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Stars CharacteristicsStars Characteristics

What are the characteristics of a star?What are the characteristics of a star?

Stars differ in…Stars differ in…

MassMass

SizeSize

TemperatureTemperature

ColorColor

LuminosityLuminosity

*Stars can be plotted using these characteristics on a chart or HR diagram

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Star Mass and Composition Star Mass and Composition

Mass and composition determine most Mass and composition determine most of the properties of a star. of the properties of a star.

The more The more massivemassive a star is, the a star is, the greater greater the gravity, the the gravity, the hotter and denserhotter and denser a star a star must be.must be.

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Star SizeStar Size

The The diametersdiameters of stars range from as of stars range from as little as 1/10 the Sun’s diameter to little as 1/10 the Sun’s diameter to hundreds of times larger. hundreds of times larger. The The massmass of stars can be from less of stars can be from less than 1/100 to 20 or more times that of than 1/100 to 20 or more times that of our Sun.our Sun.Stars over 50 times the Sun’s mass are Stars over 50 times the Sun’s mass are extremely rareextremely rare. . Our Sun is aOur Sun is a medium medium sized star.sized star.

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Star EnergyStar Energy

The enormous pressure and heat in a The enormous pressure and heat in a star’s star’s corecore converts matter into converts matter into energyenergy..

Stars consist of controlled atomic Stars consist of controlled atomic reactions called reactions called nuclearnuclear fusionfusion in which in which hydrogenhydrogen (nuclei) atoms fuse to form (nuclei) atoms fuse to form heliumhelium (nuclei) atoms. (nuclei) atoms.

During each step of the process, During each step of the process, massmass is lost and is lost and energyenergy is released. is released.

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Star Energy cont.Star Energy cont.The The energyenergy released causes the star (Sun) to released causes the star (Sun) to shineshine and gives the star its high and gives the star its high temperature.temperature.

Star stability – the Star stability – the energyenergy from fusion stabilizes from fusion stabilizes a star by producing the outward a star by producing the outward pressurepressure needed to counteract the inward force of needed to counteract the inward force of gravitygravity..

Fusion>Gravity = expansionFusion>Gravity = expansion

Fusion<Gravity = contractionFusion<Gravity = contraction

Fusion=Gravity = stableFusion=Gravity = stable

NOTE: Stable stars are plotted on the HR diagram. NOTE: Stable stars are plotted on the HR diagram.

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Star TemperatureStar Temperature

Stars in the sky show tinges of different Stars in the sky show tinges of different colorscolors which reveal the stars’ which reveal the stars’ temperaturestemperatures. .

BlueBlue stars shine with the stars shine with the hottesthottest temperatures and temperatures and redred stars shine with the stars shine with the coolestcoolest temperatures. temperatures.

Our Sun is a Our Sun is a yellowyellow star having a surface star having a surface temperature of about 5,500ºCtemperature of about 5,500ºC

Stop and ThinkStop and Think

The hottest flame color in a campfire is?The hottest flame color in a campfire is?

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Star Spectral ClassificationStar Spectral Classification

Astronomers place stars in Astronomers place stars in spectralspectral (color) class categories (color) class categories based on their based on their surface surface temperaturetemperature..

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The Spectral Class Letters are The Spectral Class Letters are OBAFGKMOBAFGKM

OOh h BBe e AA FFine ine GGirl/Guy irl/Guy KKiss iss MMe e

Star colors from Hottest to Coolest:Star colors from Hottest to Coolest: Blue, Blue-White, White, Yellow-White, Yellow, Orange, RedBlue, Blue-White, White, Yellow-White, Yellow, Orange, Red

OO or or blueblue is the is the hottesthottest (>25,000 (>25,000ooC) and C) and

MM or or redred is the is the coolestcoolest (3,500-2,000 (3,500-2,000ooC) C)

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Classification of StarsClassification of StarsColorColor Surface Surface

temp (ºC)temp (ºC)ExamplesExamples Spectral ClassSpectral Class

BlueBlue Above 30,000Above 30,000 10 Lacertae10 Lacertae OO

Blue-whiteBlue-white 10,000 – 30,00010,000 – 30,000 Rigel, SpicaRigel, Spica BB

WhiteWhite 7,500 – 10,0007,500 – 10,000 Vegas, SiriusVegas, Sirius AA

Yellow-whiteYellow-white 6,000 – 7,5006,000 – 7,500 Canopus, Canopus,

ProcyonProcyon FF

YellowYellow 5,000 – 6,0005,000 – 6,000 Sun, CapellaSun, Capella GG

OrangeOrange 3,500 – 5,0003,500 – 5,000 Arcturus, Arcturus,

AldebaranAldebaran K K

RedRed Less than 3,500Less than 3,500 Betelgeuse, Betelgeuse,

AntaresAntares MM

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Star Luminosity/BrightnessStar Luminosity/Brightness

The The energyenergy output from the surface of a output from the surface of a star per second measured in star per second measured in wattswatts..The The brightnessbrightness of a star depends upon of a star depends upon the the distancedistance and its and its luminosity.luminosity.

Stop and ThinkStop and ThinkThe star Rigel in Orion is about 60,000 The star Rigel in Orion is about 60,000 times larger than our Sun.times larger than our Sun.Why does our Sun appear brighter than Why does our Sun appear brighter than Rigel?Rigel?

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Stop and ThinkStop and Think

If person A has a pen flashlight and If person A has a pen flashlight and person B has a mega flashlight and person B has a mega flashlight and both were standing at the front of the both were standing at the front of the classroom, which light would appear classroom, which light would appear brighter?brighter?

If person A (pen light) stood in the classroom and If person A (pen light) stood in the classroom and person B (mega light) stood at the baseball fields person B (mega light) stood at the baseball fields across the street, which light would appear brighter?across the street, which light would appear brighter?

You cannot tell by looking in the sky how bright a star You cannot tell by looking in the sky how bright a star truly is. The farther away the star is, the less bright ittruly is. The farther away the star is, the less bright it

will appear.will appear.

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Apparent MagnitudeApparent Magnitude

The brightness of a star as it The brightness of a star as it appears appears from Earthfrom Earth with the with the nakednaked eye. 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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Absolute MagnitudeAbsolute Magnitude

The The truetrue brightness of a star if all stars were brightness of a star if all stars were at a at a uniform distance uniform distance fromfrom Earth Earth

The absolute magnitude of stars is reported in The absolute magnitude of stars is reported in comparison to our comparison to our SunSun..

Ex. AM of Sun = 1Ex. AM of Sun = 1

AM < 1 : AM < 1 : brighter than Sunbrighter than Sun

AM > 1 : AM > 1 : less bright than Sunless bright than Sun

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

Star 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 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.

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

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The Hertzsprung-Russell DiagramThe Hertzsprung-Russell DiagramAn H-R diagram plots stars according An H-R diagram plots stars according to their to their luminosityluminosity and and temperaturetemperature (or spectral class)(or spectral class)

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Location of Stars on an H-R DiagramLocation of Stars on an H-R Diagram

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1. 1. Main Sequence StarsMain Sequence Stars

Upper left stars are Upper left stars are

LargeLarge, , hothot, , luminousluminous

Ex. Blue GiantsEx. Blue Giants

Lower right stars areLower right stars are

SmallSmall, , coolcool, , dimdim

Ex. Red DwarfsEx. Red Dwarfs

90%90% of stars occur along the main sequence of stars occur along the main sequence

Middle aged, Middle aged, stablestable stars stars

Main Sequence runs from Main Sequence runs from upper leftupper left to to lower rightlower right

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Locate Main Sequence StarsLocate Main Sequence Stars

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2. 2. Upper Right StarsUpper Right Stars

Red Red GiantsGiants & & Super GiantsSuper Giants (Super Super Giants, too)(Super Super Giants, too)

LargeLargeCoolCoolBrightBright1% of stars1% of stars

Our sun in 5 billion years will be Our sun in 5 billion years will be 2000 times brighter and 100 2000 times brighter and 100 times larger (large enough to times larger (large enough to expand past Mars orbit)expand past Mars orbit)

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Locate Red Giants & Super GiantsLocate Red Giants & Super Giants

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3. 3. Lower Left StarsLower Left Stars

White White DwarfsDwarfs (9% of stars) (9% of stars)

SmallSmall

HotHot

DimDim

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Locate White DwarfsLocate White Dwarfs

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Star MassStar MassPosition on H-R depends on Position on H-R depends on massmass

The larger the amount of The larger the amount of massmass in a star, in a star, the more the more luminousluminous it is. it is.

Giants: Giants: moremore mass, mass, moremore luminous luminous

Dwarfs: Dwarfs: lessless mass, mass, lessless luminous luminous

During its lifetime a star will evolve on the During its lifetime a star will evolve on the H-R diagram depending on its mass. H-R diagram depending on its mass.

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