Download Stellar Evolution

ASTR 1120
Questions:
• What does the
Hertzprung-Russell
diagram mean?
• How do stars form?
Chapter 16 & start of
Chapter 17: `Star Stuff’
Phil Armitage, standing in today for Andrew Hamilton
ASTR 1120: Fall 2005
luminosity of the star
What properties can
we measure for
stars?
(1) color - recall this
tells us about the
temperature at the
surface of the star
(2) how bright the star
appears to us
temperature
If we know the distance (hard!) we can convert
the apparent brightness to a luminosity (power
in watts) and plot luminosity vs temperature
for a collection of stars
ASTR 1120: Fall 2005
Ejnar Hertzprung
Henry Russell
ASTR 1120: Spring 2005
luminosity
Giant and
supergiant
stars
Main
sequence
Dead stars,
white dwarfs
increasing surface temperature
ASTR 1120: Spring 2005
Clicker Q: suppose you observe a binary star
where the 2 stars (assume they are identical)
are too close to resolve in the telescope (they
appear as a single point of light). Where does
the binary appear:
A: on the main sequence
B: above the main sequence
C: below the main sequence
D: as a supergiant
ASTR 1120: Spring 2005
Clicker Q: suppose you observe a binary star
where the 2 stars (assume they are identical)
are too close to resolve in the telescope (they
appear as a single point of light). Where does
the binary appear:
A: on the main sequence
B: above the main sequence
C: below the main sequence
D: as a supergiant
ASTR 1120: Spring 2005
Small amount above
the main sequence:
- twice as much light
(luminosity) from the
2 stars
-same color (since the
stars are the same)
ASTR 1120: Fall 2005
What does the HR diagram represent?
1) An evolutionary sequence - perhaps
stars start out hot and bright and become
cool and dim, moving along the main
sequence as they go?
2) A sequence of stars of different mass each mass star has `its place’ in the diagram
and remains there throughout its (mainsequence) life?
ASTR 1120: Fall 2005
Almost always, stars evolve very slowly, so even
if stars did move along the main sequence we
wouldn’t observe that
Very beautiful exception - Eta Carina
ASTR 1120: Fall 2005
Debate was resolved
by measuring the
masses of stars at
different points along
the main sequence
Even harder than
measuring distances,
get mass from motion in binary star systems
e.g. the binary Sirius A/B
ASTR 1120: Fall 2005
Main sequence
Understand the main sequence as stars of
different masses fusing hydrogen into helium
Higher mass - larger inward
force of gravity
gravity
pressure
gradient
To balance this, need large
central pressure - high
central temperature
High T means faster nuclear
reactions, generating more
luminosity
ASTR 1120: Spring 2005
luminosity
With higher mass, the
surface temperature
increases and the
luminosity increases
rapidly
increasing surface
temperature
Roughly, the luminosity
of a main sequence star
scales as the fourth
power of the mass:
LµM
4
ASTR 1120: Spring 2005
Main sequence lifetime
More massive stars have larger reservoir of fuel
(hydrogen) for nuclear fusion
BUT they burn it much much faster
Massive stars have shorter lives
than low mass stars
Quantitatively:
Fuel µ Mass
Luminosity µ Mass 4
Fuel supply
1
Lifetime µ
µ 3
Luminosity M
e.g. a 2 Solar mass star lives for roughly 1 / 23
(one eighth) as long as the Sun
†
†
ASTR 1120: Spring 2005
Main sequence lifetime
Some numbers from computer models of stellar
evolution:
0.25 Solar masses: 1000 billion (a trillion!) years
1 Solar mass: 10 billion years
10 Solar masses: 10 million years
Short lifetimes of massive stars are crucial to
the existence of the Earth - allow products of
nuclear burning to be recycled into the Galaxy
and form new generations of stars
ASTR 1120: Spring 2005
Star clusters
Observational clues to what happens at the end
of stars’ main sequence lives come from star
clusters
Most stars form in such
clusters - e.g. in the Orion
nebula
ASTR 1120: Spring 2005
Star formation:
a large cloud of
gas (thousands
of Solar masses)
collapses under
gravity to form
a star cluster
ASTR 1120: Spring 2005
Star clusters come in 2 types:
Open clusters
Modest groupings
of stars - up to
several thousand
Example: the
Pleiades cluster
in Taurus
ASTR 1120: Spring 2005
Star clusters come in 2 types:
Globular clusters
Can contain a million
stars in a region ~100
light years across
In our galaxy, globular
clusters are all old,
whereas new open
clusters are being
formed now
ASTR 1120: Spring 2005
Why clusters are important
For both open and globular clusters:
• all the stars are at about the same distance
from the Earth
• all the stars formed at about the same time
Determining the distance and age to a globular
cluster is much easier than trying to find the
distances and ages of a million random stars!
Laboratories for understanding how
stars of different masses evolve
ASTR 1120: Spring 2005
What happens when a star, fusing hydrogen
into helium on the main sequence, exhausts
the hydrogen in the core?
• hydrogen burning stops, star loses energy
• core contracts, and gets hotter
• higher temperature allows new nuclear
reactions to start that are very slow
at the ~15 million K temperature
maintained by hydrogen fusion
Star leaves the main sequence, becomes a
red giant!
ASTR 1120: Fall 2005
In a H-R diagram
star:
luminosity
- moves to the
right - i.e. a lower
surface T and a
redder color
- moves upward,
to much higher
luminosity
increasing surface temperature
ASTR 1120: Spring 2005
Atmospheres of the largest giants can (just) be
resolved with HST
ASTR 1120: Spring 2005
Can date a star cluster by looking for the most
massive star that is still on the main sequence:
Young
Old
ASTR 1120: Fall 2005
Point where stars are leaving the main
sequence is called main sequence turnoff,
it moves to cooler (lower mass) stars with age
ASTR 1120: Fall 2005