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ASTR100 (Spring 2008) Introduction to AstronomyClassifying Stars
Prof. D.C. Richardson
Sections 0101-0106
What is a Hertzsprung-Russell Diagram?
Temperature
Luminosity
An H-R diagram plots the luminosities and temperatures of stars.
Most stars fall somewhere on the main sequence of the H-R diagram.
Stars with lower T and higher L must have larger radius R:
giants and supergiants.
Large Radius
L = 4R2 T4
Stars with higher T and lower L must have smaller radius R:
white dwarfs.
L = 4R2 T4
Small Radius
Giants and Supergiants
White Dwarfs
Add luminosity class to spectral class:
I - supergiantII - bright giantIII - giantIV - subgiantV - main sequence
Examples: Sun – G2 VSirius – A1 VProxima Centauri – M5.5
VBetelgeuse – M2 I
H-R diagram depicts:
Temperature
Color
Spectral
Type
Luminosity
Radius
Temperature
Luminosity
Temperature
Luminosity
A
BC
D
Which star is the hottest?
Temperature
Which star is the hottest?
A
BC
D
A
Luminosity
Temperature
Which star is the most luminous?
A
BC
D
Luminosity
Temperature
Which star is the most luminous?
A
BC
D C
Luminosity
Temperature
Which star is a main-sequence star?
A
BC
D
Luminosity
Temperature
Which star is a main-sequence star?
A
BC
D D
Luminosity
Temperature
Which star has the largest radius?
A
BC
D
Luminosity
Temperature
Luminosity
Which star has the largest radius?
A
BC
D C
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.
Low Mass
High Mass
The mass of a normal, hydrogen-burning star determines its luminosity and spectral type!
Low Mass
High Mass
The core pressure and temperature of a higher-mass star need to be higher in order to balance gravity.
A higher core temperature boosts the fusion rate, leading to higher luminosity.
Sun’s life expectancy: 10 billion years.
Life expectancy of 10 MSun star:
10 times as much fuel, uses it 104 times as fast.
10 million years.
Until core hydrogen(10% of total) is used up.
Mass & Lifetime
Sun’s life expectancy: 10 billion years.
Life expectancy of 10 MSun star:
10 times as much fuel, uses it 104 times as fast.
10 million years.
Life expectancy of 0.1 MSun star:
0.1 times as much fuel, uses it 0.01 times as fast.
100 billion years.
Until core hydrogen(10% of total) is used up.
Mass & Lifetime
Main-sequence Star Summary
High mass: High luminosity
Short-lived Large radius Blue
Low mass: Low luminosity
Long-lived Small radius Red
Concept Check
Two stars have the same surface temperature but different luminosities. How can that be?
Answer: one is bigger than the other!
Why? Thermal radiation law: objects at a given temperature emit a certain luminosity per unit surface area.
Hence the more luminous star has a larger surface area, and so a larger radius.
What are giants, supergiants, and white dwarfs?
Off the Main Sequence
Stellar properties depend on both mass and age: stars that have finished fusing H to He in their cores are no longer on the main sequence.
All stars become larger and redder after using up their core hydrogen: giants and supergiants.
Most stars end up small and white after fusion has ceased: white dwarfs.
Main-sequence stars (to scale)
Giants, supergiants, white dwarfs
Temperature
Luminosity
Which star is most like our Sun?
A
B
C
D
Temperature
Luminosity
Which star is most like our Sun?
A
B
C
DB
Temperature
Luminosity
Which star will have changed the least 10 billion years from now?
A
B
C
D
Temperature
Luminosity
Which star will have changed the least 10 billion years from now?
A
B
C
D
C
Temperature
Luminosity
Which star can be no more than 10 million years old?
A
B
C
D
Temperature
Luminosity
Which star can be no more than 10 million years old?
A
B
C
D
A
What are the two types of star clusters?
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.
The Pleiades cluster now has no stars with life expectancy less than around 100 million years.Main-sequence
turnoff
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.
Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old…
Surprise Quiz!! (10 points)
Take out a piece of paper, print your name and section number on it.
Sketch an H-R diagram…1. Label the temperature & luminosity axes.2. Sketch the main sequence.3. Plot a point representing the Sun.4. Plot a main-sequence B star.5. Plot a main-sequence M star.6. Indicate where giants & supergiants are
found.7. Indicate where white dwarfs are found.
Giant and Supergiants
White Dwarfs
Here’s what your sketch should look like!
Sun (G2 V)
B Star
M Star
