E5 Fusion & Stars

Fusion & Stars

E5.1 Nuclear fusion

Joining of 2 small nuclei to produce a larger nucleus.

Eg. ²₁H (deut.)+ ³₁H (trit.) → ⁴₂He + ¹₀n + ENERGY

HUGE amounts of energy released in reaction. This energy e.g. provides radiation pressure (prevent star from collapsing under gravity) → provide fuel for star to continue burning.

He mass < total nucleon mass

E5.2 Strong nuclear force

For 2 nuclei to fuse, both nuclei must have high E_k, to overcome repulsive electric forces (protons), and because strong nuclear binding nucleons has very short range→ need nuclei to get very close tg. for F_nuclear to be activeactive → extremely hot/dense env needed to achieve fusion. Binding energy per nucleon increases.

E5.3 Energy released in fusion

mass defect between parent nuclei vs daughter: energy released.

³₁H + ²₁H → ⁴₂He + ¹₀n + 2γ

E5.4 Star Formation

Need high E_k : high T, high pressure and density.

4 Hydrogen nuclei (protons) → 1 He nucleus + 2γ + 2v + 2β⁺

Momentum of γ outward = radiation pressure = outward.

E5.5 Equilibrium in stars

Stable when in- out-ward forces in eq. in gravity vs out gas

T increases: out pressure increases; volume decreases: gravity (p>F_g: expand)

gas + rad pressure = opposite and equal to balance F=mg

T up: expand. when pressure>F_g

E5.6 Life cycle of star

Predictable;

4 Initial stages: after this either low core mass, or high mass

1. Nebula: giant cloud of H-gas + dust: grav attraction; denser clumps: grav collapse

2. Protostar: collapse cause H to heat and glow. T up. Detected as infrared radiation

3. Fusion of H → He begins → grav field attract more gas/dust → increase T and pressure of core.

4. Main star sequence: stable state, F_out = F_in . As T up, V down, gas pressure up.

Low mass stars:m<4m_S→red giant→plan neb→white dwarf:

red giant: H supply runs out. Star shrinks→hotter core →He→Be

plan. nebu: He core run out→ not hot enough→outer layers release

E5.7 Life cycle of low mass stars

E5.8 Life cycle of massive star

m>4m_S→red supergiant→supernova→neutron star/black hole

Red supergiant: same red giant: H runs out,make element→Fe

Supernova: iron core collapses: outer shell is blown into explosiv

Neutron star/black hole; collapsed core either in tact or collapse

E5.9 Hertzsprung russel diagram (HR)

Size (up) vs surf Temperature (decreasing!!) vs Luminosity (up)

All relative to sun. Stars are clustered in distict areas.

Most in main sequence, for most of them, luminosity inc w. surf T

Red giants+supergiants, white and black dwarfs.

Go up from right bottom to mid top, then down to left bottom.

E5.10 Continuous spectrum of star

photons emitted from core containing all f and λ of EM spectrum. Hot dense sources, core of star.

E5.11 Emission spectration

When electron transitions from high E-level to lower; emit photon. each transition corresponds to diff λ of light: line on spec

Discrete λ represented by coloured line on black background. Low pressure, hot gases.

E5.12 Absorption spectra

absorption of a photonl cool, low pressure gases. see rainbow-black lines.

E5.13 Chemical composition of stars and their colour emitted

Hotter: white or blue

Cooler: red or yellow

Give absorption line spectrum to identify (core=contin.)

E5.14 Stellar paralax

AU = astronomical units: mean dist. centre earth-sun: 1.5×10¹¹m

Light years: ly ; dist trav light 1 year = 9.5×10¹⁵ m

Parsecs: pc ; ¹/_{parallax angle in sec}. d=¹/_p ,1pc=3.26ly = 3.1×10¹⁶m

E5.15 Parallax calculations

How position of an object appears to change depending on where observed from: seen in like mountains, when moving diff angles form objects.

Stellar parallax: used to measure distances nearby stars; apparent shifting in position of nearby star against background distant star when viewed from diff position of earth in sun orbit.

Paralax angle, when with time interval 6 months: orbit diameter, diff apparent position of star, use p = ¹/_d.

arcsec vs arcminute: 60arcsec = 1 arcmin

E5.16 Stellar radius

L = 4πr²σT⁴ = AσT⁴ easyyyy