Download WHAT MAKES A STAR SO SPECIAL Abstract

WHAT MAKES A STAR SO SPECIAL
Veselka S. Radeva
EAAE Summer School Working Group (Bulgaria)
Abstract
The main goal of this workshop is to help students understand the connection between
the characteristics of stars by creating a “spectrum-luminosity” diagram. Questions
about the connection between the characteristics of stars, the position of stars with
various masses, the place of the Sun and the stars around which terrestrial planets can
be found will be considered during the class discussion. Based on creation of a
“spectrum-luminosity” diagram and the discussion students will determine what makes
a star so special.
INTRODUCTION
More than 300 years ago when Isaac Newton separated white light into different
coloured rays using a prism, he discovered spectral lines. This makes Newton the first
astro-spectroscopy scientist.
Star spectra are analyzed with the help of spectrographs that are placed in the focal
plane of telescopes. In astrophysics, in order for star spectra to be classified, scientists
use the M-K system named after the scientists Morgan and Kinan who introduced it. In
this system a star is classified according to two of its characteristics – spectral class and
luminosity class.
• Spectral classes
Star spectra are divided into groups called classes. The main spectral classes are named
as follows: O, B, A, F, G, K and M. The hottest stars are from class O, while the coldest
stars are from class M. Each class is divided into 10 subclasses that are named with
numbers from 0 to 9, which are added after the letter of the class. The spectral class of
the Sun is G2.
Spectral classification is the classification of star spectra depending on the temperature
of the star’s photosphere. Stars from a specific class have very similar temperatures.
Table 1 shows the main characteristics of different spectral classes.
• Luminosity classes
Some stars have higher luminosity than other stars with the same temperature. i.e. from
the same spectral class. Taking into consideration that L = 4πR2σT4, if the temperature is
a constant, then the higher luminosity is due to bigger radii of stars (L ~ R2) – we know
that there are dwarf stars and giant stars. The spectral lines of giants are much narrower
than the spectral lines of dwarfs from the same spectral class. This is so because the
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Table 1. Main characteristics of different spectral classes
Spectral
class
O
Temperature of the
photosphere (К)
25 000 – 60 000
B
10 000 – 25 000
Blue-white
Neutral helium; there are hydrogen
lines
A
7500 – 10 000
Blue
Strong hydrogen lines from the
Balmer series
F
6000 –7500
Yellow-white
Hydrogen lines are weak; there are
ionized calcium(CaII) lines
Colour
Spectral characteristics
Blue
Ionized helium (He II)
G
4500 – 6000
Yellow
Ca II lines are very strong, there are
lines of other strong metals like
iron; hydrogen lines are much
weaker
K
3500 - 4500
Yelloworange
Strong and numerous lines of metals
(calcium, iron, magnesium)
M
2 000 - 3500
Red
Absorption strips of molecules (for
example TiO)
Table 2. Star characteristics and their range
Characteristics
Sign
Luminosity
L
Photosphere
temperature
T
Radius
R
Mass
M
Value for the Sun
26
LSun = 3,83.10 W
Range in solar units
10-5 – 106 LSun
TSun = 5780 K
8
RSun = 6,96.10 m =
7.105 km
MSun = 6,96.108 m
1000-100 000 К
10-2 – 103 RSun
10-2 – 102 МSun
104 – 1010 years
Age
Chemical contents
(amount of heavy elements compared to the amount of
the same elements in the Sun)
0,05 – 2
(or 0,15 - 6% heavy
elements)
atmospheres of the giants are bigger and their density is less than those of the dwarfs
stars. Thus, every spectral class can be classified on the basis of “the sharpness” of
spectral lines. There are seven Luminosity classes which are written down with Roman
numbers: I – Supergiants; II.- Bright giants; III.- Giants; IV. - Subgiants; V. - Dwarfs
from the main sequence; VI. - Subdwarfs; VII. - white dwarfs. Some luminosity classes
are further divided into subclasses a, ab and b.
The luminosity classification is actually a star classification that depends on the radii
and the atmosphere density of stars.
Giant stars have big radii and low density, while dwarf stars are just the opposite.
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Thus, the complete spectral classification of the Sun in the MK system is G2 V (the Sun
is a yellow dwarf). A red supergiant, for example the star Betelgeuse, α from the
constellation Orion, is classified as M2 Iab.
• Observable star characteristics
Stars have a wide range of observable parameters. These parameters characterize them
as space objects. There are two fundamental dependences between their parameters that
connect the luminosity of the star with its temperature and mass. In this workshop we
will learn more about how stars are grouped according to similarities in stars main
characteristics. Star world is diverse. The main characteristics of stars and their range
are shown in Table 2.
• The Hertzsprung- Russell-Diagram (HRD)
We know that the number of stars in the Milky Way is around 2 x 1011. Stars are not
equally distributed according to their spectral class and luminosity. For every star with
high luminosity from class B or O there are 350 stars from class A or K with average
luminosity, or 1000 red stars from class M with low luminosity. This statistics shows
that the formation of stars with low masses is a very common process.
If we plot the spectral classes (or the temperature, which is the same) along the x-axis
and the luminosity (or stars’ absolute magnitude) along the y-axis, we will create the
“spectrum-luminosity” diagram. The relationship between the luminosity of stars and
their spectra carries the name of two astronomers who created this diagram –
Hertzsprung and Russell (figure 1).
Figure 1. The Hertzsprung- Russell-Diagram
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Stars do not fill in the HRD evenly, but are grouped in sequences. Around 90% of all
stars are positioned in a diagonal strip called the main sequence. The upper end of the
diagram has two almost horizontal sequences of stars with high luminosity called Giants
and Super-Giants. At the bottom end of the diagram there is an area with a small
number of stars that are very hot but with low luminosity – this is the area of White
Dwarfs. Stars from the bottom part of the main sequence are also dwarfs, but they are
yellow or red depending on their temperature. Our Sun is an ordinary star – Yellow
Dwarf from the main sequence. The HRD contains fundamental information for the star
world.
PEDAGOGICAL INSTRUCTIONS
Educational goal:
To introduce students to the large number of different coloured stars and to help
students recognise the connection between the colour and the temperature, and the mass
and the age of stars.
Activity:
1. Students will use star circles of different colours (dark-blue, blue-white, white,
yellow-white, yellow, yellow-orange and red). They will have to write down three
characteristics on each circle – brightness, temperature and age, while considering
the data about the temperature of the star and the colour of the circle – figure 2.
Figure 2
2. Each student will place their star circles on the big “spectrum-luminosity” diagram
according to the brightness and spectrum of a star (the student will have to think
about what star colour corresponds to what spectral class). Thus, students will create
their HRD that will contain around 100 stars.
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3. Students will participate in a discussion during which they will consider the
connection between the mass and the colour, the temperature and the colour, and the
age of the stars.
Educational method:
This lesson uses one of the interactive methods called “brain storm”. This means that
after the creation of the HR diagram, students will participate in a discussion that will
stimulate group creativity. The discussion’s topic is “What Makes a Star So Special”.
The method tries to make students come up with ideas and suggestions without any
limitations or criticism, to develop creativity, and to make students find new ways for
solving a problem. All suggestions and solutions are written down but no criticism is
allowed even though the ideas can sound absurd. After a certain time the suggested
ideas are discussed or are given to a group of students – experts who choose the best
suggestions.
Practical preparation:
1. The teacher has to make 100 circles with different colours - blue, blue-white, white,
yellow-white, yellow, yellow-orange and red.
2. The teacher prepares 5 stars and a table with information about 5 stars for every
student [refer to Appendix 1].
3. The teacher makes a large “spectrum-luminosity” diagram on which students will
place their stars.
4. The teacher prepares a list with questions for the discussion.
Organization of the lesson:
Steps:
1. The teacher introduces the physical characteristics of stars and the spectral
classification to the students (you can use the information from the Introduction
section as a supplement to the lesson about Star characteristics in the textbook
you are using).
2. The teacher gives every student an envelope with 5 star circles and a table with
the characteristics of 5 stars.
3. Students have to write down the information about the star in the circles taking
into consideration what temperature corresponds to what colour.
4. Students have to put their star circles on the “spectrum-luminosity” diagram.
5. Once the diagram is built, you can start the discussion. The class is divided into
two groups – a working group and a group of experts. There are 5 students in the
group of experts.
Example questions for the discussion:
Is there a connection between the colour and the brightness of stars?
How does the colour change taking into consideration the temperature
plotted along the x-axis? What colour corresponds to hot stars and what
colour to cold stars?
Are there any exceptions in the diagram? How would you describe the
colour and the brightness of the stars – exceptions (are they red and faint
or yellow and bright)?
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What is the position of the Sun on the diagram?
What would you say for its place? What are the colour and the brightness
of the most commonly seen stars? Which stars are rarely seen?
What is the relationship between the colour and the life time of stars
from the main sequence?
How would you explain the fact that one type of star is more common
than another type of star? Which type of stars can we see and explore
most easily?
Around which type of stars can we look for planets where life might
exist and why?
6. Students answer the questions – all answers are discussed and accepted. At the
end all answers are given to the group of experts who use the answers in order to
answers to the question: “What makes a star so special?”
Conclusion:
The lesson is very dynamic, with the participation of all students. During the
preparation of the star circles, students assimilate their knowledge about the star
characteristics. During the creation of the “spectrum-luminosity” diagram, students
realize and understand the meaning of the HRD: the relationship between spectrum and
luminosity, temperature and brightness, life time and the mass of a star. During the
discussion students assimilate their knowledge about stars from the different areas of
the diagram: the main sequence, and the groups of Giants, Super-Giants, and Dwarfs.
Students understand that every star has its own characteristics that determine its place
on the “spectrum-luminosity” diagram, and that all these characteristics make a star
very special.
References
Николов Н., Радева В., Илиева Е., Астрономия, Педагог 6.
http://en.wikipedia.org/wiki/Constellation
http://www.nasa.gov/worldbook/star_worldbook.html
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APPENDIX
Name of the
star
Luminosity, LSun
1
APAC617
0.02
4,100 K
1010 yrs
2
Phaet
480
20,000 K
108 yrs
3
Proxima
0.00005
2,900 K
1011 yrs
4
XXR42
19
9,200 K
109 yrs
5
K073491
120
35,000 K
1010 yrs
№
Temperature
Time of life
K
years
Colour circles: yellow-orange, blue-white, red, white, dark blue
№
Name of the
star
Luminosity, LSun
1
BDE10298
2
Temperature
Time of life
K
years
0.01
3,900 K
1010 yrs
Mintaka
10,000
40,000 K
104 yrs
3
Barnard’s
0.0003
3,000 K
1011 yrs
4
XL9876-1
0.5
5,500 K
1010 yrs
5
XXR49
22
9,900 K
109 yrs
Colour circles: yellow-orange, blue, red, yellow, white
№
Name of the
star
Luminosity, LSun
1
APAC424
2
Temperature
Time of life
K
years
0.01
3,900 K
1011 yrs
Sun
1
5,800 K
1010 yrs
3
Sheratan
25
12,000 K
109 yrs
4
Ross 154
0.0005
3,000 K
1011 yrs
5
WZ443
60
9,000 K
107 yrs
Colour circles: yellow-orange, yellow, blue-white, red, white
№
Name of the
star
Luminosity, LSun
1
DC0032864
2
Temperature
Time of life
K
years
0.06
4,200 K
1010 yrs
Proxima
0.00005
2,900 K
1011 yrs
3
Acamar
110
20,000 K
106 yrs
4
XXR42
19
9,200 K
109 yrs
5
XL9876-1
0.5
5,500 K
1010 yrs
Colour circles: yellow-orange, red, blue-white, white, yellow
7
№
Name of the
star
Luminosity, LSun
1
KO65430
2
Temperature
Time of life
K
years
0.01
3,500 K
1010 yrs
Mirach
480
3,200 K
3
XL9473-1
0.6
6,000K
1010 yrs
4
Tau Cetus
0.5
5,200 K
1010 yrs
5
PY7Z91
80
9,600 K
107 yrs
Colour circles: red, red, yellow-white, blue-white, white
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
DC0029876
0.04
4,300 K
1011 yrs
2
Wolf
0.00002
2,900 K
1011 yrs
3
XXR86
66
10,000 K
109 yrs
4
ARN93227
0.7
5,000 K
1010 yrs
5
Polaris
5495
6 100 K
Colour circles: yellow-orange, red, white, white, yellow
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
JAC76582
0.04
3,800 K
1011 yrs
2
L726-8A
0.00006
3,000 K
1010 yrs
3
GL86715
1.1
5,900 K
1010 yrs
4
ARN93227
0.7
5,000 K
1010 yrs
5
SAC5430
28
9,700 K
Colour circles: yellow-orange, red, yellow, yellow, white
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
CRTP987
0.08
3,600 K
1011 yrs
2
Ross 248
0.0004
2,800 K
1011 yrs
3
YEZ8732
0.8
5,400 K
1010 yrs
4
Rigel Orion
40 000
11 000 K
Super giant
(radius 70 Rsl)
Vega
52.5
5
9 900K
Colour circles: yellow-orange, red, yellow, blue-white, white
8
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
JAC39672
0.02
3,900 K
1011 yrs
2
BD591915A
0.0006
3,100 K
1011 yrs
3
SL9-67294
1.6
6,200 K
1010 yrs
4
Deneb
60 000
8 400 K
106 yrs
5
Sirius
25 000 K
109 yrs
Colour circles: yellow-orange, red, yellow-white, white, blue
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
JAC12967
0.09
4,200 K
1011 yrs
2
CD3615693
0.0001
3,300 K
1011 yrs
3
SAC2243
29
9,400 K
109 yrs
4
PL0089
0.3
4,800 K
1010 yrs
5
Altair
11
8 000 K
<109 yrs
Colour circles: yellow-orange, red, white, yellow, white
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
DAAC329
0.09
4,200 K
1011 yrs
2
Ross 128
0.0003
3,200 K
1011 yrs
3
G757
2.3
5,800 K
1010 yrs
4
LP033276
0.001
11,100 K
5
RTL6439
81
26,000 K
107 yrs
Colour circles: Yellow, yellow-orange, yellow, blue-white, dark blue
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
JAC43928
0.07
4,100 K
1011 yrs
2
L7896 ABC
0.0001
3,200 K
1011 yrs
3
RTL9847
2
6,000 K
1010 yrs
4
Bellatrix
4 000
21 500K
5
Ross614B
0.00001
2,900 K
№
1011 yrs
Colour circles: yellow-orange, red, yellow, blue-white, red
9
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
BD+362147
0.005
3,500 K
1011 yrs
2
L37258
0.0001
3,200 K
1011 yrs
3
YEZ8872
88
10,400 K
109 yrs
4
DAAC328
1.6
6,200 K
5
K073491
120
35,000 K
1010 yrs
Colour circles: red, red, blue-white, yellow-white, blue
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
Kapteyn’s
0.004
3,500 K
1011 yrs
2
CST6128
1.9
5,800 K
1010 yrs
3
PG000017
0.0002
10,200 K
4
JAC64932
101
38,000 K
5
BLG9982
0.009
3,000 K
1011 yrs
Colour circles: red, yellow, blue-white, blue, red
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
Epsilon Eri
0.28
4,200 K
1010 yrs
2
BD591915B
0.0005
3,200 K
1011 yrs
3
APAC529
1.7
6,100 K
1010 yrs
4
JAC94320
90
22,000K
5
Sirrah
200
13,000K
Colour circles: yellow-orange, red, yellow-white, blue-white, blue-white
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
Epsilon Ind
0.16
4,200 K
1010 yrs
2
BD51668
0.0006
3,200 K
1011 yrs
3
BSC3430
3
5,900 K
1010 yrs
4
Bellatrix
4 000
21 500 K
5
CD3914192
0.03
3,600 K
Colour circles: yellow-orange, red, yellow, blue-white, orange
10
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
L72532
0.0002
2,900 K
1011 yrs
2
BDE00017
1.0
5,900 K
1010 yrs
3
DAAC444
0.09
4,100 K
1010 yrs
4
Alioth
108
9 400K
5
JAC7632
0.8 S
5,900 K
Colour circles: red, yellow, yellow-orange, white, yellow
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
CD2510553
0.00004
3,000 K
1011 yrs
2
Dubhe
300
4 500 K
3
Arcturus
110
4 300K
4
BD51668
0.0006
3,000 K
5
Ross 614A
0.00007
3,000 K
1011 yrs
Colour circles: red, yellow, yellow-orange, red, red
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
MGU82067
0.0001
3,100 K
1011 yrs
2
Kruger 60A
0.0006
3,200 K
1011 yrs
3
JAC57492
1.2
5,900 K
4
Antares
9120
3,340K
5
Kruger 60B
red 0.00008
3,100 K
1011 yrs
Colour circles: red, red, yellow, red, red
№
Name of the
star
Luminosity,
LSun
Temperature
Time of life
K
years
1
Alinlam
375,000
25,000 K
2
Rigel
66,0000
11,000 K
3
Saiph
57,500
26,000 K
4
Etamin
600
4,000 K
5
Shedar
855
4,530 K
Colour circles: blue, blue-white, blue, yellow, yellow
11
1011 yrs
№
Temperature
Time of life
star
Luminosity,
LSun
K
years
1
Regulus
350
10,300 K
107 yrs
2
Zosma
23
8,350 K
1011 yrs
3
Vega
51
9,300 K
108 yrs
4
Aldebaran
150
4,100 K
5
Nath
700
13,600 K
Name of the
Colour circles: blue-white, white, white, yellow, blue-white
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