Assignment #5– The H-R diagram – Finding out about stars.
In the early part of the 20th century Ejnar Hertzsprung and Henry Norris Russell worked
at plotting the properties of stars. To their amazement, they discovered that stars do not
fall randomly on these plots, but fall into distinct groupings when absolute magnitude or
luminosity is graphed versus temperature/spectral type/colour. In fact it was found that
90% of stars fall along a distinct line we now refer to as the main sequence.
The main groupings on the HR diagram are referred as ‘luminosity classes’. They are as
follows
Luminosity classes 
Ia – most luminous supergiants
Ib – less luminous supergiants
II – luminous giants
III – normal giants
IV – subgiants
V - main sequence
Our sun for example is a G2V star. That means its spectral type is G2, and since it is on
the main sequence it is also a V.
This diagram became a very important tool for stellar astronomers. The grouping of the
stars was a key to the evolution of stars. It also allows us to deduce stellar characteristics
based on where a star falls on the chart. If we examine a star’s spectra we can then figure
out its age, size, mass, temperature and colour.
The different luminosity classes tell us where the star is in its lifespan. Also we have
discovered more recently an additional class ‘white dwarfs’ that are at the end of a stars’
lifespan, and that are in the lower left hand corner of the HR diagram.
The following assignment allows students to create their own HR diagram by plotting
some of the nearest and brightest stars on their own graph. The HR diagram for this
exercise will plot the absolute magnitude versus spectral class.
Absolute magnitude is a measure of how bright a star actually is, irrelevant of how far it
is. (Just as a 100W light bulb is certain brightness, but depending on your distance it can
be very faint or very bright). The magnitudes often listed for observers, or on your star
charts are visual magnitudes; this is how bright a star actually appears to the observer
standing on Earth. For example our sun’s visual magnitude is around –26, while it’s
absolute magnitude is only around 4. (Please note the magnitude scale is backward, the
lower the number the brighter the star, this dates from Hipparchus, when 1 was a bright
star and 9 a dim star)
Spectral type is a classification based on the appearance of a star’s spectra. There are
seven main types, these are O, B, A, F, G, K, M. These types are subdivided into 0-9.
The shape of the spectra of these objects is examined, and based on certain key features,
including the Hydrogen lines, a spectral type is found.
The following chart gives the common name, absolute magnitude (y-axis) and the
spectral type (x-axis) for stars for the HR diagram you will plot. A graph on which you
will plot the stars will be handed out in class. Although you do not have to number all of
the stars, it is suggested that you should number some in order to be able to answer the
questions.
Once you have plotted the diagram you will answer questions about it, and the stars
plotted on it.
Common
Name
Abs.
Mag.
Spec
type
Sirius A
1.45
A1
Canopus
-5.53
Arcturus
Common
Name
Abs.
Mag.
Spec
type
Common Name
Abs. Spec
Mag. type
Bellatrix
-2.72
B2
 Centauri B
5.70
K1
F0
Miaplacidus
-0.99
A2
Barnard's Star
13.24
M4
-0.31
K1
Alnilam
-6.38
B0
Ross 154
13.00
M3
Rigel
Kentaurus
4.34
G2
Alnair
-0.73
B7
 Eridani
6.18
K2
Vega
0.58
A0
Alioth
-0.21
A0
Lacaille 9352
9.76
M0
Capella
-0.48
G5
Kaus Australis -1.44
B9
FI Virginis
13.50
M4
Rigel
-6.69
B8
Mirphak
-4.50
F5
61 Cygni A
7.49
K5
Procyon A
2.68
F5
Dubhe
-1.08
K0
Procyon
2.68
F5
Betelgeuse
-5.14
M1
Wezen
-6.87
F8
61 Cygni B
8.33
K7
Achernar
-2.77
B3
Sargas
-2.75
F1
GX
Andromedae
10.33
M1
Hadar
-5.42
B1
Avior
-4.58
K3
 Indi
6.89
K4
Altair
2.20
A7
Menkalinan
-0.10
A2
 Ceti
5.68
G8
Acrux
-4.19
B0
Atria
-3.62
K2
YZ Ceti
14.25
M4
Aldebaran
-0.63
K5
Delta Velorum -0.01
A1
Luyten's Star
11.94
M3
Spica
-3.55
B1
Alhena
-0.60
A0
Kapteyn's Star
10.89
M0
Antares
-5.28
M1
Peacock
-1.81
B2
AX Microscopii
8.71
M2
Pollux
1.09
K0
Polaris
-3.64
F7
70 Ophiuchi A
5.50
K0
Formalhaut
1.74
A3
Mirzam
-3.95
B1
 Draconis
5.87
K0
Deneb
-8.73
A2
Alphard
-1.69
K3
 Cassiopei o
4.59
G0
Regulus
-0.52
B7
Hamal
0.48
K2
van Maanen's
Star
14.15
wd
Castor
0.59
A1
36 Ophiuchi C 7.45
K5
Sirius B
11.34
wd
Gacrux
-0.56
M3
37 Bootis
G8
Procyon B
12.98
wd
5.41
Questions
1. Plot the stars on the provided diagram. Hand diagram in.
2. Label the following on the diagram - main sequence, red giants.
3. To which group ( e.g. main sequence or red giants would be an example of a
group) to the following stars belong? (this should include a colour as part of the
answer)
a.
b.
c.
d.
e.
Sun
Procyon A
Atria
YZ Ceti
Luyten’s Star
4. Find an example (other than stars in question 3) for the following categories.
a.
b.
c.
d.
e.
White dwarf
Red main sequence star
Red giant
Yellow main sequence star
K type star
5. Why do you think Sirius and Procyon have both an ‘A’ and a ‘B’ star?
6. Which is the hottest star on the diagram?
7. Which is the coolest star on the diagram?
Name:
Answer Sheet - Exercise # 4
Student Number:
1 & 2 – Hand in HR diagram you have plotted
3. Star groups
a. Sun______________________________
b. Procyon A____________________________
c. Atria ________________________
d. YZ Ceti____________________________
e. Luyten’s Star_____________________________
4. Example of star type
a. White dwarf_____________________________
b. Red main sequence star____________________
c. Red giant_______________________________
d. Yellow main sequence star____________________
e. K type star_______________________________
5. Why ‘A’ and ‘B’?
6. Hottest Star?
7. Coolest star?
.