Astrophysics

Unit for Gravitational Field Strength

Newton per Kilogram (N/kg)

Universe

large collection of billions of galaxies

Galaxy

large collection of billions of stars

Our Solar System

Milky Way Galaxy

Explain Variations in Gravitational Field Strength

• earth's gravitational field strength is different than that on other planets and the moon

• larger mass → greater gravitational field strength

• smaller radius → greater gravitational field strength at surface

Gravitational Force causes:

• moon to orbit planets

• planets to orbit the sun

• artificial satellites to orbit the Earth

• comets to orbit the Sun

Differences in Orbits

• comet: very elliptical

• planets: elliptical

• moon: circular

Orbital Speed Equation

orbital speed = (2 x π x orbital radius)/ time period

v = (2 x π x r)/ T

Classification of Stars According to Colours

• star's colour is related to its surface temperature

• hot stars looks blue, medium stars looks yellow, cooler stars look red.

• OBAFGKM (hottest to coldest) - Oh Be A Fine Guy, Kiss Me

Evolution of Stars of Similar Mass to Sun

Nebula → Main Sequence Star → Red Giant → White Dwarf Star

Process of Evolution of Stars from Nebula to Main Sequence Star

• nebula is cloud of dust

• gravity causes cloud to collapse

• temperature of cloud increases

• hydrogen atoms in nebula move towards each other due to gravitational force

• KE store increases, increase temperature

• nuclear fusion of hydrogen starts

• becomes main sequence star

Process of Evolution of Stars from Main Sequence Star to White Dwarf

• fuses H into He in core

• outward pressure from fusion balances inward gravitational force — star is stable

• hydrogen in core runs out

• core contracts and heats up

• outer layers expand and cool

• becomes a red giant

• helium fuses into heavier elements in core

• core becomes white dwarf

Evolution of Stars with Mass Larger than Sun

Main Sequence Star → Red Supergiant → Supernova

Nebula → Neutron Star or Black Hole

Process of Evolution of Stars with Mass Larger than Sun

• fusion stops in core of main sequence star

• core collapses (increases temp in layer surrounding core)

• restarts fusion in core

• star contracts, surface temp decreases

• star becomes red supergiant

• fusion of heavier elements stops in core

• star explodes as supernova

• core collapses to form a neutron star or black hole

Absolute Magnitude

brightness of star at standard distance

Hertzsprung-Russel Diagram

A → White Dwarfs

B → Main Sequence

C → Giants

D → Supergiants

Big Bang Theory

• expansion (of space)

• continuing (expansion)

• from very hot/dense start

• from a point

• origin of Universe

Evidence Supporting Big Bang Theory

• Red-Shift

• Cosmic Microwave Background (CMB) Radiation

Red-Shift of Galaxies

• change in frequency or wavelength of radiation

• wavelength increased/ frequency decreased

• as star moves away (from us)

• big bang has expanding universe

• with galaxies moving away (from each other)

• further the galaxy is (from Earth), the greater the red-shift;

• larger redshift means faster movement of galaxies

• the further away, the faster the galaxy moves away

• galaxies moving apart from each other implies expansion from a single point or since the Big Bang;

CMB Radiation

• radiation that comes from all over the sky

• the Big Bang theory has a initial explosion

• that releases radiation

• CMBR appears to be the same in all directions / is everywhere

• (which) implies all parts of the Universe were in contact a long time ago

• wavelength has increased (as the universe has expanded);

• Universe was (significantly) hotter long ago;

• Doppler Effect

Relationship between Change in Wavelength and Reference Wavelength

Red-Shift in Light Received from Galaxies at Different Distances

• light shifted to red end of specctrum

• light waves stretched so wavelength increases

• black lines move to red end