Download ASTR2050 Spring 2005 • In this class we will cover: Brief review

ASTR2050 Spring 2005
Lecture 10am 1 March 2005
Please turn in your homework now!
In this class we will cover:
• The Main Sequence: Brief review
• Stellar evolution off the Main Sequence
• Cepheid variable stars: Measuring distance
1
The Main Sequence: Brief review
40M!
1M!
2
0.2M!
Properties of the Main Sequence
by mass M
• Position determined
3
• Luminosity L∝M (approximately) 2
M/L∝1/M
• Mass is fuel, so1 lifetime
4
• Power from 4 H→ He + energy 2 4
Model:
Central
pressure
P
≈GM
/R
•
C
Ideal
Gas:
Central
Temp
T
=mP
/ρk
C
C
•
3
Example: Model calculation for The Sun
Central pressure is
about 1011 atmospheres
Central temperature is
about 15 Million K
Most of the radiation is
generated inside 20% of
the solar radius
Compare Kutner Fig.9.11 and associated discussion
4
Evolution off the Main Sequence
First: “Low mass” stars (M < 5M!)
Hydrogen in core is used up.
Temperature in core is not
high enough to burn helium.
Hydrogen burns in “shell”.
Helium core is “degenerate”.
H burning
He core
is very
dense
H envelope
5
Red giant phase for “low mass” stars
Helium “flash” occurs when core contracts enough.
H burning
He burning
H envelope
6
Different behavior for “high mass” stars (M > 10M!)
Helium can burn before becoming degenerate
Core continues to shrink,
gets hotter, and burns heavier
and heavier elements.
Eventually iron (Fe) reached
and it can burn no more...
7
Example: A Globular Cluster
Very old, so none of the high mass stars are left
Note: Brighest stars are “red”8
Cepheid Variable Stars
Some relatively
massive stars
evolve through the
“instability strip”:
• Type I (classical)
cepheids
• Type II (W Vir)
cepheids
• RR Lyrae stars
See Kutner Fig.10.4
9
Interlude: Naming stars
Ordinary stars
Greek letter (in order of brightness) then constellation
e.g. α-Orionis is brightest star in Orion (aka Betelgeuse)
δ-Cephei is fourth brightest star in Cepheus
Variable stars
Listed in order of discovery, starting with “R”, then “S”
and on through “Z”, then “RR..RZ...SS...SZ...ZZ”, and
then “AA...AZ...” (but no J’s) ending at “...QZ” which is a
total of 334 variable star names. After that, “V335...”
e.g. W-Virginis, RR-Lyrae, etc...
10
Cepheid Pulsation
The star oscillates in both size
and in temperature. It is most
bright at high temperature,
whichis near minimum radius.
Short period
11
Long period
Cepheid Mechanism: The
+
He
valve
Surface of the star
... and then
falls back in.
Helium zone
traps radiation
Radiation pushes out,
but the star’s surface
overshoots equilibrium...
Temperature falls and
helium zone breaks
12
Period-Luminosity Relation
Depth of the helium zone is very sensitive to mass
This is an interesting physical
phenomenon, but it also makes
cepheids a very useful tool!
13
Measuring distance
The Period tells you the Absolute Magnitude!
You will see
this first hand
in studio lab
this Friday.
You will use
cepheids to find
the distance to
galaxy M81.
14