What is a black hole?
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Loosely speaking, a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. Since our best theory of gravity at the moment is Einstein's general theory of relativity, we have to delve into some results of this theory to understand black holes in detail, but let's start of slow, by thinking about gravity under fairly simple circumstances.

Suppose that you are standing on the surface of a planet. You throw a rock straight up into the air. Assuming you don't throw it too hard, it will rise for a while, but eventually the acceleration due to the planet's gravity will make it start to fall down again. If you threw the rock hard enough, though, you could make it escape the planet's gravity entirely. It would keep on rising forever. The speed with which you need to throw the rock in order that it just barely escapes the planet's gravity is called the "escape velocity." As you would expect, the escape velocity depends on the mass of the planet: if the planet is extremely massive, then its gravity is very strong, and the escape velocity is high. A lighter planet would have a smaller escape velocity. The escape velocity also depends on how far you are from the planet's center: the closer you are, the higher the escape velocity. The Earth's escape velocity is 11.2 kilometers per second (about 25,000 m.p.h.), while the Moon's is only 2.4 kilometers per second (about 5300 m.p.h.).

Now imagine an object with such an enormous concentration of mass in such a small radius that its escape velocity was greater than the velocity of light. Then, since nothing can go faster than light, nothing can escape the object's gravitational field. Even a beam of light would be pulled back by gravity and would be unable to escape.

The idea of a mass concentration so dense that even light would be trapped goes all the way back to Laplace in the 18th century. Almost immediately after Einstein developed general relativity, Karl Schwarzschild discovered a mathematical solution to the equations of the theory that described such an object. It was only much later, with the work of such people as Oppenheimer, Volkoff, and Snyder in the 1930's, that people thought seriously about the possibility that such objects might actually exist in the Universe. (Yes, this is the same Oppenheimer who ran the Manhattan Project.) These researchers showed that when a sufficiently massive star runs out of fuel, it is unable to support itself against its own gravitational pull, and it should collapse into a black hole.

In general relativity, gravity is a manifestation of the curvature of spacetime. Massive objects distort space and time, so that the usual rules of geometry don't apply anymore. Near a black hole, this distortion of space is extremely severe and causes black holes to have some very strange properties. In particular, a black hole has something called an 'event horizon.' This is a spherical surface that marks the boundary of the black hole. You can pass in through the horizon, but you can't get back out. In fact, once you've crossed the horizon, you're doomed to move inexorably closer and closer to the 'singularity' at the center of the black hole.

You can think of the horizon as the place where the escape velocity equals the velocity of light. Outside of the horizon, the escape velocity is less than the speed of light, so if you fire your rockets hard enough, you can give yourself enough energy to get away. But if you find yourself inside the horizon, then no matter how powerful your rockets are, you can't escape.

The horizon has some very strange geometrical properties. To an observer who is sitting still somewhere far away from the black hole, the horizon seems to be a nice, static, unmoving spherical surface. But once you get close to the horizon, you realize that it has a very large velocity. In fact, it is moving outward at the speed of light! That explains why it is easy to cross the horizon in the inward direction, but impossible to get back out. Since the horizon is moving out at the speed of light, in order to escape back across it, you would have to travel faster than light. You can't go faster than light, and so you can't escape from the black hole.

(If all of this sounds very strange, don't worry. It is strange. The horizon is in a certain sense sitting still, but in another sense it is flying out at the speed of light. It's a bit like Alice in "Through the Looking-Glass": she has to run as fast as she can just to stay in one place.)

Once you're inside of the horizon, spacetime is distorted so much that the coordinates describing radial distance and time switch roles. That is, "r", the coordinate that describes how far away you are from the center, is a timelike coordinate, and "t" is a spacelike one. One consequence of this is that you can't stop yourself from moving to smaller and smaller values of r, just as under ordinary circumstances you can't avoid moving towards the future (that is, towards larger and larger values of t). Eventually, you're bound to hit the singularity at r = 0. You might try to avoid it by firing your rockets, but it's futile: no matter which direction you run, you can't avoid your future. Trying to avoid the center of a black hole once you've crossed the horizon is just like trying to avoid next Thursday.

Incidentally, the name 'black hole' was invented by John Archibald Wheeler, and seems to have stuck because it was much catchier than previous names. Before Wheeler came along, these objects were often referred to as 'frozen stars.' I'll explain why below.

How big is a black hole?
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There are at least two different ways to describe how big something is. We can say how much mass it has, or we can say how much space it takes up. Let's talk first about the masses of black holes.

There is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if you compress it to a high enough density. We suspect that most of the black holes that are actually out there were produced in the deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star. A typical mass for such a stellar black hole would be about 10 times the mass of the Sun, or about 10^{31} kilograms. (Here I'm using scientific notation: 10^{31} means a 1 with 31 zeroes after it, or 10,000,000,000,000,000,000,000,000,000,000.) Astronomers also suspect that many galaxies harbor extremely massive black holes at their centers. These are thought to weigh about a million times as much as the Sun, or 10^{36} kilograms.

The more massive a black hole is, the more space it takes up. In fact, the Schwarzschild radius (which means the radius of the horizon) and the mass are directly proportional to one another: if one black hole weighs ten times as much as another, its radius is ten times as large. A black hole with a mass equal to that of the Sun would have a radius of 3 kilometers. So a typical 10-solar-mass black hole would have a radius of 30 kilometers, and a million-solar-mass black hole at the center of a galaxy would have a radius of 3 million kilometers. Three million kilometers may sound like a lot, but it's actually not so big by astronomical standards. The Sun, for example, has a radius of about 700,000 kilometers, and so that supermassive black hole has a radius only about four times bigger than the Sun.

Pluto Part II

Pluto orbits beyond the orbit of Neptune (usually). It is much smaller than any of the official planets and now classified as a "dwarf planet". Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).

        orbit:    5,913,520,000 km (39.5 AU) from the Sun (average)
       diameter: 2274 km
       mass:     1.27e22 kg

In Roman mythology, Pluto (Greek: Hades) is the god of the underworld. The planet received this name (after many other suggestions) perhaps because it's so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percival Lowell.

Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.

After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no Planet X. But that doesn't mean there aren't other objects out there, only that there isn't a relatively large and close one like Planet X was assumed to be. In fact, we now know that there are a very large number of small objects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size as Pluto.

Pluto has not yet been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). A spacecraft called New Horizons was launched in January 2006. If all goes well it should reach Pluto in 2015.

Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies.

In late 2005, a team using the Hubble Space Telescope discovered two additional tiny moons orbiting Pluto. Provisionally designated S/2005 P1 and S/2005 P2, they are now known as Nix and Hydra. They are estimated to be between 50 and 60 kilometers in diameter.

Pluto's radius is not well known. JPL's value of 1137 is given with an error of +/-8, almost one percent.

Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and basic physics) the individual masses of Pluto and Charon are difficult to determine because that requires determining their mutual motions around the center of mass of the system which requires much finer measurements -- they're so small and far away that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157; more observations are underway but we won't get really accurate data until a spacecraft is sent.

Pluto is the second most contrasty body in the Solar System (after Iapetus).

There has recently been considerable controversy about the classification of Pluto. It was classified as the ninth planet shortly after its discovery and remained so for 75 years. But on 2006 Aug 24 the IAU decided on a new definition of "planet" which does not include Pluto. Pluto is now classified as a "dwarf planet", a class distict from "planet". While this may be controversial at first (and certainly causes confusion for the name of this website) it is my hope that this ends the essentially empty debate about Pluto's status so that we can get on with the real science of figuring out its physical nature and history.

Pluto has been assigned number 134340 in the minor planet catalog.

Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Pluto rotates in the opposite direction from most of the other planets.

Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than Neptune's. Its orbital inclination is also much higher than the other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really doesn't and they will never collide. (Here is a more detailed explanation.)

Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit.

The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). The "warmer" regions roughly correspond to the regions that appear darker in optical wavelengths.

Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays.

Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. NASA mission planners want to arrive at Pluto while the atmosphere is still unfrozen.

The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud. Like the Earth's Moon, Charon may be the result of a collision between Pluto and another body.

Pluto can be seen with an amateur telescope but it is not easy. There are several Web sites that show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several days will be required to reliably find it. Suitable charts can be created with many planetarium programs.

Charon ( "KAIR en" ) is Pluto's largest satellite:

        orbit:    19,640 km from Pluto
       diameter: 1206 km
       mass:     1.52e21 kg

Charon is named for the mythological figure who ferried the dead across the River Acheron into Hades (the underworld).

(Though officially named for the mythological figure, Charon's discoverer was also naming it in honor of his wife, Charlene. Thus, those in the know pronounce it with the first syllable sounding like 'shard' ("SHAHR en").

Charon was discovered in 1978 by Jim Christy. Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together.

Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon.

Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known.

Pluto and Charon are also unique in that not only does Charon rotate synchronously but Pluto does, too: they both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very interesting.)

Charon's composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice. Interestingly, this is quite different from Pluto.

Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved.

It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon.

It is doubtful that Charon has a significant atmosphere.

Time Crystal

here’s a disaster at CERN in Switzerland and two men are absorbed by a black hole. Moments later time stops everywhere in the Universe except near fragments of mysterious blue crystal. Each one is surrounded by a bubble of time. Hold a crystal and you’re alive. Without it you’re frozen in time. Fourteen-year-old Catriona O’Brien is visiting CERN when the disaster happens. She is given a crystal and hears one of the two absorbed men saying that if she can collect all the crystal fragments and bring them down something called the ‘Time Tunnel’ then he can restart time.

So begins an adventure that will take Catriona back thru the history of the Universe to the Big Bang and beyond!

But there are other things, mysterious and dangerous things, coming up the Time Tunnel, things which want to use the people of Earth for their own evil purposes. Time Crystal, the greatest adventure in the history of the Universe, written and read by me, Wyken Seagrave. Visit www.timecrystal.c o.uk for free podcast, role-playing on-line games, competitions, prizes and far far more!

So begins Wyken Seagrave’s epic adventure which will take Catriona on a journey through time back to the Big Bang and beyond.

Time Crystal is Unique

It is the first work of fiction to:

1. use the history of the Universe as its stage
2. allow listeners to influence the next episode
3. describe a novel mechanism for the creation of a black hole
4. explore the science and art underlying the work in a simultaneous sister podcast
5. cost the author US$ 1 million to develop

This is truly the greatest adventure in the history of the Universe!

Click on the links above to visit the parts of this site, or click the links at the side to subscribe free either via iTunes or Podiobooks.

The Art & Science of Time Crystal describes the writing, development, characters and science behind the story.

http://www.timecrystal.co.uk/

Jupiter

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Jupiter is the largest planet in the solar system, the 5th from the Sun. It is not quite as bright as Venus when seen from Earth.
Measured across, Jupiter is ten times the size of the Earth and one-tenth the size of the Sun. Like the Sun and Saturn, it is mostly hydrogen and helium. There may be a small core of rock and perhaps ice at the center. This is surrounded by an ocean of liquid hydrogen and helium in the form of a metal. Above this is a layer of liquid hydrogen and helium, and above this a thick atmosphere.

Jupiter is radiating energy, possibly as a result of radioactivity. The most famous feature is the Great Red Spot, which may be a huge storm in the atmosphere. If so it must be quite a storm, as it has been recorded for 300 years!

Jupiter also has a faint ring made of dust which was only recently discovered.
Far more easy to see are the moons. The four largest of these were seen by Galileo when he made one of the first telescopes. The way they moved round the planet convinced him that the old idea, that everything moves round the Earth, must be wrong.

The following was kindly contributed by Russell Odell

The sun contains 99.9 percent of the mass in the solar system. The remaining 0.1 percent make up the other planets and their moons, and Jupiter took most of that mass. If Jupiter were a shell, all the other planets and their moons could fit inside with room to spare. Jupiter could hold 317 of our earth's.

Jupiter's diameter is 88,000 miles, thirty times the width of the United States. It orbits the sun at 8.1 miles per second (mps), or 19,160 miles per hour (mph). It is 485 million miles from the sun making it a very cold planet in the range of minus 240 degrees F. However, due to the enormous pressure of its mass, the center is estimated to be 54,000 degrees F., or five times hotter than the surface of the sun. (The interior of the sun is 40 million ºF.)

The earth rotates on its axis in 24 hours establishing day and night. Jupiter rotates in 9 hours, 54 minutes, giving it the shortest day and night of any of the planets. The surface speed of the earth at its equator is a little over 1,000 mph. Jupiter's surface speed at its equator is 30,000 mph.

Jupiter is in full view six months of the year. It is very bright. Only the planet Venus is brighter. If you compare the brightness of Jupiter to the stars, only one star, Sirius, is brighter.

Jupiter, like Saturn, Uranus and Neptune is a ball of gas while the other four planets are solid (Mercury, Venus, Earth & Mars).

Galileo discovered Jupiter and three of its moons on January 7, 1610. Six nights later, January 13th , he discovered the fourth moon. He named them, J1, J2, J3, and J4.

A few nights later the Dutch astronomer Simon Marius, quite independently, saw the moons through his telescope. Marius gave them names from mythology after the sons and daughters of the Greek gods naming them Io, Europa, Gannymede and Callisto. Later, when Marius announced his discovery he was ridiculed by the followers of the great Galileo who was a patron of the Grand Duke of Tuscany. Marius was ostracized and no one would honor the names he had given to the moons. As other moons were discovered Galileo's method of numbering prevailed over giving names to the moons. These first four moons of Jupiter are known as the Galilean Moons. However, later, to recognize the work of Marius, the names he gave the moons are the names we know them by today.

The 14th moon was discovered in October, 1975, by the American astronomer Charles T. Kowal. Its distance from the planet has not yet been determined.

Jupiter's 12th moon is the incredible distance of 14, 880,000 miles from the planet. With such a distance, it takes the moon two years to orbit its planet.

The 16 moons of Jupiter is a story by itself. For example, its four outer most moons are 15 million miles from the planet and orbit in the opposite direction of Jupiter’s other moons. .

One of the mysteries of Jupiter is its big red spot. The Space Probe Pioneer Ten flew by Jupiter on December 3, 1973. It was followed a year later by Pioneer Eleven. In March of 1979 Voyager One sent back pictures of the big Red Spot. Voyager Two sent back additional pictures in July (1974). The Red Spot was first reported by Robert Hooke in 1664. Several years later it was also noted by Cassini, the Italian-French astronomer. Drawings of it appeared in 1831 by William Dawes and later by other astronomers. We have known about the great Red Spot of Jupiter for more than 300 years.

We do not know why this Red Spot grows and diminishes in size from 7000 miles wide and 13,000 to 30,000 miles long. We know it rises and sinks in the gaseous clouds but don't know why. Neither do we know why it drifts east and west but never north or south.

Did you ever wonder how or why the planet Jupiter came into existence? What or who determined its size? What keeps it rotating, where does the energy come from to keep it zooming around the sun at 19,160 mph? Why does it rotate on its axis anti-clockwise while the planet Venus rotates clockwise? Why does Jupiter have 16 moons but Mercury and Venus have none? Science says this all happened purely by chance over a period of billions of years.

However, we have many theories that explain its creation from many different viewpoints, but, which theory is right?

The gravitational fields of Jupiter are so strong they disintegrated the Shoemaker-Levy 9 Comet into 20 fragments before it hit the planet.

Revealing the mysteries of galaxy formation

This image provides a view of the Large Magellanic Cloud in visible light. The distribution of stars can be clearly seen in this image. The red outlined area corresponds to AKARI's far-infrared image. The green area shows the location of the near- and mid-infrared image. AKARI's far-infrared image reveals that interstellar clouds cover the entire galaxy, while the stellar distribution is concentrated in the lower part of the image. This galaxy is located in the constellation Dorado, in the southern sky. When looking at the night sky, the Large Magellanic Cloud can be seen with a smaller neighbouring galaxy, called the Small Magellanic Cloud. Both appear as dim clouds in the sky. The name 'Magellanic' is taken from the great 16th century navigator Magellan, who observed the two 'clouds' during his voyage around the world. Credits: Mr. Motonori Kamiya

As the AKARI satellite nears completion of its All Sky Survey, it has released two stunning images of the Large Magellanic Cloud. AKARI is an infrared astronomical satellite from the Japan Aerospace Exploration Agency (JAXA) with involvement from the UK, the Netherlands and the European Space Agency.

Mars, The Stars & Planets - Did the Universe from from - God? New Angle on a Tired Old Debate - CosmicFingerprints.com

Dr Stephen Serjeant of the Open University, commenting on the new images said, "AKARI has given us a superb view of the Large Magellanic Cloud - a dwarf galaxy which our own galaxy is consuming. All galaxies cannibalize each other; in time, our own galaxy will be subsumed within the Andromeda galaxy."

The Large Magellanic Cloud is a dwarf galaxy orbiting our own galaxy. It lies at a distance of about 160,000 light years, having about 1/20 the diameter of our galaxy and 1/10 the number of stars. It is believed that the LMC was once a barred spiral galaxy that was disrupted by the Milky Way to become somewhat irregular.

The Large Magellanic Cloud contains about 10 billion stars and is at a distance of 160,000 light years, extremely close by astronomical standards. The Large Magellanic Cloud is located in the constellation Dorados in the southern sky. The name "Magellanic" is taken from the great 16th century navigator Magellan who observed the clouds during his voyage around the world.

This false-colour view of the Large Magellanic Cloud is a composite of images taken by AKARI at far-infrared wavelengths (60, 90 and 140 microns). The Large Magellanic Cloud is a neighbour galaxy to the Milky Way. Interstellar clouds in which new stars are forming are distributed over the entire galaxy. The bright region in the bottom-left is known as the 'Tarantula Nebula' and is a very productive factory of stars. This figure is a part of a data set covering a sky portion of about 17 000 by 17 000 light-years. Credits: JAXA

AKARI (formerly known as ASTRO-F) was launched in February 2006 and began its All Sky Survey in May. It is the first new infrared survey satellite since the 1980s and the data it collects will shed new light on the dusty Universe – such as areas where stars are born and die.

Dr Chris Pearson, European Space Agency support astronomer for AKARI, said, "The spectacular images of the Large Magellanic Cloud in the Far-Infrared give us a first mouth watering taste of what we can expect from the AKARI All-Sky Survey as the satellite continues to sweep out great swathes of the sky."

Dr Seb Oliver of the University of Sussex added "AKARI has now mapped virtually the whole sky. This is a tremendous achievement by all concerned. These examples show the impressive quality that we can expect from the first far infrared sky survey for 20 years."

During the survey observations, AKARI investigated one of the most important targets for studies of the formation of galaxies, the Large Magellanic Cloud, over more wavelength bands than has ever been possible in the past. Using the data taken by AKARI astronomers will be able to unlock the secrets of how both the Large Magellanic Cloud and our own Galaxy have formed and have grown to their current state.

Your Zodiac Horoscope - Insert Your Birthdate & Get Answers about Past-Present and Future. Free - AboutAstro.com/horoscope

Professor Glenn White of the Open University / CCLRC Rutherford Appleton Laboratory said "this stunning AKARI image traces the far -infrared emission from a noticeable bar of stars, some of which, including S Doradus, are extremely luminous. The Large Magellanic Cloud is rich in a variety of diffuse nebulae, including the spectacular Tarantula Nebula, planetary nebulae, open clusters, globular clusters, which suggest that it has experienced early and late bursts of star formation."

Since most stars are formed deep in dusty clouds of interstellar gas, their starlight is not directly visible in the optical, but light from the young stars heats the surrounding dust, which in turn radiates at infrared wavelengths that can escape the dusty clouds. However, if dust is very widespread and thick throughout galaxies, the stars which heat the surrounding dust may not be just the young stars, so the observed infrared radiation would not be the good measure of star formation rates that it is usually assumed to be. Clarifying the dust content of typical galaxies is important to understanding how they evolve over time, as interstellar gas is transformed into stars. These AKARI far infrared surveyor observations are designed to trace the distribution of warm dust close to hot stars, and the associated star-formation rate. Understanding this in one of the Local Group galaxies in our own backyard is crucial to our understanding of the formation and evolution of more distant galaxies.

Dr Rich Savage of the University of Sussex said, "These latest images are a great example of the new views of the cosmos that AKARI is giving us."

Dr Elysandra Figueredo of the Open University said, "I'm really very impressed with these images but much more impressed about the superb scientific insights this data bring. It's very exciting to see the structure and the content of the compact star-forming regions distributed throughout our neighbour galaxy. These regions are crowded with newly born stars that are probably still inside their cocoons. This is a new view Large Magellanic Cloud and it will certainly help us a great deal in understanding the birth of stars like our Sun, and also bring new insights about our own Galaxy."

Dr David Clements from Imperial College London said, 'The last all sky far infrared survey, made by the IRAS satellite, was completed in 1983 but astronomers still refer to it on a daily basis. These early results from AKARI show just how much of an advance the new survey is going to be - we'll be using AKARI data daily for at least the next 20 years!'

http://www.physorg.com/news81599877.html

The Galaxies

Galaxies

The Milky Way system

The Sombrero Galaxy (M104), an almost edge-on spiral galaxy in the constellation of Virgo.
© Peter Barthel (Kapteyn Institute), Very Large Telescope, European Southern Observatory. On a dark night we can often see a band of light stretching across the sky. If we look at this band with binoculars or a small telescope we see that it is partially resolved into stars. This band we call the Milky Way and indeed it is composed of a band of stars most of which are too faint to be resolved so that we see their combined light as a faint glow.

This band is the plane of the disk of our galaxy. The Sun is one, rather faint, example of approximately 200,000,000,000 stars that make up our galaxy. These stars are mostly grouped into a flattened disk which has a bulge at its centre. The Sun is in this disk about two thirds of the way from its centre to its edge. When we look at the night sky we see the Milky Way when we look along the plane of this disk whereas when we look in other directions, out of the plane, we see far fewer stars.

There is a spherical component to our galaxy which contains very old stars and spherical clusters of old stars. These are often referred to as Population 2 objects. Population 1 being the objects found in the disk.

The size of our galaxy is huge; light would take about 100,000 years to cross the Galaxy.

Spiral galaxies

NGC 253, a nearly edge-on spiral galaxy (type Sc) in the constellation Sculptor. NGC 253 lies about 8 million light years away. © European Southern Observatory Wide Field Imager, Max-Planck-Institut für Astronomie, Osservatorio Astronomico di Capodimonte. Our galaxy has arms of younger stars and gas that appear to spiral out from the centre. In fact the objects in these spiral arms are in almost circular orbits about the centre of the Galaxy. The Sun takes about 200 million years to complete one orbit around the centre. About 30 percent of all galaxies have spiral arms. Some have arms that spiral directly from the nucleus while others have a linear feature, called a bar, from whose ends the arms originate.

Spiral galaxies are rich in gas and dust. Some are viewed face-on so that the spiral arms are easily seen whereas others are viewed edge-on. These show the presence of dust lanes which obscure the starlight coming from near the midline of the disk. We see this in our galaxy where the Milky Way is divided into two portions for much of its length. Indeed the centre of the Milky Way galaxy is invisible in ordinary light because the interstellar dust in that direction is so thick. Infrared light, however, penetrates the dust and recent measures have allowed astronomers to `see' the Galactic centre.

Elliptical galaxies

The majority of galaxies show no spiral features, nor are they flattened disks; they take the form of ellipsoids. They show only small evidence for young stars, dust or gas. They are very different in size ranging from giant ellipticals with masses of about 1 million million times that of the Sun to dwarf ellipticals with masses closer to those of the globular clusters.

Irregular galaxies

NGC 253, a nearly edge-on spiral galaxy (type Sc) in the constellation Sculptor. NGC 253 lies about 8 million light years away. © European Southern Observatory Wide Field Imager, Max-Planck-Institut für Astronomie, Osservatorio Astronomico di Capodimonte. Some galaxies are neither ellipsoidal nor are they spirals. Some of these are obviously objects which have been tidally distorted by the presence of another near-by galaxy but there are some, such as the Magellanic Clouds (see below), which have little symmetry to their structure.

Active galaxies

Some galaxies show evidence for the generation of enormous amounts of energy from the vicinity of their nucleus. These are often strong radio emitters and often show complex lobe structure extending for millions of light years. Other galaxies have such energetic nuclei that we only see the bright nucleus and not the underlying galaxy; we call these objects quasars (quasi-stellar objects).

The presence of black holes at the centres of these objects is thought necessary by many astronomers to explain their nature. Because they are the brightest objects known in the universe it is not surprising that quasars are the objects that have been traced out furthest from us. The furthest known are so far away that the light we see coming from them must have originated when the Universe was only one tenth of its present age.

Clusters of galaxies

There are many clusters of galaxies. Members of some of the closest can be seen with a small telescope in the constellations Virgo and Coma Berenices. We can trace clusters of galaxies out to the furthest distances that we can reach. Some of these clusters contain thousands of galaxies. Near their centres giant ellipticals are often found and it is thought that these arise from the collision of several galaxies which have combined.

X-ray studies have shown that there is very hot gas between the galaxies in a cluster but this gas does not solve one of the great puzzles in astronomy which is that these clusters require a certain total mass to explain how they are held together but we only can account for one tenth of this mass. This is known as the `missing mass problem'.

Nearby galaxies

Unfortunately those of us who live in the northern hemisphere cannot see the two closest galaxies, called the Magellanic Clouds, which are rather like two satellite galaxies to the Milky Way.

They can easily be seen by the naked-eye and their brightest stars can be seen with binoculars. These two galaxies are much smaller than the Milky Way and are about 200,000 light years away.

In the northern sky we can see two galaxies with the naked-eye. The Andromeda galaxy, M31, is a faint fuzzy patch that appears, with binoculars, as a lens shaped object. It is a galaxy rather like ours at a distance of about 2 million light years. It has two dwarf elliptical satellites which can be seen with a small telescope.

The other galaxy (M33 in Triangulum) is much harder to see although it is at a similar distance to the Andromeda galaxy. This is because it is smaller and less bright intrinsically. It too is a spiral galaxy.

ANDROMEDA GALAXY PART 2

• GALEX Team, CalTech, NASA
  
  
  
  
  Larger ultraviolet image.
  
  
  
  
  This ultraviolet image highlights
  a 150,000-ly-wide ring of young
  and hot, blue stars that surrounds
  Andromeda's central bulge (more
  from APOD and GALEX).
  

  
  

  On October 18, 2006, astronomers using NASA's Spitzer Space Telescope announced the discovery of two dust rings (or "holes") in Andromeda's dust disk using infrared light that provide evidence of an ancient head-on collision with neighboring dwarf galaxy along its polar axis Messier 32 (M32) some 210 million years ago. Computer simulations support the hypothesis that the passage of the much smaller galaxy created violent waves of gravitational interactions that left rings of gas and dust propagating outward from the site of the impact. Since Andromeda is much more massive than M32, the larger galaxy was not substantially disrupted, but M32 lost more than half its initial mass in the course of the collision (more).

  
  

  
  Pauline Barmby, CfA, JPL, NASA -- larger infrared image
  
  Holes in Andromeda's disk may be from an ancient collision with satellite galaxy M32 (more).
  

  
  

• Active Galactic Nucleus

  
  

  STScI, NASA
  
  
  Larger image.
  
  
  
  The central 30 light-years of Andromeda
  contains two galactic nuclei, which suggests
  that the great spiral consumed a major
  galactic companion whose substance has
  been mostly merged except for its central
  core (more from APOD and STScI).
  

  
  

  In the 1990s, astronomers using the Hubble Space Telescope found that Andromeda has a nucleus with a double structure. The "nuclear hot-spots" are located close together, considering that the galaxy's spiral disk has been estimated to be anywhere from 150,000 to more than 200,000 ly across while the observed central area measures only around 30 ly wide. Subsequent ground-based observations led some astronomers to speculate that two galactic nuclei do indeed exist, are moving with respect to each other, and that one nucleus is slowly disrupting the other through tidal forces. As a result, some astronomers believed that that one nucleus may be the remains a smaller satellite galaxy that was "eaten" by Andromeda (Corbin et al, 2001; Gerssen et al, 1995; and Lauer et al, 1993). In 2005, astronomers using the Hubble Space Telecope announced that the two two bright blobs are actually composed of an elliptical ring of older red stars and a smaller, brighter, and denser disk of young blue stars of around 200 million years old around the galaxy's central black hole (NASA press release).

  
  

  Michael Garcia, Stephen Murray,
  Palomar Sky Survey
  
  
  
  
  
  Larger image.
  
  
  
  
  
  The black square in the center
  of Andromeda's spiral disk has
  been observed with x-rays to
  reveal a supermassive black
  hole, as as well as smaller
  ones (more from CXC).
  

  
  

  Andromeda's core has a supermassive central black hole of around 140 million Solar-masses (latest NASA press release). Recent observations with the Chandra X-Ray Observatory also reveal numerous other bright X-ray sources, most of which are probably due to binary systems where a star is feeding gas into a neutron star and black hole. A very cool X-ray source has been identified about 10 light years south of the galactic center. A second, hotter X-ray source was found to be at a position consistent with the position of the super-massive black hole.

  
  

  Garcia et al, 2001;
  T. Brown et al, 2001;
  CXC, SAO, NASA
  
  
  
  Larger x-ray image.
  
  
  
  The blue dot is an unusually "cool"
  million degree X-ray source that
  lies just below an black hole
  (yellow) that may be X-ray bright
  from matter swirling toward a
  supermassive black hole of 30
  million Solar-masses (more
  from CXC).
  

  
  

  Andromeda's satellite (or "companion") galaxies include M32 and M110, two bright dwarf elliptical galaxies that are the brightest of a swarm of smaller companions. By late 1999, however, at least 10 satellite galaxies of Andromeda were known, including NGC 185 (which was discovered by William Herschel), and NGC 147 (discovered by Heinrich Ludwig d'Arrest, 1822-1875) as well as the very faint dwarf systems And I, And II, And III, possibly And IV (which may be a cluster or a remote background galaxy), And V, And VI (also called the Pegasus dwarf), and And VII (also the Cassiopeia dwarf).

  Satellite galaxy M32 may be interacting to distort the disk structure of Andromeda itself, whose spiral arms of neutral hydrogen are displaced from those consisted of stars by around 4,000 light-years and so cannot be continuously followed in the area closest to its smaller neighbor. Computer simulations have shown that such disturbances can be modelled by assuming a recent close encounter with a small companion of the mass of M32, which also suggest M32 has lost many stars from such an encounter to be spread out in Andromeda's halo.

  
  

• Very Large Galactic Halo

  Using the Hubble Space Telescope, astronomers had previously announced in 2003 that they had obtained the deepest visible-light image ever taken of the sky to resolve approximately 300,000 stars in Andromeda's luminous halo. By capturing both faint dwarf stars and bright giant stars, astronomers were able to estimate the age of many members of Andromeda's halo population by analyzing color and brightness distributions. Describing an initial hypothesis subsequently contested in January 2007, the astronomers indicated that they had found many stars spanning a wide range of ages from six to 13 billion years old, which is much wider than that of the stellar population of the Milky Way's halo where 11- to 13-billion-year-old, metal-poor stars reside. (More discussion and close-up images from Alan M. MacRobert at Sky and Telescope).

  
  

  STScI, NASA
  
  
  
  
  Larger image.
  
  
  
  
  Although Andromeda's luminous halo
  was thought to include many younger
  stars around six to 13 billion years
  old in 2003, new observations of
  old red giant, halo stars up to
  500,000 light-years away from
  Andromeda's core were announced
  in 2007 (2003; and 2007 findings).
  

  
  

  On January 7, 2007, astronomers announced finding old low-metallicity, red giant stars up to some 500,000 light-years from Andromeda's center which suggests that the galaxy is up to five times larger than originally thought, so that its luminous halo may actually overlap with that of the Milky Way. The new finding also suggests that previous observers mis-identified relatively metal-rich red giants in Andromeda's galactic bulge as halo stars. Based on observations of the Milky Way and other galaxies, the metallicity of stars farther from the galactic center should fall with distance from the core (more).

  
  

  Ann Feild, STScI, NASA
  
  
  
  
  Larger image.
  
  
  
  
  
  Although a 2003 study of 300,000 stars
  in Andromeda's halo indicated that their
  age range was wider than those found in
  the Milky Way's (more from STScI and
  APOD), this observation has been revised
  by subsequent observations that suggest
  that Andromeda's luminosity may be five
  times larger than originally thought (more).
  

  
  

  In addition, a giant stream of metal-rich stars was recently detected in Andromeda's halo (Ibata et al, 2001). The presence of younger stars in Andromeda's halo may the result of a more violent phase of the galaxy past involving mergers with smaller satellite galaxies. Furthermore, numerical simulations of the movements of Andromeda and the Milky Way suggest that the two big spiral galaxies themselves may eventually collide and merge within five to 10 billion years.

  
  

• Old but Bright Globular Cluster

  A small and compact satellite of Andromeda, G1 is the brightest globular star cluster in the Local Group. Also known as Mayall II, G1 contains at least 300,000 old stars. Despite its globular appearance, however, G1 may actualy be the stripped down core of a dwarf spheroidal galaxy (like SagDEG, the Milky Way's satellite) that has been "shredded" by its larger host. G1 may have at least 10 to 18 million Solar-masses, at least twice the mass of Omega Centauri, the Milky Way's largest globular (Meylan et al, 1998). It is located around 130,000 to 170,000 ly from Andromeda's nucleus.

  
  

  Michael Rich, Kenneth Mighell,
  James D. Neill, Wendy Freedman,
  Columbia University, Carnegie
  Observatories, STScI, NASA
  
  
  Larger image.
  
  
  
  One of Andromeda's more compact satellites
  is G1, the brightest globular star cluster in
  the Local Group (more from STScI and APOD).
  

  
  

  G1 appears to be nearly as old as the oldest of the roughly 250 known globulars in the Milky Way Galaxy and so probably was formed shortly after the birth of the first stars at the beginning of the universe. Unlike many other globulars, it has a "rather high mean metallicity of [Fe/H] = --0.95, somewhat similar to 47 Tucanae" which may be the result of self enrichment during an early phase of cluster evolution (Meylan et al, 2001). Recently, some astronomers detected a 20,000 Solar-mass black hole in G1's core (more from STScI and Gebhardt et al, 2002).

  
  

• Satellite Galaxies

  On January 11, 2006, astronomers announced their discovery that many of Andromeda's faint companion galaxies lie within a thin sheet running perpendicular through the galaxy's Andromeda disk. Nine out of 14 low-mass satellites lying with 1.3 million light-years from Andromeda are found within this sheet, whose typical width is only 52,000 light years (about two percent of the distance between Milky Way and Andromeda). The sheet runs through Andromeda's core and is almost exactly aligned with its polar axis.

  
  

  Eva Grebel,
  Andreas Koch,
  University of Basel,
  NOAO/AURA/NSF,
  Keck Observatory
  
  
  Larger illustration.
  
  
  Many of Andromeda's
  satellite galaxies
  are located within
  a plane pendicular
  to its disk (more).
  

  
  

  Similar polar planes containing contain many of the Milky Way's companion galaxies were found around three decades ago by William Kunkel and Donald Lynden-Bell. One hypothesis is that such satellite galaxies are tiny left-overs from the break-up of a more massive galaxy which has since been swallowed by their host but still move within the orbital plane of their predecessor, as galactic mergers are believed to be a main mechanism of galactic growth. A second possibility is that the observed alignment with the poles of spiral galaxies' disks traces the otherwise invisible distribution of non-luminous, so-called dark matter around these massive galaxies. Finally, it is also possible that the observed orientation along a plane is a consequence of the infall of satellites along dark matter filaments, as cosmological models predict density fluctuations or matter concentrations which would attract neighboring clumps and continued growth that lead to streams of dark and luminous matter along filamentary features of the so-called cosmic web. Small galaxies forming in dark matter streams could end up in preferred sheets determined by their infall direction toward massive galaxies. Indeed, the Andromeda satellite plane points to the nearby spiral galaxy M33 as well as to the M81 group of galaxies (more).

  
  

• Other Information

  • More information and images of Andromeda may be available at NASA and IPAC's Extragalactic Database.

  • Up-to-date technical summaries on Andromeda may be available at: NASA's ADS Abstract Service for the Astrophysics Data System; the SIMBAD Astronomical Database mirrored from CDS, which may require an account to access; and the NSF-funded, arXiv.org Physics e-Print archive's search interface.

  • In Greek mythology, Andromeda was rescued from Cetus, the Whale, by Perseus who also married her. This constellation is most easily seen in Autumn for observers in the Northern Hemisphere, but may be visible from June through February. For more information on stars and other objects in Constellation Andromeda and an illustration, go to Christine Kronberg's Andromeda. For another illustration, see David Haworth's Andromeda.


http://www.solstation.com/x-objects/andromeda.htm

ANDROMEDA GALAXY PART 1

© Robert Gendler,
Astroimaging Gallery
(Used with permission)

Larger mosaic (more).


Andromeda is the
largest member of
the Local Group of
galaxies, which
includes the Milky
Way and its satellite
galaxies. Andromeda's
own satellites include
M32, at center left, and
M110, at lower right
(more from APOD).

Breaking News

Stefan Immler,
Erin Grand, Swift,
GSFC, NASA

Larger UV image.

This new high-resolution
UV image highlights the
hottest and youngest stars
and energetic stellar
remnants in Andromeda's
spiral arms, densest
clusters, and violent
core around its central
black hole (more).

On September 16, 2009, NASA's Swift Satellite Mission released the highest resolution ultra-violet (UV) image of Andromeda available. Covering an area some 200,000 light-years (ly) wide and 100,000 ly high, the image was compiled from 330 UV images made at wavelengths of 192.8, 224.6, and 260 nanometers. The result covers some 20,000 UV sources among the hottest and youngest stars and stellar remnants in Andromeda's spiral arms, densest clusters, and violent core around its central, supermassive black hole (more from NASA and Astronomy Picture of the Day).

McConnachie et al, 2009;
PAndAS




Larger composite image and
composite illustration
.




Andromeda has been growing
by pulling stars (as well as
gas) from smaller satellite
galaxies, such as bright
neighbor Triangulum (more).

On September 2, 2009, a team of astronomers (using the Canada-France Hawaii Telescope for the Pan-Andromeda Archaeological Survey or PAndAS) announced the discovery of stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of Andromeda (M31). They also found that the halo of the giant spiral galaxy has non-native stars and "coherent structures" (such as stellar streams or "tidal tails" and a gaseous as well as stellar warp in its spiral disk). M31's nearest bright neighbor, satellite galaxy Triangulum (M33), moreover, has a stellar structure (including a warped disk) suggestive of a recent encounter with M31 around 2.5 billion years ago, and model simulations indicate that the two galaxies will have an even more violent encounter in about 2 billion years). Their observations supports the hierarchical galaxy formation model through the apparent interactions of M31 and M33 in the continuing growth of galaxies in the modern era (more from PAndAS; UBC news release; Lisa Grossman, New Scientist, September 2, 2009; McConnachie et al, 2009; and Martin et al, 2009).

Courtesy of PAndAS





Larger illustration (more).





The two galaxies will have an
even more violent encounter
in about 2 billion years (more).

A Large Spiral Galaxy

Wider and possibly brighter than our own Milky Way, the Andromeda Galaxy was once thought to be the dominant member of the Local Group of galaxies. Although it is Milky Way's nearest large galactic neighbor, this large spiral galaxy (type Sb with two arms) lies around 2.52 ± 0.14 million light-years (ly) from the Solar System (Ribas et al, 2005). It can be found in (0:40:27+40:40:12, J2000; and 0:42:44.3+41:16:9.4, ICRS 2000) Constellation Andromeda, the Chained Maiden. It is located northwest of Mu and Beta Andromedae (Mirach); west of Nu Andromedae; northeast of Theta and Sigma Andromedae; north of Pi, Delta, and Epsilon Andromedae; and south of Theta and Omega Cassiopeiae. Andromeda can be seen by Human eyes from Earth without a telescope as a "little cloud" (see Akira Fujii's photo to better relate the galaxy's location to the brightest stars of Constellation Andromeda).

Andromeda has a bright disk that is now believed to span as much as 228,000 ly in width (Chapman et al, 2005). In 2005, astronomers announced that Andromeda's disk actally extends far further out, so that the disk spans at least 260,000 light-years -- almost twice the size of the bright disk seen in photographs (Ibata et al, 2005). The outer disk emits nearly 10 percent of the galaxy's total light and may be comprised of metal-poor stars stripped from smaller galaxies that strayed too close. On January 7, 2007, a team of astronomers announced the discovery of low-metallicity, red giant stars up to some 500,000 light-years from Andromeda's core which suggests that the galaxy is much larger than originally thought, so that Andromeda's luminous halo may actually overlap with that of the Milky Way (BBC News -- more below).

© Jason Ware,
www.galaxyphoto.com
(Used with permission)




Larger image (more).




Andromeda has a bright
yellowish nucleus, dark
winding dustlanes, and
bluish spiral arms and
star clusters (more
from APOD).

In the venerable Star Names: Their Lore and Meaning, Richard Hinckley Allen noted that: "... the Great Nebula, the Queen of the Nebulae, ..., is said to have been known as far back as A.D. 905; was described by [Abd-al-Rahman] Al Sufi as the Little Cloud before 986; and appeared on a Dutch star-map of 1500." According to Robert Burnham, Jr. (1931-93): "The first hint of the true nature of the Andromeda Galaxy came late in 1923 when several [C]epheid variable stars were identified in the system [by Edwin Powell Hubble (1889-1953) who thus] ... definitively established the great spiral as an extra-galactic object ...."The galaxy is frequently referred to as M31 because it was the 31st object in the Messier Catalogue of diffuse objects that Charles Messier (1730-1817) found not to be comets. Subsequently, the "nebula" was also designated as NGC 224 by John Louis Emil Dreyer (1852-1926) in his New General Catalogue (NGC) of Nebulae and Clusters of Stars, which was first published in 1887 and later supplemented with Index Catalogue (IC) I in 1895 and IC II in 1907.

Bill Schoening, Vanessa Harvey,
REU program, NOAO/AURA/NSF




Larger red, green, and blue composite image.





Recent observations indicate that,
although the spiral disk of Andromeda
may be much larger than that of the
Milky Way, the galaxy appears to be
much less dense, with a smaller mass
counting its dark matter halo (box
view at page bottom -- more at NOAO).

Careful estimates of Andromeda's angular diameter obtained using 2-inch binoculars by Robert Jonckhere from 1952 to 1953 indicated that Andromeda's disk had a diameter of over 200,000 ly (assuming a distance of 2.9 million ly). Hence, Andromeda's spiral disk may as much as twice as large as the Milky Way's. Although Andromeda was long thought to be the most massive galaxy in the Local Group, recent data suggest that Andromeda's visible mass may total around 300 to 400 billion Solar-masses. This is considerably less than more recent estimates of the Milky Way's visible mass of as much as 600 billion or more Solar-masses, which suggests that the Milky Way may be much denser than Andromeda. These results apparently have been confirmed by recent estimates of the total halo masses of the two spirals that account for the gravitational effects of their invisible dark matter, which suggest that Andromeda has a total of around 700 billion to 1.2 trillion Solar-masses compared to 0.93 to 1.9 trillion or more for the Milky Way (more discussion from (Xue et al, 2008; Evans et al, 2000; and Evans and Wilkinson, 2000).

IRAS, IPAC, NASA




Larger infrared image.






Young stars are probably being born
in many dusty regions of Andromeda
that are bright in infrared wavelengths,
with many short-lived but massive, blue
stars in the more intense white and
yellow areas (more from IPAC).

The brightest star cloud in Andromeda has its own NGC number, NGC 206. One of the largest star-forming regions known in the Local Group of galaxies, Sir William Friedrich Wilhelm Herschel (1738-1822, portrait) noted it in his catalog as H V.36 on his discovery of the diffuse object on October 17, 1786. Located next to a dark nebula towards the southwestern, outer edge of Andromeda's spiral disk (another photo), the cloud's bright blue stars give an indication of the massive star cluster's youth (more from Astronomy Picture of the Day).

B.J. Mochejska (Warsaw University),
The DIRECT Project, FLWO, MDM



Larger blue, visual, and infrared,
composite image.




One of the largest clusters of young blue stars
in the Local Group of galaxies, NGC 206 is
located in one of Andromeda's dusty spiral arms
(more from APOD and CfA).

So far, only one supernova has been recorded in the Andromeda Galaxy, but it was the first to be detected outside the Milky Way. Known as Supernova 1885 for the year of its appearance, it has also been designated as S Andromedae. Ernst Albrecht Hartwig (1851-1923) observed it on August 20, 1885 at Dorpat Observatory in Estonia. While found independently by several other observers, only Hartwig realized its significance. The supernova reached a magnitude of six between August 17th and 20th but then faded to magnitude 16 by February 1890.

Philip Choi, Puragra Guhathakurta, UCSC, KPNO -- larger blue and infrared image.

Andromeda has an "extreme" warp in its outer spiral disk, possibly from interactions
with satellite galaxies (more from UCSC and UCOLICK), as well as debris trails
from past mergers with other galaxies (more).

Astronomers have been finding evidence of a warp in Andromeda's spiral disk for some time. The faint outer parts of a spiral galaxy are more susceptible to warping because they are less strongly bound by the gravitational and other forces that keep disk stars in a plane and are also more susceptible to the influence of neighboring galaxies. As a result, the outer regions of a rotating body of stars and gas can deviate from the plane of the disk, like an old record album exposed to too much heat. Such a warp tends to occur at the outer edges, while the inner part of the spiral disk continues to look reasonably flat. Andromeda's warp is especially pronounced on the northeast (left) side of its major axis. Such galactic warps are very difficult to demonstrate conclusively because the outer portions of a spiral disk are extremely faint compared to their bright central regions. However, the warp in Andromeda may be the most extreme case of a warped spiral galaxy found thus far. Possible causes of the warp include interactions between Andromeda and its smaller satellite galaxies (more discussion).

The Facts OF GALAXIES

The Facts

1) Main Types of Galaxies: 3;

• Spiral
• Elliptical
• Irregular (Ellipticals and Irregulars exist in both normal and 'dwarf' sizes)

2) Number of Galaxies Visible: 100 billion

3) Most Abundant Type of Galaxy: Dwarf Ellipticals

4) Distribution of Normal Size Galaxies in Hubble Classification: Spiral-75%; Elliptical-20%; Irregular-5% (Edwin Hubble's data was skewed because spirals are generally brighter than any other galaxies, and he found more of them. Dwarfs are dim and were not found until bigger telescopes were built.)

5) Spiral Galaxy Nearest Our Milky Way Galaxy: Andromeda - 2.6 million light years away. (The Magellanic Clouds - dwarf irregulars--are only an average 200,000 light years away, but they are more giant star clusters than galaxies.) (See - Survey of Our Nearest Galaxies)

6) Distance of Visible Galaxies Fatherest From Us: Appx. 14 billion light years.

7) Size of Typical Galaxy: 3,260 light years to 326,000 light years across.

8) Number of Stars in Average Galaxy: 40 billion

9) Number of Stars in Typical Large Galaxy (such as our Milky Way): 200 billion to 400 billion.

10) Number of Galaxies in Local Group: Appx: 40 (there may be dwarfs so dim we can't see them).

11) Largest Galaxy in Local Group: Andromeda

12) Smallest Galaxy in Local Group: Leo T, a dwarf irregular 600 light years across

13) Number of Galaxies in Average Galactic Group: <50

14) Fewest Number of Galaxies in Known Group: 4; Seyfert's Sextet in the constellation Serpens (in the photo it appears there are 6 galaxies, but closer study reveals that one is much farther way, and one is not a galaxy at all but a wisp of stars pulled from one of the other galaxies by gravitational forces).

Galaxies Amazing Facts

1) Massive black holes may be at center of large galaxies - It appears that most, perhaps all, spiral and elliptical galaxies hide a massive black hole at their core. This was suspected for years, but the Hubble telescope has given us direct evidence that it is a fact. Visual, infrared and ground based radio telescope images have produced clear images of high speed jets of electrons and gas shooting from the core of a number of galaxies. The Hubble also has shown that the core of these galaxies are rotating at extremely high rates. That could be caused only by a massive gravitational field--far greater that that of the stars in the core of the galaxy. Some theories suggest that black holes are the engine that form galaxies.

Our Milky Way contains such a black hole at its core. Our neighbor Andromeda may contain two, as it appears to have two distinct cores.

2) Galaxies like company - Galaxies invariably form groups, and groups form clusters, and clusters form superclusters. Gravity is at the core of this tendency. When a large galaxy forms, its massive gravitational field captures smaller galaxies that have formed in its vicinity. These smaller galaxies often actually become satellites of the large galaxy. Our Milky Way has several small satellite galaxies that orbit around us. Sometimes two galaxies get so close they collide.

Then our local group, which includes Andromeda (our group is fairly unusual in having two large galaxies), has a gravitational connection with several other groups that together form the Virgo Cluster, so called because they all appear in the constellation Virgo. Then the Virgo Cluster teams up with two other clusters to form the Virgo Super Cluster, a collection of 100 groups of gravitationally attached galaxies approximately 200 light years across.

Read more: http://www.brighthub.com/science/space/articles/12504.aspx#ixzz0Yb4vFPrM