Download 8 clusters stellar evo

Extra credit!
Get out your clickers.
The following questions are worth 2 points each.
What direction does the Sun rise
in Sydney?
A.
B.
C.
D.
E.
In the East
In the West
In the North
In the South
Cannot conclude
Looking south in Sydney, what
type of stars would you see?
A.
B.
C.
Seasonal
Circumpolar
Neither
Looking North in Sydney, what
type of stars would you see?
A.
B.
C.
Seasonal
Circumpolar
neither
Sydney’s circumpolar stars rotate
A.
B.
C.
D.
Counterclockwise
Clockwise
Rise in east, set in west
Rise in west, set in east
Agenda



Reading: Finish Unit 5, if you haven’t
already.
Star clusters
Stellar life cycles (stellar evolution)
Star clusters


Easier to observe overall evolution
than of one star
Stars in a cluster



Formed at the same time
Have similar composition
They will differ only in mass
Globular
cluster
M 15
Thousands to millions
of stars
NASA
Globular cluster Tucanae 47
NASA
Open cluster: Pleiades
Hundreds of stars
Case Western
Open Cluster: Jewel box
Jordell Observatory
HR diagram of a cluster

Color (B – V) on
horizontal axis


Equivalent to
OBAFGKM
Apparent magnitude
(V) on vertical axis


All stars at the same
distance
Easy to convert to
absolute magnitude
Evolution and the HR diagram


High mass (higher luminosity) stars
progress through life more quickly
Lower mass stars take longer to be
born, consume their fuel more slowly.
Which HR diagram shows the
older cluster?
A.
C. Cannot conclude
B.
Young cluster ~80 million yrs
U. of Sheffield
Older cluster
U. of Oregon
Compare the HR diagrams
Many young, hot (blue) stars
Many older red giants
Star clusters — summary


Stars in one cluster are of different
types but the same age.
Observing many clusters tells us about
star life cycles



HR diagram
Old stars leave the main sequence
Cluster age <=> turnoff point
Stellar “evolution” (first part)
What we found in star clusters:
 Small stars live longer
 Very massive stars live hard and die
young
 Old stars leave the main sequence to
become red giants.
Raw
materials
for star birth
Interstellar clouds.
This is a star cluster
in the making!
Raw
materials
for star birth
In a Stellar nursery



Raw materials collapse
Protostar begins to spin
Eventually, fusion of H into He begins
Life as a star

Zero-age main sequence


when a star first starts fusing H into He
Stars do this for 90% of their lives
Big stars don’t live long!


Massive stars burn
very fast.
They soon run out
of fuel!
Wikipedia
Time on main sequence
versus initial stellar mass
Large stars live and die very quickly!
200 billion years!
~12 billion years (Sun)
50 million years
0.4
0.8
1.1
Initial stellar mass (MSun)
1.7
3.3
16
40
1 million years!
H runs out: star becomes a
red giant
Exterior expanding
H fusing
He core
contracting
This is how stars
leave the main sequence!
Our star is tiny compared
to a red giant!
Wikipedia
Further evolution


Helium all used up, gravity takes over
again
Much mass is spewed into space
Mass now determines death



Low mass: White dwarf
Medium mass: Neutron star or pulsar
High mass: Black hole
Summary



Stars are born from Hydrogen
Stars spend 90% of their lifetime
fusing Hydrogen into Helium
Stars leave the main sequence and
become red giants
Next time
Death of stars:
Black holes, neutron stars, Relativity