Part I: The Life and Death of Stars

1
Part I The Life and Death of Stars

• Earth / Space Science

2
What is a star?

• The simplest (although not the most accurate)
  description of a star is that it is a massive
  ball of gas held together by the collective
  gravity of its innumerable atoms.

3
What is a star?

• Recall the composition of a star
• Inward gravitational force, outward pressure
• The pressure squeezes the gas and heats it
• The rising temperature increases the outward
  pressure
• Hydrostatic equilibrium is achieved
• Gravity and pressure are in balance

4
What is a star?

• Hydrostatic Equilibrium

5
What is a star?

• Luminosity is the amount of energy radiated per
  second by an astronomical body.
• Very similar to the wattage of a light bulb
• The high temperature in the stars core causes
  heat to flow from the core to the surface,
  eventually escaping as starlight.

6
Birth of a star

• Stars begin life as interstellar clouds.
• A cold, dark mass of gas drifting through the
  galaxy.
• Made up of roughly 71 Hydrogen and 27 Helium
• Also contains solid, microscopic particles of
  silicates, carbon, and iron compounds

7
Birth of a star

• The cloud somehow gets irritated by outside
  gravity
• Perhaps some space debris or another cloud comes
  into contact with it.
• Gas and dust begin to form clumps
• These clumps encounter outside forces and
  collapse
• Gasses drawn inward by the increasing gravity

8
Birth of a star

• The increasing gravity begins to compress and
  heat the gas
• The cloud of gas begins to rotate
• This rotation causes the cloud to flatten into a
  disk
• This disk is called either a circumstellar disk
  or a protostellar disk

9
Birth of a star

• At the center of this disk, a protostar begins to
  form.
• This happens approximately 1,000,000 years after
  the cloud first gets irritated.
• This protostar is a small, hot, dense core
• It is hotter than the original cloud of gas
• It is not yet as hot as a star

10
Birth of a star

• Eventually, the protostar gets hot enough to
  support nuclear fusion.
• The proton-proton chain begins as hydrogen is
  fused to form helium and release energy.
• Strong outflows of gasses begin
• Jets of leftover gas stream out into space from
  the protostars poles bipolar flows
• These bipolar flows begin to clear away gas and
  dust from around the protostar.

11
Birth of a star
12
Birth of a star

• Actual protostar in the Orion Nebula

13
Birth of a star

• The amount of gas and dust in the original cloud
  will determine what kind of star will be born.
• The more massive the protostar
• The shorter the stars life span
• The hotter the star
• The more luminous the star
• This called the mass-luminosity law

14
Birth of a star

• This example of the H-R diagram shows where
  protostars of different masses will end up on the
  main sequence
• It also shows approximately how long they will
  live

15
Life of a star

• Main sequence stars
• As learned earlier, energy leaks out of a star as
  heat and light.
• Stars need to replenish this energy in order to
  sustain their incredible masses.
• This is achieved by converting their hydrogen
  into helium (remember energy is released when
  this happens)

16
Life of a star

• Main sequence stars
• Most average stars have a few billion years
  worth of hydrogen to use.
• Our sun has about 5 billion years of H left.
• When this hydrogen is used up, the star begins
  its long road to death.

17
Life of a star

• Once all of the hydrogen in a stars core is used
  up several things happen
• It begins to burn helium instead
• The core shrinks and becomes denser
• The pressure in the core rises, therefore
  increasing the temperature

18
Life of a star

• Extreme waves of heat are released, pushing the
  outer layers of the star further out from the
  core.
• This expansion causes the outer layers of the
  star to cool
• The cooler outer layers now look red
• This huge, red star is now called a red giant.

19
Death of a star

• When the helium is exhausted, the star begins
  burning nitrogen, and so on
• As each fuel is burned up, the star adjusts its
  structure
• It grows brighter, larger, and cooler (on the
  surface its core is still extremely hot)
• Very old red giants have an onion structure
  with extremely heavy elements such as iron at the
  core.

20
Death of a star

• The internal structure of an old red giant

21
Death of a star

• The increasing waves of released energy push dust
  outward from the red giant
• Once the fuel is used up, the dust creates a
  slowly expanding shell around a hot collapsed
  core.
• This core is now known as a white dwarf.

22
Death of a star

• The expanding shell of gas and dust is known as a
  planetary nebula
• New stars are born from this cloud of gas and
  dust.

23
Death of a star

• Extremely massive stars
• Different things happen to stars with
  significantly more mass than an average star.
• These stars have cores that are hot enough to
  fuse carbon into higher elements such as oxygen
  and iron

24
Death of a star

• Supernovae
• Once a stars core turns to iron, the iron cannot
  fuse anymore to make heavier elements.
• The immense mass of the iron core causes it to
  collapse in on itself.
• This triggers an incredible explosion that blasts
  the outer layers of the star into space.

25
Death of a star

• One thing that can happen at this point
• The protons and electrons in the iron core can
  merge into neutrons.
• This causes the core collapse into a ball about
  20km across made up of nothing but neutrons with
  a mass not too much more than our sun.
• This is known as a neutron star

26
Death of a Star

• Top image shows the composition of a neutron star
• Bottom image shows a neutron surrounded by the
  supernova remnant

27
Death of a Star

• The other thing that can happen
• If the star is massive enough (the most massive
  stars of all), the iron core can collapse into a
  black hole

28
Intermission
29
Part II Types of Stars and Stellar Remnants

• Along with some other stuff

30
Types of stars

• Yellow giants
• These stars lie between the hot, luminous, blue,
  main sequence stars and the red giants on the H-R
  diagram.
• Yellow giants tend to pulsate
• Their size changes, thus making their luminosity
  change.

31
Types of stars

• Pulsating yellow giants
• The atmospheres of yellow giants trap some of the
  radiated energy from the core
• This energy heats the outer layers, thus raising
  pressure and causing expansion
• The expanding gas eventually cools, the pressure
  drops, and gravity pulls the layers back down
• The cycle goes on and on

32
Types of stars

• White dwarfs
• Hot, compact stars
• Masses comparable to our Sun
• Diameter about the size of Earth
• Very dim (100 times fainter than the Sun)
• In about 10 million years, white dwarves cool
  enough so as not to emit any visible light
• These are known as black dwarves
• About ½ of the galaxys mass is composed of white
  dwarf stars

33
Other stuff

• Supernovae
• The spray of elements created by nuclear fusion
  flies from the core of the star at more than
  10,000 km/sec.
• Supernovae enrich the interstellar gas with heavy
  elements
• Free neutrons combine with other atoms to create
  heavier elements such as gold, platinum, and
  uranium

34
Other stuff

• Supernovae
• When a star goes supernova, more energy is
  released in that few minutes than the star
  released during its whole lifetime.
• Supernovae can be several billion times more
  luminous than our sun.

35
Other stuff

• Black holes
• Collapsed stars whose gravitational field is so
  intense that not even light can escape from them.
• The escape velocity is greater than the speed of
  light
• Black holes cannot be seen directly
• Only their effect on their environment can be
  observed

36
Other stuff

• Black holes
• A star collapses until its mass becomes a point
  of zero radius and infinite density
• This is known as a singularity
• Surrounding this singularity is a region of space
  from which light cannot escape
• The radius of this region is determined by the
  objects mass.
• This is called the Schwarzschild Radius

37
Other Stuff

• Black holes
• The Schwarzschild Radius is also known as the
  event horizon
• Outside observers are unable to see any events
  occurring inside the event horizon
• Black holes were discovered by observing light
  from more distant objects bending as it passed by.

38
Summary

• The life cycle of a low mass star (less than 10
  solar masses)
• Protostar (gravity supplies energy)
• Main sequence (hydrogen burns in core)
• Core H is eventually consumed
• Red Giant (hydrogen burns in shell)
• Core contracts, heats, ignites helium
• Yellow Giant (star pulsates)

39
Summary

• Red Giant (helium burns in shell)
• Outer layers eventually ejected
• Planetary Nebula
• The core cools
• White Dwarf
• Black Dwarf

40
Summary

• The life cycle of a high mass star (more than 10
  solar masses)
• Protostar (gravity supplies energy)
• Main sequence (hydrogen burns in core)
• Core H is eventually consumed
• Yellow Giant (helium burns in core and star
  pulsates)

41
Summary

• Red Supergiant (heavier elements burn in a series
  of shells in the stars core)
• Buildup of iron in the core causes collapse
• Supernova
• Outer shell is blasted off
• Either a neutron star or a black hole is left,
  depending on the original mass of the star

42
Our Sun

• Our sun is a main sequence star that will have a
  total lifetime of about 5-10 billion years
• Most likely, our sun will not go supernova
• It will probably expand into a red giant and then
  collapse into a white dwarf.

43
For comparison
44
For comparison
45
For comparison