18. Stellar Birth• Star observations & theories aid understanding

• Interstellar gas & dust in our galaxy

• Protostars form in cold, dark nebulae

• Protostars evolve into main-sequence stars

• Protostars both gain & lose mass

• Star clusters reveal formation & evolution details

• Protostars can form in giant molecular clouds

• Supernovae can trigger star birth

Stellar Observations & Theories• Fundamental observational difficulties

– Stars exist far longer than astronomers• Star lifetimes range from millions to billions of years

– Stellar birth, life & death observed as stages• Each observation is an extremely brief snapshot

• Fundamental observational simplicity– Every star is far simpler than any living organism

• The materials are very simple• The processes are very simple

• Basic physical processes– Gravity tends to gather matter closer

together• Gravity is determined by distance between atoms

– Pressure tends to disperse matter farther apart• Pressure is determined by temperature of atoms

Interstellar Gas & Dust in Our Galaxy• Emission nebulae

– Fluorescence similar to common light bulbs• Emission lines depend on material & temperature

• Reflection nebulae– Characteristic blue color

• Selective scattering of continuous spectra from stars• Dust particles comparable in size to blue wavelengths

• Dark nebulae– Characteristic blocking of background light

• May be partial or total blocking• Thermal infrared can penetrate some dark nebulae

Initiation of Star Formation• Compression of interstellar medium is essential

– Gentle mechanisms from low-mass star death• Gently expanding shell of gas called a “planetary nebula”

• Weak shock wave may initiate compression

• Gas adds low-mass elements to the forming stars– Usually limited to Carbon & Silicon

– Violent mechanisms from high-mass star death• Rapidly expanding gas shell is a “supernova remnant”

• Strong shock wave will initiate formation of O & B stars

• Gas adds high-mass elements to the forming stars– May include elements as heavy as Uranium

The Orion Nebula: A Close-Up View

Emission, Reflection & Dark Nebulae

Reflection Nebula In Corona Australis

Interstellar Reddening by Dust Grains

Weakly scattered

Strongly scattered

Spiral Galaxies: Two Perspectives…Face-on

Edge-on…

Protostars Form in Cold Dark Nebulae• Basic physical processes

– Gravity effects must exceed pressure effects– Highest probability for star formation

• Extremely low temperatures minimizepressure

• Extremely close atoms maximize gravity– Only dark nebulae have high enough density

• Large Barnard objects– A few thousand M☉ & ~ 10 pc in diameter

• Small Bok globules– Resembles the core of a Barnard object

• Basic chemical composition (by mass)– ~ 74% hydrogen– ~ 25% helium– ~ 1% “metals”

All elements heavier than helium

Bok Globules: Opaque Dust & Gas

Anglo-Australian Observatory

Protostar Details• Earliest model

– Henyey & Hayashi 1950’s• Stage 1 Cool nebula several times Solar System

size• Stage 2 Continued contraction raises the

temperature

Kelvin-Helmholtz contraction• Stage 3 Still quite large, the cloud begins to glow

Convection move heat outward

Low temperature + Huge surface = Very bright

• A protostar the mass of the Sun– After 1,000 years of contraction…

• Surface temperature is ~ 2,000 K to 3,000 K• Diameter is ~ 20 times > the Sun• Luminosity is ~ 100 times > the Sun

Evolutionary Track of Protostars• High- mass stars

– Approximately a horizontal line on an H-R

diagram• Progression is toward the leftCool to hot

• Solar- mass stars– Approximately a V-shaped line on an H-R

diagram• Progression is toward the leftCool to hot

• Low- mass stars– Approximately a vertical line on an H-R

diagram• Progression is toward the bottom Bright to

dim

Pre-Main-Sequence Evolutionary Tracks

Progress of Star Formation• A positive feedback process

– Gravity & pressure increase as the nebula shrinks• Pressure increases µ d• Gravity increases µ d2

Gravity overwhelms pressure

– Magnetism could disrupt this in the earliest stages

• Additional characteristics– Angular momentum is conserved

• The shrinking nebula spins faster & faster

– Original 3-D cloud deforms into a donut-like disk• Material spins inward very rapidly• Much of this material is ejected at the protostar’s poles

Culmination of Star Formation• A negative feedback process

– High core pressure & temperature sustain H fusion• A new & intense source of heat energy

• Core pressure rises dramatically

– Gravitational collapse ends• Thermal & hydrostatic equilibrium established

• A new star stabilizes on the main sequence

Protostars Become Main-Sequence Stars• Protostar temperature changes

– Surface Little temperature change• Minimal increase for 15 M☉ protostars

• Slight increase for 5 M☉ protostars

• Slight decrease for 2 M☉ protostars

• Significant decrease for 1 M☉ protostars

• Dramatic decrease for 0.5 M☉ protostars

– Core Dramatic temperature increase• Increasing temperature ionizes the protostar’s interior

– Energy is transmitted outward by radiation

• Temperatures > several million kelvins initiate fusion– This event marks the “birth” of a true star

Protostar Evolution is Mass-Dependent• Very-low- mass stars M < 0.8 M☉

– Core temperatures too low to ionize interior• Convection characterizes the entire interior of the star

• Low- mass stars 0.8 MSun < M < 4 M☉– Core temperatures high enough to ionize interior

• Radiation characterizes the region surrounding the core• Convection characterizes the region near the surface

• High- mass stars M > 4 M☉– Hydrogen fusion begins very early

• Convection characterizes the region surrounding the core• Radiation characterizes the region near the surface

Main-Sequence Stars of Different Mass

Brown Dwarfs: Failed Stars• A minimum mass is required for fusion

– Pressure & temperature cannot get high enough• Minor lithium fusion can occur• Surface temperature may reach ~ 2,000 K

• Brown dwarf characteristics– Mass between 1028 kg & 84 . 1028 kg

• ~ 10 to 84 times the mass of Jupiter• The lower mass limit is sometimes set at ~ 14 times MJup

– Continues to cool & contract– Detectable only at thermal infrared wavelengths

• Many brown dwarfs exhibit irregular brightness changes– Possible storms far more violent than on Jupiter

Protostars Both Gain & Lose Mass• Protostar formation is extremely dynamic

– Matter is drawn inward along an accretion disk– Matter is hurled outward perpendicular to this disk

• T Tauri stars– 20th brightest star in the constellation Taurus– Exhibit both emission & absorption spectral lines

• Surrounded by hot low-density gas• Doppler shift indicates a velocity of 80 km . sec-1

– Luminosity varies irregularly over several days– Mass ~ 3 M☉

• Herbig-Haro objects– Bipolar outflow compresses & heats interstellar gas

• May last only ~ 10,000 to 100,000 years

Herbig-Haro Objects: Bipolar Outflow

Clusters Reveal Formation & Evolution• Star clusters never have stars of uniform mass

– High-mass stars evolve very quickly• O & B spectral class stars emit abundant UV radiation

– Low-mass stars evolve very slowly• K & M spectral class stars emit abundant IR radiation

• The destiny of excess gas & dust– H II regions

• H I regions are neutral (non-ionized) hydrogen• H II regions are singly-ionized hydrogen

– Hydrogen has only 1 electron Result is free protons & electrons⇒• Produce red emission nebulae

– Dust regions• Resist dissipation by strong UV radiation from O & B stars• Produce blue reflection nebulae

A Star Cluster With An H II Region

H-R Diagram of a Young Star Cluster

The Pleiades & Its H-R Diagram

Protostars In Giant Molecular Clouds• Characteristics of molecular clouds

– 195 different molecules identified in space• ~ 10,000 H2 molecules for every CO molecule

– The Milky Way contains ~ 5,000 molecular clouds• These include several star-forming regions• 17 molecular clouds outline the local arm of our galaxy

– Orion nebula’s parent cloud contains ~ 500,000 M☉

• Spectral emission lines– Cold dark interstellar hydrogen clouds

• Emission in the UV, visible & IR regions of the spectrum

– Molecular interstellar gas clouds• Emission in the microwave region of the

spectrum

Carbon Monoxide Molecular Clouds

Molecular Clouds in the Milky Way

O & B Stars Trigger Star Formation

Supernovae Can Trigger Star Birth• Supernova remnants are common

– High-mass stars exhaust their H2 supply very quickly• Many old star clusters have supernova remnants

• Supernova remnants are violent– High-mass stars die in tremendous explosions

• Spherical shock wave goes outward at supersonic speeds• This compresses interstellar gas & dust clouds

– Often results in associations rather than clusters• New stars are moving too fast to stay gravitationally bound• New stars quickly disperse in various directions• Probably the situation when our Sun formed

Supernova Remnant in the Cygnus Loop

• Interstellar gas & dust– Emission, reflection & dark nebulae– Potential birthplace of stars

• Stages of star formation– Initiation

• Coldest & densest regions are ideal• Contest between gravity & pressure• Compression mechanism required

– Progress• Positive feedback: Gravity > Pressure• Collapse accelerates until fusion

– Culmination• Heat from fusion increases pressure• Equilibrium is established

• Protostar evolution depends on mass– Very-low- mass

< 0.8 times MSun

– Low- mass< 4 times MSun

– High- mass> 4 times MSun

• Mass gain & loss in protostars– Circumstellar accretion disk inflows– Bipolar outflows

• T Tauri [variable] stars• Herbig-Haro objects

• Star clusters give evolution details– Few clusters have same-age stars– Luminosity & color on H-R diagram

• Stellar models fit observations well

• Star formation in molecular clouds– ~ 5,000 in the Milky Way galaxy– 17 define our galactic spiral arm

• Compression mechanisms– UV emissions from OB associations– Supernova explosions

Important Concepts