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Home , Fox Fur Nebula, Interstellar cloud

The and Formation Interstellar Nebulae Absorption Emission Neutral Molecular H II Regions Reflection Star Formation Star Forming Regions Stages of Formation Examples The Fox Fur The Interstellar Medium (ISM) The space between the is not completely empty, but filled with a very dilute 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 from stars behind them Interstellar Matter The interstellar medium consists of gas and dust.

. Atoms, mostly and helium, and small 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

. Infrared radiation is hardly scattered at all Barnard 68 . Foreground interstellar clouds make the background stars appear Visible Infrared redder Interstellar Matter

Reddening can interfere with blackbody 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 glows by excitation from hot stars. . Some nebulae are not clouds. They M20: The will come up later. Dark or Absorption Nebulae

Dense clouds of gas and dust absorb the light from the stars behind;

They appear dark in front Barnard 86 of a bright background 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 . 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 of 21 centimeters, which is in the radio portion of the . . Actual 21-cm spectra are complex because the lines are Doppler-shifted and broadened. . The Doppler shift is caused by the 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 emit radio- 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 (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 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 emission lines. The Fox Fur Nebula TheNGC Trifid 2246 Nebula Reflection Nebulae These images illustrate a 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 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 -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 (“ 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 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

Only one of the Infraredtrapezium image: stars ~ 50 is veryX-ray young, hotimage: enough cool, ~ 1000 low to - veryionize young,mass hydrogen stars hot stars in The Orion Nebula the Orion nebula