General Astronomy Project

Astro 102

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Nida Ababtain 430201040

The Origin of Solar System

Photo from: thetechherald.com

A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed and collapses under its own gravity. The disturbance could be, for example, the shock wave from a nearby supernova. As the cloud collapses, it heats up and compresses in the center. It heats enough for the dust to vaporize. The initial collapse is supposed to take less than 100,000 years. The center compresses enough to become a protostar and the rest of the gas orbits/flows around it. Most of that gas flows inward and adds to the mass of the forming star, but the gas is rotating. The centrifugal force from that prevents some of the gas from reaching the forming star. Instead, it forms an "accretion disk" around the star. The disk radiates away its energy and cools off. First brake point. Depending on the details, the gas orbiting star/protostar may be unstable and start to compress under its own gravity. That produces a double star. The gas cools off enough for the metal, rock and (far enough from the forming star) ice to condense out into tiny particles. (i.e. some of the gas turns back into dust). The metals condense almost as soon

as the accretion disk forms (4.55-4.56 billion years ago according to isotope measurements of certain meteors); the rock condenses a bit later (between 4.4 and 4.55 billion years ago). The dust particles collide with each other and form into larger particles. This goes on until the particles get to the size of boulders or small asteroids. Run away growth. Once the larger of these particles get big enough to have a nontrivial gravity, their growth accelerates. Their gravity (even if it's very small) gives them an edge over smaller particles; it pulls in more, smaller particles, and very quickly, the large objects have accumulated all of the solid matter close to their own orbit. How big they get depends on their distance from the star and the density and composition of the protoplanetary nebula. In the solar system, the theories say that this is large asteroid to lunar size in the inner solar system, and one to fifteen times the Earth's size in the outer solar system. There would have been a big jump in size somewhere between the current orbits of Mars and Jupiter: the energy from the Sun would have kept ice a vapor at closer distances, so the solid, accretable matter would become much more common beyond a critical distance from the Sun. The accretion of these "planetesimals" is believed to take a few hundred thousand to about twenty million years, with the outermost taking the longest to form. Two things and the second brake point. How big were those protoplanets and how quickly did they form? At about this time, about 1 million years after the nebula cooled, the star would generate a very strong solar wind, which would sweep away all of the gas left in the protoplanetary nebula. If a protoplanet was large enough, soon enough, its gravity would pull in the nebular gas, and it would become a gas giant. If not, it would remain a rocky or icy body. At this point, the solar system is composed only of solid, protoplanetary bodies and gas giants. The "planetesimals" would slowly collide with each other and become more massive. Eventually, after ten to a hundred million years, you end up with ten or so planets, in stable orbits, and that's a solar system. These planets and their surfaces may be heavily modified by the last, big collision they experience

The Planets and Its Properties

Photo from: scinewsblog.blogspot.com

Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

Distance (km) 57,910 108,200 149,600 227,940 778,330 1,426,940 2,870,990 4,497,070 5,913,520

Radius ( km) 2439 6052 6378 3397 71492 60268 25559 24764 1160

Mass (kg) 3.30e23 4.87e24 5.98e24 6.42e23 1.90e27 5.69e26 8.69e25 1.02e26 1.31e22

All eight planets can be seen with a small telescope, or binoculars. And large observatories continue to provide much useful information. But the possibility of getting up close with interplanetary spacecraft has revolutionized planetary science.

# Planet Classification:

The planets inside the orbit of the earth are called the Inferior Planets: Mercury and Venus. The planets outside the orbit of the earth are called the Superior Planets: Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. The planets inside the asteroid belt are termed the Inner Planets (or the Terrestrial Planets): Mercury, Venus, Earth, and Mars. The planets outside the asteroid belt are termed the Outer Planets: Jupiter, Saturn, Uranus, Neptune, and Pluto. The planets sharing the gaseous structure of Jupiter are termed the Gas Giant (or Jovian) Planets: Jupiter, Saturn, Uranus, and Neptune.

1- Mercury
Mercury is the closest planet to the Sun and the eighth largest. Until 1962 it was thought that Mercury's "day" was the same length as its "year" so as to keep that same face to the Sun much as the Moon does to the Earth. But this was shown to be false in 1965 by doppler radar observations. It is now known that Mercury rotates three times in two of its years. Photo from: solarsystem.nasa.gov Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K to 700 K. The temperature on Venus is slightly hotter but very stable. Mercury is in many ways similar to the Moon, its surface is heavily cratered and very old; it has no plate tectonics. Mercury is the second densest major body in the solar system, after Earth. Actually Earth's density is due in part to gravitational compression; if not for this, Mercury would be denser than Earth. This indicates that Mercury's dense iron core is relatively larger than Earth's. Mercury actually has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replenished. Radar observations of Mercury's north pole show evidence of water ice in the protected shadows of some craters. Mercury has a small magnetic field whose strength is about 1% of Earth's. And it has no known satellites. It is often visible with binoculars or even the unaided eye, but it is always very near the Sun and difficult to see in the twilight sky.

2- Venus
Venus is the second planet from the Sun and the sixth largest. Venus' orbit is the most nearly circular of that of any planet, with an eccentricity of less than 1%. Venus's rotation is somewhat unusual in that it is both very slow (243 Earth days per Venus day, slightly longer than Venus' year) and retrograde. Venus is only slightly smaller than Earth (95% of Earth's diameter, Photo from: solarsystem.nasa.gov 80% of Earth's mass). Both have few craters indicating relatively young surfaces. Their densities and chemical compositions are similar. Because of these similarities, it was thought that below its dense clouds Venus might be very Earthlike and might even have life. But, unfortunately, more detailed study of Venus reveals that in many important ways it is radically different from Earth. It may be the least hospitable place for life in the solar system. There are no small craters on Venus. It seems that small meteoroids burn up in Venus' dense atmosphere before reaching the surface. The interior of Venus is probably very similar to that of Earth, an iron core about 3000 km in radius, a molten rocky mantle comprising the majority of the planet. Recent results from the Magellan gravity data indicate that Venus' crust is stronger and thicker than had previously been assumed. Venus has no magnetic field, perhaps because of its slow rotation. It has no satellites, and thereby hangs a tale. On June 8 2004, Venus passed directly between the Earth and the Sun, appearing as a large black dot travelling across the Sun's disk. This event is known as a "transit of Venus" and is very rare: the last one was in 1882, the next one is in 2012 but after that you'll have to wait until 2117. While no longer of great scientific importance as it was in the past, this event was the impetus for a major journey for many amateur astronomers.

3- Earth
Earth is the third planet from the Sun and the fifth largest. Mir space station and Earth's limb Earth, of course, can be studied without the aid of spacecraft. Nevertheless it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous Photo from: solstation.com help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful. The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km): Depth (km) 0- 40 40- 400 400- 650 650-2700 2700-2890 2890-5150 5150-6378 Layer Crust Upper mantle Transition region Lower mantle D'' layer Outer core Inner core

The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. Taken as a whole, the Earth's chemical composition (by mass) is: 34.6% Iron, 29.5% Oxygen, 15.2% Silicon, 12.7% Magnesium, 2.4% Nickel, 1.9% Sulfur and 0.05% Titanium The Earth is the densest major body in the solar system. There are (at present) eight major plates:

 North American Plate - North America, western North Atlantic and Greenland Earth's Plate Boundaries delineated by earthquake epicenters South American Plate - South America and western South Atlantic Antarctic Plate - Antarctica and the "Southern Ocean" Eurasian Plate - eastern North Atlantic, Europe and Asia except for India African Plate - Africa, eastern South Atlantic and western Indian Ocean Indian-Australian Plate - India, Australia, New Zealand and most of Indian Ocean Nazca Plate - eastern Pacific Ocean adjacent to South America Pacific Plate - most of the Pacific Ocean (and the southern coast of California!) The Earth's surface is very young. In the relatively short (by astronomical standards) period of 500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years are rare. 71% Percent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface. Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar system today. Earth's atmosphere seen at the limb The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants. The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements. The oxygen in Earth's

atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen. The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 18-hour days in a year. Earth has a modest magnetic field produced by electric currents in the outer core. The interaction of the solar wind, the Earth's magnetic field and the Earth's upper atmosphere causes the auroras. Irregularities in these factors cause the magnetic poles to move and even reverse relative to the surface; the geomagnetic north pole is currently located in northern Canada. (The "geomagnetic north pole" is the position on the Earth's surface directly above the south pole of the Earth's field.)

# Moon
The Moon is the only natural satellite of Earth. It is the second brightest object in the sky after the Sun. As the Moon orbits around the Earth once per month, the angle between the E arth, the Moon and the Sun changes, we see this as the cycle of the Moon's phases. The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) Photo from: sos.noaa.gov since the Earth moves a significant distance in its orbit around the Sun in that time. Due to its size and composition, the Moon is sometimes classified as a terrestrial "planet" along with Mercury, Venus, Earth and Mars. The Moon's crust averages 68 km thick and varies from essentially 0 under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon's mass). Unlike the Earth, however, the Moon's interior is no longer active. Curiously, the Moon's center of mass is offset from its geometric center by about 2 km in the direction toward the Earth. Also, the crust is thinner on the near side. With no atmosphere and no magnetic field, the Moon's surface is exposed directly to the solar wind. Over its 4 billion year lifetime many ions from the solar wind have become embedded in the Moon's regolith. Thus samples of regolith returned by the Apollo missions proved valuable in studies of the solar wind.

4- Mars
Mars is the fourth planet from the Sun in the Solar System.

There are a lot of fascinating features of the Red Planet (so called because of the high quantity of rust, or iron-oxide, on the surface), including the largest mountain in the solar system (Olympus Mons) and the largest valley (Valles Marineris). Some of the solar systems biggest storms take place on Mars as well, sometimes engulfing the entire planet. Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth's plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun. Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for months. Mars' thin atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5 degrees (K); much less than what we see on Venus and Earth. Early telescopic observations revealed that Mars has permanent ice caps at both poles; they're visible even with a small telescope. We now know that they're composed of water ice and solid carbon dioxide ("dry ice"). Mars has two tiny satellites which orbit very close to the martian surface, Phobos and deimos .

Photo from: windows2universe.org

5- Jupiter
Jupiter is the fifth planet from the Sun and by far the largest. Jupiter is more than twice as massive as all the other planets combined (the mass of Jupiter is 318 times that of Earth). Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Photo from: boingboing.net Venus). The gas planets do not have solid surfaces, their gaseous material simply gets denser with depth What we see when looking at these planets is the tops of clouds high in their atmospheres (slightly above the 1 atmosphere level). Jupiter is about 90% hydrogen and 10% helium with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium. Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses. The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts. Recent experiments have shown that hydrogen does not change phase suddenly. Therefore the interiors of the jovian planets probably have indistinct boundaries between their various interior layers. Farthest from Jupiter (outward of the Galilean moons) are the eight tiny moons, The moons fall into two groups: Leda, Himalia, Lysithea and Elara at about 11 million km from Jupiter and Ananke, Carme, Pasiphae and Sinope at about 23 million km.

6- Saturn
Saturn is the sixth planet from the Sun and the second largest. Saturn is visibly flattened (oblate) when viewed through a small telescope; its equatorial and polar Photo from: arcadiastreet.com diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not so much so. Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water. Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and "rock", similar to the composition of the primordial Solar Nebula from which the solar system was formed. Saturn has 53 named satellites (as of spring 2010): The three pairs Mimas-Tethys, Enceladus-Dione and Titan-Hyperion interact gravitationally in such a way as to maintain stable relationships between their orbits: the period of Mimas' orbit is exactly half that of Tethys, they are thus said to be in a 1:2 resonance; Enceladus-Dione are also 1:2; Titan-Hyperion are in a 3:4 resonance. See Scott Sheppard's site for the latest about recently discovered moons (there are lots). There are 9 more that have been discovered but as yet not named. Major moons: Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas, Enceladus, Tethys, Telesto, Calypso, Dione, Helene, Rhea, Titan, Hyperion ,Iapetus ,Phoebe Saturn has many rings such as D-Ring, C-Ring, B-ring, A-Ringetc

7- Uranus
Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune.
Photo from: picturesoftheplanets.net Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed. Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane.

Uranus' blue color is the result of absorption of red light by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer. Uranus has 27 named moons: Unlike the other bodies in the solar system that have names from classical mythology, Uranus' moons take their names from the writings of Shakespeare and Pope. They form three distinct classes: the 11 small very dark inner ones discovered by Voyager 2, the 5 large ones , and the newly discovered much more distant ones. Most have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic); the outer 4 are much more elliptical. Uranus has many rings such as alpha, beta, gamma, epsilon.

8- Neptune
Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus. After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit.

Photo from: nssdc.gsfc.nasa.gov

Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years. Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane. Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yetunidentified chromophore that gives the clouds their rich blue tint. Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour. Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun. Neptune has 13 known moons; 7 small named ones and Triton plus four discovered in 2002 and one discovered in 2003. Neptune's Rings: diffuse, inner, plateau and main.

Pluto
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. Long considered to be the smallest, coldest, and most distant planet from the Sun, Pluto may also be the largest of a group of objects that orbit in a disk-like zone of beyond the orbit of Neptune called the Kuiper Belt. This Photo from: aididza.netne.net distant region consists of thousands of miniature icy worlds with diameters of at least 1,000 km and is also believed to be the source of some comets. Pluto is the second most contrasty body in the Solar System (after Iapetus). Charon is Pluto's largest satellite. 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.

3 Comets
Photo from: solarsystem.nasa.gov

Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town. When a comet's orbit brings it close to the sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. Each comet has a tiny frozen part, called a nucleus, often no bigger than a few kilometers across. The nucleus contains icy chunks and frozen gases with bits of embedded rock and dust. The nucleus may have a small rocky core. A comet warms up as it nears the sun and develops an atmosphere, or coma. The sun's heat causes ices on the nucleus surface to change to gases so that the coma gets larger. The coma may be hundreds of thousands of kilometers in diameter. The pressure of sunlight and high-speed solar particles (solar wind) blows the coma materials away from the sun, forming a long, and sometimes bright, tail. Comets actually have two tails -- a dust tail and a plasma (ionized gas) tail.

The Origin of Comets

The origin of comets is still uncertain. Modern theories suggest they were formed during the formation of the solar system and are permanent members of it. According to the storage-cloud hypothesis

of J. H. Oort, a spherical shell of more than 100 billion comets surrounds the solar system at a distance of 75,000150,000 astronomical units (1 astronomical unit (AU) being the mean distance from the earth to the sun). While the comets move very slowly in this huge storage cloud, a passing star may change their orbits enough to force some of them into the inner part of the solar system.

The Kinds Of Comets

Comets are divided into two types by the period of their orbit: short period comets complete their orbit in 200 or less years and long period comets take more than 200 years to orbit the Sun. Of the two types, short period comets have less elliptical orbits. Short period comets, like asteroids and meteoroids, are left over bits that were never incorporated into a planet during planet formation. The Kuiper Belt, which is 30-100 AU from the Sun, is a reservoir of these short period comets. During the formation of the Solar System, the Kuiper belt was on the outer part of the pre-planetary disk. Since the outer part of the disk was less dense than the inner part, only small comets could be formed, not large planets. Long period comets, which have very elliptical orbits, usually originate from the Oort Cloud. The Oort Cloud is located 3,000 50,000 AU from the Sun and is thought to contain as many as 100 billion comets. Occasionally, a star passes by the Oort Cloud and disturbs the orbits of the comets within it. Some of these comets change course and enter the Solar System.

The Origin Of Stars

Photo from: spacetelescope.org

Stars are hot bodies of glo wing gas that start their life in Nebulae. They vary in size, mass and temperature, diameters ranging from 450x smaller to over 1000x larger than that of the Sun. Masses range from a twentieth to over 50 solar masses and surface temperature can range from 3,000 degrees Celsius to over 50,000 degrees Celsius. The color of a star is determined by its temperature, the hottest stars are blue and the coolest stars are red. The Sun has a surface temperature of 5,500 degrees Celsius, its color appears yellow.

Stars are born in a region of high density Nebula, and condense into a huge globule of gas and dust and contracts under its own gravity. A region of condensing matter will begin to heat up and start to glow forming Protostars. If a protostar contains enough matter the central temperature reaches 15 million degrees centigrade. At this temperature, nuclear reactions in which hydrogen fuses to form helium can start. The star begins to release energy, stopping it from contracting even more and causes it to shine. It is now a Main Sequence Star. A star of one solar mass remains in main sequence for about 10 billion years, until all of the hydrogen has fused to form helium. The helium core starts to contract further and reactions begin to occur in a shell around the core. The core is hot enough for

the helium to fuse to form carbon. The outer layers begin to expand, cool and shine less brightly. The expanding star is now called a Red Giant. The helium core runs out, and the outer layers drift of away from the core as a gaseous shell, this gas that surrounds the core is called a Planetary Nebula. The remaining core (80% of the original star) is now in its final stages. The core becomes a White Dwarf the star eventually cools and dims. When it stops shining, the now dead star is called a Black Dwarf.

Stars Types And Properties

Black Hole: A Black Hole is a region of space where a star used to be. It is believed that when a star achieves enough mass it collapses inward, possibly after first exploding as a Supernova, creating a region of space that exists outside the known universe. No matter or energy can escape a black hole. The lack of emitting electromagnetic radiation makes it invisible. Binary Star: A Binary Star is actually a two-star system; two stars orbit each other around their center mass. Blue Straggler: The theory behind a Blue Straggler star is that it is a binary star in the process of merging to become one star. A Blue Straggler is so named because it is hotter and bluer than other stars with the same luminosity, and also is found off the normal curve of proposed stellar evolution. Neutron Star: Another possible outcome from a Supernova, a Neutron Star is what a star becomes at the end of its life cycle if it does not have enough mass to become a Black Hole. Magnetar: This is a Neutron Star with an extremely powerful and active magnetic field. Pulsar: Another type of Neutron Star, a Pulsar (short for Pulsating Star) emits radio waves with precise regularity, and the first one discovered was code named LGM-1, as scientists wondered if it was a beacon from Little Green Men.

The Black Holes

Photo from: scinewsblog.blogspot.com

A black hole is a region in space that is so dense that not even light can escape its gravitational pull. The name comes from the fact that light cant emanate from it, and if seen against a lighter background it would appear to be a dull black disk. Black holes can be created in a few ways, theoretically, but general it is thought that when a star reaches a critical mass it collapses in upon itself, and if the mass is sufficiently high it becomes a black hole. The idea that black holes can capture light itself has been around since the 18th century, but it wasnt until Albert Einstein and his theory of general relativity in 1916 that the model of black holes currently accepted as fact was established. Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter. Astronomers have identified numerous stellar black hole candidates in binary systems, by studying their interaction with their companion stars. There is growing consensus that supermassive black holes exist in the centers of most galaxies. In particular, there is strong evidence of a black hole of more than 4 million solar masses at the center of our Milky Way.

6 The constellations

Photo from: flickr.com/photos/iconolith

The constellations are totally imaginary things that poets, farmers and astronomers have made up over the past 6,000 years (and probably even more!). The real purpose for the constellations is to help us tell which stars are which, nothing more. On a really dark night, you can see about 1000 to 1500 stars. Trying to tell which is which is hard. The constellations help by breaking up the sky into more managable bits. They are used as mnemonics, or memory aids. Astronomers officially recognize 88 constellations covering the entire sky in the northern and southern hemispheres. Currently, 14 men and women, 9 birds, two insects, 19 land animals, 10 water creatures, two centaurs, one head of hair, a serpent, a dragon, a flying horse, a river and 29 inanimate objects are represented in the night sky (the total comes to more than 88 because some constellations include more than one creature.) It is important to realize that the great majority of star patterns bear little, if any, resemblance to the figures they are supposed to represent and whose name they bear. The ancient constellation-makers probably

meant for them to be symbolic, not literal, representations of their favorite animals or fabled heroes, a kind of celestial "Hall of Fame."

Constellations names
The ancient Greek tradition was to name stars by their position within a constellation. For example, Ptolemy refers to one star by the description "the reddish one on the southern eye," a star we now know as Aldebaran in the constellation of Taurus the Bull. But these descriptions could get quite involved. Ptolemy refers to another star in the obsolete constellation of Argo the Boat as "the northernmost of two stars close together over the little shield in the poop," a bit cumbersome if you are trying to learn the names of many stars. When Al-Sufi, one of the greatest Arabic astronomers, published his own version of Ptolemy's Almagest in the tenth century, he introduced many individual star names. For centuries, bedouin Arabs had given names to bright stars -- for example Aldebaran and Betelgeuse -- since they regarded single stars as representing people and animals. Many of the original meanings of the names had been forgotten even in Al-Sufi's time, but some were direct translations of Ptolemy's descriptions. For example, the star name Fomalhaut (in the constellation of Pisces) comes from the Arabic for "mouth of the southern fish," which is how Ptolemy described it in the Almagest. After the tenth century, the works of Ptolemy and others were reintroduced into Europe by the Islamic Arabs, and the Greek books were translated from Arabic into Latin, the scientific language of the day. Thus we know Ptolemy's work from its Arabic translation, The Almagest, not by its original Greek title. And it explains why we have a system of Greek constellations with Latin names containing stars with Arabic names.

The Sun
Photo from: solarsystem.nasa.gov

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 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, 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. The surface of the Sun, called the photosphere, is at a temperature of about 5800 K.

Sun Spot
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. Sunspots are temporary phenomena on the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature.

Eclipse
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 slightly 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.

The Quiet and Non-Quiet Sun

Every 11 years solar activity surges. Sunspots pepper the Photo from: solarsystem.nasa.gov sun; they explode; massive clouds of gas known as "CMEs" hurtle through the solar system. Earth gets hit with X-rays and protons and knots of magnetism. This is called solar maximum. There's nothing mythical about "Solar Max." During the most recent episode in 2000 and 2001, sky watchers saw auroras as far south as Mexico and Florida; astronomers marveled at the huge sunspots; satellite operators and power companies struggled with outages. Now the sun is approaching the opposite extreme of its activity cycle, solar minimum, due in 2006. We can relax because, around solar minimum, the sun is quiet.

The Galaxies

Photo from: nasa.gov

A galaxy is an organized concentration or clumping of stars held together by mutual gravitational interaction in an aggregate containing millions to billions of discrete individual stellar objects grouped into specific geometric arrangements (spiral; elliptical; irregular). Typical galaxies range from dwarfs with as few as ten million stars, up to giants with a hundred trillion stars, all orbiting the galaxy's center of mass. Galaxies may contain many star systems, star clusters, and various interstellar clouds. The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.

Types of Galaxies
Galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). Milky Way The Milky Way is the galaxy which is the home of our Solar System together with at least 200 billion other stars (more recent estimates have given numbers around 400 billion) and their planets, and thousands of clusters and nebulae. The Milky Way Galaxy belongs to Galactic center of Milky Way the Local Group, a smaller group of Photo from: home.comcast.net 3 large and over 30 small galaxies, and is the second largest (after the Andromeda Galaxy M31) but perhaps the most massive member of this group. M31, at about 2.9 million light years, is the nearest large galaxy, but a number of faint galaxies are much closer. The elliptical galaxy Which has an ellipse-shaped light profile. The spiral galaxies Are disk-shaped assemblages with dusty, curving arms. Irregular galaxies Are galaxies with irregular or unusual shapes, and typically result from disruption by the gravitational pull of neighboring galaxies.
typical spiral galaxy Photo from: nasa.gov Starburst galaxy Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy.

Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.

Reference:
astro-tom.com apod.nasa.gov windows2universe.org solstation.com astronomyewpert.co.uk arcadiastreet.com nssdc.gsfc.nasa.gov scinewsblog.blogspot.com/ solarsystem.nasa.gov universetoday.com flickr.com/photos/iconolith physics.csbsju.edu wikipidia.org Thenineplanets.org