Lecture Notes:
Lives of Stars: Main Sequence, Giants and
Supergiants
text: Chapter 13 (sections 13.3 - 13.7)
Main Sequence:
stars spend most of their lives as Main Sequence stars
Structure of Main Sequence stars depends on MASS
we can divide the main sequence into
- lower main sequence (F, G, K, M)
- low mass, low luminosity, low temperature
- like the Sun
- upper main sequence (O, B, A)
- higher mass, higher luminosity, higher temperature
- different nuclear fusion process
- still H to He
- different steps which require higher
temperatures
- different structure of star
- convection layer next to smaller core
- then radiative layer OUTSIDE convective layer
- apparently no chromosphere or corona
- stronger stellar winds
Stars are on main sequence as long as they have Hydrogen to
burn:
- very massive, hot stars: about 1 million years
- lots of fuel, but burn it VERY quickly
- stars like Sun: about 10 billion years
- very low mass, cool stars: about 200 billion years
- not much fuel, but burn it VERY slowly
Giants and Supergiants - - - -
[CORRECTIONS IN BOLD
ITALICS]
[Terminology varies
depending on what book you use. Here I am using the terminology of
our text book (Arny). Other books may refer to "Yellow Giants" as
the "horizontal branch of Red Giants". (In fact, I have not found
any other books so far that use the term "Yellow
Giants".]
What happens when H burning stops ? Giants
Now no heat source in core
=> core will contract (to about 1/10 of original) and
heat up (to about 100 million K)
shell just outside core gets hot and dense enough for H fusion
to ignite in shell
outer layers of star expand (about 100x) and cool (about
3500K)
larger size = more luminosity (even with lower temperature) (about
2000x brighter)
cooler surface = redder color
so call these "Red Giants"
As the outer layers of high
mass stars are cooling toward the "Red Giant" stage - they pass
through a short phase as "Yellow Giants" (slightly smaller and
slightly hotter). We will discuss this intermediate stage more
shortly.
New fuels:
at higher temperatures and densities: other nuclear fusion
reactions can occur:
|
REACTION
|
TEMPERATURE NEEDED
|
|
hydrogen to helium
|
10 million K
|
|
helium to carbon
|
100 million K
|
|
carbon to oxygen
|
600 million K
|
|
OTHERS
|
|
|
silicon to iron
|
1 - 3 billion K
|
NO FURTHER REACTIONS ARE POSSIBLE after iron
Helium Fusion
High mass stars only need to contact a little to achieve
conditions suitable for Helium fusion to start
gradually core becomes main energy source again
outer layers contract and heat up
star remains a Red giant but
increases in temperature (may also re-enter yellow giant
stage)
Low mass stars (like the Sun) need to contract a lot !!
when a gas of atoms is compressed this much it becomes a
"degenerate gas"
behavior is VERY different from normal gas
no longer expands when you heated (gas pressure is no longer
connected to temperature)
Helium flash occurs - entire core ignites in minutes
huge release of energy changes core gas back to normal
gas
star's core expands and outer layers shrink and heat up
star becomes a Yellow Giant
Yellow Giants - Pulsating
Stars
when low mass
stars start He fusion
intermediate stage prior to He
fusion for high mass stars (can also re-occur later in
life)
these stars often pulsate - swell and shrink in a regular
repeating pattern
unstable stage in star's life
radiation gets trapped in star's outer layers causing
them to heat and expand
expanded gas cools and is pulled back in by gravity
process repeats
Only happens under certain conditions of size and
temperature (so only in certain region of the H-R diagram.
Instability strip. (see Figure 13.14 in the text)
several types:
- long period variables
- change luminosity over months or years
- Cepheid variables
- change luminosity in 1-100 day cycles
- luminosity about 1 million x Sun
- RR Lyrae variables
- change luminosity in cycles lasting < 1 day
- mass similar to Sun
- yellow giants with luminosity about 40 x Sun
Stars spend about a million years pulsating before their
temperature or size change enough to make them stable again.
They may become unstable several times during their lives.
Period-Luminosity Law
Larger stars have weaker gravity and so are slower to pull the
expanded layers back in.
Larger stars also tend to be more luminous (if temperatures are
similar).
So more luminous stars tend to have longer pulsation periods.
Use the relationship between period and luminosity to determine
distances.
especially useful for Cepheid variables since they are
very bright
calibrate using nearby stars whose distances are known
use period to determine luminosity
also measure brightness and find distance from relationship of
brightness to luminosity and distance
Later stages
Low mass stars:
Burning Helium into Carbon at increasing rate
temperature increases and star expands
star becomes more luminous = Supergiant (star could extend out
near the orbit of Earth)
Outer layers cool to about 2500 K
C and Si atoms condense out to form dust grains
grains are pushed OUT of the star by radiation, drag some
gas with them
material mainly pushed out in a bi-polar manner
star now has two "cones" of gas coming out
gas cones are heated by radiation from hot core and
glow
called a planetary nebula
(planetary nebula = "fuzzy disk-like object")
See Figure 13.16 in text. Link
to Figure 13.16
contain quite a bit of mass - probably entire outer layers
of star
cones of gas gradually diffuse into the interstellar medium
leave behind the hot core of the star = "white dwarf"
High mass stars:
more massive stars can produce conditions in the core to
create other nuclear fusion reactions
can fuse carbon into oxygen
can fuse other elements as well
All the heavy elements (other than H and He) were probably
made in stars by these reactions
star contracts and heats up with each change in fuel
contraction allows outer shells to start earlier fusion
processes leading to a set of shells of different fusion
reactions. See Figure 13.18 in the text. [Link
to Figure 13.18]
stars become SuperGiants as outer shells expand (could extend
out beyond orbit of Jupiter)
Once iron is formed in the core, no additional fusion
reactions are possible.
Core can no longer support force of gravity.
Atoms are crushed together and protons and electrons
merge to create neutrons.
Star's core becomes closely packed neutrons (about 10 km
radius)
Outer layers of star crash into core.
Gas is heated and explode back out = SUPERNOVA
Figure 13.20 shows pictures of the spreading gas from
supernovas. [Link
to Figure 13.20]
Remaining core is a neutron star.
Some VERY massive stars may have such strong gravity that the
material which explodes out as a supernova gets slowed down and
can NOT escape.
The material is pulled back in and the core is crushed down
beyond a neutron star.
Now NO forces in physics exist to oppose gravity and the star
becomes a black hole.
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