Classification of Stars – HR diagram
Objectives:
• understand the differences between near and bright
visible stars
• learn how to use the HR-diagram to classify stars
(temperature vs. brightness)
• use the HR-diagram to observe the evolution of stars
(when a star is born and dies)
Lab Books:
pages 52-54
worksheet, page 115
HR-diagram – classifies stars using temperature
magnitude (brightness)
HR-diagram – Hertzbrung-Russell diagram
Star color and Temperature
Lets review the visible color spectrum.
“What do our eyes see”?
Low Energy
High Energy
wavelength
R O Y
G
B
I
V
Star Color and Temperature
Yellow
Blue
RED
2,500
6,000
30,000 K
Hot stars
Cool stars
Long wave lengths
R
O
Y
Short wavelengths
G
B
I
V
Measuring the Brightness of a Star
“Stellar Brightness”
Magnitude – the brightness of a star
Magnitude is measured using (-) and (+)
numbers
the more (-) the number, the brighter
the more (+), the dimmer the star
Apparent Magnitude
• a stars brightness as it appears from earth
• factors controlling apparent magnitude:
• how big the star
• how hot the star
• how far away star
Which is brighter?
-5 star
or
5 star ?
Apparent Magnitude of various objects
dim
bright
OBJECT
SUN
Full
Moon Venus
Apparent
Magnitude -26.5 -12.5
- 4.1
North Naked Pluto HST
Sirius Star
eye
-1.4
2.0
6.0
15
28
Absolute Magnitude
• measures the “actual” brightness
• two stars with the same absolute magnitude
would not be the same apparent magnitude
WHY?
• To compare absolute brightness
• use a standard distance of 32.6 light-years
away from earth
Example:
Apparent Magnitude
Absolute magnitude
bright
dim
-26.7
+5
Earth
32.6 ly
Distance, Apparent Magnitude, and Absolute
Magnitude of Some Stars
View from Earth 32.6 ly
Name
Distance
Light-years
Apparent
Magnitude
Absolute
Magnitude
Sun
NA
-26.7
5.0
Alpha Centauri
4.27
0.0
4.4
Sirius
8.70
-1.4
1.5
Arcturus
36
-0.1
-0.3
Betelgeuse
520
.08
-5.5
1600
1.3
-6.9
Deneb
So, which star is biggest to smallest?
Brightness scale – Using Absolute Magnitude
-7
-5
Bright
-3
-1 0
1
3
5
7
9
Dim
The brighter, the bigger the star
How are stars classified? – the Hertzsprung-Russell
Diagram (HR-diagram)
-10
O
B
A
F
G
K
M
Absolute Magnitude
Very Bright
-5
0
+5
+10
Very Dim
+20
20,000 14,000 10,000 7000
5000
Surface Temperature (K)
3000
Spectral class: based on their spectrum (color they emit
resulting from the chemical make-up)
O
B
A
F
G
K
M
Oh, Be A Fine Guy (or Gal), Kiss Me
O0O1…..O9 B0…..B9 A0…..A9 F……………
Hottest
Coolest
HR-Diagram
Cool, bright
Hot, dim
20,000 14,000 10,000 7000
5000
Surface Temperature (K)
3000
O
B
A F GK M
Stellar evolution
HR-Diagram
Variable
Star stage
Red Giant
Stage
Protostar
Planetary
Nebula
Stage
White Dwarf
Main Sequence
Star
Black
Hole Stage
20,000 14,000 10,000 7000
5000
Surface Temperature (K)
3000
Dust and
gasses