Download ASTR2050 Spring 2005 •

ASTR2050 Spring 2005
Lecture 11am 25 January 2005
In this class we will cover:
• Spectral lines in the laboratory and in stars
• Spectral classifications
• The origin of spectral lines in stars
• The Hertzprung-Russell (HR) diagram
1
Spectral Lines
So far, we have only discussed “continuous radiation”
from stars. We see that it is connected to temperature
and we approximate it by “blackbody radiation”.
However, there is also “discrete radiation”
obvious in stellar spectra. These “spectral lines”
give clues to the elements which make up the star.
Historically, this is how astronomers came to classify
stars and study the correlation between “spectral
classification” and luminosity. Now, we know better, but...
2
Recall from last week...
“Spectral lines”
are
clearly evident
on top of the
blackbody
spectrum.
3
On a “screen”:
Hydrogen spectrum
In a “detector”:
4
... but in stars we see absorption lines!
5
Kutner Fig.3.3b
Spectral classifications
Developed before astronomers understood
what caused spectral lines and what they mean.
Nevertheless, we are stuck with the names today!
Based on the strength of hydrogen lines:
A = strongest, B = next, G = not so strong, etc...
Space between is divided up, e.g. A0, A1, ..., A9, B0...
Today: We order them correctly by temperature
and only seven major classifications survive
6
The Spectral Classification Sequence
Class
Mnemonic
T (App. E)
O
Oh
35,000K
B
Be
21,000K
A
A
9700K
F
Fine
7200K
G
Girl/Guy
6000K
K
Kiss
4700K
M
Me
3300K
7
Appearance of different spectral lines
8
See Kutner Fig.3.9
O6.5
B0
B6
A1
A5
F0
F5
G0
G5
K0
K5
M0
M5
Metals!
9
The origin of spectral lines
Basis is in the energy levels of atoms. Schematically:
“Emission Lines”
“Absorption Lines”
All atoms have energy
levels, but the only atom
for which we can calculate
them is hydrogen!
10
Energy levels of hydrogen
The Bohr model (see Kutner) gets it right, but it’s wrong!
Quantum mechanics is right, but it’s too involved for now.
hcRH
13.6 eV
En = − 2 = −
n
n2
!E = h" = hc/#
1
!
1
1
=R
− 2
2
!n→m
m n
11
"
12
Emission lines
Absorption lines
(with continuum)
13
What regulates the intensity
of absorption lines?
How does hydrogen get excited into the n=2 level
so that we can observe absorption lines from it?
Temperature!
n j g j −(E j−Ei)/kT
= e
ni gi
The excitation energies determine the
line intensity for a given temperature.
14
Hertzprung-Russell diagrams
Stars entered as “dots” on a “scatterplot”:
Horizontal axis is color or spectral class or temperature
Vertical axis is (absolute) magnitude or luminosity
Patterns emerge that are crucial for understanding stars!
Definition of “luminosity classes”:
Main Sequence (V): “Normal” stars
Supergiants (Ia, Ib):Very bright, even at low temperature
Red giants (III): Cool, but still bright; abundant
White dwarfs: Very hot, but not bright
We will be using these diagrams as we study the insides of stars.
15
See also Kutner Figure 3.11
16