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.

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

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

The Sun II

Our Sun is a normal main-sequence G2 star, one of more than 100 billion stars in our galaxy.

        diameter:    1,390,000 km.
       mass:        1.989e30 kg
       temperature: 5800 K (surface)
                    15,600,000 K (core)

The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest).

It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun.

The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol.

The Sun is, at present, about 70% hydrogen and 28% helium by mass everything else ("metals") amounts to less than 2%. This changes slowly over time as the Sun converts hydrogen to helium in its core.

The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it's as much as 36 days. This odd behavior is due to the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body.

Conditions at the Sun's core (approximately the inner 25% of its radius) are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the center of the core the Sun's density is more than 150 times that of water.

The Sun's power (about 386 billion billion megaWatts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation.

The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field.

A small region known as the chromosphere lies above the photosphere.

The highly rarefied region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during a total solar eclipse (left). Temperatures in the corona are over 1,000,000 K.

It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stay home, you're likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel half way around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it's time to sleep. And you can see the solar corona. It is well worth a major journey.

The Sun's magnetic field is very strong (by terrestrial standards) and very complicated. Its magnetosphere (also known as the heliosphere) extends well beyond Pluto.

In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind which propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis.

Recent data from the spacecraft Ulysses show that during the minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers per second, than it does at lower latitudes. The composition of the solar wind also appears to differ in the polar regions. During the solar maximum, however, the solar wind moves at an intermediate speed.

Further study of the solar wind will be done by the recently launched Wind, ACE and SOHO spacecraft from the dynamically stable vantage point directly between the Earth and the Sun about 1.6 million km from Earth.

The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft.

Spectacular loops and prominences are often visible on the Sun's limb (left).

The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun's output has increased by about 40%.

The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth (and probably the creation of a planetary nebula).

The Sun's satellites

There are eight planets and a large number of smaller objects orbiting the Sun. (Exactly which bodies should be classified as planets and which as "smaller objects" has been the source of some controversy, but in the end it is really only a matter of definition. Pluto is no longer officially a planet but we'll keep it here for history's sake.)

http://nineplanets.org/sol.html

Fact About Pluto

Pluto is the farthest planet from the Sun (usually) and by far the smallest. 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 tenth planet.

Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface.

Fact About The SUN

The Sun is one out of billions of stars. The Sun is the closest star to Earth. The Sun rotates once every 27 days. The Sun is now a middle-aged star, meaning it is at about the middle of its life. The Sun formed over four and a half billion years ago. You may think the Sun will die soon, but it will keep shining for at least another five billion years.

The Sun’s surface is called the photosphere. The temperature of the photosphere is about 10,000° Fahrenheit. Its core is under its atmosphere. The temperature at the core, or very middle, of the Sun, is about 27 million° Fahrenheit. That’s pretty hot!

The Sun’s diameter is about 870,000 miles wide. The Sun is 109 times wider than Earth, and is 333,000 times heavier. That means if you put the Sun on a scale, you would need 333,000 objects that weigh as much as the Earth on the other side to make it balance.

The Sun is only one of over 100 billion stars. In ancient times, the people believed the Sun was a burning ball of fire created by the gods. Later, people thought it was a solid object, or a liquid ball. Over one million Earths could fit inside the Sun. Looking directly at the Sun can permanently damage your eyes because it is so bright. A star mostly gives off light and heat. The larger the star, the hotter its temperature. A super giant star can get to be 400 times larger than our Sun, which is almost a million miles in diameter. The Sun is tilted.

Without the Sun, Earth could not support life. The Sun gives off heat and light that the Earth needs to support life (us). If you lived on the Sun, and you built a spacecraft, it would have to go over 618.2 kilometers per second to escape the Sun’s gravitational pull. The Sun is 695,000 kilometers at its equator. The Sun is the largest mass in our Solar System.

Sun loops are large loops caused by the Sun’s magma (molten rock) shooting off of the Sun’s surface. These loops can fly millions of miles into space. Our Sun is approximately 25,000 light-years from the galactic core of our galaxy (the Milky Way). It is like a really big star. It is a million times bigger than the biggest.

Did you know that the Sun is made out of 92% hydrogen, 7% helium and the rest is other low number gasses? The Sun’s core is the hottest part of its matter. It is 27 billion° Fahrenheit. The Sun does not rise or set. It just looks like it does because the Earth is moving. The Earth orbits the Sun every 365 space days. Can you believe that the Sun can burn over seven million tons of natural gas every second? A star can live for over three billion years. If the Sun was hollow, you could fit 333,000 Earths inside! The Sun rotates, too. It rotates every 25-36 days. It seems as if stars always stay in the same position night after night, year after year, but they actually do move over time. They helped scientists to develop a reference system for charting a planet’s movement.

The moon does not give off light of its own. It is the Sun that gives light to the Moon. The Moon reflects the Sun’s light. A star is the only body in space that emits its own light; everything else reflects light from the closest star. Can you believe that it is over 4.24 light-years to the nearest star? Did you know that about 65% of all “stars” are actually double stars? They are stars that look like one, but when viewed through a telescope, they are actually two stars. Stars vary in sizes. They can be as small as 7,000 miles in diameters, or as large as 900 billion miles in diameter. Some stars change in brightness over a period of time. They do this when the star’s temperature dramatically drops. These stars are called Variable Stars.

A star has many different characteristics, such as their position, motion, size, mass, chemical ingredients and temperature. No two stars are exactly alike. The number of stars in the known Universe exceeds one billion.

http://library.thinkquest.org/J002231F/Sun/factsaboutthesun.htm

Moon Facts

How did the moon form? Why do we always see only one side of it? Why does the lunar day last one Earth month? Scroll down for the answers—and other facts about our moon. • How did the moon form? According to the "giant impact" theory, the young Earth had no moon. At some point in Earth's early history, a rogue planet, larger than Mars, struck the Earth in a great, glancing blow. Instantly, most of the rogue body and a sizable chunk of Earth were vaporized. The cloud rose to above 13,700 miles (22,000 kilometers) altitude, where it condensed into innumerable solid particles that orbited the Earth as they aggregated into ever larger moon lets, which eventually combined to form the moon.

• By measuring the ages of lunar rocks, we know that the moon is about 4.6 billion years old, or about the same age as Earth.

• The distance between the Earth and its moon averages about 238,900 miles (384,000 kilometers). The diameter of the moon is 2,160 miles (3,476 kilometers). The moon's mass—the amount of material that makes up the moon—is about one-eightieth of the Earth's mass.

• Because the force of gravity at the surface of an object is the result of the object's mass and size, the surface gravity of the moon is only one-sixth that of the Earth. The force gravity exerts on a person determines the person's weight. Even though your mass would be the same on Earth and the moon, if you weigh 132 pounds (60 kilograms) on Earth, you would weigh about 22 pounds (10 kilograms) on the moon.

• The rotation of the moon—the time it takes to spin once around on its own axis—takes the same amount of time as the moon takes to complete one orbit of the Earth, about 27.3 days. This means the moon's rotation is synchronized in a way that causes the moon to show the same face to the Earth at all times. One hemisphere always faces us, while the other always faces away. The lunar far side (aka the dark side) has been photographed only from spacecraft.

• The shape of the moon appears to change in a repeating cycle when viewed from the Earth because the amount of illuminated moon we see varies, depending on the moon's position in relation to the Earth and the sun. We see the full moon when the sun is directly behind us, illuminating a full hemisphere of the moon when it is directly in front of us. The new moon, when the moon is darkened, occurs when the moon is almost directly between Earth and the sun—the sun's light illuminates only the far side of the moon (the side we can't see from Earth).

• The moon orbits the Earth at an average speed of 2,300 miles an hour (3,700 kilometers an hour).

• The moon's gravitational pull on the Earth is the main cause of the rise and fall of ocean tides. The moon's gravitational pull causes two bulges of water on the Earth's oceans—one where ocean waters face the moon and the pull is strongest and one where ocean waters face away from the moon and the pull is weakest. Both bulges cause high tides. These are high tides. As the Earth rotates, the bulges move around it, one always facing the moon, the other directly opposite. The combined forces of gravity, the Earth's rotation, and other factors usually cause two high tides and two low tides each day.

• The airless lunar surface bakes in the sun at up to 243 degrees Fahrenheit (117 degrees Celsius) for two weeks at a time (the lunar day lasts about a month). Then, for an equal period, the same spot is in the dark. The dark side cools to about -272 degrees Fahrenheit (-169 degrees Celsius).

• The rocks and soil brought back by Apollo missions are extremely dry; the moon has no indigenous water. However, the moon is bombarded by water-laden comets and meteoroids. Most of this water is lost to space, but some is trapped in permanently shadowed areas near both poles of the moon.

• To the unaided eye, the bright lunar highlands and the dark maria (Latin for "seas") make up the "man in the moon." A telescope shows that they consist of a great variety of round impact features—scars left by objects that struck the moon long ago. The largest scars are the impact basins, ranging up to about 1,500 miles (2,500 kilometers) across. The basins were flooded with lava some time after the titanic collisions that formed them. The dark lava flows are what the eye discerns as maria.

• On the moon there are no mountains like the Himalaya, produced by one tectonic plate bumping into another. There is no continental drift on the moon. Everywhere, the moon is sheathed by rocky rubble created by constant bombardment by meteoroids, asteroids, and comets. • No cheese has ever been found on the moon.

Adapted from the National Geographic Atlas of the World (Seventh Edition) and Exploring Your World: The Adventure of Geography, both published by the National Geographic Society.

From : http://news.nationalgeographic.com/news/2004/07/0714_040714_moonfacts.html