properties of stars ii the hurtzprung-russell diagram how long do stars live? star clusters
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Properties of Stars II
• The Hurtzprung-Russell Diagram
• How long do stars live?
• Star clusters.
How stars age (evolve)
• Suppose we want to study how people change over their life time.
• How could we do this.• We could follow a person from birth to death.• That would take a long time.• But there are a lot of people in the world so …we
could study people of different ages so we don’t have to wait a lifetime for our research to finish.
• For obvious reasons we cannot wait for stars to change .
Temperature
Lum
inos
ity
An H-R diagram plots the luminosities and temperatures of stars.
THIS IS PROBABLY THE MOST IMPORTANT DIAGRAM IN ASTRONOMY.
The Hertzsprung-Russell Diagram
Most stars fall somewhere on the main sequence of the H-R diagram.
Stars with lower T and higher L than main-sequence stars must have larger radii:
giants and supergiants
These stars have no H to He fusion going on in their core (as they have run out of H fuel).
They are fusing elements heavy than H in their cores (for example He to C). They are also fusing H to He in a shell outside their core but not in the core.
large radius
small radius
Stars with higher T and lower L than main-sequence stars must have smaller radii:
white dwarfs
These are dead stars that have no nuclear fusion of any kind.
Temperature
Lum
inos
ity
H-R diagram depicts:
Temperature
Color
Spectral type
Luminosity
Radius
Which star is the hottest?
A
B C
D
Which star is the most luminous?
A
BC
D
Which star is a main-sequence star?
A
BC
D
Which star has the largest radius?
A
BC
D
What is the significance of the main sequence?
Main-sequence stars are fusing hydrogen into helium in their cores, like the Sun.
Luminous main-sequence stars are hot (blue).
Less luminous ones are cooler (yellow or red).
Mass measurements of main-sequence stars show that the hot, blue stars are much more massive than the cool, red ones.
High-mass stars
Low-mass stars
The mass of a star is its key property.
A star’s mass determines where it will rest on the main sequence. Its temperature, radius, luminosity and lifetime on the main sequence are all determined by its mass only.
The reason for this is that a star’s mass determines the rate of H to He fusion.
High-mass stars
Low-mass stars
The core temperature of a higher-mass star needs to be higher in order to balance gravity.
A higher core temperature boosts the fusion rate, leading to greater luminosity.
Hydrostatic Equilibrium
Stellar Properties Review
Luminosity: from brightness and distance
10-4LSun– 106LSun
Temperature: from color and spectral type
3000 K – 50,000 K
Mass: from period (p) and average separation (a) of binary-star orbit
0.08MSun – 100MSun
Radius: from blackbody radiation the L = constant * T4 * R2 (we will not use this formula or discuss this method on the course).
0.1RSun – 10RSun (on the main sequence)
Stellar Properties Review
Luminosity: from brightness and distance
10-4LSun–106LSun
Temperature: from color and spectral type
3000 K–50,000 K
Mass: from period (p) and average separation (a) of binary-star orbit
0.08MSun–100MSun
(0.08MSun) (100MSun)
(100MSun)(0.08MSun)
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Until core hydrogen(10% of total) is used up
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Life expectancy of a 10MSun star:
10 times as much fuel, uses it 104 times as fast
10 million years ~ 10 billion years × 10/104
Until core hydrogen(10% of total) is used up
Mass and Lifetime
Sun’s life expectancy: 10 billion years
Life expectancy of a 10MSun star:
10 times as much fuel, uses it 104 times as fast10 million years ~ 10 billion years × 10/104
Life expectancy of a 0.1MSun star:
0.1 times as much fuel, uses it 0.01 times as fast100 billion years ~ 10 billion years × 0.1/0.01
Given the age of the Universe is 14 billion years no star of 0.1MSun has ever die of old age.
Until core hydrogen(10% of total) is used up
Main-Sequence Star Summary
High-mass:
High luminosity Short-lived Large radius Blue
Low-mass:
Low luminosity Long-lived Small radius Red
What are giants, supergiants, and white dwarfs?
These are Off the Main Sequence• Off the main sequence stellar properties depend on both
mass and age.• These stars have finished fusing H to He in their cores are
no longer on the main sequence.• They may be fusing He to Carbon in their core or fusing H
to He in shell outside the core … but there is no H to He fusion in the core.
• All stars become larger and redder after exhausting their core hydrogen fuel: giants and supergiants.
• Most stars end up small and white after all fusion has ceased: white dwarfs.
• The white dwarf stage is the final stage for most stars.
Which star is most like our Sun?
A
BC
D
Which of these stars will have changed the least 10 billion years from now?
A
BC
D
Which of these stars can be no more than 10 million years old?
A
BC
D
Star Clusters
Our goals for learning:
• What are the two types of star clusters?
• How do we measure the age of a star cluster?
Open cluster: A few thousand loosely packed stars
Globular cluster: Up to a million or more stars in a dense ball bound together by gravity
How do we measure the age of a star cluster?
Massive blue stars die first, followed by white, yellow, orange, and red stars.
To measure a star clusters age we make one important assumption,
ALL STARS IN A CLUSTER ARE BORN AT THE SAME TIME.
Visual Representation of a Star Cluster Evolving
How do we measure the age of a star cluster?
Pleiades now has no stars with a life expectancy less than around 100 million years.
Main-sequenceturnoff
The main-sequence turnoff point of a cluster tells us its age.
To determine accurate ages, we compare models of stellar evolution to the cluster data.
Using the H-R Diagram to Determine the Age of a Star Cluster
Globular cluster are very old.
Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old.
• Now do the lecture tutorial section on HR Diagram.
• When you have finished do the lecture tutorial section on Star Formation and Lifetimes.