The Interstellar Medium

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 cm3 • 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 Lecture 19

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 Lecture 19

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 Lecture 19

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 Lecture 19

Trifid Nebula • 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 Lecture 19

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 Lecture 19

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 Lecture 19

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 Lecture 19

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 Lecture 19

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 Lecture 19

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 Lecture 19

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 Lecture 19

Dark Nebula at Different Wavelengths Lecture 19

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 Lecture 19

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 Lecture 19

Portrait of Interstellar Reddening • Red light passes through because • Dust tends to scatter blue light leaving more red light to reach the observer Lecture 19

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 Lecture 19

Portrait of an Interstellar Dust Grain • Note that interstellar grains cannot be studies with emission lines (they are solids) Lecture 19

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 Lecture 19

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 Lecture 19

Molecular Clouds Giant columns of cool, dense gas in the Eagle Nebula • 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 (104 to 105 atoms/cm3) • Most of the gas exists as molecules • Perfect conditions for gravity to compress the material and produce densities and temperatures high enough to ignite a star Lecture 19

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 Lecture 19

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 Lecture 19

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) Lecture 19

Winds and Jets HH30 photographed by HST The disk of the flattened cloud around the protostar is seen edge-on • 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 Lecture 19

The H-R Diagram and Stellar Evolution Protostar forming in Orion nebula • 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 Lecture 19

Formation of Planets around Stars 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 • 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 Lecture 19

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 Lecture 19

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 extra-solar planets are known Lecture 19

Characteristics of Extra Solar Planets Artist’s conception of giant planet close to a Sun-like star • 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 Lecture 19

The Interstellar Medium