STAR BIRTH

1. STAR BIRTH Road to the Main Sequence © Sierra College Astronomy Department

2. Star Birth – Road to the Main SequenceA Brief Woodland Visit • An Alien Visit • If you were alien from a treeless world and were sent to Earth for one day to gather data from a forest, what do you think your chances are of developing the correct theory for the growth history of a tree? • How would your chances change if you were given a basic knowledge base of Earth biology? • Astronomers are in a Similar Position • The life cycle of stars takes a minimum of millions of years • Astronomers tackle the problem by observing tremendous numbers of stars in various stages of development. • Combining this observational data with laboratory measurements and theoretical models, the life cycle of a star is pieced together. © Sierra College Astronomy Department

3. Star Birth – Road to the Main SequenceStellar Nurseries – Where do Stars Form? • A First Look • The youngest star clusters, determined by their main sequence turnoff points, are always associated with dark clouds of gas and dust • The interstellar medium must be the birthplace of stars • The interstellar clouds are of a special type: cold and dense - an indicator of the competing forces of gravity and pressure • A simple count shows that 2-3 new stars form each year in our part of the Milky Way © Sierra College Astronomy Department

4. Star Birth – Road to the Main SequenceStellar Nurseries – Where do Stars Form? • The Interstellar Medium (ISM) • The ISM is the gas and dust between the stars • 70% H, 28% He, 2% heavier elements (referred to as heavy elements or metals by astronomers) • These percentages are by mass and for our region of the Milky Way • Gas at beginning of Universe just H and He • Over time, stars have created the heavier elements • Composition stays the same, but density and temperature may vary considerably from place to place © Sierra College Astronomy Department

5. Star Birth – Road to the Main SequenceStellar Nurseries – Where do Stars Form? • Star-Forming Clouds • Stars are born in the coldest and highest density interstellar clouds • 10-30 K, 300 molecules/cm3 (an average with high density regions hundreds of times denser) • Usually called molecular clouds since temperatures are low enough to form molecules • H2 is the most abundant molecule – difficult to detect due to lack of emission lines at low temperatures • More than 120 other molecules • CO is the most abundant and usually its emissions are used to study a molecular cloud’s physical characteristics • Others: Water (H2O), ammonia (NH3), ethyl alcohol (C2H5OH) © Sierra College Astronomy Department

6. Star Birth – Road to the Main SequenceStellar Nurseries – Where do Stars Form? • Interstellar Dust • About half the atoms of elements more massive than helium are found in tiny, solid grains of interstellar dust • This is about 1% of the mass of an interstellar cloud • The grains are usually a micron or smaller in size (smaller than a single cell of bacteria, more like smoke than sand) • Composition varies from mostly carbon to combinations of silicon, oxygen, and iron © Sierra College Astronomy Department

7. Star Birth – Road to the Main SequenceStellar Nurseries – Where do Stars Form? • Interstellar Dust (continued) • Interstellar dust scatters light more and more effectively as the wavelength of light decreases • This leads to interstellar reddening whereby objects with intervening interstellar dust appear redder than in the absence of the dust (the complete obscuring of star light by dust is sometimes referred to as interstellar extinction) • Interstellar reddening is easily distinguished from Doppler redshifting (the spectral lines are not significantly shifted and the stellar thermal spectrum lacks to correct proportion of light from the blue end) • The amount of reddening of a star of a known spectral type allows • The determination of the amount of dust between Earth and the star • With other such measurements, a determination of the dust distribution in our region of the Milky Way © Sierra College Astronomy Department

8. Star Birth – Road to the Main SequenceStellar Nurseries – Where do Stars Form? • Interstellar Dust (continued) • Observing in the infrared allows observations of protostars within and stars beyond the densest dust clouds • Dust grains absorb the visible light and even some infrared light of the protostars • The dust then emits the energy in the infrared and microwave bands • Clouds that are dark in the visible “glow” in the longest wavelength infrared light and this glow characterizes the temperature distribution of the cloud © Sierra College Astronomy Department

9. Star Birth – Road to the Main SequenceStellar Nurseries – Why do Stars Form? • Gravity vs Pressure • Gravity can create stars from a gas cloud only if it can overcome the gas pressure, which depends on temperature and density through the ideal gas law P = nkT where P is the thermal pressure (to distinguish it from degeneracy pressure), T is the temperature, n is the number density, and k is Boltzmann’s constant • Only when the density of a cloud is high enough and the temperature low enough will gravitational contraction start the stellar birthing process © Sierra College Astronomy Department

10. Star Birth – Road to the Main SequenceStellar Nurseries – Why do Stars Form? • Gravity vs Pressure (continued) • The critical mass of an interstellar cloud, above which a cloud can collapse to form a star is called the Jeans mass: Mbalance = 18Msun(T3/n)1/2 • For typical clouds with T = 30 K and n = 300 cm-3, we find that Mbalance = 171Msun • Most star-forming clouds easily exceed this value • For T = 30 K and n = 300,000 cm-3, Mbalance = 5.4Msun • Clouds are typically lumpy and thus each lump can collapse separately from the much larger cloud © Sierra College Astronomy Department

11. Star Birth – Road to the Main SequenceStellar Nurseries – Why do Stars Form? • Preventing a Pressure Buildup • As a gas contracts, typically the temperature will rise and eventually prevent further contraction • Molecular clouds avoid this fate through gas collisions and photon emission • The collisions transform thermal energy into the excited states of vibrational and rotational energy levels in the molecules • The molecules return to their ground states producing emission lines in the infrared and radio portions of the spectrum, the photons of which escape the cloud © Sierra College Astronomy Department

12. Star Birth – Road to the Main SequenceStellar Nurseries – Why do Stars Form? • Clustered Star Formation • Most stars are born in clusters of thousands of stars • This is a contradiction since the Jeans mass and the radiative emission from the gas molecules would suggest that cloud collapse would occur at a few hundred solar masses, well before the size indicated by observed star clusters • Jeans mass calculation does not include any other dynamical interactions other than pressure and gravity • Proposed mechanisms to prevent early contraction • Turbulent gas motion wherein gas clumps are moving with much faster random directions and require stronger gravity to slow to contraction speeds • Magnetic fields providing a friction for the ionized atoms which in turn inhibit the contraction of the neutral gas atoms and molecules © Sierra College Astronomy Department

13. Star Birth – Road to the Main SequenceStellar Nurseries – Why do Stars Form? • Fragmentation of the Molecular Cloud • A massive enough molecular cloud (hundreds if not thousands of solar masses) will overcome thermal pressure, turbulence, and magnetic friction • Collapse will evolve through fragmentation of denser molecular cloud cores, which in turn can form star systems of one, two or more stars © Sierra College Astronomy Department

14. Star Birth – Road to the Main SequenceStellar Nurseries – Why do Stars Form? • Isolated Star Formation • Small clouds of a few solar masses (isolated from the more typical and larger molecular clouds) can collapse if the temperature is closer to 10 K and the gas number density is about a few tens of thousands • Such small dense clouds are observed, but it is not clear how such low temperatures and high densities are achieved © Sierra College Astronomy Department

15. Star Birth – Road to the Main SequenceStellar Nurseries – Why do Stars Form? • The First Generation of Stars • The composition of the ISM after the Big Bang was almost solely H and He. • H2 is not a very efficient emitter for cooling a collapsing molecular cloud • Consequently, the collapsing clouds could only cool to about 100 K and only very large stars (at least 30 solar masses) could form • These massive stars have very short lifetimes and the proportion of heavy elements rose quickly in the young Universe © Sierra College Astronomy Department

16. Star Birth – Road to the Main SequenceStages of Star Birth – Slowing Contraction • First Stage of Star Formation • A cloud core (cloud fragment) becomes isolated from the rest of molecular cloud • Thermal energy, initially radiated away, eventually becomes trapped in the increasingly dense (in gas and dust) center • Potential energy of collapse now goes into heating the center and the increased temperature slows contraction © Sierra College Astronomy Department

17. Star Birth – Road to the Main SequenceStages of Star Birth – Slowing Contraction • First Stage of Star Formation (continued) • A protostar forms • A starlike clump of gas with a surface temperature and luminosity of a true star • Fusion has not yet commenced (the marking of a main-sequence star) • The protostar continues to grow until the gas supply ceases due to • The protostar’s increasing radiation emission • The protostar’s increasing stellar wind • The stellar winds of nearby stars © Sierra College Astronomy Department

18. Star Birth – Road to the Main SequenceStages of Star Birth – Effects of Rotation • Gas Contraction onto a Central Protostar • All molecular clouds have some rotation • From the conservation of angular momentum, the cloud cores will have rotation and this rotation increases with the contraction • A protostellar disk will form around a protostar, and the protostar grows by gas accretion from the disk (also called an accretion disk) • Accretion occurs due to gas friction which can transfer angular momentum away from the infalling gas enabling the protostar to grow more massive • The accretion disk may coalesce into a planetary system © Sierra College Astronomy Department

19. Star Birth – Road to the Main SequenceStages of Star Birth – Effects of Rotation • Gas Contraction onto a Central Protostar (continued) • A protostar’s magnetic field • Will interact with the particles in the protostellar disk and slow the rotation of the protostar • May generate a strong stellar wind – an outward flow of particles – which can carry additional angular momentum away from the protostar • This explains observation that older stars rotate slower than young stars © Sierra College Astronomy Department

20. Star Birth – Road to the Main SequenceStages of Star Birth – Effects of Rotation • Gas Contraction onto a Central Protostar (continued) • High-speed gas streams or jets are observed in young protostars • Two jets are seen aligned in opposite directions along the protostar’s spin axis • May be generated through the twisting of magnetic fields into “ropes” embedded in protostellar disk • A protostar’s wind and jets will clear away the cocoon of gas surrounding a forming star eventually revealing the protostar within © Sierra College Astronomy Department

21. Star Birth – Road to the Main SequenceStages of Star Birth – The Onset of Fusion • From Protostar to Main Sequence • Only half the thermal energy released by gravitational contraction is radiated away • This energy comes from the surface and induces further contraction • The other half of the energy goes into heating the protostar’s interior • When the core temperature hits 10 million K, fusion occurs via the proton-proton chain and a main-sequence star is born © Sierra College Astronomy Department

22. Star Birth – Road to the Main SequenceStages of Star Birth – The Onset of Fusion • Summary - Birth Stages on a Life Track • Stage 1 – Assembly of a Protostar • Collapsing cloud fragment concealed in shroud of gas and dust – creation of protostellar disc • Protostellar winds and jets disrupt shroud and reveal protostar • Photosphere temperature at 3000 K with surface much larger than Sun and a luminosity 10-100 times that of the Sun • Stage 2 – Convective Contraction • 3000 K surface maintained primarily by convective energy transport to surface • Luminosity decreases as radius decreases © Sierra College Astronomy Department

23. Star Birth – Road to the Main SequenceStages of Star Birth – The Onset of Fusion • Summary - Birth Stages on a Life Track (continued) • Stage 3 – Radiative Contraction • Surface temperature rises as energy transport switches to radiative diffusion within the protostar • Even with radius decreasing, luminosity increases slightly • Fusion in core commences • Stage 4 – Self-Sustaining Fusion • Core temperature continues to rise as does rate of fusion • Hydrostatic equilibrium is reached • Star settles into main-sequence life • The duration of a life track (evolutionary track) from protostar to main sequence varies with the eventual mass of the star (from less than one million years to well over 100 million years) © Sierra College Astronomy Department

24. Star Birth – Road to the Main SequenceMasses of Newborn Stars – The Smallest • The Least Massive Stars • A star must be massive enough to initiate fusion • Degeneracy pressure, which only depends on density and not temperature, prevents protostars less than 0.08 MSun (which is 80 times more massive than Jupiter), from reaching the fusion temperature • Brown dwarfs are “failed stars” with masses between that of a planet and 0.08 MSun. • The spectral classification of a brown dwarf is T and these objects are sometimes called T dwarfs with a surface temperature of less than 1400 K • L dwarfs include brown dwarfs and hydrogen-burning stars with surface temperatures of 1400-2200 K © Sierra College Astronomy Department

25. Star Birth – Road to the Main SequenceMasses of Newborn Stars - The Largest • The Most Massive Stars • Radiation pressure will eventually dominate gravity’s ability to accrete gas and dust onto a forming star, effectively blowing away any extra mass in their outer layers into space • Theoretical maximum is about 150 Msun the observed mass of the Pistol Star is about this massive) • Mass Distribution of Newborn Stars © Sierra College Astronomy Department