1. Stars Characteristics
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2. What are the characteristics of a star?
Stars differ in…
Mass
Size
Temperature
Color
Luminosity
*Stars can be plotted using these
characteristics on a chart or HR diagram

3. Star Mass and Composition
Mass and composition determine most of the properties of a star.
The more massive a star is, the greater the gravity, the hotter and denser a star must be.
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4. Star Size
The diameters of stars range from as little as 1/10 the Sun’s diameter to hundreds of times larger.
The mass of stars can be from less than 1/100 to 20 or more times that of our Sun.
Stars over 50 times the Sun’s mass are extremely rare.
Our Sun is a medium sized star.
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6. Star Energy
The enormous pressure and heat in a star’s core converts matter into energy.
Stars consist of controlled atomic reactions called nuclear fusion in which hydrogen (nuclei) atoms fuse to form helium (nuclei) atoms.
During each step of the process, mass is lost and energy is released.
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7. Star Energy cont.
The energy released causes the star (Sun) to shine and gives the star its high temperature.
Star stability – the energy from fusion stabilizes a star by producing the outward pressure needed to counteract the inward force of gravity.
Fusion>Gravity = expansion
Fusion<Gravity = contraction
Fusion=Gravity = stable
NOTE: Stable stars are plotted on the HR diagram.
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8. Star Temperature
Stars in the sky show tinges of different colors which reveal the stars’ temperatures.
Blue stars shine with the hottest temperatures and red stars shine with the coolest temperatures.
Our Sun is a yellow star having a surface temperature of about 5,500ºC
Stop and Think
The hottest flame color in a campfire is?
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9. Star Spectral Classification
Astronomers place stars in spectral (color) class categories based on their surface temperature.
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10. The Spectral Class Letters are OBAFGKM Oh Be A Fine Girl/Guy Kiss Me
Star colors from Hottest to Coolest:
Blue, Blue-White, White, Yellow-White, Yellow, Orange, Red
O or blue is the hottest (>25,000oC) and
M or red is the coolest (3,500-2,000oC)
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11. Classification of Stars
Color
Surface temp (ºC)
Examples
Spectral Class
Blue
Above 30,000
10 Lacertae O
Blue-white
10,000 – 30,000
Rigel, Spica B
White
7,500 – 10,000
Vegas, Sirius A
Yellow-white
6,000 – 7,500
Canopus, Procyon F
Yellow
5,000 – 6,000
Sun, Capella G
Orange
3,500 – 5,000
Arcturus, Aldebaran K
Red
Less than 3,500
Betelgeuse, Antares M
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13. Star Luminosity/Brightness
The energy output from the surface of a star per second measured in watts.
The brightness of a star depends upon the distance and its luminosity.
Stop and Think
The star Rigel in Orion is about 60,000 times larger than our Sun.
Why does our Sun appear brighter than Rigel?
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15. Stop and Think
If person A has a pen flashlight and person B has a mega flashlight and both were standing at the front of the classroom, which light would appear brighter?
If person A (pen light) stood in the classroom and person B (mega light) stood at the baseball fields across the street, which light would appear brighter?
You cannot tell by looking in the sky how bright a star truly is. The farther away the star is, the less bright it will appear.
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16. Apparent Magnitude
The brightness of a star as it appears from Earth with the naked eye.
As you can see, the magnitude numbers are bigger for faint stars, and magnitudes are negative for very bright stars.
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17. Absolute Magnitude
The true brightness of a star if all stars were at a uniform distance from Earth
The absolute magnitude of stars is reported in comparison to our Sun.
Ex. AM of Sun = 1
AM < 1 : brighter than Sun
AM > 1 : less bright than Sun
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18. Apparent and Absolute Magnitudes for the ten brightest stars in the night sky
Apparent mag.
Absolute mag.
Sirius
-1.44
1.45
Canopus
-0.62
-5.53
Arcturus
-0.05
-0.31
Rigel Kent
-0.01
4.34
Vega
0.03
0.58
Capella
0.08
-0.48
Rigel
0.18
-6.69
Procyon
0.40
2.68
Betelgeuse
0.45
-5.14
Achernar
-2.77
The larger the negative magnitude a star has, the brighter it is;
but the larger positive magnitude, the fainter the star.
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19. What is the Absolute Magnitude and Luminosity of Procyon? Rigel? Sun?
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20. The Hertzsprung-Russell Diagram
An H-R diagram plots stars according to their luminosity and temperature (or spectral class)
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21. Location of Stars on an H-R Diagram
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22. 1. Main Sequence Stars Upper left stars are Large, hot, luminous
90% of stars occur along the main sequence
Middle aged, stable stars
Main Sequence runs from upper left to lower right
Upper left stars are
Large, hot, luminous
Ex. Blue Giants
Lower right stars are
Small, cool, dim
Ex. Red Dwarfs
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23. Locate Main Sequence Stars
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24. 2. Upper Right Stars
Red Giants & Super Giants (Super Super Giants, too)
Large
Cool
Bright
1% of stars
Our sun in 5 billion years will be 2000 times brighter and 100 times larger (large enough to expand past Mars orbit)
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25. Locate Red Giants & Super Giants
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26. 3. Lower Left Stars
White Dwarfs (9% of stars)
Small
Hot
Dim
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27. Locate White Dwarfs
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28. Star Mass Position on H-R depends on mass
The larger the amount of mass in a star, the more luminous it is.
Giants: more mass, more luminous
Dwarfs: less mass, less luminous
During its lifetime a star will evolve on the H-R diagram depending on its mass.
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30. Bibliography
en-US:official%26sa%3DN
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