Download Astronomy Activity: The Life-Line of the Stars

Astronomy Activity: The Life-Line of the
Stars
Activity:
Using the concepts of stellar brightness and color, you will graphically analyze
the relationships between bright stars, near stars, and temperature (color).
Materials:
 graph paper (provided)
 2 different colored pencils, pens, or crayons
Exploration:
When you look up at the stars of the night-time sky, you will immediately notice
that all of the stars do not have the same apparent brightness. Here are some
possible hypothesis for you to consider:
Hypothesis 1: All stars are the same size, brightness, and temperature, it's just
that dim stars are far away and bright stars are close.
Hypothesis 2: All stars are the same distance away, but some are naturally bright
while others are just dim.
Hypothesis 3: Bright stars are far away and dim stars are close.
Which one do you think is correct and why? Write your answer in the form of a
sentence, then compare your answer with another student.
Concept Introduction:
A. Background: Brightness of Stars
The brightness that a star has as seen from the Earth is called the apparent
brightness . Stars which are very bright are called magnitude 1 stars . The next
brightest are magnitude 2 stars. Then comes magnitude 3, 4, 5, and down to the
very faintest stars visible with the naked eye, magnitude 6 stars*.
All stars are not the same distance away. Obviously, a star which is far away will
appear dimmer than a similar star which is closer. We adjust for the distance of
stars by giving stars an absolute brightness . The absolute magnitude of a star is
the apparent magnitude that a star would have at a distance of 32.6 light years
(10 parsecs). Stars that are really bright are also very large!
B. Background: Colors of Stars
Just like a fire flame (like a burning match) has different colors, so do the stars.
This is because stars have different temperatures. Really hot stars are blue or
white-hot. Cool stars are red or redish-orange in color.
Astronomers classify stars based on what they are made of (in addition to
hydrogen). An "A" class star has lots of Iron, a "B" class star has lots of calcium,
and so on. This classification system is called a spectral class system . IF you
arrange the stars from hot to cold, the order of the stars are:
OBAFGKM
C. Directions for the Graph:
1. Using the list of Bright Stars in Table 1 and the list of Near Stars in Table 2,
plot their absolute magnitude vs. spectral class on the attached graph. Use
different colors for the near stars and for the bright stars.
2. What general trends or concentrations do you see in the data? Are there
generalizations you can make about bright stars? How are they different from
stars that are close to the Earth? Write your answers using sentences.
Concept Application:
3. Our star, the Sun, is a G2 spectral class star with an absolute magnitude of 4.8
. How does it compare to the locations of the Near Stars on the diagram? How
does it compare to the locations of the Bright Stars on the diagram? Write your
answers using sentences.
4. Which spectral class is most common? Which spectral class is the least
common?
5. Most of the stars seem to be along a line from the upper left corner to the
lower right corner of the HR Diagram. Stars which fall into this category of stars
are called main sequence stars . Does our Sun fit into this category?
6. Consider the extra stars in the upper right hand corner ( bright-cool stars) and
the lower left hand corner (hot- dim stars). What would you name the bright-cool
stars? What would you name the hot-dim stars? (Compare your answer with
another student).
Name the bright-cool stars:
Name the hot-dim stars:
7. Why is it that black holes do not appear on the HR Diagram?
Table 1 Bright Stars
(Stars which appear very bright as seen from the Earth)
Star Name
Spectral Absolute
Class
Magnitude
Sirius B
B8
+11.5
Canopus
F2
-3.1
Arcturus
K2
-0.3
Capella
G6
-0.7
Rigel A
B8
-6.8
Betelgeuse
M2
-5.5
Antares
M8
-4.5
Spica
B4
-3.6
Deneb
A2
-6.9
Procyon B
A10
+13.0
Table 2 Near Stars
(Stars which are very close to the Earth)
Star
Name
Spectral
Class
Absolute
Magnitude
Sun
G2
+4.8
Centari
Alpha
G2
+4.4
Sirius A
A6
+1.4
Ross
154
M4
+13.3
Ross
248
M8
+14.7
Luyten
789-6
M4
+14.7
Ross
128
M2
+13.8
61
Cygnus
A
K4
+7.5
61
Cygnus
B
K8
+8.3
Procyon
A
F6
+2.7
Note : Several of the star names appear twice followed by an A or a B. These are
mostly double stars - two stars which are revolving around each other together in
space. They seem to frequently have similar spectral types. Can you guess why
they have similar spectral types?
Background Information for Activity
* Stellar Magnitude
For historical reasons, astronomers still call the brightest stars magnitude 1 stars,
the next brightest 2, then 3, 4, 5, and on down to the dimmest stars, magnitude 6
stars. This backwards scale is now defined as mathematically as:
brightness of star 1 = difference in magnitude
brightness of star 2 (2.512)
Using this sort of scale, we can actually describe the brightness of stars in real
numbers. Really bright objects, like the full moon have a negative magnitude.
Object
Apparent
Magnitude
Sun
-26.5
Full Moon
-12.5
Venus
-4.1
Sirius
-1.4
North Star
2.0
limit of
naked eye
6
Pluto
15
limit of HST
28
** Spectral Classes
Stars are easily classified by the amount of hydrogen which they contain as
revealed to us by the intensity of their spectrum. Stars with the most hydrogen
are called A stars, the remaining letters designate varying amounts of different
chemical stellar compositions.
If we consider the temperature of stars, as revealed to us by their color, we find
that the O stars, are hottest, followed by B stars, A stars, F, G, K, and M stars. A
star which has an intermediate temperature between an O star and a B star is
called an O5 star. This layout of temperature is given from hottest to coolest as:
O0, O1, O2, O3, O4, O5, O6, O7, O8, O9, B0, B2, B3, B4, B5, B6......
Astronomers sometimes humorously teach their students to remember this
sequence from hottest to coolest as:
O h, b e a f ine g uy (or gal), k iss m e!
Our Sun is G2 star, which means that it is a rather intermediate temperature star.