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Students-I ApJ 717, L66 : Dhanraj ApJ 691, 1042, YoungHyun Lee ApJ 739, 12, Shu Woo Young ApJ 712, 139, Park Keun Hong Mid-infrared observations of the transitional disks around DH Tau, DM Tau, and GM Aur (Grafe et al., 2011) Song Donguk IR Universe 1. Our Solar System All of the planets and moons in our solar system emit strongly in the infrared. This infrared emission is the heat from atmospheres and surfaces which peaks in the mid to far infrared (15 to 100 microns). Solar system objects also reflect infrared radiation from the Sun. This reflected radiation peaks in the near infrared at about 0.5 microns. For planets and moons which have atmospheres, infrared studies can show us how the abundance and composition of atmospheric gases as well as the temperature of the atmosphere vary with depth. In addition, infrared astronomy has led to the discovery of new comets and asteroids and bands of dust which lie in our solar system. Planets Planets Infrared Image of the Sun Venus Both images were taken by the Galileo satellite : the infrared image of the clouds of Venus on the left with the visible light image of Venus on the right. Sunlight passing through the atmosphere of Venus is absorbed by its surface which causes it to heat up, giving off radiation which is primarily infrared. The carbon dioxide in Venus's thick atmosphere traps much of the infrared heat, giving Venus a surface temperature of 750K (890 degrees Fahrenheit) which is hot enough to melt lead. By studying the infrared spectra of Venus, astronomers found that sulfuric acid droplets exist in its atmosphere. Mars Some observations of Mars may be made with PACS and SPIRE, but these instruments are so sensitive that Mars will cause the detectors to be overexposed. But the HIFI instrument is ideally suited to investigating the thin martian atmosphere, and it can detect the presence of molecules such as methane and water in the atmosphere. JUPITER an infrared image of Jupiter and one of its moons, Io ( has several active volcanoes and is heated by tidal forces caused by the gravitational pull of Jupiter and the other Galilean moons.) Jupiter radiates ~1.6 times as much heat, in the form of infrared energy, as it receives from the Sun. This indicates that Jupiter has an internal source of energy - probably heat created by Jupiter's collapse when it was formed. By studying the infrared emission from Jupiter we have learned much about its cloud structure. Jupiter's belts (its dark horizontal bands) are brighter in the infrared than its zones (its bright horizontal bands) This indicates that the belts are regions of hotter gas. The temperature of Jupiter also increases towards its center, so the zones are at higher levels in Jupiter's atmosphere than the belts. SATURN: The HST/NICMOS infrared image of Saturn shows the details of its cloud structure. Saturn also radiates about twice as much radiation in the infrared as it received from the Sun. ISO was recently used to study the concentrations of heavy and ordinary hydrogen in the atmosphere of Saturn. These measurements give information about the composition of the original cloud of gas and dust from which the sun and planets formed. TITAN a set of four near infrared pictures of Titan visible light image of Titan on the right taken by Voyager 2. In 1944, Gerard Kuiper discovered that Titan, the largest moon of Saturn, had an atmosphere. He detected the methane in Titan's atmosphere by studying its infrared emission. In 1994, astronomers using the Wide Field Planetary Camera on the Hubble Space Telescope made the first images of the surface of Titan. These images were made in the near-infrared since infrared radiation is able to penetrate the hazy atmosphere of Titan. The infrared images of Titan's surface show a bright area which is a surface feature that is about 2,500 miles across (about the size of Australia). Uranus and Neptune Emission from Uranus and Neptune at different wavelengths in the Herschel range comes from different levels in their atmospheres, so will be sensitive to properties such as temperature, pressure, and composition and how they vary with altitude. Tracing molecules at various altitudes provides information on how convection and winds move gases in the atmospheres – in other words, we can examine the weather deep inside the atmospheres of the giant planets. Gases like water, methane, carbon monoxide and ammonia are present and will show up through characteristic spectral features at particular wavelengths. Neptune The diagram shows a theoretical model of what the spectrum of Neptune may look like as observed by SPIRE. By comparing the actual observations with such models, we will learn a lot about the structure and composition of Neptune’s atmosphere. Uranus Uranus will be used as a standard “calibration” source by the SPIRE instrument – in other words, the brightness of other sources observed by SPIRE (galaxies, stars, comets, etc.) will be measured by comparing them to Uranus. The reason for the choice of Uranus as the standard calibration source is that we already know quite accurately how warm it is. It turns out to be the best choice, since Uranus was first discovered by William Herschel himself. ASTEROIDS: Asteroids are rocky-metallic objects which range in size from about 1 to 1000 km. They orbit the Sun and are thought to be leftover material from the formation of the planets in our solar system. Most of the asteroids are found in the Asteroid Belt which lies between the orbits of Mars and Jupiter. Astronomers have also identified a group of asteroids whose orbits cross Earth's orbit. The infrared radiation from an asteroid can be used to get information about its location, composition, rotation and its shape and size. The IRAS mission discovered over 400 new asteroids and Kuiper Belt objects, dwarf planets Although most of the well-known asteroids are to be found between the orbits of Mars and Jupiter, it has recently been discovered that there is a large population of asteroids much further out from the Sun, past the orbit of Neptune. These are so-called , Kuiper Belt objects, trans-Neptunian objects, about which we do not know very much, but we believe them to be remnants of the primitive planetesimal disk from which the solar system formed. Some of them are large enough to have been given the name “dwarf planets”. In fact, Pluto, was recently relegated to the status of a dwarf planet. The pictures below show the location of the trans-Neptunian belt and the sizes of some known dwarf planets in Kuiper Belt objects, dwarf planets Fig the location of the trans-Neptunian belt and the sizes of some known dwarf planets in comparison to that of the Earth. The trans-Neptunian asteroids and dwarf planets are very cold (about 40 degrees above absolute zero) – but that’s actually warm enough for them to emit light at Herschel wavelengths. The diagram to the left shows the expected thermal spectrum that Herschel could measure for dwarf planets of different sizes and distances from Earth. COMETS Comets are basically dusty snowballs which orbit the Sun. They consist of an icy nucleus surrounded by a large cloud of gas and dust (called the coma). The coma is created as the ice in the nucleus is warmed and vaporizes. Comets have 2 tails, a straight gas tail and a curved dust tail. The gas tail is created by the solar wind whose magnetic fields pull the gas away from the comet's coma. The dust in the coma is not affected by magnetic fields but is pushed out by the Sun's radiation. The dust in the tail reflects sunlight and radiates in the infrared. The infrared emission from comets can be used to get information on the nature of the dust they contain as well as on the rate at which material is being lost from the nucleus. comets IRAS found that dust from comets fills the Solar System and that comets are dustier than they were thought to be. Many of the meteors may be the larger pieces of this comet dust. IRAS discovered 6 new comets including comet IRAS-Araki-Alcock and collected infrared data on 25 previously known comets. Comets are balls of dust, rock and ice left over from the formation of the solar system. We can learn about the raw material from which the Sun and the planets formed. Herschel will look at comets, detecting the thermal radiation from the nucleus and the spectral features of the gases that evaporate from the nucleus when the comet gets close to the Sun. Water is especially important to reconstruct the early development of the solar system, and determine whether comets were the source of water and pre-life chemicals on primitive Earth. It is another pleasing connection with the Herschel family that Caroline Herschel, sister of William, became famous herself as the discoverer of eight. COMET IMPACT WITH JUPITER In 1994 several fragments of comet Shoemaker-Levy 9, which had broken apart during its previous orbit in 1992, collided with the planet Jupiter. These impacts released a tremendous amount of energy into Jupiter's atmosphere. Spectacular images of comet Shoemaker-Levy's collision with Jupiter were taken in the infrared. The image shows the impact of fragment A (Courtesy of NASA/NSSDC,) The image is an infrared color composite showing fragments A,E F,G and H impacting Jupiter (Courtesy of NASA/NSSDC). ZODIACAL DUST BANDS IRAS also discovered bands of infrared emission that girdle our solar system. Called the zodiacal dust bands, these are likely to be debris from colliding asteroids. Two bands appear 9 degrees above and 9 degrees below the ecliptic, which result from debris in an orbit about the sun that is inclined by 9 degrees to the ecliptic. The bands result from the particles spending more time at the extremes of their orbit, causing an apparent increase of density at plus and minus 9 degrees. Another one is found in the ecliptic plane. The infrared emission of these bands show a temperature of 165-200K and a distance of 2.2 - 3.5 AU from the Sun This places these dust bands between Mars and Jupiter in the region of the asteroid belt. 2. Star Formation Dense molecular clouds are always observed around starforming regions. New stars condense out of these clouds through gravitational collapse triggered by an instability (e.g. Jeans instability for self-gravitation) or shock wave (e.g. from an exploding supernovae). The formation of protostars and their evolution to the main-sequence of the Hertzsprung-Russell diagram are main topics of stellar astrophysics. But we will be focuss in here on the observable effects of star formation on the ISM: photoionisation of surrounding gas, absorption and re-radiation of starlight by dust. protostar Many of the most interesting infrared objects are associated with star formation. Stars form from collapsing clouds of gas and dust. As the cloud collapses, its density and temperature increase. The temperature and density are highest at the center of the cloud, where a new star will eventually form. The object that is formed at the center of the collapsing cloud and which will become a star is called a protostar Since a protostar is embedded in a cloud of gas and dust, it is difficult to detect in visible light. Any visible light that it does emit is absorbed by the material surrounding it. Only during the later stages, when a protostar is hot enough for its radiation to blow away most of the material surrounding it, can it be seen in visible light. Until then, a can be detected only in the infrared. star forming regions The light from the protostar is absorbed by the dust surrounding it, causing the dust to warm up and radiate in the infrared. Infrared studies of star forming regions will give us important information about how stars are born and thus on how our own Sun and Solar System were formed. Infrared telescopes have cataloged thousands of hot, dense cores within clouds of gas and dust which could be newly forming stars. Two infrared images of star forming regions: an IRAS view of the constellation Orion in which there are several regions active star formation, and an image from the new Spitzer Space Telescope of newborn stars in the nebula NGC 7129. T-Tauri stars and Herbig-Haro objects Protostars which are starting to blow away the gas and dust surrounding them are called T-Tauri stars. The warm dust remaining around T-Tauri stars still radiates in the infrared. There is evidence that the remaining dust and gas surrounding T-Tauri stars form rotating disks which may mark the beginnings of planetary systems. Herbig-Haro objects, which are also associated with newly forming stars, can be studied in the infrared. These are small nebulae which vary in shape and brightness over a period of only a few years. Both Herbig-Haro objects and T-Tauri stars are found in regions of active star formation. It is thought that these nebulae represent high speed gas flowing from young stars colliding with interstellar clouds. The study of T-Tauri stars will help us understand the details on how T-Tauri stars and KleinmannLow Nebula an infrared adaptive optics image of a T-Tauri stars (UY Aur). an infrared image of the Kleinmann-Low Nebula, a region of intense star formation in the constellation Orion. CISCO, Subaru 8.3-m Telescope, NAOJ newborn stars In visible light much of this region is hidden by dust however in the infrared you can see the effects of the hot winds produced by newly formed massive stars. These hot winds heat up the surrounding gas and cause them to radiate strongly in the infrared. The winds will eventually clear much of the gas and dust surrounding the stars. The infrared image shows stars and glowing interstellar dust heated by the intense starlight of the newborn stars. Here is a comparison of a visible light (left) and an infrared (right) view of OMC-1 (OMC stands for Orion Molecular Cloud) taken by the Hubble Space Telescope Globules Globules are also found in star forming regions. These are small clouds (~ 1 light year in diameter) which contain 10 to 1000 solar masses of gas and dust. In visible light, globules are seen in dark silhouette against bright nebulae. They produce no visible light of their own and are thought to be collapsing clouds which will produce stars. Infrared observations from IRAS showed that some globules contain protostars. an infrared image of the globule in the nebula IC 1396 taken by the new Spitzer Space Telescope. Spitzer's sensitive infrared detectors unveiled the brilliant hidden interior of this opaque cloud of gas and dust and revealed, for the first time, a glowing stellar nursery with never-before-seen young stars and stars still in the process of formation. The inset shows a visible light view of the same region. 3. Stars Infrared observations have led to the discovery of a large number of stars which are too cool to be detected by their visible light or are hidden behind obscuring dust. Infrared observations have also led to the discovery of several stars which have orbiting material. Above is an image of infrared point sources in the entire sky as seen by the Infrared Astronomical Satellite (IRAS). The plane of our Galaxy runs horizontally across the image. Blue sources are cool stars within our Galaxy, which show an obvious concentration to the galactic plane and center. Yellow-green sources are galaxies which are basically uniformly distributed across the sky, but show an enhancement along a great circle above the galactic plane. This enhancement is caused by galaxies in the Local Supercluster. Reddish sources, the infrared cirrus, are extremely cold material close to us in our own Galaxy. Black areas were not surveyed by IRAS. the brightest stars in our galaxy In 1997, the infrared camera on the Hubble Space Telescope (NICMOS) revealed one of the brightest stars in our galaxy .This star, which is 10 million times more radiant than our Sun, was discovered in the center of our galaxy where it was hidden from visible light telescopes by thick dust. In the image you can see two expanding shells of gas being ejected by the star in one of the largest stellar eruptions ever seen. Don F. Figer (UCLA) and NASA, AURA/STScI Wolf-Rayet 104, a "spiral" star A "spiral" star which was discovered in the infrared, Wolf-Rayet 104 is 3 times the size of our sun and 100,000 times brighter. Because it is so large and radiates so intensely, part of its outer atmosphere is being blown off. Wolf-Rayet 104 is a binary star system - its companion is a smaller OB star. The material being ejected from Wolf-Rayet 104 is swept into a spiral pattern by the stellar winds of both stars. UC Berkeley Space Sciences Laboratory/WM Keck Observatory New Star Cluster Stars are often form in groups called star clusters. The stars in a cluster will usually move slowly away from each other as they age. Many new star clusters are still partially hidden by the dust and gas leftover from star formation, making them more difficult to view in visible light. Infrared light, however, can penetrate this dust and provide us with a deeper view of the cluster. an infrared view of the Quintuplet Star Cluster which lies near the center of our Milky Way Galaxy. Since the center of our galaxy is a very dusty place, infrared observations are often the best way to view objects embedded in and hidden by this dust. This image taken with the NICMOS instrument on the Hubble Space Telescope is the clearest view yet of this cluster. Don Figer (STScI) et al., Brown Dwarfs brown dwarfs - objects whose mass is between twice that of Jupiter and the lower mass limit for nuclear reactions (0.08 times the mass of our sun). They can be discovered by infrared telescopes, even deep within thick clouds. An infrared image of the Trapezium star cluster in the Orion Nebula, was part of a survey done at the United Kingdom Infrared Telescope ( UKIRT ) in which over 100 brown dwarf candidates were identified in the infrared. an infrared image of the Trapezium star cluster in the Orion Nebula. 4. Extrasolar Planets In the 1980's, astronomers using IRAS data discovered about two dozen stars which had infrared-emitting dust surrounding them, extending hundreds of astronomical units from the stars. This discovery inspired astronomers to make more detailed observations of these stars. What they found around these stars were flat, disk-shaped, areas of dust in which planets have formed or could be forming. These findings have led the way to one of the most exciting new areas of research in astronomy - the search for planets around other stars. Among the stars studied were Beta Pictoris, HL Tauri, Vega, Epsilon Eridani and Alpha Piscis Austrinus. The discovery of these disks provided the first significant evidence that other solar systems might exist. Beta Pictoris HR 4796A An infrared image of Beta Pictoris : The an infrared view of a disk around the star HR 4796A presence of a warp in this disk indicates the existence of a Jupiter-sized planet around this taken with the Hubble's NICMOS camera. the light from star. There is also evidence for the existence of the star (which is about 1000 comets around Beta Pictoris.. times brighter than the disk) is The visible light from a planet or disk of material blocked so we can better see the ring of material around the is hidden by the brightness of the star that it star. orbits. In the infrared, where planets have their peak brightness, the brightness of the star is reduced, making it possible to detect a planet in the infrared. To aid in the detection of planets, use occulting disks to mask out the light from a star allowing Beta Pictoris : J.-L. Beuzit for a better view of possible planets. et al. . (Grenoble Obs.), ESO 5. Our Galaxy Our galaxy, the Milky Way, radiates about half of its luminosity in the infrared. The Milky Way is a spiral galaxy containing over 100 billion stars. It is over 100,000 light years in diameter and has a disk containing spiral arms and a dense central sphere or bulge. The center of our galaxy is not visible at optical wavelengths because it is hidden behind numerous clouds of gas and dust. However we can view the center of our galaxy in the infrared, since infrared rays can penetrate gas and dust. The image is a combination of infrared data from the 2MASS. The Galactic plane runs horizontally along the image, and the Galactic center is the bright (yellow) object near the middle. In the blue regions (2MASS) many stars invisible to optical telescopes can be seen in the infrared. The red areas (MSX) show the distribution of dust near the center of our galaxy. 2MASS Project, Umass, IPAC/Caltech, our galaxy from 2MASS from the COBE satellite The center of our galaxy is one of the brightest infrared sources in the sky. It is about one thousand times brighter in the infrared than at radio wavelengths. Infrared observations show that the center of our galaxy consists of a very dense crowding of stars and that stars and gases near the center are orbiting very rapidly (probably due to the existence of a black hole). The image, a near-infrared view of the center of our Milky Way Galaxy from 2MASS, shows multitudes of otherwise hidden stars. The central core of our galaxy is the brighter region at the upper, left portion of the image. An infrared image of the entire sky from the COBE satellite shows the bright band in the middle of the image, our Milky Way galaxy. This is how our galaxy appears from our vantage point in the sun's orbit about the center of our galaxy. Our solar system is located far out in the disk of our galaxy at a distance of about 30,000 light years from the center Galaxies The infrared emission from galaxies comes primarily from three sources: stars, interstellar gas, and dust. The emission from stars peaks in the near-infrared (1-3 microns). Emission from atoms and molecules in interstellar gas makes up only a few percent of the infrared output of galaxies. The primary source of infrared radiation beyond 3 microns is thermal emission from dust grains heated by starlight. An infrared image of the spiral galaxy M83 (European Southern Observatory) showing the infrared glow of stars in the spiral arms. Starburst Galaxies The brightest infrared galaxies are usually the ones which have a lot of dust (in star-forming regions for example). Astronomers using the IRAS satellite observed 20,000 galaxies in the infrared. Many of these were starburst galaxies galaxies which are forming enormous numbers of new stars, and are thus extremely bright in the infrared. Further infrared studies of these galaxies may find the cause of this star-forming frenzy. About half of the luminosity of an average spiral galaxy is radiated at far-infrared wavelengths. Elliptical galaxies are faint in the infrared because they no longer have much gas and dust. An infrared image of the starburst galaxy M83. Spiral galaxies, which are rich in gas and dust, are strong infrared sources and are still forming new stars. Galaxies infrared images of three : Andromeda galaxy taken by the IRAS satellite (Notice the regions where young stars are forming shown in yellow and red). The barred spiral galaxy NGC 1364 (see the bright areas of star formation, as well as young star clusters) taken in 1999 (shows the improvement in resolution since the days of IRAS (sixteen years earlier). The edge on spiral galaxy NGC891(clearly see the lanes of dust along the edge of the galaxy). Infrared images of the Andromeda galaxy (NASA/IPAC/IRAS), the barred spiral galaxy NGC 1364 (C. Marcella Carollo (JHU, Columbia U.), NASA, ESA), and the edge-on spiral galaxy NGC 891 (JC Barentine (PSI) et al., KPNO, NOAO). (JC Barentine (PSI에) 외., KPNO, NOAO). Star formation in spiral galaxies Star formation in spiral galaxies is concentrated in the spiral arms. This is evident from the observed Balmer recombination emission (red) from photoionised regions around hot young stars, interspersed with dark regions obscured by dense dust and molecular clouds. Messier 81 The magnificent, dusty, star-studded arms of a nearby spiral galaxy, Messier 81 : infrared image from NASA's Spitzer Space Tele. shows us old stars, interstellar dust heated by star formation activity, and embedded sites of massive star formation within this galaxy. The bluish-white central bulge of the galaxy contains older stars and only a little dust. Winding outward from the bulge are the grand spiral arms which are very rich in infrared emitting dust. The infrared-bright clumpy knots within the spiral arms show the places where massive Spitzer view of Messier 81 stars are being born in giant molecular clouds. The greenish areas are regions dominated by the infrared light radiated by hot dust that has Antennae galaxies Sometimes galaxies, each containing billions of stars, collide with each other. These collisions trigger star formation in these galaxies They do this by compressing gas and dust to the point where this material starts to collapse under its own gravity. Due to a high rate of star formation, colliding gas-rich galaxies radiate very strongly in the infrared. an infrared image of two galaxies (called the antennae galaxies) in collision. the bright areas of intense star formation and the glow from the centers of the two galaxies. (Bernhard Brandl and the WIRC team (Cornell), Palomar Observatory) Hubble Deep Field : HST During December 1995, the Hubble Space Telescope scanned a small area of the sky to make the deepest image of the sky ever - this area is called the Hubble Deep Field. In 1996, Astronomers using ISO found that many of the faint galaxies detected by the Hubble Space Telescope in the Hubble Deep Field radiate most of their energy in the infrared and are going through a period of very active star formation. Recently, several new galaxies were discovered behind the Milky Way in Infrared View of the 30 Largest near-infrared 2MASS images. Galaxies (2MASS) 5. ISM Much of the space between the stars is filled with gas (primarily hydrogen and helium) and tiny pieces of solid particles or dust (composed mainly of carbon, silicon and oxygen). In some places this interstellar material is very dense, forming nebulas. In other regions the gas and dust density is very low. The image shows an infrared view of the gas and dust in our galaxy along the plane of our Milky Way galaxy. Here you can see areas of dense gas and dust as well as areas which are nearly empty. Most of this gas and dust originates from the death of stars which either exploded (supernova) or blew off their outer layers, returning their material to interstellar space. W. Waller and F. Varosi (GSFC), IRAS, SkyView, NASA Often, the gas and dust between the stars can be detected only in the infrared. By using infrared detectors, astronomers can penetrate the often invisible interstellar gas and dust clouds and gain much information about their composition and structure By using infrared detectors, astronomers can penetrate the often invisible interstellar gas and dust clouds and gain much information about their composition and structure dying stars Below are two infrared images taken by NICMOS of material being ejected into space by dying stars. Credits (Left Image): Rodger Thompson, Marcia Rieke, Glenn Schneider, Dean Hines (University of Arizona); Raghvendra Sahai (Jet ProInstrument Definition Team, and NASA/AURA/STScI Credits (Right Image): Credits: William B. Latter (SIRTF Science Center/Caltech) and NASA/AURA/STScI pulsion Laboratory); NICMOS "infrared cirrus A surprise discovery from the IRAS mission was that space is filled with faint wisps of dust which cannot be seen in visible light.This has been given the name "infrared cirrus" because it resembles the cirrus clouds in the Earth's atmosphere. Infrared cirrus is very cold (15-30 K) and can only be detected in the infrared. IRAS image of cirrus at the south celestial pole and ISO-LWS spectra of infrared cirrus Astronomers using ISO discovered emission lines from interstellar water vapor in a variety of sources including star form ing regions, planetary nebulae and near formed stars. ISO also discovered for the first time hydrogen cyanide ice molecules in a dusty cloud surrounding a newly forming star. ISM The ISM is a multi-component, multi-phase medium. Its main constituent is hydrogen gas, but all components produce distinctive emission and absorption spectral signatures. Not all phases are in pressure equilibrium. Interstellar dust, whilst comprising < 0.1% of the ISM, plays an exceptionally important role in the physics and chemistry of the ISM, star formation, and our interpretation of spectra from astrophysical sources. gas component phase neutral molecular cold warm ionised diffuse HII coronal T (K) N (m−3) 50 − 100 109 100 25 × 106 8 × 103 0.25 × 106 8 × 103 0.03 × 106 104 106−10 5 × 105 6 × 103 M(Mo) 109? 1.5 × 109 1.5 × 109 109 5 × 107 108? Dust mostly grains or aggregates of molecules containing carbon, graphite, silicates • grain sizes a range from nm to μm • intimately mixed in with gas • formed in atmospheres of old stars, novae, supernovae • destroyed by UV irradiation (evaporation) • re-radiated IR dust emission traces star formation • grains act as catalysts for chemical reactions in the ISM, providing surfaces on which molecules (e.g. H2) can form • charged grains align themselves along the ISM magnetic field B and contribute to polarisation • grains have a broad distribution of sizes, shapes,T and N l • reduction in overall optical brightness of objects seen through dust is called interstellarextinction; strong wavelength dependence on absorption and scattering – generally, l < a, implying UV/optical light suffers most from dust extinction: • absorbed UV/optical light is re-radiated in the IR Nebulae These are clouds of gas and dust in the ISM. They fall broadly into 3 categories: 1. Emission nebulae, or HII regions Appear red, violet, blue Balmer reddish in colour due to emission of recombination lines (m -> n = 2 transitions in HI, where m = 3, 4, 5 are the most important transitions). These transitions are: Ha lambda 6563 °A , Hb 4861 °A and Hg 4340 °A . Nebulae 2. Reflection nebulae: clouds illuminated by starlight; the flux is too low to photoionise hydrogen; appear blue in colour since we see scattered light which is preferentially scattered at short wavelengths by dust in the clouds 3. Dark nebulae: dense clouds which completely extinguish starlight from our line of sight; the dust in these clouds absorbs and scatters the optical/UV starlight; we could only see the stars in the IR, where there is less extinction; this is called interstellar reddening IS Gas • atomic hydrogen (HI) produces the 21 cm (1420MHz) radio signal, which can be either in emission or absorption • neutral ISM gas produces interstellar absorption lines, which are much narrower than stellar absorption lines because the ISM gas is much cooler gas is shock-ionised by local novae/super-novae and emits rcan use absoprtion lines to determine abundances of elements in ISM gas • coronal radio and X-ray synchrotron radiation due to relativistic, nonthermal electrons and magnetic fields • ionised ISM gas is coupled to a highly turbulent B-field Multi-wavelength images Molecular clouds Although atomic gas contains most of the mass of the ISM, molecular gas is the densest component and is responsible for star formation. Most of what we know about ISM molecules comes from spectroscopy. Molecules can have 3 types of transitions: electronic, vibrational and rotational. The transitions are almost always seen in absorption because of the extremely low temperatures of ISM molecular gas. Electronic transitions usually occur in the far-UV, whilst the weaker vibrational and rotational transitions occur in the mm/submm wavebands (i.e. between radio and FIR). The most abundant ISM molecule is H2, although the most observed molecule is CO. Orion molecular cloud, A portion of the sub-mm spectrum of the Orion molecular cloud, showing strongest lines and atmospheric transmission. The lines are seen in emission due to the relatively high temperature of the cloud (T ~60 K), such that some highly excited vibrational levels and rotational sublevels are populated. 6. The Early Universe The billions of galaxies outside our own galaxy range in distance from hundreds of thousands to billions of light years away. For the most distant galaxies, we see them as they were billions of years ago. As a result of the Big Bang (the tremendous explosion which marked the beginning of our Universe), the universe is expanding and most of the galaxies within it are moving away from each other. All distant galaxies are moving away from us and the farther away they are, the faster they are moving. This recession of galaxies away from us has an interesting effect on the light emitted from these galaxies, the light that galaxy emits is "redshifted" , the wavelengths of light get longer and are shifted towards the red part of the spectrum. This means that infrared studies can give us much information about the visible spectra of very young, distant galaxies. The Early Universe An infrared view of some of the farthest galaxies ever seen, taken by the Hubble Space Telescope's NICMOS camera. Infrared studies have also found a potential protogalaxy (a galaxy in the process of formation) more than 15 billion light years from Earth. This object, named IRAS 10214+4724, may be a huge, contracting hydrogen cloud just beginning to shine with newborn stars. This is close to the edge of the observable universe and its light has taken since nearly the beginning of the universe to reach us. Protogalaxies provide us with a look at the era when galaxies were first coming to life.