Astronomical images!


Wolfe Creek crater (Kandimalal ), Australia

Wolfe Creek is the second largest 'fresh' meteorite crater on Earth (i.e., so recent that actual meteorite fragments have been found).  Nearly circular, with a diameter varying between 870 and 950 meters, it is only slightly smaller than the Barringer crater in Arizona.  It is young enough (some 300,000 years) to have retained most of its original structure, although the bottom 150 meters have been filled in with wind-blown sand.  Rainfall is retained in the whitish gypsum areas on the central crater floor, permitting sizable trees to grow.




Europeans discovered the crater only in 1947, but to Australian aborigines it has been known since time immemorial.  Its name in the local Djaru language is Kandimalal, and it is twined in legends, in particular that of rainbow serpents whose sinuous paths across the desert formed two nearby creeks: the crater is where one snake emerged from the ground (see the book by Peggy Reeves Sanday: Aboriginal Paintings of the Wolfe Creek Crater: Track of the Rainbow Serpent, 2007).  The location is in the remote Kimberley region in the northern part of the state of Western Australia.  It is a national park, and during the dry season the crater can be reached by 4-wheel drive vehicles from the town of Halls Creek, which is some 150 km north along the adventurous Tanami Road.  The crater has also been the setting of an Australian horror movie.


Photos by Dainis Dravins. 


    These photos have appeared in several books and magazines published in Europe, North America, Asia, and Australia.  One nice little book is Australia's Meteorite Craters, by Alex Bevan & Ken McNamara, available from the Western Australian Museum webshop in Perth.







Although called "fixed", stars move in space, tracing out paths across the sky.  This full-sky map shows stars in nearby clusters, and their paths during the next 200,000 years, computed from measurements made by the ESA space astrometry satellite Hipparcos.  Well-known clusters include the Hyades and Pleiades at top right, while the one closest in space the Ursa Major group spreads out all over the sky.  Thicker paths indicate stars closer to us: the nearest one is Sirius and the two next ones are faint red dwarfs.


The stellar paths in each cluster converge due to effects of perspective, a phenomenon used for distance determinations.  The accuracies reached in space astrometry now permit to infer also the radial motions of stars.  Each cluster moves through space with a common velocity, like a flock of birds.  When receding, the angle subtended by the cluster decreases.  By combining this change in relative size with stellar distances obtained from trigonometric parallaxes, astrometric radial velocities are obtained without using any spectroscopy, nor applying the Doppler effect.


S.Madsen, D.Dravins, & L.Lindegren: Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations, Astron.Astrophys. 381, 446 (2002).



Accretion onto a black hole in a close binary system


What phenomena can one expect accretion processes? An artist's vision, combining established knowledge with predicted phenomena unfolds in this painting. In a close binary system, matter is escaping from the dynamic and unstable outer atmosphere of an evolved red giant star and impinging into an accretion disk surrounding a black hole. The accretion flow is turbulent, with eddies on many different scales.  A collimated jet is ejected from the center, but after some distance it becomes unstable and disintegrates after suffering a supersonic collision with the surrounding circumstellar medium. At certain distances from the center, hydrodynamic instabilities appear as various  types of waves, possibly seen by a distant observer as quasi-periodic oscillations.


The angular momentum of some of the inflowing matter conspires with moving hydrodynamic shocks to form a three-dimensional structure above and around the disk. Some of these "walls" are very thin, their thickness perhaps reflecting electric current sheets. Differential gas motions inside and around the accretion disk feed a dynamo which generates a chaotic magnetic field on many different spatial scales, whose energy is released in short and energetic flares of magnetic reconnection, accompanied by local mass ejections and high-energy radiation. Some magnetic areas are cooler than their local surroundings, while others dissipate with magnetic heating and appear hotter.


At the very center, a glimpse of processes very near the black hole can be seen: the appearance is asymmetric because the flux (and wavelength) of light is altered by both the gravitational field and by the Doppler effect in the rotating gas (the side approaching the observer is brighter. Further, relativistic ray-bending permits us to view also the "back" side of the central region. All this is accompanied by infalling planetisimals and crashing comets, possibly remnants of a former planetary system, local hydromagnetic instabilities seen as vortices in the gas streams, gas ejections collimated by local magnetic fields, and many other small-scale instability phenomena. On larger scales, the whole disk is undergoing acoustic oscillations.


How much could be reality and what is fantasy? Of course, nobody knows exactly what an accretion disk looks like (and, arguably, none has ever been directly observed). However, all the phenomena depicted are inspired from predictions in the literature. Some of the processes occur over very small dimensions. In order to understand these, we are driven toward high time resolution. Even if there is no [immediate] hope for the spatial imaging, signatures of many events may be observable in the time domain, on timescales of seconds, milli-, or even microseconds.


Artist's vision by Catrina Liljegren, Bild & Form, Lund ( Dainis Dravins, Lund Observatory).


Page created by Dainis Dravins; comments are welcome to dainis @
Updated JD 2,455,775