

The universe contains a significant amount of baryonic matter that should slow down its expansion. This acceleration should therefore require a form of invisible energy, namely, dark energy.

Black body

A perfect emitter and absorber of light

The Cosmological Principle

1. The universe is homogenous. (the same everywhere)

2. The universe is isotropic. (there is no centre)

Dark matter

Undetectable matter that emits no radiation.

Dark energy

Undetectable energy that overcomes the gravitational pull of matter.

Anisotropies in the CMB

Anisotropies are minute temperature fluctuations in the isotropic reading of the universe.

They result from tiny random variations in density which result in cosmological structure (galaxies, stars, etc.).

Cosmic scale factor

A measure of the size of the universe which also describes the expansion of the universe

Black hole

Space-time singularity

Evidence of dark matter

Within spiral galaxies, the rotational speed of stars are expected to decrease as the distance from the center of the galaxy increases.

However, the velocity of the outer stars remain constant and are greater than expected.

This implies there is some large mass on the outer edge of the galaxy which increases the rotational velocity which is dark matter.

Evidence of dark energy

Evidence from type Ia supernovae acting as a standard candle shows that galaxies are moving further apart and the expansion of the universe is accelerating.

However, the universe is filled with baryonic matter which should theoretically slow down the expansion of the universe

Dark energy counteracts the forces of gravity and accelerates the rate of expansion of the universe

Neutron capture

Upon the collapse of a type II supernova, gravitational pressure overcomes electron degeneracy pressure. This causes electrons and protons to undergo inverse beta decay to produce massive amounts of neutrons.

These neutrons have no charge, hence allowing them to penetrate the charged nuclei of elements produced within the red giant, specifically, Fe.

Thus, Fe captures the neutrons and undergoes beta decay to produce heavier and higher atomic number elements.

R process

Involves rapid neutron capture and occurs at high temperatures during a type II supernova.

Multiple neutrons are captured in the nucleus of a heavy element like Fe. This creates a heavier isotope of the element (Fe).

Then the nucleus undergoes beta minus decays, turning the neutrons into protons and increasing atomic number to become stable.

As a result, nuclei with higher atomic numbers than Bismuth are produced.

S process

Involves slow neutron capture and occurs at moderate star temperatures and giant stars.

A small number of neutrons are captured in the n the nucleus of a heavy element like Fe. This creates a heavier isotope of the element (Fe).

Then the nucleus goes through beta decay to increase the atomic number of nuclei by turning neutrons into protons before another neutron is captured.

As a result, nuclei with atomic numbers higher than Fe and lower than or equal to Bismuth are produced.

Jean’s mass

The minimum mass required to form a protostar for a given radius and temperature

How do type Ia and II supernovae form

Ia:

• Forms in a binary star system with a red giant and white dwarf

• The white dwarf accretes mass from the giant until the Chandrasekhar limit at which point it explodes

II:

• A red supergiant explodes after it runs out of elements to fuse and its core collapses

Stellar evolution for a small star (M < 8 solar masses)

Nebula → Protostar → MS star → Red giant → Planetary nebula → White dwarf → Black dwarf

Stellar evolution for a large star (M > 8 solar masses)

Nebula → Protostar → (Blue) MS star → Red supergiant → Type II supernova → Black hole (M>3 at supernova) (or) Neutron star (2<M<3 at supernova)

Causes for CMBR

1. Density perturbations

2. Quantum fluctuations

Significance of Critical Density

critical density is used to predict whether the universe is flat or curved / the future of the universe

Astronomical unit

The mean distance from the centre of the Earth to the centre of the Sun

Nebula

A cloud of dust, hydrogen, helium and other ionized gases.