Colors of Stars

Stars come in many different colors.

The color tells us the star’s temperature according to Wien’s Law.

Bluer means hotter!

Masses of Stars

• Mass is the single most important property of any star.• at each stage of a star’s life, mass determines…

• what its luminosity will be• what its spectral type will be

• The mass of a star can only be measured directly by …• observing the effect which gravity from another object has on

the star• This is most easily done for two stars which orbit one

another…a binary star!

The orbit of a binary star system depends on strength of gravity

Binary Stars(two stars which orbit one another)

• Optical doubles• two unrelated stars which are in the same area

of the sky

• Visual binaries• a binary which is spatially resolved, i.e.

two stars are seen (e.g. Sirius)

Binary Stars

• Spectroscopic binaries• a binary which is spatially unresolved, i.e only

one star is seen; the existence of the second star is inferred from the Doppler shift of lines.

Binary Stars

• Spectroscopic binaries• a binary which is spatially unresolved, i.e only

one star is seen; the existence of the second star is inferred from the Doppler shift of lines.

Binary Stars

• Spectroscopic binaries• a binary which is spatially unresolved, i.e only

one star is seen; the existence of the second star is inferred from the Doppler shift of lines.

SO...

For a few thousand stars we can now find:� the distance� the total luminosity� the temperature (color or

spectral type)� the radius CAN WE FIND ANY

RHYME, REASON, OR RELATIONSHIPS?

Looking for correlations:

Height vs. IQ ?

Height vs. Weight ?

Height Height

A B

B – V (Temperature

or spectral type)

L

HOTCOOL

BRIGHT

FAINT

• A very useful diagram for understanding stars

• We plot two major properties of stars:

• Temperature (x) vs. Luminosity (y)

• Spectral Type (x) vs. Absolute Magnitude (y)

• Stars tend to group into certain areas

The Hertzsprung-Russell Diagram

Normal hydrogen-burning stars reside on the main sequence of the H-R diagram

Low-Mass Stars

High-Mass Stars

The Main Sequence (MS)

90% of all stars lie on the main sequence!

Normal hydrogen-burning stars reside on the main sequence of the H-R diagram

Low-Mass Stars

High-Mass Stars

The Main Sequence (MS)

90% of all stars lie on the main sequence!

Stars with low temperature and high luminosity must have large radius

GIANTS

SUPERGIANTS

Temperature

Lum

inos

ity

H-R diagram depicts:

Temperature

Color

Spectral Type

Luminosity

Radius

*Mass

*Lifespan

*Age

Temperature

Lum

inos

ity

Which star is the hottest?

A

BC

D

Temperature

Lum

inos

ity

Which star is the hottest?

A

BC

DA

Temperature

Lum

inos

ity

Which star is the most luminous?

A

BC

D

Temperature

Lum

inos

ity

Which star is the most luminous?

A

BC

DC

Temperature

Lum

inos

ity

Which star is a main-sequence star?

A

BC

D

Temperature

Lum

inos

ity

Which star is a main-sequence star?

D

A

BC

D

Temperature

Lum

inos

ity

Which star has the largest radius?

A

BC

D

Temperature

Lum

inos

ity

Which star has the largest radius?

C

A

BC

D

Temperature

Lum

inos

ity

Which star is most like our Sun?

A

B

C

D

Temperature

Lum

inos

ity

Which star is most like our Sun?

A

B

C

D

B

Temperature

Lum

inos

ity

Which of these stars will have changed the least 10 billion years from now?

A

B

C

D

Temperature

Lum

inos

ity

Which of these stars will have changed the least 10 billion years from now?

C

A

B

C

D

Temperature

Lum

inos

ity

Which of these stars can be no more than 10 million years old?

A

B

C

D

Temperature

Lum

inos

ity

Which of these stars can be no more than 10 million years old?

A

A

B

C

D

Regions of the H-R Diagram

RED GIANTS• Cool but VERY BRIGHT!• Betelgeuse: 3500 K (10% as

bright/unit area as Sun) but 100,000 times as luminous--must have 1 million times the area

• radius must be 1000x that of Sun!

ÿAuaKaÿelo

WHITE DWARFS• Hot but not very luminous• Sirius B: 3% as luminous as

Sun but same temp. as Spica (10,000x)--Sirius B must be 1/600 the radius of Spica

• Also much smaller than Sun

Stellar Masses on the H-R Diagram

Mass–Luminosity Relation

• All main sequence stars fuse H into He in their cores.

• Luminosity depends directly on mass because:• more mass means more weight from the star’s

outer layers• nuclear fusion rates must be higher in order to

maintain gravitational equilibrium

Mass-Luminosity Relation

L � m3.5for main sequence stars only

We use binary stars to measure directly the masses of stars of every type. We find a

• As one moves to the upper-left of the main sequence:• stars become more massive • stars become even much more luminous• stars become fewer in number

Lifetime on the Main Sequence

How long will it be before MS stars run out of fuel? i.e. Hydrogen?

How much fuel is there? M

How fast is it consumed? L � M3.5

How long before it is used up?

t = M/L = M/M3.5 = M-2.5 = 1/M2.5

Lifetime on the Main Sequence

• Our Sun will last 1010 years on the Main Sequence • MS Lifetime � = 1010 yrs / M2.5

So for example:

B2 dwarf (10 M) lasts 3.2 x 107 yrF0 dwarf (2 M) lasts 1.8 x 109 yr

M0 dwarf (.5 M) lasts 5.6 x 1010 yr

But the Universe is ~1.37 x 1010 yr old!

Every M dwarf that was ever created is still on the main sequence!!

Stellar Evolution

• Stars are like people in that they are born, grow up, mature, and die.

• A star’s mass determines what life path it will take.• We will divide all stars into three groups:

– Low Mass (0.08 M < M < 2 M)– Intermediate Mass (2 M < M < 8 M)– High Mass (M > 8 M)

• The H-R Diagram makes a useful roadmap for following stellar evolution.

Stellar Evolution• The life of any star is a battle between two forces:

– Gravity vs. Pressure

• Gravity always wants to collapse the star.• Pressure holds up the star.

– the type of star is defined by what provides the pressure

• Newton’s Law of Gravity:– the amount of gravitational force depends on the mass– gravitational potential energy is turned into heat as a star

collapses

hydrostatic equilibrium

The Stellar Womb

• Stars are born deep in molecular clouds.– cold (10 – 30 K) dense nebulae– so cold that H2 can exist

• A cold cloud can fragment– gravity overcomes thermal

pressure in dense regions– these regions (cores) become

more dense and compact

dark molecular cloud in Scorpius

Stellar Gestation

• Something happens to perturb a molecular cloud and make it begin to fragment

• As a core of gas collapses, it heats up– it radiates infrared from its surface

– protostar• The protostar collapses until its

core reaches 107 K in temperature – The proton – proton chain fusion

reaction begins and …. infrared image of Orion

A Star is Born!

MakaliÿiEagle NebulaHubble Space Telescope

Movie. Click to play.

Star Formation• As the protostar collapses, angular momentum is conserved

– the protostar rotates faster– matter falling in to the protostar flattens into a (protostellar) disk– a planetary system could form from this disk

Direct Evidence of Disks & Jets

a disk forms

Star Formation

• As the protostar heats up, enough thermal energy is radiated away from surface to allow collapse to continue.– energy is transported to surface first via convection– as core gets even hotter, transport via radiation takes over

• The protostar must rid itself of angular momentum, or it will tear itself apart– magnetic fields drag on the protostellar disk– fragmentation into binaries

• Fusion reactions begin when core reaches 107 K

Stages of Star Formation on the H-R Diagram

Arrival on the Main Sequence

• The mass of the protostar determines:– how long the protostar

phase will last– where the new-born star

will land on the MS i.e., what spectral type the star will have while on the main sequence

When a protostar ceases to accumulate mass, it, becomes a pre-main-sequence star.

It’s life path is forever determined by its initial mass

Missing the Main Sequence

• If the protostar has a mass < 0.08 M:– It does not contain enough gravitational energy

to reach a core temperature of 107 K– No fusion reactions occur– The star is stillborn!

• We call these objects Brown Dwarfs.• They are very faint, emit infrared, and have

cores made of Hydrogen– degenerate cores

Detection by

Michael Liu (IfA)

January, 2002

“Brown Dwarf”

orbiting a star

at same

distance as

Saturn in Solar

system

Life on the Main Sequence

The internal structure is different for MS stars of different masses.

The more massive a star, the faster it goes through its main

sequence phase

When core hydrogen fusion ceases, a main-sequence star becomes a giant

• The star can no longer support its weight• The enormous weight from the outer layers

compresses hydrogen in the layers just outside the core enough to initiate shell hydrogen fusion.

• This extra internal heat causes the outer layers to expand into a giant star.

Leaving the Main Sequence• The core begins to collapse

– H shell heats up and H fusion begins there– there is less gravity from above to balance this pressure– so the outer layers of the star expand– the star is now in the subgiant phase of its life