Transcript Stellar Evolution after the Main Sequence

Stellar Evolution
up to the Main Sequence
Stellar Evolution
Recall that at the start we made a
point that all we can "see" of the
stars is:
•
•
•
•
Brightness
Color (Spectra)
Position
Distance (if we are lucky or clever)
Let's see if there are any correlations
Stellar Evolution
Using distance (when we know it) we can
convert the Brightness (apparent
magnitude) into the absolute magnitude,
or even the Luminosity
To make things easy we can write the
luminosity relative to that of the Sun,
L/L
Stellar Evolution
The Color, or spectra, we can convert to
– A Spectral Class
– A Temperature
– A B-V value
• V is the visible magnitude
• B is the magnitude as seen on photographic plates
– Photographic plates are more sensitive to blue light – blue
stars will appear brighter
• B-V gives a numerical "Color" index
• For comparison
– the yellowish Sun (G2) has a B-V index of 0.656 and a
surface temperature of about 6000K
– the bluish Rigel (B8) has B-V index of -0.03 and a
surface temperature of about 11000K
Stellar Evolution
We can plot the Luminosity ratio versus the color:
1000
100
The Sun would go here
10
1
.1
.01
O
B
A
F
G
K
M
Stellar Evolution
This plot was independently discovered
by Hertzsprung and Russell
It is now called the Hertzsprung-Russell,
or H-R, Diagram
Ejnar Hertzsprung
Henry Norris Russell (1877-1957)
(1873-1967)
The HR Diagram
The 50 Nearest Stars
1000
100
The Sun
10
1
.1
.01
O
B
A
F
G
K
M
The HR Diagram
The 50 Brightest Stars
1000
100
10
1
.1
.01
O
B
A
F
G
K
M
The HR Diagram
1000
100
10
1
.1
.01
O
B
A
F
G
K
M
The HR Diagram
1000
100
There appears
to be three
main areas
where the
stars are
grouped
10
1
.1
.01
O
B
A
F
G
K
M
The HR Diagram
This curve is where 90% of the stars appear
1000
100
10
1
.1
.01
O
B
A
F
G
K
M
The HR Diagram
1000
100
10
1
These are pretty
dim, but also
very hot…white
hot
.1
.01
This implies that
they are very small
O
B
A
F
G
K
M
The HR Diagram
These are cool, but very
bright - the size must be
huge
1000
100
10
1
.1
.01
O
B
A
F
G
K
M
HR Diagram
1000
Red Giants
Blue Giants
100
10
1
.1
White Dwarfs
.01
O
B
A
Red Dwarfs
F
G
K
M
Starbirth
Protostars form
in cold, dark
nebulae
Giant Molecular Clouds
Distant dark nebulae are hard to observe, because they
do not emit visible light
However, dark nebulae can be detected using microwave
observation, because the molecules in nebulae emit at
millimeter wavelengths
Giant molecular clouds are immense nebular so cold that
their constituent atoms can form molecules.
Giant molecular clouds are found in the spiral arms of
our Galaxy.
Giant Molecular Clouds and Starforming Regions
Star-forming regions
appear when a giant
molecular cloud is
compressed
This can be caused by
the cloud’s passage
through one of the
spiral arms of our
Galaxy, by a
supernova explosion,
or by other
mechanisms
Molecular Clouds
Giant Molecular
Cloud in Orion
Infrared view
Disorderly
Complex
From IRAS satellite
Stages of Star Formation
"Cores" and
Outflows
1 pc
Molecular or
Dark Clouds
Jets and
Disks
Extrasolar System
Stages of Starbirth
#
t to Next
Stage
(yr)
Core
Temp.
(K)
Surface
Temp
(K)
Diameter
(Km)
Description
Interstellar Gas Cloud
1
2,000,000
10
10
100,000,000,000,000
2
30,000
100
10
1,000,000,000,000
Cloud Fragment
3
100,000
10,000
100
10,000,000,000
Cloud Fragment
4
1,000,000
1,000,000
3,000
100,000,000
ProtoStar
5
10,000,000
5,000,000
4,000
10,000,000
ProtoStar
6
30,000,000
10,000,000
4,500
2,000,000
Star
7
10,000,000,000
15,000,000
6,000
1,500,000
Main Sequence
Lifetime on the Main Sequence
• Luminosity basically describes how fast the star is
‘burning’ its fuel.
• This is clearly related to how much fuel there is
because the greater the mass the higher the
pressures and temperatures:
L  M3
• Lifetime is “how much fuel / how fast it’s used”
T = M/L  1/M2
Lifetime on the Main Sequence
Here are some comparison values:
Mass
(Msun)
100
Lifetime
(Tsun)
0.0001
Lifetime
(years)
1 million
10
0.01
1
1
10 billion
0.1
100
1 trillion
100 million
The Path to the Main Sequence
1000
100
10
1
.1
.01
O
B
A
F
G
K
M
The T Tauri phase
The T Tauri phase
• Gravity causes the gas/dust cloud to condense.
• The situation then usually becomes quite complex
• Some of the infalling gas is heated so much by
collisions that it is immediately expelled as an
outgoing wind.
• Jets and disks form as the infalling and outflowing gas
collide and interact with changing magnetic fields.
• Temperatures and masses are similar to the Sun, but
they are brighter
• They have fast rotation rates (few days)
• Variable X-ray and radio emission
• Not yet a 'star', but will be in a few million years
During the birth process, stars both gain
and lose mass
In the final stages of pre–main-sequence contraction, when thermonuclear
reactions are about to begin in its core, a protostar may eject large
amounts of gas into space
Low-mass stars that vigorously eject gas are called T Tauri stars (age ~ 1
million year)
Jets: A circumstellar accretion disk provides
material that a young star ejects as jets
Jets: Clumps of glowing gas are sometimes
found along these jets and at their ends
Known as Herbig-Haro Objects
A Magnetic Model for Jets
(Bipolar Outflow)
Starbirth in NCG 281
M16 in Infrared
Bok Globules