Stars Characteristics

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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

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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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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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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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. 05/03/2023

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 5,500ºCStop and Think• The hottest flame color in a campfire is?

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Star Spectral Classification

•Astronomers place stars in spectral (color) class categories based on their surface temperature.

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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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Classification of StarsColor 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 AYellow-white 6,000 – 7,500 Canopus,

Procyon F

Yellow 5,000 – 6,000 Sun, Capella GOrange 3,500 – 5,000 Arcturus,

Aldebaran K

Red Less than 3,500 Betelgeuse,

Antares M05/03/2023

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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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Stop and Think

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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?

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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.

Apparent Magnitude • The brightness of a star as it appears from

Earth with the naked eye.

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As you can see, the magnitude numbers are bigger for faint stars, and magnitudes are negative for very bright stars.

How is star luminosity measured?• Stars with the same absolute magnitude may have

different apparent magnitudes.

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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 = 1AM < 1 : brighter than SunAM > 1 : less bright than Sun

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Apparent and Absolute Magnitudes for the ten brightest stars in the night sky

Star Apparent mag. Absolute mag.

Sirius -1.44 1.45Canopus -0.62 -5.53Arcturus -0.05 -0.31Rigel Kent -0.01 4.34Vega 0.03 0.58Capella 0.08 -0.48Rigel 0.18 -6.69Procyon 0.40 2.68Betelgeuse 0.45 -5.14Achernar 0.45 -2.77

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The larger the negative magnitude a star has, the brighter it is; but the larger positive magnitude, the fainter the star.

What is the Absolute Magnitude and Luminosity of Procyon? Rigel? Sun?

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The Hertzsprung-Russell Diagram

• An H-R diagram plots stars according to their luminosity and temperature (or spectral class)

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Location of Stars on an H-R Diagram

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1. Main Sequence Stars

Upper left stars are • Large, hot, luminous• Ex. Blue Giants

Lower right stars are• Small, cool, dim

• Ex. Red Dwarfs

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90% of stars occur along the main sequence

Middle aged, stable stars

Main Sequence runs from upper left to lower right

Locate Main Sequence Stars

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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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Locate Red Giants & Super Giants

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3. Lower Left Stars• White Dwarfs (9% of stars)• Small• Hot • Dim

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Locate White Dwarfs

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Star MassPosition 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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