Astronomy Assignment #9: Stellar Evolution I
Review Questions from the first half of Chapter 13: Lives and Deaths of Stars
1. What fundamental property of stars determines their evolution?
Mass is the fundamental property that determines the evolution of stars. The mass of a star determines the central
pressure of the star which in turn is the leading term in establishing the luminosity of the star which in turn
determines the stars lifetime.
2. Why do massive stars last for a short time as main sequence stars but low-mass stars last a long time in the main
sequence stage?
Massive stars last for such a short time as main sequence stars because the higher central pressures in those stars
drive faster fusion rates and created higher luminosities. The higher luminosities “burn” mass faster and the star
will then “burn” through its core reserves of hydrogen faster. Low mass stars have slower fusion rates because
the fusion rate is slower due to the lower central pressure of these low mass stars. Thus they take longer to
“burn” through ther core reserves of hydrogen.
3. How can you detect protostars if the surrounding gas and dust blocks visible light?
Protostars emit the maximum of their radiation at infrared wavelengths due to the heat they generate from
converting gravitational energy while they collapse into thermal energy. IR radiation has the characteristic of
being able to penetrate through dust and gas. Thus, even though the protostars are invisible behind a cloud of gas
and dust, their IR radiation penetrates the gas and dust and they can been “seen” at IR wavelengths.
4. How do T-Tauri stars get rid of the surrounding gas and dust from which they formed?
T-Tauri stars are young pre-main sequence stars that are experiencing a strong stellar wind as they make the final
gravitational collapse to the ZAMS line. The thermal energy created by the rapid gravitational collapse creates a
strong outflow of stellar material away from the star – the stellar wind. This stellar wind is what sweeps away the
gas and dust surrounding the young star.
5. What is happening in the core of a main sequence star and why is it so stable?
In the core of the Sun the p-p chain is producing energy at a rate of 3.84 x 1026 Watts (Joule per second). The
rate of the p-p chain depends on temperature, and pressure in the core.
The main reason why the Sun is so stable is that the physical relation among the pressure, temperature, and
fusion rate creates a natural thermostat which keeps the center of the Sun (and the center of any other main
sequence star) at a steady temperature.
A thermostat is any feedback device which acts to keep the temperature of a system nearly constant. (If you've
ever been curious how the thermostat in a home heating system works, you can go to the ``How Stuff Works''
web site.) Basically, when the temperature drops too low, the thermostat increases the rate at which heat is
generated. When the temperature rises too high, the thermostat decreases the rate at which heat is generated.
How does a star's natural thermostat work? Consider what would happen if you increased the fusion rate in a
star's core:





(1) Core temperature increases
(2) Core pressure increases
(3) Core expands
(4) Core density & temperature decrease
(5) Fusion rate decreases
Thus, increasing the fusion rate sets a chain of events into action whose end result is to decrease the fusion rate
again.
Now consider what would happen if you decreased the fusion rate in a star's core:





(1) Core temperature decreases
(2) Core pressure decreases
(3) Core contracts
(4) Core density & temperature increase
(5) Fusion rate increases
6. What happens to a main sequence star that has stopped fusing hydrogen in its core?
When a main sequence stars stops fusing hydrogen in its core, it stops producing energy and can no longer hold
the weight of the envelope and it leaves the main sequence area on the HR diagram. The core is compressed by
the weight of the stars envelope and a thin shell of previously un-fusable hydrogen is dragged deeper into the star
with the collapsing core and inner envelope and this shell of hydrogen can now begin fusing. The shell Hburning creates shell gamma rays that are able to deposit more energy into the star’s envelope (since they do not
originate in the core) and this causes the star’s envelope to swell. The star becomes a giant star.
7. Are all red giants or supergiants very massive stars? Why are red giants so big and red? What is going on inside
the giants?
All red giant and supergiants evolve from relatively massive stars. Any main sequence star that hotter than midK spectral type can become a giant star. So, not all giant stars are very massive. There are many giant stars the
mass of the Sun. These stars are giants in radius, not in mass. They become giant stars due to shell fusion that
produces shell gamma rays. The shell H-burning creates shell gamma rays that are able to deposit more energy
into the star’s envelope (since they do not originate in the core) and this causes the star’s envelope to swell. The
star becomes a giant star. All Giant stars have shell fusion occurring inside them.
8. What is the evolution sequence for stars around the mass of our Sun? How long is the Sun's main sequence
lifetime?
The stages of evolution for stars around the mass of the Sun are
1. Main Sequence Star where energy is produced by core H-burning for about 10 billion years.
2. Red Giant Star where energy is produced by Shell H-burning for about 100 Million years
3. Horizontal Branch Star where energy is produced by core He-burning for about 50 million years
4. Red Supergiant Star where energy is produced by shell He-burning in Helium Flashes for about 10,000
years
5. Planetary Nebula where no energy is produced.
6. White Dwarf Stellar Remnant where no energy is produced.
2
Instructor Assigned Topic
Complete an evolutionary track for the Sun on the blank HR diagram attached using the data in the table attached.
Label each section of the sun’s post-main sequence evolutionary track with the sun’s current method of energy
production as well as the name of the phase at the endpoint of each segment.
Post-Main Sequence Evolution of the Sun
Stage
Energy
Production
Method
Main
Sequence
Core
Hydrogen
Burning
Red Giant
Shell
Hydrogen
Burning
Horizontal
Branch
Core
Helium
Burning
Red Super
Giant
Shell
Helium
Burning
Planetary
Nebula
None
3,000 K
White
Dwarf
None
50,000 K
Spectral
Type
M
G2
+4.8
(1 L)
M3
-3.6
(2,350
L)
K1
0
(100 L)
M3
-3.9
(3,000
L)
Surface
Temperature
Radius
in Solar
Radii
Core
Temperature
Lifetime
15 Million K
10
Billion
Years
166
50 Million K
100
Million
Years
5,000 K
10
200 Million
K
50
Million
Years
3,000 K
180
250 Million
K
10,000
years
300 Million
K
Short
100 Million
K
Very
Long
5,800 K
3,500 K
1
0.01
3
HR Diagram
30,000 K
-10
9,500 K
7,200 K
6,000 K
5,250 K
3,800 K
1,000 R
60 M
-5
17.5 M
100 R
Absolute Magnitude
5.9 M
Planetary Nebula creation that exposes core after the last shell He flash
2.9 M
0
1.8 M
10 R
1.2 M
0.1 R
1.0 M
5
Sun
Red Supergiant Star
Shell He-burning in
Flashes
Horizontal Branch Star
Core He-burning
Red Giant Star
Shell H-burning
.67 M
1 R
0.01 R
10
.21 M
0.001 R
Main Sequence Star
Core H-burning
15
20
O
B
A
White Dwarf Stellar Remnant with
no energy production.
F
G
K
M
Spectral Type
4