The Interstellar Medium About 99% of the material between the stars is in the form of a gas The remaining 1% exists as interstellar grains or interstellar dust If all the interstellar gas were spread evenly, there would be about 1 atom per cm 3 Dust grains are even scarcer Although the density is low, the total amount of interstellar matter is huge 5% of the matter in the Milky Way galaxy Papillon Nebula The Papillon nebula is located in the Large Magellanic Cloud which is the site of young massive stars The red in this true color picture is from the hydrogen and the yellow from high excitation ionized oxygen 1 3 Interstellar Gas Some of the most beautiful sights in the sky are created by interstellar gas heated by nearby stars Can be heated to 10,000 K and glows with the characteristic red of hydrogen gas (Balmer line) Interstellar hydrogen gas near very hot stars is ionized by the radiated UV H II region A single I means neutral, two II means ionized Light is emitted when protons and electrons recombine to form atomic hydrogen UV to visible light Fluorescence Neutral Hydrogen Clouds Ionized hydrogen makes pretty pictures but most interstellar hydrogen is not ionized We can study these clouds by absorption First done using binary stars to help isolate the absorption lines from the interstellar clouds Sodium and calcium (Z=11 and 20) have distinctive absorption lines and are easily seen using visible light Hydrogen, oxygen,nitrogen absorb in the UV have been seen with satellite based observations Interstellar gases are depleted in elements that can easily condense Aluminum, calcium, titanium, iron, silicon, magnesium These elements become dust rather than gas 2 4
Red glow comes from excitation of hydrogen Blue comes from scattering of light by interstellar dust Black regions are thick clouds of dust that absorb all the light Trifid Nebula Ultra-hot Interstellar Gas Regions of ultra-hot interstellar gas have been observed with temperatures up to 1 million K The heat source is supernovae Exploding stars A supernova occurs about every 25 years in our galaxy The shock wave spreads out and heats the gas between the cold hydrogen clouds Any given point is heated once every 2 million years 5 7 Radio Observation of Cold Clouds Most of the interstellar material is cold hydrogen Hydrogen atoms can make a transition from electron spin up to electron spin down radiating photons with a wavelength of 21 cm Radio waves! (1400 MHz) Observations at 21 cm show that the neutral hydrogen in our galaxy is confined to a flat layer less than 300 LY thick that extends throughout the flat disk of the Milky Way Hydrogen is located in cold clouds with diameters ranging from 3 to 30 LY Masses range from 1 to 1000 times the mass of the Sun About 20% of the interstellar hydrogen exists as warm clouds 6 Interstellar Molecules A number of molecules (not just atoms) have been observed in the interstellar medium Many complex molecules have been observed, including progenitors of the basic amino acids required to build life These complex molecules can only survive in space when they are shielded by dense, dark, giant clouds containing dust These giant clouds are interesting structures that provide the raw material for stellar birth 8
The Eagle Nebula The Eagle Nebula consists of clouds of molecular hydrogen and dust that have survived the UV radiation from nearby hot stars As the pillars are eroded by the UV light, small globules of denser gas buried within the pillars are uncovered EGGs Evaporating Gaseous Globules Embryonic stars Picture taken by HST, April 1, 1995 Interstellar Matter around the Sun A region of where the density of interstellar matter is low surrounds the Sun Local Bubble Extends to 300 LY We should have observed about 2000 interstellar clouds in the Local Bubble but we see very few The Sun itself seems to be inside a cloud Local fluff One sizable warm cloud is known 60 LY from us toward the center of the galaxy 9 11 Structure and Distribution of Interstellar Clouds Models for interstellar gas clouds required that the pressure of the clouds and the interstellar material must be the same Pressure depends on density and temperature These clouds are embedded in a thin gas with a temperature of 1 million K from exploding stars The outer layers can be heated to a few 1000 K If the could is large enough, the inner core can stay cool and dense Stars form from collapsing, dense clouds of gas and dust Dark Nebula Dark nebula absorb light and block the view of stars behind them We can only see them visually when they block out light from behind Dark nebula absorb in the visible and UV Dark nebula radiate in the infrared In the Milky Way there are dark nebula throughout the plane of the galaxy Visible in infrared Infrared cirrus 10 12
Dark Nebula at Different Wavelengths Interstellar Reddening Dust grains absorb and scatter light and make distance stars appear to be dimmer Interstellar extinction Some stars appear to be redder than they are because of interstellar dust Short wavelengths are absorbed and scattered more strongly Sunlight looks redder at sunset The sky looks blue Because long wavelengths penetrate better, infrared astronomy can study stars that are more than twice as far away 13 15 Reflection Nebula Some dense clouds are close to luminous stars and scatter enough light to become visible This example comes from stars in the Pleiades cluster The bluish hue comes about because the dust particles are small and scatter blue most efficiently This cloud is moving through the Pleiades system and small dust particles are being slowed down faster than large particles Streamers and wisps Portrait of Interstellar Reddening Red light passes through because Dust tends to scatter blue light leaving more red light to reach the observer 14 16
Interstellar Grains Interstellar gas is transparent An enormous amount of interstellar gas would be required to account for the absorption and scattering we observe Small solid or liquid particles are much more efficient at scattering light than gas molecules Interstellar grains are about the size of the wavelength of light 10 to 100 nm There are many types of interstellar grains Silicates, carbon Probably formed by material ejected from stars Cosmic Rays High speed particles coming to Earth from space are called cosmic rays Cosmic rays are high speed atomic nuclei, electrons, and positrons Most are protons The abundances of the elements in cosmic rays are similar to those on earth except there is much more lithium, beryllium, and boron (Z=3,4,5) These elements are produced by fragmenting carbon, nitrogen, and oxygen nuclei (Z=6,7,8) Cosmic rays that reach the surface of the Earth are muons 17 19 Portrait of an Interstellar Dust Grain Note that interstellar grains cannot be studies with emission lines (they are solids) Origin of Cosmic Rays Cosmic rays are charged particles and their motion is affected by magnetic fields Difficult to pinpoint the origin of cosmic rays The galactic magnetic field is strong enough to keep cosmic rays from leaving the galaxy From the abundance of Li,Be,B we can estimate how far the cosmic rays have traveled 30 times around the galaxy, 10 million years The best candidates for the source of cosmic rays are supernova explosions 18 20
Molecular Clouds Giant molecular clouds contain enough gas and dust to make 100 to 1,000,000 Suns These clouds are 50 to 200 LY in diameter The cores of these clouds are cool (10 K) and dense (10 4 to 10 5 atoms/cm 3 ) Most of the gas exists as molecules Perfect conditions for gravity to compress the material and produce densities and temperatures high enough to Giant columns of cool, dense gas in the Eagle Nebula ignite a star 21 Star Birth in the Orion Nebula A progression of star formation has been moving through the molecular cloud On one end of the cloud, there are old stars (near the western shoulder of the hunter) about 12 million years old The stars in Orion s belt are 8 million years old The stars in the Trapezium cluster are 0.3 to 1 million years old 23 The Orion Molecular Cloud The closest and best studied stellar nursery is in the constellation of Orion about 1500 LY away The Orion nebula can be seen with binoculars along the sword of Orion In infrared light, the full extent of the nebula can be seen Star Formation First step is the formation of cold cores in the cloud (a) A protostar forms with a surrounding disk of material (b) Stellar wind breaks out along the poles of the star (c) The solar wind sweeps away the cloud material and halts the accumulation of more material and a newly formed star is visible surrounded by a disk (d) 22 24
Winds and Jets Jets thought to form along the poles of the protostar These jets of material collide with existing material and cause ionization Herbig-Haro objects (HH) On the right is a very young star (HH30, 100,000 years old) obscured by a dust clouds The jets along the poles of the star are clearly visible HH30 photographed by HST The disk of the flattened cloud around the protostar is seen edge-on 25 Formation of Planets around Stars Planets outside our solar system are difficult to detect Planetary searches are done indirectly One method is to study protoplanetary disks About 50% of known protostars are surrounded by by disks Picture taken by HST of a developing star called AB Aurigae Clumps of dust and gas are visible that may be leading to planet formation 27 The H-R Diagram and Stellar Evolution A star forms at a particular size and luminosity which places is on the H-R diagram As the star ages, it moves on the H-R diagram When a protostar forms, it contracts and heats up until it reaches the main sequence Protostar forming in Orion nebula Evidence for Planets Separated zones can form in protoplanetary disks if there is some large body like a planet that would stop the inevitable fall of the material into the star A visible dust ring around a star is evidence for an unseen planet In the HST picture on the right, a very young star HR 4796A is surrounded by a dust ring 26 28
Search for Planetary Orbital Motion One method would be to see the wobble of the star as the planet orbited around it No success so far Another method would be to study the Doppler shift of the light of the star as it wobbled from the effects of the planet This method has been successful More than 50 extrasolar planets are known 29 Characteristics of Extra Solar Planets We are only able to detect very large, Jupiter size planets Many of these planets are very close to their stars Hot Jupiters Planetary systems have been found but no Earth like planets have been found The future is infrared interferometry Artist s conception of giant planet close to a Sun-like star 30