The death of stars like the sun Astronomy Midterm

How does a star become a main sequence star

Once you start fusion, gravity is still winning

• As it shrinks more and gets hotter, it settles into hydrostatic equilibrium

• MAIN SEQUENCE STAR

  • Generates energy by hydrogen to helium (fusion) AND is in equilibrium

Summary of these stars so far and the formations

• Molecular cloud → Protostar → Star → Main sequence star

  • A cold, dense molecular cloud collapses & breaks up

  • When the center is hot and dense enough it becomes opaque (protostar)

  • When fusion of hydrogen into helium begins, a star is formed

  • When the star reaches hydrostatic equilibrium, it is now a member of the main sequence

Higher mass vs lower mass stars

• Massive stars

  • more gravity, die fast but born quickly

• Lower mass

  • less gravity, born slowly

Brown Dwarfs

Failed stars

• Never gets hot and dense enough to produce fusion, so it collapses

• center of the star gets solid, so can’t shrink to get energy

• Able to fuse deuterium for a little, BUT NEVER hydrogen

Angle Momentum

Has to spin faster to not collapse, but stuff can fall from above or below that wouldn’t affect it

Disk formation

Causes the gas around a protostar to settle into a flat, rotating disk

• disk that grows because material is falling on it from outer parts of the cloud

• Happens from massive black holes

• Molecules hit each other they stay together and make bigger

In what way do disc spins

Counterclockwise

• This is why the planets were spinning counterclockwise

• All the planets orbit in the same plane in the same direction

Evolution

Describes how a SINGLE object changes overtime

• NOT population

Burning

Nuclear fusion

• Ex: main sequence stars burn hydrogen into helium

Mass

The main determinant of where a star will be on the main sequence

• L ∝ M^4

• determines the evolution of the star

when do stars start leaving the Main Sequence

When all the hydrogen in a star’s core is consumed, the star begins to leave the Main Sequence

• Low mass stars like the sun GO QUIETLY

• High Mass stars GO OUT EXPLOSIVELY

• Luminosity goes up, and the star gets larger + cooler

  • RED GIANT

Shell burning

Hydrogen fusion happens in a shell outside the inner core

Departures from equilibrium

A change in a star’s condition leads to a change in its structure (temperature/density/size) until the hydrostatic equilibrium is restored

• When the core heats up, pressure goes up, and outer layers expand and cool

  • Makes the star bigger

Structure of a red giant

• Center is made of helium

• Supported by degeneracy pressure

2 kinds of gas pressure

• Ordinary pressure

  • the force exerted by particles bouncing off each other

• The Pauli Exclusion Principle (degeneracy pressure)

  • requires that no 2 identical particles of ordinary matter (like two electrons) ever be in the same state

Degeneracy Pressure

It resists being compressed any further by pushing material bacj

• Independent of temperature, but not density

• Stops creating energy which is a problem

A new equilibrium

After helium fusion starts, the structure of the star readjusts itself bc there’s enough energy to re-expand

• Ends up as a Horizontal Branch star

  • Spread horizontally on the HR diagram bc it has abt the same luminosity

  • Burns Helium stably (not expanding or shrinking) at the core & is in hydrostatic equilibrium

What is LESS efficient to fuse, Helium or Hydrogen?

Helium

The death of a sun-like star

There is no more fusion energy being generated in the central core, which is now only carbon and oxygen, and continues to contract

• “Flashes” of intense Helium fusion occur

  • 50% stays with object, 50% is blown out. Then everything settles down and it happens again

Planetary nebula

Gas from the outer part of a red giant ejected during its last stages of evolution (The reminisce of the star)

• More gas at the edge, so that’s why it is brighter and looks like a ring

• HAS NOTHING TO DO WITH THE FORMATION OF PLANETS

Life after death

• Planetary Nebula: The outer parts of the star are now a cloud of gas (BIG)

• White dwarf: The small, extremely dense carbon/oxygen CORE remains, containing about half the star’s mass and supported by electron degeneracy pressure

What force prevents white dwarfs from collapsiing?

Electron degeneracy pressure

White dwarf is born

The remaining core is extremely dense and extremely hot but very small

• Very hot, but cools off quickly

• Stays the same size bc it is a solid and just cools down

• ONLY Luminous bc of the high temperature

• Can't collapse bc of the degeneracy pressure

End of planetary nebula

As the dead core of the star cools, the nebula continues to expand and eventually dissipates into the interstellar medium

Fate of a white dwarf

As it cools it does not change size

• Gets cooler and dimmer until it is extremely faint (“Black dwarf”)

  • black dwarf is a white dwarf that cooled so much you can’t really see it, it’s almost invisible

Summary of Solar Evolution

Molecular cloud → Protostar → main sequence star (star) → red giant→ white dwarf → planetary nebula