Stars: Stars: Their PropertiesTheir Properties

T. K. PrasadT. K. Prasad

http://www.cs.wright.edu/http://www.cs.wright.edu/~tkprasad~tkprasad

(Adapted from a lecture by Daniel (Adapted from a lecture by Daniel Wang of UMass)Wang of UMass)

Fundamental Problem and Fundamental Problem and Solution ApproachSolution Approach

No direct access to star! No direct access to star! Large distance Large distance

Nearest stars: Sun (8 light minutes), Proxima Centauri (4 Nearest stars: Sun (8 light minutes), Proxima Centauri (4 light years), etc.light years), etc.

High temperature and radiationsHigh temperature and radiationsWill melt/vaporize probes; interfere with detector electronics Will melt/vaporize probes; interfere with detector electronics even if we try to send them closer!even if we try to send them closer!

What information can we glean from remote observation?What information can we glean from remote observation? What information do we ultimately need to understand start What information do we ultimately need to understand start

evolution?evolution? STEMSTEM Quantitative vs qualitative informationQuantitative vs qualitative information

Fundamental Problem and Fundamental Problem and Solution ApproachSolution Approach

Only Remote Access Only Remote Access FeasibleFeasibleWhat information can we glean What information can we glean

using remote observation?using remote observation?What information do we need What information do we need

to understand star properties to understand star properties and evolution?and evolution?

STEMSTEM

ScienceScienceE.g., Physics, Chemistry, BiologyE.g., Physics, Chemistry, Biology

TechnologyTechnologyE.g., Vacuum tubes vs Transistors, E.g., Vacuum tubes vs Transistors,

Nanotechnology, 3D-Printing, Nanotechnology, 3D-Printing, Virtual RealityVirtual Reality

STEMSTEM

EngineeringEngineeringE.g., Mechanical, Electrical, E.g., Mechanical, Electrical,

Computer, Civil, Aeronautical, Computer, Civil, Aeronautical, Materials, etc.Materials, etc.

MathematicsMathematicsGeometry, Trigonometry, Geometry, Trigonometry,

Calculus, Algebra, Probability and Calculus, Algebra, Probability and Statistics, etc.Statistics, etc.

What can we observe?What can we observe? Apparent brightness (engineering + Apparent brightness (engineering +

technology)technology) Color (spectroscopy)Color (spectroscopy) Distance (telescopes + mathematics)Distance (telescopes + mathematics)

What do we need?What do we need? Luminosity (actual brightness) (total Luminosity (actual brightness) (total

energy)energy) Temperature (from Color)Temperature (from Color) Composition (phy + chem)Composition (phy + chem) Size (…)Size (…)

Apparent brightness + distance Apparent brightness + distance

=> Actual brightness=> Actual brightness Nuclear physics will explain the real source Nuclear physics will explain the real source

of this energy (e.g., Sun : Hydrogen fusion)of this energy (e.g., Sun : Hydrogen fusion) Ultimately Mass dictates the entire Ultimately Mass dictates the entire

lifecycle of a star and its volume (size)lifecycle of a star and its volume (size) Physics : Gravity, Optics, Velocity and its Physics : Gravity, Optics, Velocity and its

impact on spectrum (Dopplerimpact on spectrum (Doppler’’s effect), …s effect), … Tech : Materials to computers, …Tech : Materials to computers, … Engineering : Right device …Engineering : Right device … Math for precision and accuracy + Math for precision and accuracy +

quantitative analysisquantitative analysis

StarsStarsTwinkle, twinkle, little star,How I wonder what you are.Up above the world so high,Like a diamond in the sky.

Are Stars similar to our Sun?How far away are they?Where did they come from?What do they do?Do they live forever?

Panorama view of the sky Panorama view of the sky

The Four Basic Parameters of The Four Basic Parameters of a Stara Star

LuminosityLuminositySizeSizeMassMassSurface Surface TemperatureTemperature

To infer these parameters, we need to know the distance!

Luminosity and Apparent Luminosity and Apparent BrightnessBrightness

LuminosityLuminosity is the measure of energy is the measure of energy radiated by a star per second over all radiated by a star per second over all wavelengths. (Cf. Visual Luminosity)wavelengths. (Cf. Visual Luminosity) Luminosity depends both on temperature and surface Luminosity depends both on temperature and surface

area. area.  This cannot be determined by direct observation.This cannot be determined by direct observation.

Apparent brightnessApparent brightness is the amount of  is the amount of energy coming from the star per square energy coming from the star per square meter per second, as measured on Earth. meter per second, as measured on Earth. (cf. Flux)(cf. Flux) This can be determined by direct observation.This can be determined by direct observation.

(cont(cont’’d)d) LuminosityLuminosity is an intrinsic property of a is an intrinsic property of a

star, while star, while apparent apparent brightnessbrightness depends on the  depends on the distancedistance to to the observer. the observer.

LuminosityLuminosity is is how bright a star really is, while apparent brightnessapparent brightness is how  is how bright a star appears to an observer. bright a star appears to an observer.

Inverse Square LawInverse Square Law

Distances by Distances by TriangulationTriangulation

We can measure We can measure distances by distances by comparing the comparing the position of position of objects observed objects observed from two ends of from two ends of the the ““baselinebaseline”” of of a triangle.a triangle.

ParallaxParallax

1.1. Hold your thumb up, steadily in front of you.Hold your thumb up, steadily in front of you.

2.2. Move your head from side to side and note Move your head from side to side and note the shift of your thumb with respect to the shift of your thumb with respect to background objects—this background objects—this angular shift is angular shift is called called parallaxparallax..

3.3. Now look at your thumb while keeping your Now look at your thumb while keeping your head steady but first closing one eye then the head steady but first closing one eye then the other.other.

4.4. Move your thumb closer to you—does it shift Move your thumb closer to you—does it shift more or less with respect to the background?more or less with respect to the background?

StereovisionStereovision

You use parallax constantly to estimate You use parallax constantly to estimate distances.distances.

Close your eyes. Have a neighbor dangle a Close your eyes. Have a neighbor dangle a pen in front of you, then open just one eye. pen in front of you, then open just one eye. Without moving your head, bring your hand Without moving your head, bring your hand in from the side and try to touch the pencil in from the side and try to touch the pencil with just the tip of another pen.with just the tip of another pen.

Your brain processes the information from Your brain processes the information from each eye and compares the angles to allow each eye and compares the angles to allow you to judge distances.you to judge distances.

The Geometry The Geometry of Parallaxof Parallax

We use the Earth’s whole orbit as our baseline.

D (in Parsecs) = 1 (AU)

P (in arcseconds)

DA

DB

PB

PA

=

p

1 parsec (pc) = 3.26 ly. Other useful units: kpc and Mpc

Parallax ExampleParallax Example

Parallax from a Different Parallax from a Different PlanetPlanet

If we lived on Mars, orbiting 1.5 If we lived on Mars, orbiting 1.5 times farther away from the times farther away from the Sun, the parallax would beSun, the parallax would be

1.1. the same as from Earththe same as from Earth

2.2. 1.5 times smaller than from 1.5 times smaller than from EarthEarth

3.3. 1.5 times bigger than from 1.5 times bigger than from EarthEarth

Digression:Digression:Proper Proper Motion of Motion of Stars (Very Stars (Very Slow)Slow)

100,000 yrs ago

Now

100,000 yrs in future

Big Dipper

Surprising Fact:It is easier to measure radial velocity using Doppler Effect than transverse velocity!

Stellar ParallaxStellar Parallax Since ancient Greek times, astronomers Since ancient Greek times, astronomers

expected that if the Earth moved through space, expected that if the Earth moved through space, we would see the stars shifting due to parallax.we would see the stars shifting due to parallax. If the Copernican model is correct, parallax of stars If the Copernican model is correct, parallax of stars

was a necessary consequence, but it was undetected was a necessary consequence, but it was undetected until the 1830until the 1830’’s because of the huge distances of s because of the huge distances of stars.stars.

The nearest stars shift by only about 0.7 arcsec The nearest stars shift by only about 0.7 arcsec

1 / 0.7 = 1.4 parsec1 / 0.7 = 1.4 parsec

This is about 4.3 light yearsThis is about 4.3 light years

or about 27,000,000,000,000 miles !or about 27,000,000,000,000 miles !

Survey Question: Stellar Survey Question: Stellar ParallaxParallax

Suppose a star has a parallax of Suppose a star has a parallax of 0.01 arc seconds. How many 0.01 arc seconds. How many parsecs away is it?parsecs away is it?distance (in parsecs) = 1 / parallax (in distance (in parsecs) = 1 / parallax (in

arcsec)arcsec)

Answer: 100

Brightness, Distance, and Brightness, Distance, and LuminosityLuminosity

L=4D2 B

luminosity distance

apparent brightness

There is a Big Range of There is a Big Range of Stellar Luminosities Out Stellar Luminosities Out

there!there!

StarStar Luminosity Luminosity (in units of (in units of

solar solar luminosity)luminosity)

SunSun 11Proxima CentauriProxima Centauri 0.00060.0006

Rigel (Orion)Rigel (Orion) 70,00070,000Deneb (Cygnus)Deneb (Cygnus) 170,000170,000

Apparent Brightness vs Luminosity

• LuminosityLuminosity depends on BrightnessBrightness & DistanceDistance

B

AEarth

AB

b = L b = L // 4 4πdπd22

A appears brighter

A appears brighter

How to measure the surface How to measure the surface temperature of a star?temperature of a star?

Overall spectral shape (the peak of the blackbody continuous spectrum) is Overall spectral shape (the peak of the blackbody continuous spectrum) is related to its temperature byrelated to its temperature by

Wien’s Displacement Law:

T = 2.9 × 106 K

λmax (nm)(nm)

More accurately, spectroscopicallyMore accurately, spectroscopically

WeinWein’’s Laws LawPeak frequency of radiation from a Peak frequency of radiation from a

(star) blackbody is proportional to its (star) blackbody is proportional to its (surface) temperature(surface) temperature

Spectral Types of StarsSpectral Types of Stars

Spectral types are defined by the:Spectral types are defined by the:• existence of absorption lines belonging existence of absorption lines belonging

to various elements, ions, & molecules to various elements, ions, & molecules in a starin a star’’s spectrums spectrum

• the relative strengths of these linesthe relative strengths of these lines However, spectral type is not However, spectral type is not

determined by a stardetermined by a star’’s composition.s composition.• all stars are made primarily of all stars are made primarily of

Hydrogen & HeliumHydrogen & Helium

Reason for Spectral TypesReason for Spectral Types

• temperature dictates temperature dictates the energy states of the energy states of electrons in atoms electrons in atoms

• temperature dictates temperature dictates the types of ions or the types of ions or molecules which existmolecules which exist

• this, in turn, this, in turn, determines the number determines the number and relative strengths and relative strengths of absorption lines in of absorption lines in the starthe star’’s spectrums spectrum

Spectral type is determined by a starSpectral type is determined by a star’’s s surface temperature.surface temperature.

Spectral Type Classification Spectral Type Classification SystemSystem

O B A F G K M

Oh Be A Fine Girl/Guy, Kiss Me!

50,000 K 3,000 K Temperature

(L T)

Other Mnemonics: e.g., Officially, Bill always felt guilty kissing Monica Lewinsky tenderly

 V Mag.

(m)Bayer designation

Proper nameDistance

(ly)Spectral class

0 −26.74   (Sun)0.000 01

6G2 V

1 −1.46 α CMa Sirius 8.6 A1 V2 −0.72 α Car Canopus 310 F0 Ia

3−0.04

varα Boo Arcturus 37 K1.5 III

4 −0.01 α Cen A (α1 Cen) Rigil Kentaurus, Toliman

4.4 G2 V

5 0.03 α Lyr Vega 25 A0 V6 0.12 β Ori Rigel 770 B8 Iab7 0.34 α CMi Procyon 11 F5 IV-V8 0.42 var α Ori Betelgeuse 640 M2 Iab9 0.50 α Eri Achernar 140 B3 Vpe

10 0.60 β Cen Hadar, Agena 530 B1 III

11 0.71 α1 Aur Capella A 42 G8 III

12 0.77 α Aql Altair 17 A7 V13 0.85 var α Tau Aldebaran 65 K5 III

14 0.96 α2 Aur Capella B 42 G1 III

15 1.04 α Vir Spica 260 B1 III-IV, B2 V

Stellar SizeStellar Size

Stars are spherical so we Stars are spherical so we characterize a starcharacterize a star’’s size by its s size by its radius.radius.

R

Stellar Radii vary in sizefrom ~1500 RSun for a large Red Giant to 0.008 RSun for a WhiteDwarf.

How do we determine the radius of a star?

Angular Radius of StarAngular Radius of Star

The angular radius of the Sun is about 10The angular radius of the Sun is about 1033 arc seconds. If another star like the arc seconds. If another star like the Sun was 5 parsecs away (about 10Sun was 5 parsecs away (about 1066 AU), what would its angular radius be?AU), what would its angular radius be?

101099 arc seconds arc seconds 101000 arc seconds arc seconds 1010-3-3 arc seconds arc seconds 1010-9-9 arc seconds arc seconds

Temperature, Luminosity, Temperature, Luminosity, and Size – pulling them all and Size – pulling them all

togethertogether

Stefan-Boltzmann Law

Luminosity Stellarradius

Surfacetemperature

L=4πR2 σT4

A star’s luminosity, surface temperature, and size are all related by the Stefan-Boltzmann Law:

1) 10 times more luminous

2) 100 times more luminous

4) 1/10th as luminous

5) 1/100th as luminous

Two stars have the same surface temperature, butthe radius of one is 10 times the radius of the other.The larger star is

L=4πR2 σT4

1) 1/2 as great

2) 1/4 as great

4) 4 times

5) 16 times as great

Suppose two stars are at equal distance and have the sameradius, but one has a temperature that is twice as great as theother. The apparent brightness of the hotter star is ____ as the other.

L=4πR2 σT4L=4πD2 B

Measurements of Star PropertiesMeasurements of Star Properties

Apparent brightness (B) DistanceLuminosity

TemperatureRadius

Direct measurentParallaxDistance + apparent brightness( L=4D2 B)Spectral type (or color)Luminosity + temperature(L=4R2 T4)

Luminosity and temperature are the two independent intrinsic parameters of stars.

Finding Star PropertiesFinding Star Properties

How do you weigh a How do you weigh a star?star?

Mass is the single most important Mass is the single most important property in how a starproperty in how a star’’s life and death s life and death will proceed.will proceed.

The mass of a star can only be The mass of a star can only be measured directly by observing the measured directly by observing the effect of its gravity on another objecteffect of its gravity on another object

This is most easily done for two stars This is most easily done for two stars which orbit one another --- a binary which orbit one another --- a binary star!star!

NewtonNewton’’s Version of Keplers Version of Kepler’’s s Third LawThird Law

Newton was able to derive Kepler’s Third Law from his own Law of Gravity. In its most general form:

P2 (mA + mB) = a3

The orbital period of two objects (P) depends on the distance between them (a) and the sum of the masses of both objects (mA + mB).

So if P and a can be measured, mA + mB can be estimated.

Each star in a binary system moves in its own orbit around the system's center of mass.

a

Star A

Star B

Orbits and Masses of Orbits and Masses of BinariesBinaries

We get the sum of the masses unless we see both stars moving.

The primary importance of binaries is that they allow us to measure stellar parameters (especially mass).

Visual Binaries

But for most binaries, one cannot separate the stars even with most powerful telescopes. For them, we need to use the spectroscopic information.

Visual Binary Star Visual Binary Star ImagesImages

Albireo – The “Cal” star

Mizar – in the handle of theBig Dipper.

Sirius – the brightest star in the sky.

Recall: Doppler Shift tells only if it is moving toward or away

1. The total spread (size) of the Doppler shift gives velocities about center of mass orbit sizes, a

2. The time to complete one repeating pattern period, P

1

2

3

4

5

Spectroscopic Binaries

Eclipsing Binaries

From the eclipse duration, and orbital speed, we can also find the size of the star.Thus one typically can tightly constrain the star masses in eclipsing binaries.

• Binary orbiting edge-on to our line of sight.

• The stars alternately eclipse each other changing the apparent brightness.

Eclipsing Binary : Eclipsing Binary : Algol, The Algol, The ““DemonDemon”” Star Star

In ReviewIn Review There are four principal There are four principal

characteristics of a star:characteristics of a star: LuminosityLuminosity Surface TemperatureSurface Temperature SizeSize MassMass

How may we classify stars?We can take a census of stars and see what’s out there.

But first, let’s do some sociology in the classroom.

Star A has a parallax that is twice Star A has a parallax that is twice that of Star B. What is the that of Star B. What is the relationship between their relationship between their distances?distances?

Star A is closer than Star BStar A is closer than Star B Star B is closer than Star AStar B is closer than Star A The stars are at the same The stars are at the same

distancedistance Not enough information is givenNot enough information is given

Stellar ClassificationStellar Classification

Discussion QuestionDiscussion Question

Make a plot that shows the Make a plot that shows the generalgeneralrelationship between height and relationship between height and weight for humans.weight for humans.- now add to your plot the population of basketball players who are very tall and very thin.

- now add the population of obese children

How can we classify starsHow can we classify stars

1) Collect information ona large sample of stars.

2) Measure their luminosities(need the distance!)

3) Measure their surface temperatures(need their spectra)

Star clusters: Globular Star clusters: Globular vs Open vs Open

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

Around 1910, Ejnar Hertzsprung (Dane) and Henry Norris Russell (American) had the idea of plotting the luminosity of a star against its spectral type. For a star cluster, all the stars are at the same distance. So, apparent brightness vs spectral type is basically the same as luminosity vs temperature. They found that stars appeared only in certain parts of the diagram.

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

HOT COOL

BRIGHT

FAINT

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

The Main Sequence ~90% of all stars

are in the main sequence (MS)

~90% of all MS stars are cooler spectral types than the Sun (i.e., at the lower MS)

All MS stars fuse H into He in their cores.

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

Mass-Mass-Luminosity Luminosity Relation:Relation:

L M3.5

For example, if the mass of a star is doubled, its luminosity increases by a factor 23.5 ~ 11.

Mass of MS StarL M3.5

The relation is for main sequence stars only!

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

Red Giants

- Red Giant starsare very large, cooland quite bright.

e.g., Betelgeuse is150,000 times moreluminous than the Sunbut is only 3,500K onthe surface. It’s radiusis 1,000 times that of the Sun.

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

Supergiants

L=4πR2 σT4 Size of Star:

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

White Dwarfs

- White Dwarfsare hot, but sincethey are so small,they are not veryluminous.

Main Main Sequence Sequence LifetimeLifetime

All M-S stars have temperatures sufficient All M-S stars have temperatures sufficient to fuse H into He in their cores.to fuse H into He in their cores.

Luminosity depends directly on mass:Luminosity depends directly on mass: more mass = more pressure from upper layersmore mass = more pressure from upper layers fusion rates must be high to maintain fusion rates must be high to maintain

equilibriumequilibrium Lifetime Lifetime (Amt. of Fuel)/(Rate of Burning) (Amt. of Fuel)/(Rate of Burning)

M / L M / L M / M M / M3.53.5 1 / M 1 / M2.52.5

Higher mass stars have shorter lives!Higher mass stars have shorter lives!

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

Lifetimeof Star

Shorter

Longer

More mass,more fuel,very fast burning.

Less mass,less fuel,slow, steady burning.

ThinkThinkSUV vs Honda CivicSUV vs Honda Civic

Review Questions: The H-R Review Questions: The H-R DiagramDiagram1.1. Where are most stars?Where are most stars?

2.2. What is the common What is the common characteristics of MS characteristics of MS stars?stars?

3.3. What determines the What determines the location of a star in the location of a star in the MS?MS?

4.4. Where do you find the Where do you find the largest stars? largest stars?

5.5. The smallest?The smallest?6.6. The most massive one?The most massive one?7.7. The coolest stars?The coolest stars?8. How do we know the age

of a star?

1. MS, 2. H He, 3. M, 4. upperright, 5. lowerleft, 6. upperleft, 7. lowerright, 8. normally we don’t

Luminosity classesLuminosity classes

Mass-Lifetime Relation Mass-Lifetime Relation (MS)(MS)

• High mass stars have more fuel but they burn it much faster

0.5

18

6

3

1.71.00.8

40

Long Long

Lifetime

Lifetime

Short lifetime

Short lifetime

Lifetime

Lifetime

ML

MM3.5ttlifelife ~ ~ ~ MM–2.5–2.5

e.g. for a 4 Msun star (e.g. Vega) L = 43.5 = 128 Lsun

tlife = 4–2.5 = 0.03tsun = 300 Myr

1 Myr

30 Myr

10

300 Myr

1 Gyr

10 Gyr

60 Gyr

O5 V (40Ms) : 1 Myr G2 V (Sun) : 10 GyrM5 V (0.2 Ms) : 500 Gyr

• No star with M < 0.9Ms has yet died (tUniverse = 13.7 Gyr)

L/L

s

Radius of StarsRadius of Stars

MS lasts until H is MS lasts until H is exhausted in the exhausted in the core.core.

Clues to the next stage Clues to the next stage are visible in older star are visible in older star clusters.clusters.

The brightest stars are The brightest stars are gone, replaced by red gone, replaced by red giants.giants.

Aging of a cluster of starsAging of a cluster of stars

Why are clusters useful Why are clusters useful to astronomers?to astronomers?

All stars in a cluster are at about All stars in a cluster are at about same distance from Earth.same distance from Earth.

All stars in a cluster are of about All stars in a cluster are of about the same age.the same age.

Clusters therefore are natural Clusters therefore are natural laboratory in which mass, rather laboratory in which mass, rather than age, of stars is only than age, of stars is only significant variable.significant variable.

The Hertzsprung-Russell The Hertzsprung-Russell DiagramDiagram

We can date a cluster by observing itspopulation ofstars.

The oldest clustersknown have beenmeasured to be ~13 billion years old.

All these stars in the cluster have burned themselves out!

Hydrogenfuel

Hydrogenburning core

Helium“ash”

Anatomy of a Main Sequence Star

shell

Up the red giant branchUp the red giant branchAs hydrogen in the core is being used up, it starts to contract, raising temperature in the surrounding. Eventually, hydrogen will burn only in a shell. There is less gravity from above to balance this pressure. The Sun will then swell to enormous size and luminosity, and its surface temperature will drop, a red giant.

Sun todaySun in ~5 Gyr

Helium fusion at the center of a Helium fusion at the center of a giantgiant

While the exterior layers expand, the helium core While the exterior layers expand, the helium core continues to contract, while growing in mass, and continues to contract, while growing in mass, and eventually becomes hot enough (100 million Kelvin) for eventually becomes hot enough (100 million Kelvin) for helium to begin to fuse into carbonhelium to begin to fuse into carbon

Carbon ash is deposited in core and eventually a helium-Carbon ash is deposited in core and eventually a helium-burning shell develops. This shell is itself surrounded by a burning shell develops. This shell is itself surrounded by a shell of hydrogen undergoing nuclear fusion.shell of hydrogen undergoing nuclear fusion.

For a star with M< 1 Msun, the carbon core never gets hot For a star with M< 1 Msun, the carbon core never gets hot enough to ignite nuclear fusion.enough to ignite nuclear fusion.

In very massive stars, elements can be fused into Fe.

The Sun will expand and cool again, becoming a red giant. Earth will be engulfed and vaporized within the Sun. The Sun’s core will consist mostly of carbon.•Red Giants create most of the Carbon in the universe (from which organic molecules—and life—are made)

How can two stars have the same How can two stars have the same temperature, but vastly different temperature, but vastly different

luminosities?luminosities?

• The luminosity of a star depends The luminosity of a star depends on 2 things:on 2 things:• surface temperaturesurface temperature• surface area (radius)surface area (radius)

L = L = T T44 4 4 R R22

• The stars have different sizes!!The stars have different sizes!!• The largest stars are in the upper right The largest stars are in the upper right

corner of the H-R Diagram.corner of the H-R Diagram.

L = L = T T44 4 4 R R22. If Star A is twice as . If Star A is twice as hot and one fourth the radius of Star hot and one fourth the radius of Star B, then it should be…B, then it should be…

(1) 1/4 as luminous as Star B(1) 1/4 as luminous as Star B

(2) just as luminous as Star B(2) just as luminous as Star B

(3) (3) 16 times as luminous as Star B16 times as luminous as Star B

(4)(4) 64 times as luminous as Star B64 times as luminous as Star B

LuminosityLuminosity

Review: The Hertzprung-Russell Review: The Hertzprung-Russell (H-R) Diagram (H-R) Diagram

One of the most One of the most important diagrams in important diagrams in astronomy for star astronomy for star classificationclassification

Spectral Types of Spectral Types of Stars Stars OBAFGKM: O=hottest, OBAFGKM: O=hottest,

M=coolest.  M=coolest.  spectral type carries spectral type carries

almost same info as almost same info as color or temperature color or temperature

H-R Diagram: main H-R Diagram: main sequencesequence

Most (about 90%) stars -- including the Sun -- appear to lie on the main sequence (MS).

Mass determines location of star on MS:

L M3.5 -- Luminosity depends very strongly on mass.

The defining characteristic of a MS star is that it is fusing H to He.

Other starsOther stars

Red Giants are bright because they're Red Giants are bright because they're big, even though cool. big, even though cool. Appear near the upper right section of the HR

Diagram. They have a bigger radius than the stars of the

same temperature which gives them a higher luminosity.

White Dwarf's are faint because they're White Dwarf's are faint because they're tiny, even though hot. tiny, even though hot. Appear in the lower left section of the HR Diagram. They are extremely hot, yet appear very dim due to

their extremely small size.

Why are there no Main Why are there no Main Sequence M-class stars Sequence M-class stars visible to the naked eye?visible to the naked eye?

(1) they are very rare and all very far (1) they are very rare and all very far away.away.

(2) they are so cool that they only emit in (2) they are so cool that they only emit in the infrared.the infrared.

(3) they are too dim to be seen even if (3) they are too dim to be seen even if they are only a few light years away.they are only a few light years away.

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