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Star Classification
Stars are classified by their spectra (the elements that they absorb) and their temperature.
There are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K, and
M. O and B stars are uncommon but very bright; M stars are common but dim.. An easy
mnemonic for remembering these is: "Oh be a fine girl, kiss me."
The Sun
is a as a
G2V type
star, a
yellow
dwarf and
a main
sequence
star.
Hertzsprung - Russell Diagram
The Hertzsprung -Russell (H-R) Diagram is a graph that plots stars color (spectral type or surface
temperature) vs. its luminosity (intrinsic brightness or absolute magnitude). On it, astronomers plot stars'
color, temperature, luminosity, spectral type, and evolutionary stage. This diagram shows that there are 3
very different types of stars:
•
•
•
Most stars, including the sun, are "main sequence stars," fueled by nuclear fusion converting
hydrogen into helium. For these stars, the hotter they are, the brighter. These stars are in the most
stable part of their existence; this stage generally lasts for about 5 billion years.
As stars begin to die, they become giants and supergiants (above the main sequence). These stars
have depleted their hydrogen supply and are very old. The core contracts as the outer layers
expand. These stars will eventually explode (becoming a planetary nebula or supernova,
depending on their mass) and then become white dwarfs, neutron stars, or black holes (again
depending on their mass).
Smaller stars (like our Sun) eventually become faint white dwarfs (hot, white, dim stars) that are
below the main sequence. These hot, shrinking stars have depleted their nuclear fuels and will
eventually become cold, dark, black dwarfs.
Spectral Classes
Averag
Averag
Approximat
e
Average
Star
e Mass
Main
e Surface
Radius Luminosit
Typ Color
(The
Characteristic Examples
Temperatur
(The y (The Sun
e
Sun =
s
e
Sun =
= 1)
1)
1)
O
B
A
Blue
over 25,000
K
Blue
11,000 25,000 K
Blue
7,500 11,000 K
F
Blue
6,000 - 7,500
to
K
White
G
White
to
5,000 - 6,000
Yello
K
w
K
Orang
3,500 - 5,000
e to
K
Red
M
under 3,500
K
Red
60
18
3.2
1.7
15
7
2.5
1.3
Singly ionized
helium lines (H
I) either in
104
emission or
HIGHEST
absorption.
Strong UV
continuum.
10333
Neutral helium
lines (H II) in
absorption.
Rigel
Spica
80
Hydrogen (H)
lines strongest
for A0 stars,
decreasing for
other A's.
Sirius,
Vega
6
Ca II
absorption.
Metallic lines
become
noticeable.
Canopus,
Procyon
Absorption
lines of neutral
metallic atoms
and ions (e.g.
once-ionized
calcium).
Sun,
Capella
1.2
1.1
1.1
10-1
0.8
0.3
10
Lacertra
0.9
0.4
10-2
Metallic lines,
Arcturus,
some blue
Aldebaran
continuum.
0.4
0.04
(very
faint)
10-4
Some
molecular
Betelgeuse
bands of
, Antares
titanium oxide.
Name _________________________________________Date _________________
The Hertzsprung-Russell Diagram
Student Worksheet
answer the questions on the back of this page using the above HR Diagram
Let’s Practice plotting some stars from the drawing above the information
will come from the group copy “Star Classification” paper.
Rigel, Sirius, Arcturus, Procyon, Betelgeus
THE H-R DIAGRAM
1.
What is the approximate surface temperature of the sun?
___________________
2.
Would the surface temperature of white dwarf stars be higher or lower
than red supergiants? (Circle one of the bold words)
3.
What is the color of the stars with the highest surface temperature?
___________________
What is the color of the stars with the lowest surface temperature?
_____________________________________________________
4.
List the color of the stars from hottest to coldest:
HOT_______________________________________________________
______________________________________________________COLD
5.
Most of the stars on the HR Diagram are classified as which type of star?
___________________________________________________________
6.
How is it possible for white dwarf stars to have lower luminosity than the
sun even though the sun is cooler than white dwarfs?
___________________________________________________________
___________________________________________________________
___________________________________________________________
___________________________________________________________
7.
Plot the following stars.
Star A = 4,000 0C and low/medium brightness
Star B = 6,000 0C and high brightness
Star C = 20,000 0C and low/medium brightness
Star D = 6,000 0C and medium brightness
Identify the type/color for each star:
Star:
Star A
Star B
Star C
Star D
8.
Color:
Type:
What is the name of our most popular star located at point D?
_________________