HEASARChas a web site (the source of the above image) describing ActiveGalactic Nuclei. AnotherHEASARC web page is the source of these optical and radio images ofNGC 4261:
BradleyPeterson has written a book, An Introduction to Active GalacticNuclei (Cambridge 1997). Peterson says: "... The two largestsubclasses of AGNs are Seyfert galaxies and quasars, and thedistinction between them is to some degree a matter of semantics. Thefundamental difference between these two subclasses is in the amountof radiation emitted by the compact central source; in the case of atypical Seyfert galaxy, the total energy emitted by the nuclearsource at visible wavelengths is comparable to the energy emitted byall of the stars in the galaxy (i.e., ~ 10^11 Lsun), but in a typicalquasar the nuclear source is brighter than the stars by a factor of100 or more. ...".
CannonBall pulses of Accreting Matter could form some GammaRay Bursts.
According to the HubbleHeritage website: "... one of the brightest radio sources in thesky, Virgo A, ...[is]... associated with M87 (alsoknown as NGC 4486 ... an ordinary giant elliptical galaxy; one ofmany ellipticals in the nearby Virgo cluster of galaxies ...)... andits jet. ... Lying at the center of M87 is a supermassive blackhole, which has ... a mass equivalent to 2 billion times the massof our Sun. ... The jet originates in the disk of superheated gasswirling around this black hole and is propelled and concentrated bythe intense, twisted magnetic fields trapped within this plasma. Thelight that we see (and the radio emission) is produced by electronstwisting along magnetic field lines in the jet, a process known assynchrotron radiation, which gives the jet its bluish tint. ...
... At a distance of 50 million light-years, M87 is too distantfor Hubble to discern individual stars. The dozens of star-likepoints swarming about M87 are, instead, themselves clusters ofhundreds of thousands of stars each. An estimated 15,000 globularclusters formed very early in the history of this galaxy and areolder than the second generation of stars, which huddle closer to thecenter of the galaxy. ...".
According to astro-ph/0008063,by Biermann, Ahn, Medina-Tanco, and Stanev:
"... a simple Galactic wind model patterned after the solar wind...[shows]... that back-tracing the orbits of the highestenergy cosmic events suggests that they may all come from theVirgo cluster, and so probably from the active radio galaxyM87. ...
... a very simple model for a Galactic wind rather analoguous tothe Solar wind ... may allow particle orbits at 10^20 eV to be bentsufficiently to allow "super-GZK" particles to get here from M87, andalso explain the apparent isotropy in arrival directions. ...
... we will consider for reference a model which has a fieldstrength near the Sun of 7 microGauss; this is close to the bestestimates for the total local field ... The assumption of thesymmetry of the magnetic field above and below the Galactic disk isimportant. The value of the magnetic field, here adopted as 7microGauss, for the wind near the Sun, isa key parameter. If themagnetic field were considerably weaker, the focussing would belargely removed. ...
... The ... distance to which this wind extends ... is moreuncertain: Our Galaxy dominates its near environment well past ourneighbor, M31, the Andromeda galaxy, and might well extend its sphereof influence to half way to M81. Therefore we will adopt as outer thehalo wind radius half the distance to M81, 1.5 Mpc. ... The scale ofthe Galactic wind here 1.5 Mpc, is not a critical parameter, sincethe calculations show that most of the bending happens within thefirst few 100 kpc. ...
... To follow the particle trajectories in the Galactic halo wetrace protons backwards, e.g., ... from their arrival direction atEarth. We use the 14 published cosmic ray events above 10^20 eV ...There is a big uncertainty with the energy estimate of the highestenergy Yakutsk ... event, which we therefore exclude from the presentanalysis, and hence we arrive at a final tally of 13 events used. ...the directions of all tracks point North. All events are consistentwith arising originally from Virgo A. Since these particles areassumed to be accelerated out of cosmic gas, about 1/10 of allparticles may be Helium nuclei with the same energy per particle. Ifthe two highest energy events are in fact He nuclei, all 13 eventspoint within 20 degrees of Virgo A. If Virgo A is indeed theacceleration site of the highest energy cosmic ray events, they allrequire additional systematic bending at a ten to twenty degreelevel. Such bending could be easily accomodated within the plausiblemagnetic field strength within the supergalactic sheet from here toVirgo ...
... If the model proposed here could be confirmed, then it wouldconstitute strong evidence that all powerful radiogalaxies producehigh energy cosmic rays, and that they do this at a good fraction oftheir total power output. ... ".
( adapted from the program Deep Space Explorer1.0, for Mac ) shows our
and ( reddish, near the lower right border of theimage ) the Large Magellanic Cloud satellite galaxy. Thegreen arrow points to the location ofour Solar System.
According to a6 January 2003 BBC web article: "... A ring of ... severalhundred million stars - about 1% of the total number of stars in ourgalaxy ... stars surrounds the Milky Way ...
... The ring, which has the appearance of a giant doughnut, couldbe the remains of a satellite galaxy. ...". According to an articleby D. T. on page 22 of the April 2003 issue of Skyand Telescope: "... Astronomers ... have discovered a new part ofour galaxy: a dim, sparse, but enormous ring encircling the MilkyWay some 60,000 light-years from its center, just beyond theouter edge of the galaxy's disk ... most likely the remnant of asmall galaxy that collided with andmerged with the Milky Way in the very distant past.... the ring contains 20 million to 500 million stars in all -perhaps as many as in the Small Magellanic Cloud. ... To maintain thestars' observed fast motions along the ring's wide, circular orbit,immense amounts of dark massmust be present around the Milky Way ..." [ and/or MONDmight be an accurate theory of gravity on the galactic scale].
According to an article by Angelle Tanner on pages 44-50 of theApril 2003 issue of Sky andTelescope:
"... astronomers ... can now see clear to the Milky Way's center ...
... the thin "threads" ..., observed only at the galactic center, glow in radio by means of synchrotron radiation: emission from charged particles spiraling at relativistic speeds arond magnetic-field lines ...
... symbols show the positions of .. six stars for [2002 and] each of the previous 7 years ... The white orbits are best fits to these positions ... SO-16 swung a mere 60 astronomical units from the black hole ... at more than 9,000 km per second! Kepler's laws of motion tell us that the mass ...[of Sgr A*]... amounts to at least 3 million solar masses ... Sgr A* ... X-ray variations ... in as little as 10 minutes prove that the X-ray source can be no bigger than about 10 light-minutes (about 1 a.u.) wide ... such a large mass in such a small volume cannot avoid ending up as a black hole. ...".
According to a20 September 2000 article by David Whitehouse of the BBC:
"... Sagittarius A* (Sgr A*), about 26,000 light-years from Earth ...
... is located at the exact point around which our galaxy revolves. ... the 10-metre Keck telescope atop the dormant Mauna Kea volcano on Hawaii, ... took snapshots of Sgr A* ... This showed three stars orbiting about 16 billion km (10 billion miles) from Sgr A*. Their movement around the suspected black hole revealed not only their velocity but allowed their acceleration to be deduced. And this gave a better analysis of the mass of the central black hole at Sgr A*, which is now estimated to be 2.6 million times that of the Sun. This extraordinary object is calculated to be about 400 million km (640 million miles) across. ... Supermassive black holes are believed to be common at the cores of galaxies. In 1994, the Hubble space telescope saw evidence for such an object at the centre of the galaxy M87. ... This was calculated to have a mass equal to two to three billion Suns and occupy a space no larger than our Solar System. Sgr A* is nothing like as big. It would extend from the Sun to the orbit of Mars. ... the three stars seen orbiting Sgr A* were accelerating so quickly that their complete orbit of the black hole may be as short as a few decades and in one case as little as 15 years. ...".
According to a21 February 2002 NASA Science web page:
"... Chandra ... pinpointed a source of X-rays that coincided with Sagittarius A*. ...[ According to a Harvard Chandra web page: "... This 400 by 900 light-year mosaic
of several Chandra images of the central region of our Milky Way galaxy reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The supermassive black hole at the center of the Galaxy is located inside the bright white patch in the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high). ... An analysis of the X-ray data showed that the temperature of the gas does not have to be 100 million degrees Celsius, as previously thought. Rather, a relatively mild 10 million degrees will do. ...". ]
... the X-rays were only a fifth the intensity that theory predicted. In other words, Sagittarius A* was faint -- strange, given that active galactic nuclei are so brilliant. ... Ten thousand years ago a supernova exploded very close to Sagittarius A*. The fast-expanding gases swept away much of the local interstellar gas and dust, preventing material from falling into the Milky Way's supermassive black hole, thereby "starving" it. Less material falling into the black hole meant fewer X-rays being emitted. Nevertheless, some material is still infalling. In 2001, right before Chandra's vigilant X-ray eye, Sagittarius A* suddenly brightened. Within minutes it was 45 times its normal intensity. Then it faded back to its pre-flare level about three hours later. The energy released corresponded to the black hole suddenly having engulfed a chunk of material with the mass of a comet or asteroid! Moreover, from the specific way the X-rays brightened and dimmed, astrophysicists calculated that Sagittarius A* was only about 15 million kilometers across-less than a quarter the diameter of the orbit of the planet Mercury around our Sun. This observational evidence of small size coupled with enormous mass seemed to clinch the case for its being a supermassive black hole. ...".
According to a Universityof Toronto web page: "... The nearest big spiral galaxy to theMilky Way is the
...[ According to aSEDS web page: "... M31
is the famous Andromeda galaxy, our nearest large neighbor galaxy...". ]...
... Appearing as a smudge of light to the naked eye in theconstellation Andromeda, this galaxy is about twice as big as theMilky Way but very similar in many ways. At the moment it is about2.2 million light years away from us but the gap is closing at 500000km/hour. Andromeda is the only big spiral galaxy galaxy movingtowards the Milky Way and the best explanation is that the Milky Wayand Andromeda are in fact a bound pair of galaxies in orbit aroundone another. Both galaxies are thought to have formed close to eachother shortly after the Big Bang initially moving apart with theoverall expansion of the universe. But since they are bound to oneanother, they are now falling back back together and one veryplausible scenario puts them on a collision course in 3 billionyears. ...
In about 3 billion years, the two galaxies will collide and thenover about 1 billion years after a very complex gravitational dancethey will merge to form an elliptical galaxy. ...".
An interesting fact on the timing is that the Sun will still beburning brightly when this collision occurs and maybe life of somesort will still be around on Earth at that time. So what would peoplesee in the night sky during this billion year galactic dance? AsAndromeda approaches, it will grow in size and just before thecollision the night sky will be filled by a giant spiral galaxy. Whenthe two galaxies intersect, our familiar Milky Way arch over the skywill be joined by a second intersecting arch of stars but this willonly last for 100 million years or so and will be a very confusingstate of affairs for galactic astronomers. Finally, when the twogalaxies merge our view will depend on which direction the Sun isthrown.
There are two possible fates of the Sun which depend closely onthe details of where it is in its galactic orbit at the time of thecollision.
"... Astrophysicists Lars Hernquist and John Dubinski ... recentlysimulated the future collision of the Milky Way and her largerneighbor, Andromeda, using model galaxies comprised of 110 millionparticles ... with the help of Blue Horizon, NPACI's 1,152-processorIBM RS/6000 SP. ... ", according to an NPACIweb page describing the computer simulation images shown above.Note that the computer simulation not only has fewer particles thanthe galaxies have stars, but it also is based on gravitationalinteractions among massive point-particles, so it may be anoversimplification of what actually might happen a few billion yearsin our future.
...
