The Cosmic Perspective Chapter 16 Lecture - Star Birth PDF

The Cosmic Perspective Ninth Edition Chapter 16 Lecture Star Birth 16.1 Stellar Nurseries Our goals for learning: – Where do stars form? – Why do stars form? Where Do Stars Form? Star-Forming Clouds Stars form in dark clouds of gas and dust in interstellar s...

The Cosmic Perspective Ninth Edition Chapter 16 Lecture Star Birth 16.1 Stellar Nurseries Our goals for learning: – Where do stars form? – Why do stars form? Where Do Stars Form? Star-Forming Clouds Stars form in dark clouds of gas and dust in interstellar space. This is the raw material for stars to form. The gas and dust between the stars is called the interstellar medium. Composition of Clouds We can determine the composition of interstellar gas from its absorption lines in the spectra of stars. 70% H, 28% He, 2% heavier elements in our region of Milky Way Composition is similar for different clouds but fluctuate drastically in environment (i.e., temperature and density) Molecular Clouds (1 of 2) a A visible-light image of the nebula. The b A radio-wave image of the nebula, showing dark (horsehead-shaped) region is a emission from carbon monoxide (CO) molecules. molecular cloud. Most of the matter in star-forming clouds is in the form of molecules (H2 ,CO, etc.). These molecular clouds have a temperature of 10–30 K and a density of about 300 molecules per cubic centimeter (i.e., cold and dense). Molecular Clouds (2 of 2) a A visible-light image of the nebula. The b A radio-wave image of the nebula, showing dark (horsehead-shaped) region is a emission from carbon monoxide (CO) molecules. molecular cloud. Most of what we know about molecular clouds comes from observing the emission lines of carbon monoxide (CO) even though molecular hydrogen (H2) is the most abundant. Interstellar Dust Tiny solid particles of interstellar dust block our view of stars in these gas clouds. Particles are < 1 micrometer (micron) or 10-6 meters in size and made of elements like C, O, Si, and Fe. These small particles are more like smoke than sand grains. Interstellar Dust Allamandola et al. 1985 As with the gas molecules, the dust particles have the chemical makeup of molecules found in everyday things on Earth. Auto-soot and emission from Orion Bar gas cloud. Hubble Space Telescope Orion Treasury Project Team Orion Auto Soot and Star Formation … Red Alert?! Interstellar Reddening (1 of 2) Stars viewed through the edges of the cloud look redder because dust blocks (shorter- wavelength) blue light more effectively than (longer-wavelength) red light. phenomenon known as a A visible-light image of the dark molecular interstellar reddening cloud Barnard 68. Interstellar Reddening (2 of 2) Longer wavelength infrared light passes through a cloud more easily than visible light. Observations of infrared light reveal stars on the other side of the cloud.  Thought Question 1 (1 of 2) Why does the moon look redder as it gets closer to the horizon? A. The gas and dust particles in the atmosphere emit red light better. B. The gas and dust particles in the atmosphere block and scatter shorter wavelength light better. C. The curvature of the Earth causes the moon to be redder. D. The angle of the sunlight reflecting off the moon causes the color to change.  Thought Question 1 (2 of 2) Why does the moon look redder as it gets closer to the horizon? A. The gas and dust particles in the atmosphere emit red light better. B. The gas and dust particles in the atmosphere block and scatter shorter wavelength light better. C. The curvature of the Earth causes the moon to be redder. D. The angle of the sunlight reflecting off the moon causes the color to change. Observing Newborn Stars (1 of 2) Visible light from a newborn star is often trapped within the dark, dusty gas clouds where the star formed. JWST (Stellar Spectrum diagram) Observing Newborn Stars (2 of 2) Observing the infrared light from a cloud can reveal the newborn star embedded inside it. JWST (Stellar Spectrum diagram) Pillars of Creation Glowing Dust Grains (1 of 2) Dust grains that absorb visible light heat up and emit infrared light of even- longer wavelength.  Glowing Dust Grains (2 of 2) Long-wavelength infrared light is brightest from regions where many stars are ET AL. currently forming. Vol. 692 Rieke et al. 2009 of galaxies but similar to gas clouds withing a galaxy Figure 6. Family of average templates for full radio, far-infrared, and mid- infrared spectral range. The templates are keyed to the log(L(TIR)) of the Why Do Stars Form? Gravity Versus Pressure Gravity can create stars only if it can overcome the force of thermal pressure in a cloud. – Remember the ideal gas law P = nkT Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons. Mass of a Star-Forming Cloud A typical molecular cloud (T ~ 30 K, n ~ 300 particles / cm3 ) must contain at least a few hundred solar masses for gravity to overcome pressure. Emission lines from molecules in a cloud can prevent a pressure buildup by converting thermal energy into infrared and radio photons that escape the cloud. – Remember the CO image of the Horsehead nebula, which shows the radio photons escaping; and the heated dust cloud emitting long-wavelength IR light. Resistance to Gravity Molecular clouds tend to have thousands of solar masses but temperatures and densities that gravity could collapse for only a few hundred solar masses. A cloud must have even more mass to begin contracting if there are additional forces opposing gravity. Both magnetic fields and turbulent gas motions increase resistance to gravity. Fragmentation of a Cloud (1 of 4) Gravity within a contracting gas cloud becomes stronger as the gas becomes denser. Gravity can therefore overcome pressure in smaller pieces of the cloud, causing it to break apart into multiple fragments, each of which may go on to form a star. Also recall the Inverse Square Law for gravity 1/r2. – It keeps showing up! Fragmentation of a Cloud (2 of 4) A turbulent cloud containing 50 MSun of gas a The simulation begins with a turbulent gas cloud 1.2 light-years across and containing 50MSun of gas. Fragmentation of a Cloud (3 of 4) The random motions of different sections of the cloud cause it to become lumpy. Also note how some patches are brighter (hotter) than others. b Random motions in the cloud cause it to become lumpy, with some regions denser than others. If gravity can overcome thermal pressure in these dense regions, they can collapse to form even denser lumps of matter. Fragmentation of a Cloud (4 of 4) Each lump of the cloud in which gravity can overcome pressure can go on to become a star. A large cloud can make a whole cluster of stars. c The large cloud therefore fragments into many smaller lumps of matter Its easier to collapse a corresponding to the bright yellow regions in this image. Each lump can go on to form smaller cloud than one or more new stars. larger one as the clumps break off. Fragmentation of a Cloud https://www.youtube.com/watch?v=YbdwTwB8jtc Isolated Star Formation Gravity can overcome pressure in a relatively small cloud if the cloud is unusually dense and cold. Such a cloud may make only a single star. Thousands of particles per cubic centimeter and ~10K. We do observe these clouds but the mechanism in not completely clear. Thought Question 2 (1 of 2) What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A. It would continue contracting, but its temperature would not change. B. Its mass would increase. C. Its internal pressure would increase. Thought Question 2 (2 of 2) What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A. It would continue contracting, but its temperature would not change. B. Its mass would increase. C. Its internal pressure would increase. The First Stars Elements like carbon and oxygen had not yet been made when the first stars formed, only H and He existed. Without CO molecules to provide cooling, the clouds that formed the first stars had to be considerably warmer than today's molecular clouds. – Remember the emission lines we see from molecular clouds to explain the cooling. The first stars must therefore have been more massive than most of today's stars for gravity to overcome pressure. Simulation of the First Star Simulations of early star formation suggest the first molecular clouds never cooled below 100 K, making stars of ~200MSun. Some studies have suggested masses up to 500MSun. Newer simulations suggest lower masses due to stellar feedback (