Star FormationStar Formation

Or:Or:

What hydrogen can do if What hydrogen can do if given enough timegiven enough time

Common AncestryCommon Ancestry

All stars start out the same wayAll stars start out the same way No star forms in isolationNo star forms in isolation Stars vary only in initial mass and, to Stars vary only in initial mass and, to

some extent, initial chemical makeupsome extent, initial chemical makeup– The *IMF, or The *IMF, or Initial Mass FunctionInitial Mass Function, is how , is how

many of each kind of star is produced in a many of each kind of star is produced in a clustercluster

These These initial conditionsinitial conditions determine the life determine the life of the star once it achieves the of the star once it achieves the Main Main SequenceSequence

Ingredients: The Ingredients: The Interstellar MediumInterstellar MediumHIHI 1 atom/cm1 atom/cm33 T ~ 100KT ~ 100K

HH22 1 atom/cm1 atom/cm33 T ~ 10-30KT ~ 10-30K

HIIHII 0.1 atom/cm0.1 atom/cm33 T ~ 10T ~ 1044KK

Stellar RemnantsStellar Remnants Varying densityVarying density T ~ 10T ~ 1077KK

CompositionComposition::

¾ H, ¼ He¾ H, ¼ He 2% everything 2% everything else, ½ in dustelse, ½ in dust

Giant Interstellar Giant Interstellar Molecular CloudsMolecular Clouds Precursor to the star-forming *HII regionsPrecursor to the star-forming *HII regions 101055 M Msunsun, 10pc in radius, 10pc in radius Found mostly in the spiral arms of the Found mostly in the spiral arms of the

galaxygalaxy 10-20K10-20K By far HBy far H22 Also COAlso CO22, H, H22O, ChO, Ch33OH (methanol), CHOH (methanol), CH44

(methane), NH(methane), NH33 (ammonia) (ammonia) Where did these C, O, and N come from? Where did these C, O, and N come from?

Hmmmm…Hmmmm…

*singly ionized hydrogen

Varieties of NebulaeVarieties of Nebulae

Emission Nebula: HII, star forming Emission Nebula: HII, star forming regionsregions– If we lived in one the night sky would be If we lived in one the night sky would be

as bright as dayas bright as day Reflection Nebula: Hot stars have Reflection Nebula: Hot stars have

blown away gas, leaving dust to reflectblown away gas, leaving dust to reflect Dark Nebula: no stars (yet)Dark Nebula: no stars (yet)

– If we lived in one it would block out the If we lived in one it would block out the starsstars

Planetary Nebula: (misnomer, coined Planetary Nebula: (misnomer, coined by Herschel)by Herschel)

Horsehead NebulaHorsehead Nebulaboth emission and darkboth emission and dark

Stars Coalescing out of a Stars Coalescing out of a NebulaNebula

Pleiades

Felix Shih’s work

Orion Nebula

HII regions are hidden by clouds opaque to visible light

Orion FlythroughOrion Flythrough

Young Star-Forming Young Star-Forming Region: Serpens CaudaRegion: Serpens Cauda

Tarantula Nebula: if it was as close as the Orion Nebula it’d be as big as 60 full moons

Bok GlobuleBok Globule

A cool spot to grow a hot star

10-50 solarmasses, alight yearacross

Dustobscuresthe HIIregion

M16, the Eagle NebulaM16, the Eagle Nebula

Stellar ‘eggs’:Evaporating Gaseous Globules,~100AU

IR: new stars

YouTube

Pillar ~ 4LY

3 color overlay, one for O(g), H(r), S(y)

What do you need to What do you need to cook up a star?cook up a star? Low temperatureLow temperature

– Too hot, and the forming elements Too hot, and the forming elements have too much kinetic energy, i.e. have too much kinetic energy, i.e. speedspeed

Sufficient density/massSufficient density/mass– Enough stuff, close enough for Enough stuff, close enough for

gravity to do its jobgravity to do its job Not too much rotationNot too much rotation A triggerA trigger

Jealous StarsJealous Stars

If the star-forming region is too hot, If the star-forming region is too hot, the elements (HII, dust, etc.) are the elements (HII, dust, etc.) are moving too fast for gravity to moving too fast for gravity to overcomeovercome

Sometimes one big star gets started Sometimes one big star gets started before its neighbors, and it heats the before its neighbors, and it heats the region up so no other stars can formregion up so no other stars can form– However, if the first stars to turn on aren’t However, if the first stars to turn on aren’t

tootoo hot, their solar winds can be a trigger hot, their solar winds can be a trigger

Jean’s MassJean’s Mass

Minimum mass for a star forming regionMinimum mass for a star forming region Another application of hydrostatic equilibriumAnother application of hydrostatic equilibrium

– In this case, with no fusion, the mass needed to In this case, with no fusion, the mass needed to collapse a cloud of gas into a starcollapse a cloud of gas into a star

– ccss is the speed of sound in the gas is the speed of sound in the gas

– G you’ve seen, G you’ve seen, is the density is the density Less if cooler, more if hotterLess if cooler, more if hotter Generally, MGenerally, MJJ > 0.085 Solar masses > 0.085 Solar masses

– Less than that and you get no fusionLess than that and you get no fusion– Called a Brown DwarfCalled a Brown Dwarf

2/12/3

2

G

cM s

J 2/12/3

2

G

cM s

J

RotationRotation

Much like a hurricaneMuch like a hurricane The section of the The section of the

cloud that’s closer to cloud that’s closer to the galactic core the galactic core moves fastermoves faster

Differential rotationDifferential rotation If the cloud is rotating If the cloud is rotating

too fast, gravity too fast, gravity provides insufficient provides insufficient centripetal force to centripetal force to compact the materialcompact the material

Different ways to measure Different ways to measure rotationrotation

Upper Mass LimitUpper Mass Limit

If the initial mass M > 150 Solar If the initial mass M > 150 Solar Masses, it must come from a large, Masses, it must come from a large, distended globuledistended globule

Conserving angular momentum, as it Conserving angular momentum, as it grows smaller it will rotate so fast it grows smaller it will rotate so fast it that it will tear itself apartthat it will tear itself apart– In the equation above, In the equation above, is the spin is the spin

speed and speed and rr is the radius. The subscript is the radius. The subscript ii means initial and means initial and ff means final means final

– Squaring intensifies the effectSquaring intensifies the effect So 150MSo 150Msunsun > M > 0.085M > M > 0.085Msunsun

But then But then again,again,in the LMC…in the LMC…

Update: the VLT Update: the VLT has found stars up has found stars up to 300 Mto 300 Msolsol

Apparently, stars Apparently, stars can’t form if > 150 can’t form if > 150 MMsolsol, but after , but after formation mergers formation mergers can occur making can occur making super mass starssuper mass stars

See:See:– Blue stragglersBlue stragglers– Type Ia supernovaType Ia supernova

The The TriggerTrigger

Some force is needed to compact Some force is needed to compact the protostellar materialthe protostellar material

This can be:This can be:– A galactic density waveA galactic density wave– A nearby supernova explosionA nearby supernova explosion– A collision between GMCsA collision between GMCs

Once triggered, the result is Once triggered, the result is inevitableinevitable

Filaments of star formation resulting from the shock wave from a supernova

The AnimationThe Animation

So now you know:So now you know:

As the globule shrinks it:As the globule shrinks it:– Spins faster (why?)Spins faster (why?)– Grows hotter (why?)Grows hotter (why?)

The animation did not show the The animation did not show the solar nebulasolar nebula– The left-over disk of material that The left-over disk of material that

eventually would form a system of eventually would form a system of planets, moons, asteroids, and cometsplanets, moons, asteroids, and comets That’s another course!That’s another course!

Every star Every star that that achieves achieves fusion fusion lands on lands on the the H-R H-R diagram. diagram. When it When it does we does we call it ZAMScall it ZAMS

Named for Named for Ehnar Ehnar HHertzsprung ertzsprung and Henry and Henry Norris Norris RRussell, ussell, working working independently, independently, although although various forms various forms of it were of it were floating around floating around at the at the beginning of beginning of the 20the 20thth century century

Spectral Type Spectral Type and surface and surface temperature temperature are hereare here

Luminosity in Luminosity in Solar units is Solar units is herehere

Absolute Absolute Magnitude on Magnitude on the rightthe right

B-V hereB-V here

Spectral Type we’ll discuss in a few slidesSpectral Type we’ll discuss in a few slides *Luminosity is how much energy the star *Luminosity is how much energy the star

emitsemits *Absolute Magnitude is how bright the *Absolute Magnitude is how bright the

star would be if it was 10 parsecs awaystar would be if it was 10 parsecs away *B-V is a color metric, the difference in *B-V is a color metric, the difference in

magnitude between the blue magnitude between the blue astronomical filter and the visible light astronomical filter and the visible light filter filter

*see The Brightness of Stars ppt.

The Main The Main Sequence Sequence mentioned mentioned before is the before is the wavy line going wavy line going from upper left from upper left to bottom rightto bottom right

The Sun is The Sun is here:here:

There are many, There are many, many more red many more red dwarfs than sun-like dwarfs than sun-like stars, and more sun-stars, and more sun-like stars than blue like stars than blue

giantsgiants

All the other All the other regions, regions, the Red Giantsthe Red Giantsand the White and the White Dwarfs,Dwarfs,are stars that are stars that have evolved have evolved off the MSoff the MS

Pop Quiz!Pop Quiz!

Where are the Where are the big, hot stars?big, hot stars?

Where are the Where are the big, cool stars?big, cool stars?

Where are the Where are the small, hot small, hot stars?stars?

Where are the Where are the small, cool small, cool stars?stars?

Spectral TypesSpectral Types

From the Light From the Light and Telescopes and Telescopes ppt. you know ppt. you know that atoms give that atoms give off energy off energy when their when their electrons fall electrons fall from one orbital from one orbital to anotherto another

FingerprintsFingerprints

Each element has a unique set of Each element has a unique set of orbitals, therefore a unique set of orbitals, therefore a unique set of energies it can emit or absorbenergies it can emit or absorb

These energies translate to light colors: These energies translate to light colors: the higher the energy, the shorter the the higher the energy, the shorter the wavelength and the bluer the lightwavelength and the bluer the light

The sum of these energies is the The sum of these energies is the element’s element’s spectrumspectrum and it is a and it is a fingerprint for the elementfingerprint for the element

For instance, lithiumFor instance, lithium

http://jersey.uoregon.edu/vlab/elements/Elements.html

A family of spectraA family of spectra

Origin of O-B-A-F-G-K-Origin of O-B-A-F-G-K-MM

The history of The history of spectra in spectra in Astronomy began in Astronomy began in the mid-late 19the mid-late 19thth centurycentury

Pickering and Pickering and Fleming, 2 Harvard Fleming, 2 Harvard Astronomers, Astronomers, classified stars in classified stars in the 1890s based on the 1890s based on the strength of H the strength of H lineslines– A for strongest H A for strongest H

lineslines– B for H plus HeB for H plus He– C for more He, etc, C for more He, etc,

through Qthrough Q

Pickering’s ‘Computers’Pickering’s ‘Computers’

19011901

Annie Cannon, Annie Cannon, Pickering’s Pickering’s student, looked student, looked at the spectra of at the spectra of ¼ million stars ¼ million stars (!) and (!) and rearranged them rearranged them according to according to temperature, temperature, eliminating eliminating ambiguity and ambiguity and adding adding subdivisionssubdivisions

Mnemonic for The Mnemonic for The Harvard classification Harvard classification system system OOh h BBe e AA FFine *ine *GGirl irl KKiss iss MMee Also new L (2000K) and T Also new L (2000K) and T

(<1300K)(<1300K)– Oh Be A Fine Girl Kiss My Left Toe?Oh Be A Fine Girl Kiss My Left Toe?– The first phrase is risqué enough (for The first phrase is risqué enough (for

the Victorian 19the Victorian 19thth century) century)

*or Gentleman, (goat, or gorilla). L had been originally discarded by Cannon

CategoriesCategories

Cannon’s Harvard classification Cannon’s Harvard classification system is based on surface system is based on surface temperature, not on spectral temperature, not on spectral contentcontent

O, A, B were erroneously called O, A, B were erroneously called ‘early types’ and the F, G, K, and ‘early types’ and the F, G, K, and M were ‘late types’M were ‘late types’

Sub categories B0, B1…B9, A0…Sub categories B0, B1…B9, A0…A9, etc.A9, etc.

The pictures are not The pictures are not quite so prettyquite so pretty This is closer to what This is closer to what

Astronomers useAstronomers use Each dip in the line is Each dip in the line is

an absorptionan absorption Short wavelengths Short wavelengths

on the left, long on on the left, long on the rightthe right– Short, high energy hotShort, high energy hot– Long, low energy, coolLong, low energy, cool– Hint: Hint: take our lab take our lab

class!class!

How things stack upHow things stack up

You can see Pickering and Fleming’s problem with classification:H lines tail off on hotter AND cooler stars

*Luminosity Classes*Luminosity Classes

II Supergiant Supergiant

IIII Bright Giant Bright Giant

IIIIII Giant Giant

IVIV Subgiant Subgiant

VV Dwarf (Main Dwarf (Main Sequence) Sequence)

VIVI Subdwarf Subdwarf

*not shown on this HR diagram

This makes the Sun a G2V star

NearbNearby y StarsStars

So now you So now you see how see how Spectral Spectral class and class and surface surface temperaturtemperature go e go togethertogether

All fusion All fusion burning burning stars make stars make it here.it here.

What What happens happens next is next is another another storystory