Download Evolution of a Low-Mass Star

Astronomy Picture of the Day
Recall:
Luminosity - Intrinsic property of a star.
Apparent Brightness – the brightness we perceive a star to be from
Earth.
Important Relationship, Another Inverse Square Law:
Apparent Brightness α Luminosity/ Distance2
Also
Luminosity α Radius2 * Temperature4
And
We can measure the spectral type of a star (on the main sequence
– we'll get to this in a minute) we can determine its Luminosity
Color of a Star
Why are stars different colors?
Center of our Galaxy
Color of a Star
Why are stars different colors?
They are different Temperatures!
Recall – maximum intensity of
radiated E-M waves depends on
Temperature. So Astronomers
can measure the apparent
brightness of a star at several
intensities (“colors”) to determine
the temperature.
Stars are Born in Nebulae
(video clip)
Hertzsprung-Russell Diagram
Nearby stars
Brightest visible stars
- Stars begin their lives as red giants and move onto the main
sequence where they remain until H supply is exhausted.
- Red dwarfs are most common type of star.
- 90% of stars are on the main sequence.
-Most stars are members of multiple star systems (i.e. Binary)
What happens if a star isn't massive enough to begin
fusing H into He?
Brown Dwarf - a “failed” star
Question
When a star runs out of hydrogen, what happens
next?
Evolution of a Low-Mass Star
(< 8 Msun , focus on 1 Msun case)
- Helium ash collects in core.
- Too cool for He burning. Why?
Larger electric repulsion.
- Core contracts. Heats up. H burning shell
- Higher temp. => Brighter! Star expands!
- "Red Giant". Diameter ~ 1 AU!
- Does fusion rate at this stage increase or
decrease? Why?
Red Giant
Evolution of a Low-Mass Star
(< 8 Msun , focus on 1 Msun case)
Does fusion rate at this stage increase or
decrease? Why?
Rate increases. Phase lasts ~ 1 billion years
Red Giant
Creation of Heavier Elements
- Core shrinks and heats up to
108 K, => Helium fuses into
Carbon.
- All He -> C.
- Core shrinks and heats up.
- Onion-like structure
Each phase shorter than the
last.
Red Giant
Death of a Low Mass Star

What factor(s) eventually determine when this
process stops?
"Planetary Nebulae"
- Low mass star (< 8 Msun) cannot achieve 600 Million K temp.
needed for Carbon fusion
-
- Contraction stopped by the Pauli exclusion principle: two objects
cannot occupy the same space. “Supported by the resistance of its
electrons to further contraction, the core contractin stops.”
-
- Star becomes unstable. Ejects outer layers (high velocity: 10's
km/sec). "Planetary Nebula" (Historical name, nothing to do with
planets.) Shine due to ionizing radiation from the hot core of the star
embedded in a cool gas cloud.
- Carbon core called a “White Dwarf” - shines only by stored heat,
no more nuclear reactions. About the size of Earth. Cools to
become black dwarf, remaining about the size of Earth.
-
Ring
m2-9
Cat's eye
Stellar Explosions
Novae
White dwarf in
binary system
WD steals mass from companion. Eventually, a burst of fusion, H → He.
Brightens by 10'000's! Cycle may repeat every few decades => recurrent
novae.
Nova Cygni with Hubble
May 1993
Jan 1994
1000 AU
Is all of the accreted matter expelled into space during a nova?
A Carbon-Detonation or “Type I” Supernova
Despite novae, mass continues
to build up on WD.
At 1.4 MSun (the "Chandrasekhar limit"), gravity overwhelms the Pauli
exclusion pressure supporting the WD => contraction and heating.
Carbon fusion everywhere at once.
Tremendous energy makes star explode. No core remnant.
Stellar Lifetimes

Is the lifetime of a high mass star shorter or
longer than that of a lower mass star? Why?
Evolution of Stars > 8 MSun
Higher mass stars burn out
faster and fuse heavier
elements.
Eventual state of > 8 MSun star
Example: 20 MSun star lives
"only" ~10 million years.
Heaviest element made in
core of any star is iron – iron
nuclei will not fuse to release
energy.
Products of outer layers
become fuel for inner layers.
H burns for 10 million years,
Iron for less than 1 day!
Red Supergiant
Death of a Very High-Mass Star
M > 8 MSun
Iron core at T ~ 1010 K radiation
photodisintegrates iron nuclei into protons
and neutrons.
Core collapses in < 1 sec.
Neutrons “rebound”. Shock ejects outer
layers => Core-collapse or Type II
Supernova
Ejection speeds 1000's to 10,000's of km/sec!
Remnant is a “neutron star” or “black hole”.
Supernova 1987A in the Large
Magellanic Cloud
In 1000 years, the exploded debris might look something like this:
2 pc
Crab Nebula: debris
from a stellar
explosion observed
in 1054 AD.
Or in 10,000 years:
50 pc
Vela Nebula: debris
from a stellar
explosion in about
9000 BC.
Remember, carbon-detonation (Type I) and core-collapse (Type II)
supernovae have very different origins
How long will a Star Live?
Stellar lifetime α mass/luminosity
Making the Heaviest Elements

Since iron is the heaviest element that can be
made by stellar fusion, where do the heavier
elements come from?
Making the Elements
H and some He were made in Big Bang. Many made in
stars, and distributed by supernovae.
Heaviest elements made in supernovae.
Solar System formed from such "enriched" gas 4.6
billion years ago.
Testing our Theories

How can we test our theories of stellar evolution
when the lifetimes of stars are so long?
Star Clusters
Two kinds:
1) Open Clusters
-Example: The Pleiades
-10's to 100's of stars
-Young (10's to 100's of millions of years)
2) Globular Clusters
- few x 10 5 or 10 6 stars
- Billions of years old
Why are star clusters useful for stellar evolution studies?
Clusters are useful for stellar evolution studies because all of the
stars:
1) formed at about same time
2) are at about the same distance
3) have same chemical composition
The ONLY variable property among stars in a cluster is mass!