February 7, 2006 Astronomy 2010 1 Chapter 17: The Stars: A Celestial Census.

February 7, 2006February 7, 2006 Astronomy 2010Astronomy 2010 11

Chapter Chapter 17: The 17: The Stars:Stars:A A Celestial Celestial CensusCensus


The Lives of StarsThe Lives of StarsStars live for a very long time, 100 Stars live for a very long time, 100 million years and up.million years and up.

We can’t possibly observe a star this We can’t possibly observe a star this long!long!

How can we learn about the stages in a How can we learn about the stages in a star’s life?star’s life?We perform a census, getting a snapshot of We perform a census, getting a snapshot of many stars at different stages of their life.many stars at different stages of their life.

We infer the stages a star goes through from We infer the stages a star goes through from the data we assemble in the census.the data we assemble in the census.

Cuidado!Cuidado! We can be misled if the star sample We can be misled if the star sample in the census is biased. (Like political in the census is biased. (Like political surveys.)surveys.)


A Stellar CensusA Stellar CensusWe measure distances in light years (LY).We measure distances in light years (LY).

More details in Ch. 18More details in Ch. 18

Small stars are less luminous, and Small stars are less luminous, and therefore harder to see.therefore harder to see.

If not corrected for, we will have a biased If not corrected for, we will have a biased sample of stars.sample of stars.

Careful observation reveals that small Careful observation reveals that small stars (brown dwarfs) are more common than stars (brown dwarfs) are more common than large stars.large stars.

While less numerous, large stars are While less numerous, large stars are easier to see at large distances. Most of easier to see at large distances. Most of the stars visible to the naked eye are the stars visible to the naked eye are large.large.


All Stars are DifferentAll Stars are Differentcolors: blue-white colors: blue-white to redto redbrightness: bright brightness: bright to very faint to very faint Orion: Constellation Orion: Constellation with many different with many different star typesstar typesBetelgeuse: Betelgeuse: orange-red orange-red supergiantsupergiantRigel: blue-white Rigel: blue-white supergiantsupergiant


Key Properties of StarsKey Properties of Starscolorcolor = surface = surface temperaturetemperature

Rigel (blue) 10,000 Rigel (blue) 10,000 KK

Sun (yellow) 6000 KSun (yellow) 6000 K

Betelgeuse (orange-Betelgeuse (orange-red) 3,red) 3,000 K

spectrumspectrum = = chemical chemical compositioncomposition

Rigel BRigel B

Sun GSun G

Betelgeuse M Betelgeuse M

luminosityluminosity Rigel 60,000 LRigel 60,000 LSunSun

Betelgeuse Betelgeuse 50,000 L50,000 LSunSun

massmass Rigel 16 solar Rigel 16 solar massesmasses

Betelgeuse 20 MBetelgeuse 20 MSunSun

diameter diameter Rigel 7 solar Rigel 7 solar diametersdiameters

Betelgeuse 1000 DBetelgeuse 1000 DSunSun


17.2 Measuring Stellar 17.2 Measuring Stellar MassesMasses

Mass is one the key parameters of Mass is one the key parameters of starsstarsDetermines the behavior and life cycle Determines the behavior and life cycle of the starof the star

Determined for binary stars – most Determined for binary stars – most common case: spectroscopic binary common case: spectroscopic binary starsstars

orbit depends on mass of two stars


center of mass

Orbit of Binary Stars Orbit of Binary Stars MassesMassesKepler's third law: Kepler's third law: (modified)(modified)

orbital period P and orbital period P and semimajor axis D of ellipse semimajor axis D of ellipse related to masses Mrelated to masses M11 and M and M22

DD33 = (M = (M11+M+M22)P)P22 D in AU, P in years, MD in AU, P in years, M11 and M and M22 in solar masses (Min solar masses (Msunsun))

each star orbits a common each star orbits a common point – the point – the center of masscenter of mass

Star’s distance from center Star’s distance from center of mass determines the of mass determines the star’s individual mass. star’s individual mass.

D

Visual Binary: Sirius A Visual Binary: Sirius A and Band B


Visual Binaries: Wobbling Visual Binaries: Wobbling MotionMotion


Sirius A and Sirius A and BB

Sirius A normal star (class A main sequence) Sirius A normal star (class A main sequence)

Sirius B white dwarf companionSirius B white dwarf companion

orbits are drawn to scaleorbits are drawn to scale

exaggerated sizes of the two stars exaggerated sizes of the two stars

Sirius A is considerably larger than the Sun Sirius A is considerably larger than the Sun while the white dwarf Sirius B is about the while the white dwarf Sirius B is about the size of the Earth. size of the Earth.


Spectroscopic Binary StarsSpectroscopic Binary Starsmost known binaries most known binaries are are spectroscopic spectroscopic binariesbinariesdistance too great to distance too great to resolve the two stars resolve the two stars individuallyindividuallybinary nature is binary nature is indicated in the indicated in the periodic shift of periodic shift of their spectral lines their spectral lines as they orbit around as they orbit around each othereach othercan measure their can measure their speeds from the speeds from the Doppler shifted linesDoppler shifted linesspeed determines the speed determines the massmass


17.2.3The Range of Stellar 17.2.3The Range of Stellar MassesMasses

How large and small can stars be?How large and small can stars be?

Stars with 100 solar masses are known, Stars with 100 solar masses are known, and we believe there may be stars up to and we believe there may be stars up to about 200 solar masses (200 Mabout 200 solar masses (200 Msunsun).).

True stars must be heavier than 1/12 True stars must be heavier than 1/12 MMsunsun..

Objects between 1/100 MObjects between 1/100 Msunsun and 1/12 M and 1/12 Msunsun may produce energy by fusion for a short may produce energy by fusion for a short time and are called brown dwarfs.time and are called brown dwarfs.

Objects less than 1/100 MObjects less than 1/100 Msunsun are planets. are planets.


17.2.4 The Mass-Luminosity 17.2.4 The Mass-Luminosity RelationRelation

As shown in Fig. 17.8, there is a As shown in Fig. 17.8, there is a correlation between mass and correlation between mass and luminosity.luminosity.More massive stars are more luminous More massive stars are more luminous (give off more energy).(give off more energy).

For a few stars this relation is For a few stars this relation is violated. These exceptions are the violated. These exceptions are the white dwarfs.white dwarfs.

Mass vs. LuminosityMass vs. Luminosity

each point on this plot represents the absolute magnitude (luminosity) and color (temperature) of a main sequence star

Sun:•luminosity 1.0 •mass 1.0


17.3 Diameters of Stars17.3 Diameters of Stars

Moon crosses in front of star – eclipse Moon crosses in front of star – eclipse

brightness of the star decreases brightness of the star decreases gradually during the eclipsegradually during the eclipse

time for decrease depends on size of startime for decrease depends on size of star

Eclipsing binary stars Eclipsing binary stars

Time

Brightness


Eclipsing Binary SystemEclipsing Binary SystemStars orbiting each other in a plane parallel to the Stars orbiting each other in a plane parallel to the line of sight:line of sight:

orbit is seen edge-on. orbit is seen edge-on.

One star periodically eclipses the other: One star periodically eclipses the other: total brightness of the combined stars decreases during the total brightness of the combined stars decreases during the eclipse. eclipse.

The reduction in brightness depends on the luminosity and The reduction in brightness depends on the luminosity and relative size of the two stars.relative size of the two stars.


InterferometryInterferometry

Combine the light from two or more Combine the light from two or more telescopes in a special way that yields telescopes in a special way that yields the resolution of a much larger the resolution of a much larger telescope.telescope.Regularly done with radio waves, very Regularly done with radio waves, very difficult to do with light.difficult to do with light.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.


H-R DiagramH-R DiagramRelationship between Relationship between temperaturetemperature (color) (color) and and luminosity luminosity (absolute magnitude) for (absolute magnitude) for 90% of the stars90% of the stars90% of stars lie along a band called the 90% of stars lie along a band called the main sequencemain sequencePlot of luminosity vs. temperature is Plot of luminosity vs. temperature is called the called the Hertzsprung-Russell diagramHertzsprung-Russell diagram

or just or just H-R diagramH-R diagram for short. for short.

The following slides show different The following slides show different examples of H-R diagrams.examples of H-R diagrams.

HR DiagramHR Diagram

each point on this plot represents the absolute magnitude (luminosity) and color (temperature) of a star

Sun:•+4.8 magnitude•B-V color index 0.62

temperature (Kelvin)

spectral class

lum

ino

sity

(so

lar

un

its)

classes: familiar starsclasses: familiar stars


H-R DiagramH-R Diagramclasses:classes:

main sequence main sequence 90% of stars90% of stars

large blue large blue starsstars

medium yellow medium yellow starsstars

small red stars small red stars

supergiantssupergiants

giantsgiants

white dwarfswhite dwarfs


17.4.2 Main Sequence 17.4.2 Main Sequence StarsStarsmassmass

Rigel (blue giant) ~ 16x SunRigel (blue giant) ~ 16x Sun

Proxima Centauri (red dwarf) ~ 0.4x SunProxima Centauri (red dwarf) ~ 0.4x Sun

size size Rigel ~ 7x Sun, Proxima ~ 0.6x SunRigel ~ 7x Sun, Proxima ~ 0.6x Sun

colorcolor = surface temperature = surface temperatureRigel (blue) 28,000 K, Sun (yellow) 6000 Rigel (blue) 28,000 K, Sun (yellow) 6000 K, Proxima (red) 3,500 KK, Proxima (red) 3,500 K

spectrumspectrum = chemical composition = chemical compositionRigel B, Sun G, Proxima Centauri MRigel B, Sun G, Proxima Centauri M

luminosityluminosity Rigel 27,000x Sun, Proxima 0.05x SunRigel 27,000x Sun, Proxima 0.05x Sun


Main Sequence Star PropertiesMain Sequence Star Properties

colorclass

solar

mass

solar size

Temper-ature (K)

example prominent lines

bluest O 20 –

10012 - 25

40,00010

Lacertaeionized helium

bluish B 4 -

204 - 12 18,000

Rigel, Spica

neutral helium, neutral hydrogen

blue-white A 2 -

41.5 - 4

10,000 Sirius neutral hydrogen

white F 1.05 - 2

1.1 - 1.5

7,000 Canopusneutral hydrogen, ionized calcium

yellow-white

G0.8 -

1.05

0.85 - 1.1

5,500Sun,

-Centauri

neutral hydrogen, strongest ionized

calcium

orange K

0.5 - 0.8

0.6 - 0.85

4,000 Tau Cetineutral metals (calcium, iron), ionized calcium

red M0.08 - 0.5

0.1 - 0.6

3,000Proxima Centauri

molecules and neutral metals


Color-MagnitudeColor-MagnitudeH-R diagram also H-R diagram also called a called a color-color-magnitude diagrammagnitude diagram All stars visible to All stars visible to the naked eye the naked eye (magnitude =< +5) & (magnitude =< +5) & all stars within 25 all stars within 25 parsecs. parsecs. Luminous stars Luminous stars

easier to observe easier to observe rarer in the galaxy. rarer in the galaxy. Mostly in top half of Mostly in top half of the H-R diagram. the H-R diagram.

Faint stars Faint stars harder to see harder to see more common in the more common in the galaxy. galaxy. Mostly in bottom half Mostly in bottom half of the H-R diagram. of the H-R diagram.


The Other 10% of StarsThe Other 10% of StarsAbout 10% of stars About 10% of stars

don't follow the mass-don't follow the mass-luminosity relationshipluminosity relationshipdon't lie on the main sequence don't lie on the main sequence

Giant and Supergiant starsGiant and Supergiant starsupper right of the HR diagram. upper right of the HR diagram. large in diameter because very large in diameter because very luminous even though they are luminous even though they are cool. cool.

White dwarfs White dwarfs lower left of the HR diagram. lower left of the HR diagram. small diameter (Earth-sized) small diameter (Earth-sized) hot but dim hot but dim

Betelgeuse


Main Sequence: Typical Main Sequence: Typical StarsStars

Is the Sun an "average'' or Is the Sun an "average'' or "typical''?"typical''?The meaning of "average'' depends The meaning of "average'' depends on how one chooses the sample!on how one chooses the sample!Compared to the nearby stars, the Sun Compared to the nearby stars, the Sun is luminous, hot, and big.is luminous, hot, and big.Compared to the Compared to the apparentlyapparently bright bright stars, the Sun is dim, cool, and small. stars, the Sun is dim, cool, and small. Compared to the stars in globular Compared to the stars in globular clusters, the Sun is very young.clusters, the Sun is very young.Compared to the stars in open Compared to the stars in open (galactic) clusters, the Sun is very (galactic) clusters, the Sun is very old.old.


Our Sun compared to…Our Sun compared to…

From Hipparcus From Hipparcus survey. survey. Most stars that Most stars that appearappear bright in our bright in our sky are also sky are also intrinsically intrinsically luminous luminous Near stars are all Near stars are all within 7.63 parsecs within 7.63 parsecs of the Sun.of the Sun.Near stars are Near stars are mostly cool and mostly cool and faint.faint.

the 100 (apparent) brightest stars in our skythe 100 (apparent) brightest stars in our skyand the 100 nearest stars.and the 100 nearest stars.


More comparisons…More comparisons…

Bright Stars:Bright Stars:Most of the Most of the apparently bright apparently bright stars are hot and stars are hot and luminous A and B-luminous A and B-type stars. type stars. Includes a few of Includes a few of the very hot O-type the very hot O-type stars. stars. All but one of the All but one of the K-type stars in the K-type stars in the bright star sample bright star sample are giants or are giants or supergiant stars. supergiant stars. All of the M-type All of the M-type stars are giants or stars are giants or supergiants.supergiants.

Proportions of the spectral types for each group.Proportions of the spectral types for each group.


Proportions of the spectral types for each group.

Near Stars Look different from

bright stars. Majority of near stars

are cool and faint K and M-type stars.

Only one star in the entire sample is a giant star.

Rest are main sequence stars.

More comparisons…More comparisons…


Representative SampleRepresentative SampleWhich of these samples is more Which of these samples is more representativerepresentative of the entire of the entire population of stars in our galaxy? population of stars in our galaxy? A A representative samplerepresentative sample includes all includes all parts of the population of the objects parts of the population of the objects your are investigating in their proper your are investigating in their proper proportions. proportions. The relative proportion of common The relative proportion of common things will be greater than the things will be greater than the relative proportions of rare things. relative proportions of rare things. In fact, the uncommon things may not In fact, the uncommon things may not be found in a small representative be found in a small representative sample because they are so rare! sample because they are so rare!


SummarySummaryA careful census of stars leads to a A careful census of stars leads to a number of conclusions:number of conclusions:

Stars have a wide range of masses, Stars have a wide range of masses, luminosities, temperatures, and sizes.luminosities, temperatures, and sizes.The most common stars are smaller and less The most common stars are smaller and less luminous than the Sun.luminous than the Sun.The stars organize into an understandable The stars organize into an understandable pattern on the H-R diagram.pattern on the H-R diagram.

Binary star systems are common and Binary star systems are common and useful for measuring masses.useful for measuring masses.Several techniques exist for measuring Several techniques exist for measuring diameters of stars.diameters of stars.

February 7, 2006 Astronomy 2010 1 Chapter 17: The Stars: A Celestial Census.

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Transcript of February 7, 2006 Astronomy 2010 1 Chapter 17: The Stars: A Celestial Census.