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Download 14 The Interstellar Medium and Star Formation
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The Interstellar Medium and Star Formation Interstellar Matter Nebulae Absorption Emission Neutral Molecular H II Regions Reflection Star Formation Star Forming Regions Stages of Formation Examples The Fox Fur Nebula The Interstellar Medium (ISM) The space between the stars is not completely empty, but filled with a very dilute gas and dust. It produces some of the most beautiful objects in the sky. We are interested in the interstellar medium because a) dense interstellar clouds are the birth place of stars b) clouds alter and absorb the light from stars behind them Interstellar Matter The interstellar medium consists of gas and dust. Atoms, mostly hydrogen and helium, and small molecules make up the gas. The dust is more like clumps of soot or smoke (and ice?). Dust absorbs light and reddens light that gets through by scattering. This image shows distinct reddening of stars near the edge of the dust cloud Interstellar Reddening Blue light is strongly scattered and absorbed by interstellar clouds Red light can more easily penetrate the cloud, but is still absorbed to some extent Barnard 68 Visible Infrared radiation is hardly scattered at all Foreground interstellar clouds make the background stars appear Infrared redder Interstellar Matter Reddening can interfere with blackbody temperature measurement, but spectral lines do not shift Interstellar Matter Interstellar dust grains are complex in shape (left); on the right is the result of computer modeling of how a dust grain might grow. Interstellar Matter Dust grains are known to be elongated, rather than spherical, because they polarize light passing through them. They also may be slightly conductive because they polarize and rotate radio waves. Nebulae “Nebula,” Latin for cloud, is a general term used for fuzzy objects in the sky. A dark or absorption nebula is a dust cloud. An emission nebula glows by excitation from hot stars. Some nebulae are not clouds. They will come up later. M20: The Trifid Nebula Dark or Absorption Nebulae Dense clouds of gas and dust absorb the light from the stars behind; They appear dark in front of a bright background Barnard 86 Horsehead Nebula Dark or Absorption Nebulae Light from distant stars may pass through more than one nebula; it is often possible to sort out the spectra of the star and the nebulae. Structure of the ISM The ISM contains two main types of emission nebulae: HI clouds: Cold (T ~100 K) clouds of neutral hydrogen (HI) Moderate number density (n ~10 – a few hundred atoms/cm3) Size: ~100 pc—they can be detected at radio frequencies Hot intercloud medium (HII regions): Hot (T ~ a few 1000 K) Ionized hydrogen (HII) Low density (n ~0.1 atom/cm3) Gas remains ionized because of the very low density. Detecting HI clouds 21-Centimeter Radiation Interstellar gas emits low-energy radiation by means of the spin-flip transition in the hydrogen atom. 21-Centimeter Radiation The emitted photon has a wavelength of 21 centimeters, which is in the radio portion of the electromagnetic spectrum. Actual 21-cm spectra are complex because the lines are Doppler-shifted and broadened. The Doppler shift is caused by the radial velocity of the cloud. It is used to measure the radial velocity. Molecular Clouds The densest gas clouds are also very cold, around 20 K. These clouds tend to contain more molecules than atoms. Transitions between rotation states of a molecule emit radio-frequency photons unique to the molecule. Molecular Clouds Fortunately, radio waves are not strongly absorbed, so molecular gas clouds can be detected even though there may be other gas and dust clouds in the way. These clouds consist mostly of molecular hydrogen, which unfortunately does not emit in the radio portion of the spectrum. Other molecules present are CO, HCN, NH3, H2O, CH3OH, H2CO, and more than a hundred others. Molecular Clouds Here are some formaldehyde (H2CO) emission spectra from different parts of the Trifid Nebula (M20). Emission Nebulae (HII Regions) HII regions are created by UV radiation from hot stars ionizing neutral hydrogen. This induces a shock wave front which emanates from a star and penetrates the neutral hydrogen cloud until the energy of the radiation drops below the ionization energy of hydrogen Emission Nebulae (HII Regions) A hot star illuminates a gas cloud It excites and/or ionizes the gas (electrons kicked into higher energy states) Electrons recombine, falling back to the ground state to produce Hα The Fox Fur Nebula emission lines. NGC 2246 The Trifid Nebula Reflection Nebulae These images illustrate a reflection nebula and how it forms. Light from a reflection nebula is usually blue coming from scattered light. Emission and Reflection Nebulae Star-Forming Regions Star formation is ongoing; star-forming regions are seen in our galaxy as well as others. Star-Forming Regions Star formation happens when part of a dust cloud begins to contract under its own gravitational force As it collapses, the center becomes hotter and hotter until nuclear fusion begins in the core. Interstellar clouds are usually stable and some form of shock is thought to be necessary to begin collapse. Star-Forming Regions Rotation can interfere with gravitational collapse, as can magnetism; clouds may very well contract in a distorted way. Shock Waves and Star Formation Shock waves from a nearby star formation can be the trigger needed to start the collapse process in an interstellar cloud. Shock Waves and Star Formation Possible triggers that cause shock waves: Death of a nearby Sun-like star Supernova Density waves in galactic spiral arms Galaxy collisions Shocks Triggering Star Formation Henize 206 (infrared) The Formation of Stars Like the Sun As a star forms from an interstellar cloud, it goes through several evolutionary stages The Formation of Stars Like the Sun The first stage of stellar evolution Stage 1: The interstellar cloud starts to contract. As it contracts, the cloud fragments into smaller pieces. The Formation of Stars Like the Sun Stage 2: Individual cloud fragments begin to collapse. Once the density and temperature is high enough, there is no further fragmentation. Stage 3: The interior of the fragment has begun to heat from the loss of gravitational energy and the center is about 10,000 K. The Formation of Stars Like the Sun Stage 4: The core of the cloud is now a protostar, and makes its first appearance on the H–R diagram. The Formation of Stars Like the Sun Planetary formation has begun, but the protostar is still not in equilibrium – all heating comes from gravitational collapse. The Formation of Stars Like the Sun Stages 5, 6 and 7 can be followed on the H–R diagram: The protostar’s luminosity decreases even as its temperature rises because it is becoming more compact. At stage 6, the core reaches 106 K, and nuclear fusion begins. The protostar has become a star, but it is not in equilibrium. The star continues to contract and increase in temperature until it is in equilibrium. This is stage 7: The star has reached the main sequence and will remain there as long as it has hydrogen to fuse. The Contraction of a Protostar When the star first reaches the main sequence, it will be on the lower edge of the main sequence band. This is called zero age main sequence (ZAMS) From Protostars to Stars Star emerges from the enshrouding dust cocoon Ignition of fusion processes 4 1H 4He Evidence of Star Formation Nebula around S Monocerotis: Contains many massive, very young stars, including T Tauri Stars: strongly variable; bright in the infrared. Protostellar Disks and Jets Herbig-Haro Objects Disks of matter accreted onto the protostar (“accretion disks”) often lead to the formation of jets (directed outflows; bipolar outflows): Herbig-Haro objects Globules Bok globules: ~10–1000 solar masses; Contracting to form protostars Globules Evaporating gaseous globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars The Orion Nebula: An Active Star-Forming Region Location of the Great Nebula in Orion (M 42) The Trapezium The 4 trapezium stars: Brightest, very young (less than 2 million years old) stars in the central region of the Orion nebula The Orion Nebula Only one of the Infrared trapezium image:stars ~ 50is very young, hot enough cool, lowto X-ray image: ~ 1000 masshydrogen stars veryionize young, hot starsin the Orion nebula