Classification and Orders of Star
Just like we group animals, plants, cars, and many other things, scientists also classify (group) stars. When scientists classify stars, they look at two different things, its apparent magnitude and its spectral type . Before we tackle magnitude, let’s first look at spectral type.
Definitions and Concepts
Apparent Magnitude: How bright a star looks to us from
Earth The spectral type classification was perfected by a woman named
Annie J. Cannon, who worked for Harvard in the late 1880’s. Ms. Cannon’s system used the old lettering system originally created, but because there were flaws in the original data, the lettering system no longer went in order.
The scale she perfected is listed below:
Absolute Magnitude: How bright a star looks from a set distance (32.6 light years) away from the star
Spectral Type
G
K
M
O
B
A
F
Temperature (K)
30,000-60,000
10,000-30,000
7,500-10,000
6,000-7,500
5,000-6,000
3,500-5,000
<3,500
Color
Blue
Blue-white
White
Yellow-White
Yellow (our sun)
Yellow-Orange
Red
Here is a pneumonic device to help you remember the order the classification system follows:
O
B
A
F
G
K
M
Spectral Type: A combination of two properties of stars – its surface temperature (gives its color) and its Atmospheric
Pressure (tells us its size)
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Color: Blue = Hot stars to Red = Cool stars
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Pressure: High
Pressure = Giant stars,
Low Pressure = Dwarf stars.
In addition to the lettering system, she also used a numbering scale to address the stars that fell within a range of appearances. For example; a star whose spectrum analysis placed it somewhere between a G class star and a K class star would be rated as a G5 star. Her number system ran from 0 – 9. So for stars that were categorized as a G stage star, G0 would be the warmest in that category and G9 would be the coolest.
Warmest G0 G1 G2 G3 G4 G5 G6 G7 G8 G9 K0
Coolest
Not only do scientists look at the size and color of stars, but they also categorize them based on their brightness. We call this Magnitude or Luminosity . When we describe how bright something is from Earth we say that is the star’s Apparent Magnitude . To describe how bright a star actually is, scientists measure how bright the star would be from exactly 32.6 light years away from that star. This is a distance that was chosen so that all stars could be measured against each other from the same distance. This value is called its
Absolute Magnitude (Luminosity). Some stars that are very far away may have the same Apparent
Magnitude as another star that is much closer. This difference in Apparent Magnitude can be explained by its spectral type. For example a spectral type star in category O will be much brighter than a star in category K , so if the O type star is farther away it may appear just as bright as a K type star.
Magnitude can be a little bit confusing, but what’s important to remember is that the Higher the
Magnitude the fainter the star will be, and the Lower the Magnitude the brighter the star will be. For example, our Sun has an Apparent Magnitude of -26.72 while the next closest star to Earth has an Apparent Magnitude of -0.27. Obviously the sun is MUCH brighter than our next closest star, so when we see that a star has a magnitude that is smaller we know that star appears brighter to us on Earth.
Now that we have a little bit of background, let’s take some time to dig in to some data that an actual astronomer might look at and make sure you have a good understanding of constellations!!

Name: Pd:
HW : Classification of Stars and Constellations
Environmental Science
Due Date:
In the space below, you will see both a table and some graphs of values associated with the 7 brightest stars as viewed from Earth. Because of variations in visibility as stars shift from the Main Sequence to later life stages all of our data will only be looking at stars that are currently in the Main Sequence. We will be taking some time to get comfortable with what all of these numbers and values mean and hopefully get a better understanding for the stars that you can see when you look up at the sky at night!
Table 1: The 7 Brightest Stars as Viewed from Earth (in Main Sequence Stage)
Common Name Constellation
Sun n/a
Rigil Kentaurus Centaurus
Sirius
Procyon
Altair
Fomalhaut
Vega
Castor
Canis Major
Canis Minor
Aquila
Pisces
Lyra
Gemini
Graph 1: Apparent vs Absolute Magnitude
Distance
(light years)
0
4.3
8.6
11.4
16
22
25
49
Apparent
Magnitude
-26.72
-0.27
-1.46
0.38
0.77
1.16
0.03
1.57
Luminosity (Absolute
Magnitude)
4.8
4.4
1.4
2.6
2.3
2
0.6
0.5
Graph 2: The Sun vs Rigil K.
Spectral Type
G2
G2
A1
F5
A7
A3
A0
A0
10
Apparent Magnitude vs Absolute Magnitude
5
5
4,4
4
0
Sun Rigil
Kentaurus
3
2
1
0
-1
Rigil
Kentaurus
2,6
2,3
2
1,4
1,16
0,38
0,77
0,03
0,6
Sirius Procyon Altair Fomalhaut Vega
1,57
0,5
Castor
-5
-10
-15
-20
Apparent
Magnitude
Luminosity
(Absolute
Magnitude)
-25
-2
-1,46
Apparent Magnitude Luminosity (Absolute Magnitude)
-30

Using the data and the readings from the previous pages, answer the following questions to the best of your ability.
1. In looking at Graph 2 and Table 1, comparing the sun’s apparent magnitude to that of
Rigil Kentarus, which star appears brighter from Earth?
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What values from the table tell you that this is true and why?
2. Again looking at the information in Table 1 and Graph 2, answer the questions listed below.
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What is the Absolute Magnitude of Rigil Kentarus?
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What is the Absolute Magnitude of the Sun?
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What is the Spectral Type of Rigil Kentarus?
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What is the Spectral Type of the Sun?
3. Based on the information you gathered above, what are two observations you can make about the data you collected?
4. Remember a star with a bigger magnitude number is fainter/dimmer than a star with a smaller magnitude number. Based off of that information, would a star of an Apparent
Magnitude of 2.5 be brighter or dimmer than a star of Apparent Magnitude of 1.3?
5. Using the information from question number 4 look at Graph 1 and rank the 7 stars in order from brightest to dimmest according to their Absolute Magnitude.
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6. On the next page, map out the distances that the stars from Rigil Kentarus to Altair would be by measuring their light year distance as centimeters. Label each star, then above the star write its Apparent Magnitude and below the star write its Spectral Type , I have given you Rigil Kentarus as an example, continue the rest of the stars along this line.

Distance from Earth Line.
Rigil Kentarus
G2
-0.27
7. Looking at your information above, can you explain why Sirius looks brighter from Earth even though it’s farther away? (Hint: What is its spectral type compared to Rigil?)
8. What pattern can you see regarding each of the stars in your chart when comparing it’s spectral type and it’s Absolute Magnitude?
9. *EXTRA CREDIT* Pollux is a star that was not included in your information above. It is located 40 light years from Earth, and is also part of the Gemini constellation (just like
Castor). Pollux is a K spectral type star, but its Absolute Magnitude is 0.7. All of the stars in your chart above are currently in their Main Sequence , but Pollux has actually entered its Red Giant Stage , based on what you have learned so far in class, why would this K spectral Type star, have the same luminosity (Absolute Magnitude) as an A spectral
Type star? (K spectral type stars should be fainter than our sun)