Stellar evolution

What is on the x-axis of the Hertzsprung-Russel diagram?

Spectral class (OBAFGKM) or temperature

What is the range of spectral class and temperature on the x-axis of a hertzsprung-russel diagram?

• Spectral class: OBAFGKM, OBA close together and FGKM spread further apart

• 30 000K-3 000K; high temp on the right, low on the left

What is on the y-axis of the Hertzsprung-russel diagram?

Luminosity or absolute magnitude

What is the range of luminosity and absolute magnitude on the y-axis of a hertzsprung-russel diagram?

• Luminosity: 100 000 - 0.000 01 (logarithmic scale)

• Absolute magnitude: -10 → +15

Draw a hertzsprung-russel (H-R) diagram and label star areas. +explain using stefan’s law

HERE

What is the minimum mass for a cloud to become a protostar? (/Minimum temperature for thermonuclear reactions to take place)

0.1 * solar mass

What are brown dwarves?

What happens in the main sequence phase of a star?

• the inward force of gravity and the outward force due to nuclear fusion are in equilibrium. The star is stable.

• Hydrogen nuclei are fused into Helium nuclei

• The greater the mass of the star, the shorter the main sequence phase of the star.

What happens in the protostar phase of a star?

• Nebulae clumps together due to gravitational forces

• The irregular clumps rotate and a gravity/conservation of

  angular momentum spins them inwards to form a denser centre (a protostar)

• Protostar is surrounded by a circumstellar disc

• When protostar is hot enough, it fuses elements producing a stellar wind that blows away surrounding material

What is a circumstellar disc?

a disc of material (surrounding a protostar)

Red giant

• star < 3 solar masses

• hydrogen runs out, temperature of the core increases and begins fusing helium nuclei into heavier elements (carbon, oxygen, beryllium)

• Outer layers of the star expand and cool

White dwarf

• star < 1.4 solar masses

• Once all fuel is used, fusion stops and the core contracts as gravity > outward force of fusion

• Outer layers thrown off, forming a planetary nebula around the remaining core

• core becomes extremely dense

• Eventually cools to a black dwarf

Red supergiant

• star > 3 solar masses

• Once hydrogen runs out, temperature of the core increases and begins to fuse helium nuclei into heavier elements

• Fuses elements up to iron (which red grains cannot)

Supernova (stellar evolution)

• star > 1.4 solar masses

• all fuel runs out, fusion stops and the core collapses inwards suddenly and becomes rigid (cannot become more dense)

• Outer layers of the star fall inwards and rebound off the core, resulting in a shockwave

• As shockwave passes through surrounding material, elements heavier than iron are fused and released into space

• Remaining core is dependent on mass of the star

• defined by its rapidly increasing absolute magnitude

Neutron star

• 1.4 - 3 solar masses

• protons and electrons are fused to form neutrons

• stars are as dense as nuclear matter

• Pulsars are spinning neutron stars that emit beams of radiation from the magnetic poles as they spin

Black hole

• star > 3 solar masses

• its escape velocity is greater than the speed of light

• Schwarzchild radius is the radius of the event horizon

What is a type I supernova?

when a star accumulates matter from its companion star in a binary system and explodes after reaching a critical mass

What is a type 1a supernova?

• A type I supernova with a white dwarf.

How are type 1a supernovae formed?

• when the companion star in a binary system runs out of hydrogen, it expands, and the white dwarf accumulates some of its mass.

• When the white dwarf star reaches a critical mass, fusion begins and the mass increases, causing the white dwarf to explode into a supernova.

How are type 1a supernovae used as standard candles?

all type 1a supernovae occur at the same critical mass, therefore they reach a consistent peak luminosity, so their distances can be determined using their observed brightness.

• absolute magnitude is constant = -19.3

What is the typical light curve for a type 1a supernova? (draw)

time is measured starting from the peak

What is a type II supernova?

The death of a high-mass star after it runs out of fuel

(normal, stellar evolution definition)

What is a standard candle?

a celestial object with a known and predictable luminosity, allowing astronomers to determine its distance from Earth by comparing its apparent brightness to its luminosity