The Physics of the Interstellar Medium HT 2004 Introduction/Overview Presentation - Text and references to images +----+-------+ | 1 | Title | +----+-------+ Thackeray's Globules in IC 2944 (see 5) The Horsehead Nebula in Orion http://www.aao.gov.au/images/captions/aat036.html Image and text © 1984-2002, Anglo-Australian Observatory, photograph by David Malin. This curious dark nebula is one of the best-known images in astronomy, probably because of its chance likeness to a recognisable form. The horse-head shape is an extension of a large cloud of dust which fills the lower part (east) of the picture and hides the light of stars beyond. The outer surface of the dusty gas (IC 434) runs roughly north-south and is illuminated by sigma Orionis (off the top of the picture) which causes the hydrogen there to fluoresce, outlining the horse-head shape. Though conspicuous here, the Horsehead is very difficult to see visualy, even with a large telescope. A bright star is partially enveloped in the dust cloud and its scattered light is seen as the large, irregular blue reflection nebula, NGC 2023. All these objects are about 1500 light years distant. +----+--------------+ | 4 | Dark Clouds 1| +----+--------------+ The "Black Cloud" Barnard 68 http://www.eso.org/outreach/press-rel/pr-1999/phot-20-99.html © ESO Education & Public Relations Department PR Photo 20a/99 shows an unusual sky field in the Milky Way band. It is centred on one of the classical, dark globules , known as Barnard 68 (B68) after the American astronomer, Edward E. Barnard (1857 - 1923), who included it in a list of such objects, published in 1919. It appears as a compact, opaque and rather sharply defined object against a rich, background star field. Even on this image that registers many faint stars in the area, not a single foreground star is observed. This is a clear sign that this globule must be relatively nearby. Interstellar clouds consist of gas and dust, including many molecules, some of which contain carbon atoms (i.e. "organic"). For a long time considered to be "holes in the sky", molecular clouds are now known to be among the coolest objects in the Universe (the temperature is approx. 10 K, or -263 °C). Moreover, and most importantly, they are nurseries of stars and planets. It still remains a mystery how a dark cloud like Barnard 68 at some moment begins to contract and subsequently transforms itself into hydrogen-burning stars. However, deep images of these clouds, such as this one obtained by FORS1 on VLT ANTU, may provide important clues. This small cloud seems to be in its very earliest phase of collapse. It has a diameter of only 7 light-months (approx. 0.2 pc) and it is located at a distance of about 500 light-years (160 pc) towards the southern constellation Ophiuchus (The Serpent-holder). This three-colour composite was reproduced from one blue (B), one green-yellow (V) and one near-infrared (I) exposure that were obtained with VLT ANTU and FORS1 in the early morning of March 27, 1999. +----+--------------+ | 5 | Dark Clouds 2| +----+--------------+ An Unusual Globule in IC 1396 http://antwrp.gsfc.nasa.gov/apod/ap020325.html Credit & Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT Known to some as the Elephant's Trunk Nebula, gas and dust clouds, star formation region, bright young star Energetic light from this star is eating away the dust of the dark cometary globule near the top of the above image. Jets and winds of particles emitted from this star are also pushing away ambient gas and dust. Nearly 3,000 light-years distant, the relatively faint IC 1396 complex covers a much larger region on the sky than shown here, with an apparent width of more than 10 full moons. Moriarty-Schieven et al. 1996ApJ...463L.105M: IC 1396 is a relatively nearby (750 pc), large (>2 deg), H II region ionized by a single O6.5 V star and containing bright-rimmed cometary globules. We have made the first arcminute resolution images of atomic hydrogen toward IC 1396, and have found remarkable "tail"-like structures associated with some of the globules and extending up to 6.5 pc radially away from the central ionizing star. These H I "tails" may be material which has been ablated from the globule through ionization and/or photodissociation and then accelerated away from the globule by the stellar wind, but which has since drifted into the "shadow" of the globules. This report presents the first results of the Galactic Plane Survey Project recently begun by the Dominion Radio Astrophysical Observatory. CG 4, a cometary globule http://www.aao.gov.au/images/captions/aat071a.html Image and text © 1999-2002, Anglo-Australian Observatory, photograph by David Malin. Cometary globules are isolated, relatively small clouds of gas and dust within the Milky Way. This example is about 1300 light years distant, so the head is some 1.5 light years in diameter, and the tail, which fades away off the picture, appears to be about 8 light years long. The dusty cloud contains enough material to make several sun-sized stars. The head of the nebula is itself opaque, but glows because it is illuminated by light from very hot stars nearby. Their energy is gradually destroying the dusty head of the globule, sweeping away tiny particles that scatter the starlight as a faint, bluish reflection nebula. This particular globule also shows a faint red glow, probably from excited hydrogen, and seems about to devour an edge-on spiral galaxy, which in reality is hundreds of millions of light years away, far beyond CG 4. Thackeray's Globules in IC 2944 http://hubblesite.org/newscenter/newsdesk/archive/releases/2002/01/ Credits: NASA and The Hubble Heritage Team (STScI/AURA) Strangely glowing dark clouds float serenely in this remarkable and beautiful image taken with the Hubble Space Telescope. These dense, opaque dust clouds -- known as "globules" -- are silhouetted against nearby bright stars in the busy star-forming region, IC 2944. Astronomer A.D. Thackeray first spied the globules in IC 2944 in 1950. Globules like these have been known since Dutch-American astronomer Bart Bok first drew attention to such objects in 1947. But astronomers still know very little about their origin and nature, except that they are generally associated with areas of star formation, called "HII regions" due to the presence of hydrogen gas. IC 2944 is filled with gas and dust that is illuminated and heated by a loose cluster of massive stars. These stars are much hotter and much more massive than our Sun. +----+--------------+ | 9 | Red and Blue | +----+--------------+ Reflection nebula around M20, the Trifid Nebula http://www.aao.gov.au/images/captions/aat069a.html Image and text © 2000-2002, Anglo-Australian Observatory, photograph by David Malin. The spectacular Trifid nebula (Messier 20) is one of the best known in the sky. It is a striking mixture of brilliant red light emitted from excited hydrogen gas and the soft blue glow of a reflection nebula. The blue arises from starlight, scattered by dust particles between the stars. The size of the particles is minute, similar to those of smoke, which also has a bluish hue. However, the scattered light is not a pure blue, and if we see it through a medium that is yellow (i.e. absorbs blue light) some green colouration remains. This effect can be seen in some of the darker parts of the faint reflection nebula that surrounds the Trifid, where partial absorption of light by foreground dust provides a yellow 'filter', producing shades of green. This image has been photographically enhanced using an unsharp mask. NGC 6559 and IC 1274-75 in Sagittarius http://www.aao.gov.au/images/captions/uks003.html Image and text © 1979-2002, Anglo-Australian Observatory/Royal Obs. Edinburgh. This dusty region is probably associated with the brighter and better-known Lagoon and Trifid Nebulae which are nearby in the sky and part of the same molecular cloud. The soft red glow of fluorescent hydrogen is evidence that there are young hot stars associated with the dusty clouds. These bright stars also illuminate the tiny solid particles, producing blue reflection nebulae bordering some of the emission regions. The dust is also evident in silhouette, both as sinuous dark lanes winding through the luminous gas and as the dark patches obscuring the ancient, yellow stars that populate the central parts of the Milky Way. +----+------------------+ | 10 | Dusty gas clouds | +----+------------------+ Horsehead Nebula, NGC 2024 and IC 434 http://www.aao.gov.au/images/captions/uks001.html Image and text © 1980-2002, Anglo-Australian Observatory/Royal Obs. Edinburgh. This distinctive red emission nebula (IC 434) is the result of radiation from sigma Orionis interacting with the surface of a dusty cloud of gas from which projects the dark shape of the head of a horse. Sigma is the second brightest star in the picture and is at about the same distance from the Sun as the nebula. The brightest star here is zeta Orionis, easily visible to the unaided eye as the easternmost star in the line of three which form Orion's Belt. Partly obscured by the glare of zeta is the curious yellowish nebula NGC 2024, whose energy comes from stars hidden in the dark lane, while other nebulae simply reflect the light of embedded hot stars and appear blue. All these objects are about 1500 light years distant. The NGC 6188 nebula and NGC 6193 http://www.aao.gov.au/images/captions/aat075.html Image and text © 1991-2002, Anglo-Australian Observatory, photograph by David Malin. Here we see two bright stars whose radiant energy is beating down on to the surface of a dark cloud, very similar to the Horsehead in Orion. The cloud itself is opaque and because of that, very cold inside. These conditions are ideal for the formation of organic molecules on the surfaces of the chilled dust particles. The opacity of the cloud protects the fragile molecules within from the energetic radiation of nearby stars, but as its surface is gradually warmed and eroded, the delicate organic molecules are destroyed and the hydrogen released glows as a distinctive red emission nebula. +----+----------+ | 11 | Galaxies | +----+----------+ Sombrero galaxy http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/28/ Credit: NASA and The Hubble Heritage Team (STScI/AURA) One of the largest Hubble mosaics ever assembled, this magnificent galaxy has a diameter of nearly one-fifth the diameter of the full moon. The team used Hubble's Advanced Camera for Surveys to take six pictures of the galaxy and then stitched them together to create the final composite image. The photo reveals a swarm of stars in a pancake-shaped disk as well as a glowing central halo of stars. The spiral galaxy Messier 65 (NGC 3623) http://www.aao.gov.au/images/captions/aat061.html Image and text © 1989-2002, Anglo-Australian Observatory, photograph by David Malin. M65 is an elegant spiral galaxy which is highly inclined to our line of sight, revealing a prominent and bright central bulge. Apart from the bulge, the other notable feature of M65 is its dust lane, which so obviously surrounds the galaxy but is only clearly seen where it hides a bright background. Elsewhere, dust is associated with bright, clumpy knots of star formation visible along the inner spiral arm. One arm of the galaxy seems slightly displaced, as though it had undergone some interaction with other massive galaxies. This is hardly surprising, since it is a member of a small group about 30 million light years away that includes M66 and NGC 3628. Messier 100 (NGC 4321) in Coma Berenices http://www.aao.gov.au/images/captions/aat058.html Image and text © 1991-2002, Anglo-Australian Observatory, photograph by David Malin. The Virgo cluster spills over into the adjoining constellation of Coma Berenices and is the nearest moderately rich cluster of galaxies. It contains examples of almost every known kind and two quite different types are represented here. The dominant galaxy is the face-on spiral M100, one of the finest spirals in this diverse group. As might be expected in a galaxy-rich environment, the lack of perfect symmetry in its brightest spiral arms suggests that M100 has been disturbed by gravitational interaction with its neighbours. At the top of the image is the first colour picture of a much less spectacular galaxy, a so-called 'nucleated dwarf' elliptical galaxy, probably interacting with M100. Dwarf galaxies are by far the commonest kind in rich clusters such as this, but nucleated examples are quite rare. Many more can be seen on a deep image of this field. +----+----------------------------------------+ | 12 | 21cm emission line of neutral hydrogen | +----+----------------------------------------+ Higgs L.A., Tapping K.F. 2000, AJ 120, 2471, Fig. 1 +----+---------------------------------------------------------------+ | 13 | Emission of the Milky way in the 21cm line of atomic hydrogen | +----+---------------------------------------------------------------+ Lequeux, James, 2003, The interstellar Medium (A&A Library), Springer Verlag Plate 3, page 407 These maps of the whole sky result from a systematic survey with the 25-m radiotelescope at Dwingeloo (Netherlands), made with an angular resolution of 0.5 deg. They are presented in galactic coordinates and correspond to different radial velocities with respect to the Local standard of rest (LSR). Galactic longitude increases from 300 deg (=-60 deg) to the right, to 300 deg to the left, and galactic latitude increases from -90 deg (bottom) to +90 deg (top). The missing part cannot be observed from the Netherlands. At the low velocities, the emission is dominated by nearby gas. Near the galactic plane, at high velocities (in absolute value), distant regions contribute. At high galactic latitudes (mainly positive, to the top), we also observe gas falling onto the galactic plane. Note the filamentary structure of the gas, and the empty bubbles. (From Hartmann D., 1994, The Leiden/Dwingeloo Survey of Galactic Neutral Hydrogen, Ph.D. Thesis, University of Leiden, and Hartmann D., Burton W.B., 1997, Atlas of Galactic Neutral Hydrogen, Cambridge University Press, courtesy Dap Hartmann) Note also the effects of galactic rotation! +----+--------------+ | 23 | Cosmic Cycle | +----+--------------+ ISM/Protostars: Planetary systems forming in the Great Nebula in Orion http://antwrp.gsfc.nasa.gov/apod/ap961207.html Credit: C. R. O'Dell and S. K. Wong (Rice U.), WFPC2, HST, NASA Young stars: The Pleiades Star Cluster http://antwrp.gsfc.nasa.gov/apod/ap010506.html Credit & Copyright: David Malin (AAO), ROE, UKS Telescope Old stars: Core of globular star cluster NGC 6397 http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/21/ Credit: NASA and the Hubble Heritage Team (AURA/STScI) PN: Ring Nebula (M57) http://hubblesite.org/newscenter/newsdesk/archive/releases/1999/01/ Credit: Hubble Heritage Team (AURA/STScI/NASA) SNR/Neutron stars: X-ray images of supernova remnant Puppis A and point source http://antwrp.gsfc.nasa.gov/apod/ap980425.html Credit: S. Snowden, R. Petre (LHEA/GSFC), C. Becker (MIT) et al., ROSAT Project, NASA. White dwarfs: White dwarf stars in M4 http://antwrp.gsfc.nasa.gov/apod/ap000910.html Credit: H. Richer (UBC) et al., WFPC2, HST, NASA Black holes: Artist's Concept of Galactic Black Hole with Optical Jet http://hubblesite.org/newscenter/newsdesk/archive/releases/1990/29/ Credit: Dana Berry (STScI) +----+---------------------+ | 24 | Life Cycle of Stars | +----+---------------------+ Life Cycle of Stars - galactic nebula NGC 3603 http://hubblesite.org/newscenter/newsdesk/archive/releases/1999/20/ Credit: Wolfgang Brandner (JPL/IPAC), Eva K. Grebel (Univ. Washington), You-Hua Chu (Univ. Illinois Urbana-Champaign), and NASA In this stunning picture of the giant galactic nebula NGC 3603, the Hubble telescope's crisp resolution captures various stages of the life cycle of stars in one single view. This picture nicely illustrates the entire stellar life cycle of stars, starting with the Bok globules and giant gaseous pillars (evidence of embryonic stars), followed by circumstellar disks around young stars, and progressing to aging, massive stars in a young starburst cluster. The blue super-giant with its ring and bipolar outflow [upper left of center] marks the end of the life cycle. +----+------------------------+ | 25 | Gas, Dust, Young Stars | +----+------------------------+ The dust lanes in Messier 16 (Eagle Nebula, NGC 6611) http://www.aao.gov.au/images/captions/aat047.html Image and text © 1986-2002, Anglo-Australian Observatory, photograph by David Malin. There are few clearer examples anywhere of the intimate relationship between dust, gas and young stars than M16. We see a young cluster of stars (NGC 6611) which formed about 2 million years ago, illuminating a cloud of hydrogen gas. The gas is also dusty, and in places the dust is dense enough to prevent starlight from reaching the gas and making it glow with its distinctive red hue. The luminous hydrogen provides a vivid background against which are seen numerous dark lanes and discrete globules of dust. Many of the dusty clouds are edged with bright rims which indicate the direction of the exciting stars and point to the cluster as the energising source. Small region within M17 http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/13/ Credit: NASA, ESA and J. Hester (ASU) Resembling the fury of a raging sea, this image actually shows a bubbly ocean of glowing hydrogen gas and small amounts of other elements such as oxygen and sulfur. The photograph, taken by NASA's Hubble Space Telescope, captures a small region within M17, a hotbed of star formation. M17, also known as the Omega or Swan Nebula, is located about 5,500 light-years away in the constellation Sagittarius. The Lagoon Nebula, M8, NGC 6523 http://www.aao.gov.au/images/captions/uks005.html Image and text © 1979-2002, Anglo-Australian Observatory/Royal Obs. Edinburgh. The irregular distribution of light in this beautiful part of the sky is due mainly to clouds of dust that dim the light of vast clouds of stars that make Sagittarius one of the brightest parts of the Milky Way. The Lagoon nebula is an illuminated part of such a dark ('molecular') cloud and it reveals the dust as dark lanes and globules silhouetted against the luminous gas. Within the nebula is the scattered young star cluster NGC 6530, recently formed from this material, though the centre of star-forming activity has now shifted westwards from the cluster to the brightest part of the nebula, around the tiny Hourglass Nebula. The lagoon nebula is visible to the unaided eye on southern winter nights. +----+----------------------------+ | 26 | Winds from high-mass stars | +----+----------------------------+ A Wolf-Rayet star in NGC 2359 http://www.aao.gov.au/images/captions/aat016.html Image and text © 1979-2002, Anglo-Australian Observatory, photograph by David Malin. NGC 2359 is the nebula produced by a single, extremely hot star HD 56925. This is one of the rare Wolf-Rayet stars, whose intrinsic brilliance is combined with high rates of mass loss from its surface. This material is ejected from the star with velocities which approach 2000 kilometers per second. This violent activity quickly reduces the mass of the star which in turn reduces the instability. The rapidly-moving ejected material interacts with the gas and dust around the star producing the cosmic bubble seen here. Such stars are unusual, largely because extreme behaviour of this kind cannot be sustained for long. This particular example is spectacular because it happens to inhabit a part of our Galaxy rich in interstellar gas. The Henize 70 Nebula in the Large Magellanic Cloud http://www.aao.gov.au/images/captions/aat033.html Image and text © 1983-2002, Anglo-Australian Observatory, photograph by David Malin. At the centre of this remarkable bubble-like nebula nebula is a small group of extremely hot stars. Some of these stars are rapdly losing mass and have stellar winds blowing from their surfaces with velocities that approach 4000 kilometers per second. Such stars are known as Wolf-Rayet stars and are found in galaxies capable of forming massive stars. There are many such stars in the Large Magellanic Cloud (LMC) but not all of them are associated with distinctive nebulae. The outward flowing streams of energetic particles from Wolf-Rayet (and other energetic stars) eventually interact with the relatively stationary interstellar medium pervading the LMC, releasing much of their energy as a thin spherical shell of luminous material. The hollow structure accounts for the shape we see in the sky. This nebula is almost 400 light years across, about 100 times the distance from the Sun to the nearest star. The LMC is about 160,000 light years away and is the closest galaxy to the Milky Way. +----+---------------------------+ | 27 | Winds from low-mass stars | +----+---------------------------+ TT Cygni: Carbon Star http://antwrp.gsfc.nasa.gov/apod/ap010304.html Credit: H. Olofsson (Stockholm Observatory) et al. TT Cygni is a cool red giant star with a wind. This false-color picture of TT Cyg was made using a coordinated array of millimeter wavelength radio telescopes and shows radio emission from carbon monoxide (CO) molecules in the surrounding gas. The central emission is from material blown off the red giant over a few hundred years while the thin ring, with a radius of about 1/4 light-year, actually represents a shell of gas expanding outward for 6,000 years. Carbon stars like TT Cyg are so named for their apparent abundance of carbon containing molecules. The carbon is likely the dredged-up ashes of nuclear helium burning in the stellar interior. Carbon stars lose a significant fraction of their total mass in the form of a stellar wind which ultimately enriches the interstellar gas - the source of material for future generations of stars. TT Cyg is about 1,500 light-years away in the constellation Cygnus. +----+-------------+ | 28 | Dust shells | +----+-------------+ Egg Nebula http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/09/ Credit: NASA and The Hubble Heritage Team (STScI/AURA) Protoplanetary Nebula Resembling a rippling pool illuminated by underwater lights, the Egg Nebula offers astronomers a special look at the normally invisible dust shells swaddling an aging star. These dust layers, extending over one-tenth of a light-year from the star, have an onionskin structure that forms concentric rings around the star. A thicker dust belt, running almost vertically through the image, blocks off light from the central star. Twin beams of light radiate from the hidden star and illuminate the pitch-black dust, like a flashlight shining in a smoky room. Distance: About 3,000 light-years This image is 86 arcseconds (about 1 light-year or 0.4 parsec) wide. V1610 Cyg -- Post-AGB star Filters: F606W (wide V) + polarization filters: POL0V, POL60V, POL120V +----+----------------------+ | 29 | Nebulae around stars | +----+----------------------+ Nebulae surrounding stars in our own Milky Way Galaxy http://hubblesite.org/newscenter/newsdesk/archive/releases/1999/35/ Image Credit: NASA and The Hubble Heritage Team (AURA/STScI). Two of these visible-light pictures show interstellar gas and dust around young stars at the beginning of their lives, and two more show gas ejected from old stars that are nearing the end of theirs. Remarkably, in spite of the completely different evolutionary stages, the nebulae have more striking features in common, including evidence of diametrically opposed gas ejections from both the young and old stars. +----+-------------------+ | 30 | Planetary Nebulae | +----+-------------------+ Ring Nebula (M57) http://hubblesite.org/newscenter/newsdesk/archive/releases/1999/01/ Credit: Hubble Heritage Team (AURA/STScI/NASA) Astronomers using the Hubble telescope have obtained the sharpest view yet of a glowing loop of gas called the Ring Nebula (M57), first cataloged more than 200 years ago by French astronomer Charles Messier. The pictures reveal that the "Ring" is actually a cylinder of gas seen almost end-on. Such elongated shapes are common among other planetary nebulae, because thick disks of gas and dust form a waist around a dying star. This "waist" slows down the expansion of material ejected by the doomed object. The easiest escape route for this cast-off material is above and below the star. This photo reveals dark, elongated clumps of material embedded in the gas at the edge of the nebula; the dying central star is floating in a blue haze of hot gas. The Ring Nebula is about 2,000 light-years from Earth and has a diameter of about one light-year. The faint speck at its center was once a star of greater mass than our own Sun. The photo shows numerous small dark clouds of dust that have formed in the gas flowing out from the star, and are silhouetted against more distant bright gas. These dense dust clouds are too small to be seen with ground-based telescopes but are easily revealed by the Hubble telescope. Remarkably, these finger-like clouds are only seen in the outer portions of the Ring Nebula; none are seen in the central region. This proves that they are not distributed in a uniform sphere but are instead located only on the walls of the barrel. Many of the finger-like clouds point away from the central star, like spokes on a wheel, due to the forces of radiation and gas ejected from the dying object. The image is 2 arcminutes (roughly 1.3 light-years) on the veritcal side. Filters: Red: F658N ([N II]), Green: F501N ([O III]), Blue: F469N (He II) +----+------------+ | 31 | Supernovae | +----+------------+ Around supernova 1987A, before and just after the event http://www.aao.gov.au/images/captions/aat050.html Image and text © 1989-2002, Anglo-Australian Observatory, photograph by David Malin. In February 1987 a supernova exploded in the Large Magellanic Cloud, a nearby galaxy. It was the first supernova to be clearly visible to the unaided eye for 400 years. This photograph shows the field around the site of the supernova in great detail, both before the supernova exploded and a few weeks afterwards, when it was still very bright. The image of the star that exploded to create the supernova (arrowed) is clearly elongated. This does not necessarily indicate any particular peculiarity or a close companion, rather it is the effect of stars being by chance aligned along the line of sight. Several other examples can be seen in this picture and other, different, blended images are seen in the photograph of the same field taken two weeks after the supernova appeared. +----+------------+ | 32 | Light Echo | +----+------------+ The light echo of supernova 1987A http://www.aao.gov.au/images/captions/aat066.html Image and text © 1989-2002, Anglo-Australian Observatory, photograph by David Malin. When supernova 1987A was seen to explode in the Large Magellanic Cloud, the Milky Way's nearest companion galaxy, the brilliant flash of light from the self-destructing star had taken about 170,000 years to arrive. Some light was deflected by two sheets of dust near the supernova, and is seen after the star has faded away because it covers a longer path to reach us. The dust responsible for the rings seen here lies in two distinct sheets, about 470 and 1300 light years from the supernova. The colour picture, made by subtracting images on plates taken before and after the supernova, is an accurate reproduction of the colour of the extremely faint light echo, which in turn reflects the yellow colour of the supernova when it was at its brightest, in May, 1987. There's an article by David Malin and David Allen describing the phenomenon in Sky and Telescope magazine for January, 1990 (p22) and here is a diagram and caption showing how the light echoes were formed. The light echoes appear to expand with time and we were able to record this expansion in a series of six images, the last being 1440 days (almost five years) after the appearance of the supernova. +----+------------+ | 33 | Light Echo | +----+------------+ Diagram and caption from article by David Malin and David Allen 1990, Sky and Telescope 79, 22 http://www.ast.cam.ac.uk/AAO/images/image/light_echo_3.gif +----+--------------------+ | 34 | Supernova Remnants | +----+--------------------+ The Vela Supernova Remnant http://www.aao.gov.au/images/captions/uks002.html Image and text © 1978-2002, Anglo-Australian Observatory/Royal Obs. Edinburgh. About 120 centuries ago an inconspicuous star in what is now the constellation of Vela brightened by about 100 million times to rival the Moon as the brightest object in the night sky. This photograph shows a portion of the north-western quadrant of an expanding nebulous shell, which now surrounds the site of the explosion. Near the centre of the nebula (and not seen here) is the Vela pulsar, a rapidly-spinning neutron star only a few kilometres in diameter, the remnant of the star that exploded. This tiny object spins about 11 times a second and until recently was among the faintest stars ever studied at optical wavelengths, a far cry from its brief glory as one of the brightest stars ever seen. Pencil Nebula http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/16/ Image Credit: NASA and The Hubble Heritage Team (STScI/AURA) Officially known as NGC 2736, the Pencil Nebula is part of the huge Vela supernova remnant, located in the southern constellation Vela. Discovered by Sir John Herschel in the 1840s, the nebula's linear appearance triggered its popular name. The nebula's shape suggests that it is part of the supernova shock wave that recently encountered a region of dense gas. It is this interaction that causes the nebula to glow, appearing like a rippled sheet. This is an outlying wisp of the Vela supernova remnant (SNR), almost the only sign of the eastern part of this vast bubble of expanding shock wave from a stellar explosion 12,000 years ago. Though clearly detected with radio and X-ray telescopes, the Vela SNR is partially hidden to optical observers. Many fine, tangled filaments are seen in the western part of the nebula, but in the east, most of it is hidden in dust. This is one of the brighter eastern fragments. +----+-------------+ | 35 | Crab Nebula | +----+-------------+ The Crab Nebula from VLT http://antwrp.gsfc.nasa.gov/apod/ap020714.html Credit: FORS Team, 8.2-meter VLT, ESO The Crab Nebula, filled with mysterious filaments, is the result of a star that was seen to explode in 1054 AD. This spectacular supernova explosion was recorded by Chinese and (quite probably) Anasazi Indian astronomers. The filaments are mysterious because they appear to have less mass than expelled in the original supernova and higher speed than expected from a free explosion. In the above picture taken recently from a Very Large Telescope, the color indicates what is happening to the electrons in different parts of the Crab Nebula. Red indicates the electrons are recombining with protons to form neutral hydrogen, while blue indicates the electrons are whirling around the magnetic field of the inner nebula. In the nebula's very center lies a pulsar: a neutron star rotating, in this case, 30 times a second. The Crab Nebula (M1) in the radio http://www.nrao.edu/imagegallery/php/level3.php?id=335 Image courtesy of NRAO/AUI The Crab Nebula (M1) in the X-rays http://chandra.harvard.edu/photo/0052/index.html Credit NASA/CXC/SAO The Crab Nebula is the remnant of a supernova explosion that was seen on Earth in 1054 AD. It is 6000 light years from Earth. At the center of the bright nebula is a rapidly spinning neutron star, or pulsar that emits pulses of radiation 30 times a second. +----+-----------+ | 36 | Local ISM | +----+-----------+ The Local Bubble and the Galactic Neighborhood http://antwrp.gsfc.nasa.gov/apod/ap020217.html Illustration Credit & Copyright: Linda Huff (American Scientist), Priscilla Frisch (U. Chicago) What surrounds the Sun in this neck of the Milky Way Galaxy? Our current best guess is depicted in the above map of the surrounding 1500 light years constructed from various observations and deductions. Currently, the Sun is passing through a Local Interstellar Cloud (LIC), shown in violet, which is flowing away from the Scorpius-Centaurus Association of young stars. The LIC resides in a low-density hole in the interstellar medium (ISM) called the Local Bubble, shown in black. Nearby, high-density molecular clouds including the Aquila Rift surround star forming regions, each shown in orange. The Gum Nebula, shown in green, is a region of hot ionized hydrogen gas. Inside the Gum Nebula is the Vela Supernova Remnant, shown in pink, which is expanding to create fragmented shells of material like the LIC. Future observations should help astronomers discern more about the local Galactic Neighborhood and how it might have affected Earth's past climate. +----+---------------------------+ | 37 | Milky Way disk from above | +----+---------------------------+ Michael Richmond Department of Physics Rochester Institute of Technology Physics 230, "Stellar Astronomy", Fall 2004 Interstellar Medium: Gas http://spiff.rit.edu/classes/phys230/lectures/ism_gas/ism_gas.html