A14 - Stellar evolution Flashcards
Name 2 catalogues that categorise DSOs:
-Messier Catalogue
-New General Catalogue (NGC)
Name 4 types of objects that the Messier catalogue and NGC categorise:
-nebulae
-open clusters
-globular clusters
-galaxies
Describe a systematic method for naming stars within a particular constellation:
-the Bayer system
-brightest star in a certain constellation is labelled α, and the genetive form of the Latin name of the constellation is tacked onto the end
-next brightest is called β, γ, Δ, and so on
eg Betelgeuse is called “α Orionis”
How does a main sequence star form, right from the beginning?
-clouds of gas and dust (nebulae) undergo gravitational collapse
-small clumps of the dust form due to gravity, which then form the beginning of new protostars
-as more matter begins to accumulate, pressure and temperature increases, eventually becoming hot enough for nuclear fusion of hydrogen into helium
-once this happens, it is now a main sequence star
Describe the interactions between gravity and radiation pressure in a main sequence star:
-energy radiated by the core from nuclear fusion causes an outwards pressure, called radiation pressure
-balanced with gravity from the mass of the star, and will continue to be balanced throughout the main sequence (~90% of its life)
it is in hydrostatic equilibrium
Describe how a star ends its main sequence stage:
-after the MS stage, the hydrogen in the core runs out, leaving behind a denser helium core
-radiation pressure goes to 0, so the core collapses under its own gravity
-increased pressure causes hydrogen to fuse in a shell around the inert helium core, halting the core’s collapse
What determines the path a star takes after the MS?
-its mass determines the next stage
-if the star’s mass is lower than 1.4 solar masses, it is considered to be a low mass star
-otherwise it is a massive star
1.4 M☉ is known as the Chandrasekhar limit
Explain the stage directly after the MS, assuming its mass is below the Chandrasekhar limit:
-fusion reactions continue to occur around the star’s inert helium core
-makes a much higher radiation pressure, so the star expands and cools to form a red giant (it cools down because energy is dissipated over a much larger area)
Helium core may fuse into carbon or oxygen, but it depends on its mass
What happens to a star after the red giant stage? Describe the properties of the star formed:
-outer layers of star continue to expand, and are visible as a planetary nebula
-core stops collapsing due to a new outward force of electron degeneracy pressure (balanced with gravity)
-star’s leftover core is now a white dwarf, which is still very hot but much smaller (not very luminous)
cools into black dwarf after but takes too long to know if they exist
How does a massive star evolve after the MS?
Don’t go further than the end of the stage after the MS
-new fusion reactions continue to occur due to the core’s collapse
-high radiation pressure causes it to expand into a red/blue supergiant
-new elements form more inner shells with each older element being pushed into an outer shell, until it forms an iron core
-fuel is exhausted as no further elements can be fused (no radiation pressure from iron fusion), so core collapses
What dictates the end of a red/blue supergiant’s life? Describe what happens after and why:
-its mass (who would’ve guessed)
-supernova occurs
-if less than 3 M⊙, the core’s collapse from the previous stage is halted and balanced by neutron degeneracy pressure (electrons forced into protons)
-leaves behind ~20km ∅ neutron star (super dense)
-if over 3 M⊙, neutron pressure is unable to halt collapse and forms a black hole
Give some physical properties of neutron stars:
-made of neutrons and is small/very dense
-extremely strong surface gravity
-rotates very quickly
-very strong magnetic field
image from kurzgesagt
Summarise the lifecyle of a star from birth to death with a diagram:
What is an emission nebula?
emission nebulae are the first stage of new star formation, due to the abundance of hot hydrogen gas that become ionised and emits EM radiation
eg Orion Nebula
What is an absorption nebula?
absorption nebulae are seen as dark patches against the starry background of the Milky Way, as they absorb most of the light from behind it
eg Horsehead Nebula
Only the horsehead part is the absorption bit (blocks red from behind)
How do we detect black holes? Describe 3 methods:
-detecting the x-rays from its accretion disk when a star falls into it
-gravitational lensing causes 2 identical galaxies to be observed around it
-observing the motion of stars near the black hole due to their gravitational orbits around it
