Lives of Stars

1
Lives of Stars

• Stars burn Hydrogen as fuel
• The more massive a star is the more blue it is
• Also the more massive a star is the shorter its
  lifetime because it must use much more fuel to
  maintain its size
• example our sun has a main sequence lifetime of
  10 billion year, but a star of 30 Msun only has a
  lifetime of 1/30 that of our sun
• As stars reach end of lifetime they leave the
  main sequence and grow in size (some as much as
  1000 x original size) (upper right)
• Eventually run out of fuel and die and become
  white dwarf, neutron stars (lower left)

2
Lives of Stars
3
Stars

• Luminosity amount of power star radiates into
  space
• power energy / time
• Ways to measure stars distance
• 1) Apparent Vs. Absolute Luminosity
• 2) Stellar parallax
• 3) Cepheid variables
• 3) Redshift (outside galaxy)

4
Apparent Vs Absolute Luminosity

• Q- why are stars dimmer then the sun ?
• A- farther away goes by inverse square law
• Relate to a sphere (see figure 15.1)
• If a star is 2 d it has ¼ Brightness
• Lsun 3.8x 1026 watts
• Betelgeuse is 38,000 Lsun
• Alpha Cent is 0.0006 Lsun
• May use a CCD camera to measure apparent
  brightness

5
Stellar parallax

• Parallax
• apparent change in the position of a nearby
  object relative to a distant object when the
  observer moves to a new position
• Measured in parsecs
• 1 parsec 1/angle (arcsec) 1 pc 3.09 ly
• nearest star to sun is about 1 pc
• 1000 pc farthest measure by spacecraft
• 8000 pc to galaxy center
• see link

6
Redshift

• Used to measure distance to stars outside our
  galaxy
• Uses Doppler Shift
• Due to Universal expansion (Big Bang)
• More last week of class

7
Binary Stars

• Most stars are in binary systems
• May use Newtons Version of Keplers third law to
  measure mass
• Where P Period and alength of semi-major axis
• both orbit about common center of mass

8
Binary Stars
9
Types of Binary Stars

• Visual -
• Any two stars seen close to one another is a
  double star, the most famous being Mizar and
  Alcor in the Big Dipper

10
Types of Binary Systems

• Spectroscopic
• cannot be resolved as a visual binary even with
  telescopes of the highest existing resolving
  power.
• Uses Doppler shift to resolve system

11
Types of Binary Systems

• Eclipsing -
• orbital plane of the two stars lies so nearly in
  the line of sight of the observer that the
  components undergo mutual eclipses.

12
Cepheid Variable Stasrs

• Discovered in 1912 at Harvard by Henrietta
  Leavitt
• found that the brightness of these stars vary as
  a function of time
• Star is changing size
• There is a relation between the brightness of the
  star and its luminosity
• use to measure distances (more in chap 19)

13
Star Clusters

• Groups of stars
• 2 types
• open fairly modest (example Pleiades)
• closed more -densely packed
• Interesting for two reasons
• All stars in cluster are roughly same distance
  from Earth
• Stars formed around same time (within a few
  million years
• May look at stars which have left main sequence
  and use this to date the cluster
• Must use HR diagram

14
Star Stuff

• Types of stars
• Low Mass stars
• (less than 2 Msun)
• Intermediate Mass Stars
• (between 2 Msun and 8 Msun)
• High Mass Stars
• (greater than 8 Msun)

15
Star Birth

• Most stars begin life as part an interstellar
  cloud.
• Typically about 30 Kelvin
• also known as molecular clouds (allow Hydrogen
  atoms to form H2 molecules)
• This cloud begins to collapse and form a
  protostar (see fig 16-2)

16
(No Transcript)
17
Protostar

• The dust collapses into a disk due to
  conservation of angular momentum
• Angular momentum m v r
• Think of an ice skaters arms
• Thus the dust forms a flat disk as it shrinks

18
Protostar to New star

• 1) dust begins to form protostar
• fairly bright because even though it is cool, it
  has a large surface area
• 2) looses energy due to this radiation begins to
  contract and temperature rises
• 3)When Temps reach a few million K, fusion begins
  at core
• 4) For a few million years, star continues to
  decrease in size thus raising the rate of fusion.

19
Stellar Birth Weights

• Stars must have masses between 0.5 Msun and 100
  Msun.
• If too light
• not enough gravity to ignite fusion
• 0.08 - 2 Mjupiter called brown dwarfs
• first brown dwarf seen in 1995.
• If too heavy the fusion is too rapid and
  gravitation cant control
• blows itself apart (Theory)

20
Low Mass stars

• masses less then 2 Msun.
• Live much longer then more massive stars
• good for life on surrounding planet (gives time
  for life to evolve)
• our star is relatively stable, some low mass
  stars rotate faster
• Thus have higher magnetic fields
• give rise to violent flares
• Proxima Centauri is a flare star (closest star
  to sun)

21
End of Medium to Low Mass stars

• As the star exhausts its fuel of hydrogen, the
  core begins to collapse due to no outward
  pressure to resist the pull of gravity
• But the outer surface of the star grows
• Now termed a Red Giant.

22
End of Medium to Low Mass stars

• Seems paradoxical why does core shrink but star
  grows
• Answer Hydrogen shell burning
• The inner core is now helium (result from H
  burning)
• around this core is hydrogen
• This hydrogen shell is at such a high pressure
  that it burns faster then before
• results in a large outward pressure which forces
  the star to expand

23
(No Transcript)
24
End of Medium to Low Mass stars

• Cycle ends when temperature at core reach 100
  million K.
• At this temperature Helium begins to fuse
  together and form Carbon 12
• called triple alpha process
• Dumps even more energy into star, helium flash
• Causes core to expand thus lowing temperature,
  then reducing rate of fusion
• star begins to contract

25
Death of Medium to Low Mass stars

• Star may pulsate burning He for a while, but it
  cant maintain this
• Star has one last gasp and throws out outer
  layers called planetary nebula then forms a white
  dwarf.

26
Life of High Mass stars

• Have much smaller lifetimes then lower mass star
• Due to fact that they are higher temperature and
  thus burn hydrogen much faster
• In fact, the stars are so hot that heavier
  elements fuse inside of them
• CNO cycle - a more complex version of the
  proton-proton chain that produces Carbon,
  Nitrogen, and Oxygen.

27
(No Transcript)
28
End of High Mass stars

• After the core is exhausted it becomes Helium
• But now not only does the He burn but heavier
  elements are created
• These heavier elements also begin to fuse
  together forming still heavier elements.
• Each time a new element is created in the core
  the core must shrink until it has enough pressure
  (temperature) to fuse next element

29
End of the line for High Mass stars
30
End of the line for High Mass stars

• Process continues until one has a iron core (see
  fig 16.16)
• Ends at iron because it is no longer
  energetically favorable to form elements higher
  then Iron
• In other words it requires more energy to fuse
  iron and get a heavier nuclei then you get out

31
Supernova

• Now we have an iron core where the pressure is
  increasing
• There is a limit to how much we can press this
  core (degeneracy pressure)
• In a fraction of a second the core is compressed
  from size of sun to that earth
• This compression releases HUGE AMOUNTS OF ENERGY
• Causes an explosion - Supernova
• Work still goes on to model what exactly happens,
  but star will form either a neutron star or a
  black hole.

32
(No Transcript)
33
Hubble picture of Crab Nebula
34
Supernove 1987 A
35
Origin of elements

• Cant see inside a star, so why do we think heavy
  elements are created inside stars
• Look at gas in Nebula
• very old star have only 0.1 heavy elements
• younger stars have 2-3 heavy elements
• this shows that the younger stars have gained
  elements which were created in previous supernova

36
(No Transcript)