Pulsar

PULSAR are have exceed heat energy in them. They are so brigt. When they strikes any star or planet, the whole or 2-3 nearby galaxies can destroyed. Pulsar are in the shape of plus (+) and in the meet point, a bright circle is pulsar.

Let the Hitech intro about Pulsars-

A pulsar is a neutron star which emits beams of radiation that sweep through the earth's line of sight. Like a black hole, it is an endpoint to stellar evolution. The "pulses" of high-energy radiation we see from a pulsar are due to a misalignment of the neutron star's rotation axis and its magnetic axis. Pulsars pulse because the rotation of the neutron star causes the radiation generated within the magnetic field to sweep in and out of our line of sight with a regular period.

THE GAMMA RAY OF THE PULSARS

OBSERVATION OF THE PULSAR

Neutron stars have very intense magnetic fields, about 1,000,000,000,000 times stronger than Earth's own field. However, the axis of the magnetic field is not aligned with the neutron star's rotation axis. The combination of this strong magnetic field and the rapid rotation of the neutron star produces extremely powerful electric fields, with electric potential in excess of 1,000,000,000,000 volts. Electrons are accelerated to high velocities by these strong electric fields. These high-energy electrons produce radiation (light) in two general ways: (1) Acting as a coherent plasma, the electrons work together to produce radio emission by a process whose details remain poorly understood; and (2) Acting individually, the electrons interact with photons or the magnetic field to produce high-energy emission such as optical, X-ray and gamma-ray. The exact locations where the radiation is produced are uncertain and may be different for different types of radiation, but they must occur somewhere above the magnetic poles. External viewers see pulses of radiation whenever this region above the the magnetic pole is visible. Because of the rotation of the pulsar, the pulses thus appear much as a distant observer sees a lighthouse appear to blink as its beam rotates. The pulses come at the same rate as the rotation of the neutron star, and, thus, appear periodic.
Gamma-ray PulsarsPulsars are the original gamma-ray astronomy point sources. A few years after the discovery of pulsars by radio astronomers, the Crab and Vela pulsars were detected at gamma-ray energies. Pulsars accelerate particles to tremendous energies in their magnetospheres. These particles are ultimately responsible for the gamma-ray emission seen from pulsars.
By the end of 2004 there were about 1500 pulsars known through radio detections, but only seven had been detected in the gamma-rays. Gamma-ray telescopes preferentially detect young, nearby pulsars. These pulsars tend to have large magnetic fields and to be spinning rapidly. It is the loss of the pulsar's spin energy which eventually appears as radiation across the electromagnetic spectrum, including gamma-rays. Both observations and models indicate that pulsars eventually lose the ability to emit gamma-rays as the pulsar slowly takes longer and longer to rotate.

X-RAY PULSAR

Although all pulsars are neutron stars, not all pulsars shine in the same way. X-ray pulsars in particular illustrate several ways in which pulsar emission can originate:
Magnetospheric Emission: Like gamma-ray pulsars, X-ray pulsars can be produced when high-energy electrons interact in the magnetic field regions above the neutron star magnetic poles. Pulsars seen this way, whether in the radio, optical, X-ray, or gamma-ray, are often referred to as "spin-powered pulsars," because the ultimate source of energy comes from the neutron star rotation. The loss of rotational energy results in a slowing of the pulsar spin period.
Cooling Neutron Stars: When a neutron star is first formed in a supernova, its surface is extremely hot (more than 1,000,000,000 degrees). Over time, the surface cools. While the surface is still hot enough, it can be seen with X-ray telescopes. If some parts of the neutron star are hotter than others (such as the magnetic poles), then pulses of thermal X-rays from the neutron star surface can be seen as the hot spots pass through our line of sight. Some pulsars, including Geminga (see above), show both thermal and magnetospheric pulses.
Accretion: If a neutron star is in a binary system with a normal star, the powerful gravitational field of the neutron star can pull material from the surface of the normal star. As this material spirals around the neutron star, it is funneled by the magnetic field toward the neutron star magnetic poles. In the process, the material is heated until it becomes hot enough to radiate X-rays. As the neutron star spins, these hot regions pass through the line of sight from the Earth and X-ray telescopes see these as X-ray pulsars. Because the gravitational pull on the material is the basic source of energy for this emission, these are often called "accretion powered pulsars."

FOR EXTRA INFO. VISIT- http://imagine.gsfc.nasa.gov/docs/science/know_l2/pulsars.html

Nebula

Nebulae has many types. When galaxies, stars, or black holes destroys, the colorful smoke is take place in the cosmic veccum in the place of the explosion. Originally, the word "nebula" referred to almost any extended astronomical object (other than planets and comets). The etymological root of "nebula" means "cloud". As is usual in astronomy, the old terminology survives in modern usage in sometimes confusing ways. We sometimes use the word "nebula" to refer to galaxies, various types of star clusters and various kinds of interstellar dust/gas clouds. More strictly speaking, the word "nebula" should be reserved for gas and dust clouds and not for groups of stars. There are many types of it-

GALAXIES- You can read in the later issue.

GLOBULAR CLUSTERS- gravitationally bound groups of many thousands (sometimes as many as a million) of stars. They consist primarily of very old stars. Globular clusters are not concentrated in the plane of the galaxy but rather are randomly distributed throughout the halo. There are several hundred globular clusters associated with our galaxy. A typical globular cluster is a few hundred light-years across. (M 13 shown; see the SEDS Messier catalog for more examples.)

OPEN CLUSTERS- loose aggregations of dozens or hundreds of young stars. They are generally not gravitationally bound and will disperse in a relatively short period of time, astronomically speaking. They are often associated with more diffuse nebulosity, as well. Also called "galactic clusters" because they are usually found in the plane of the galaxy. A typical open cluster is less than 50 light-years across. (M 44 shown; see the SEDS Messier catalog for more examples.)

EMISSION NEBULAE- clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states (in much the same way as a neon light). These nebulae are usually red because the predominant emission line of hydrogen happens to be red (other colors are produced by other atoms, but hydrogen is by far the most abundant). Emission nebulae are usually the sites of recent and ongoing star formation.

REFLECTION NEBULAE- clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.

DARK NEBULA- Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across. ( see the Horsehead Nebula)

PLANETARY NEBULA- shells of gas thrown out by some stars near the end of their lives. Our Sun will probably produce a planetary nebula in about 5 billion years. They have nothing at all to do with planets; the terminology was invented because they often look a little like planets in small telescopes. A typical planetary nebula is less than one light-year across.

SUPERNOVA REMNANTS- Supernovae occur when a massive star ends its life in an amazing blaze of glory. For a few days a supernova emits as much energy as a whole galaxy. When it's all over, a large fraction of the star is blown into space as a supernova remnant. A typical supernova remnant is at most few light-years across.

FOR MORE INFO. SEE THE SITE- http://astro.nineplanets.org/twn/types.html

FOR PHOTOS,- M 42, M 1 .

Supernova

What are Supernovae?
A Basic Definition- Supernovae are exploding stars. They represent the very final stages of evolution for some stars. Supernovae, as celestial events, are huge releases of tremendous energy, as the star ceases to exist, with about 1020 times as much energy produced in the supernova explosion as our Sun releases every second. Our Sun, fortunately, will not end its life as a supernova.
Currently, supernovae are only seen in galaxies other than the Milky Way. We know that supernovae have occurred in our Galaxy in the past, since both Tycho Brahe and his protege, Johannes Kepler, discovered bright supernovae occurring in the Milky Way in 1572 and 1604, respectively. And, the Chinese, and others, have records of a "guest star" occurring in 1054 in the present constellation Taurus. Today, we see remnants of all three supernovae, which appear as expanding clouds of gas, where each was originally discovered. However, no supernova has been seen in our Galaxy since Kepler's.
Supernovae, when they are discovered, are designated by the year in which they are discovered, and the order in which they are discovered during that year, by using members of the alphabet. For instance, the fourth supernova discovered this last year was named SN 1998D, which occurred in the galaxy NGC 5440.
The brightest supernova since Kepler's supernova was discovered on February 23, 1987, in the nearby galaxy, the Large Magellanic Cloud (LMC). This supernova was easily seen with the naked eye throughout 1987 in the Southern Hemisphere. This supernova was named SN 1987A. This supernova is still being observed by a number of telescopes, particularly, the Hubble Space Telescope. Another bright recent supernova, observable from the Northern Hemisphere, was SN 1993J in the galaxy Messier 81 (M81).
As of 1998 January 1, 1270 supernovae have been discovered since supernovae first really began to be catalogued in 1885, when a supernova went off in the nearby Andromeda galaxy.

How Astronomers Study Supernovae- When astronomers observe supernovae, they do so today using telescopes working at various wavelengths. With optical telescopes, with which most of us are familiar, astronomers measure the amount of light being emitted by a supernova, as seen from Earth, usually through a number of light filters. From these measurements, they can determine how the luminosity, or brightness, and color of a supernova evolves, or, varies with time. Supernovae generally brighten to a maximum brightness, then decline slowly in brightness over many weeks or months.
Astronomers also pass the light through a device, like a prism, which breaks the light from the supernova into its component colors. This is known as a spectrum. A spectrum shows how the brightness of light depends on the wavelength of light. Light is not equally bright at all wavelengths for supernovae. In fact, the spectra of supernovae vary over many weeks or months, as well.
Both the "light curves," as they are known, and the spectra of supernovae tell astronomers about the physics that is occurring during, and after, the explosion. It is the nature of the explosion that is vitally important in understanding supernovae and learning which stars in galaxies blow up. Supernovae are responsible for the production of many of the chemical elements in Nature, and astronomers can study how these elements are produced, as well as estimating the amount of energy liberated in the explosion and its effects on the star.

Types of Supernovae- The appearance of the spectrum allows astronomers to classify supernovae into two main types: Type I and Type II. Basically, supernovae arise from two very different classes of stars: massive ones and old, non-massive ones. The Type II supernovae very strongly show the presence of the element hydrogen in their spectra. Type I supernovae do not show any hydrogen in their spectra. The astronomer Rudolf Minkowski discovered this distinction in 1941, and this classification scheme was used for about five decades. It was thought that Type II supernovae are the explosions of massive stars, whereas Type I supernovae arise from old, low-mass stars.
In about 1985, things got a little more complicated. Some Type I supernovae discovered and studied in the early 1980s appeared to be peculiar in nature. They did not exhibit a characteristic spectral signature, thought to be due to the presence of silicon, seen in many other Type I supernovae spectra. Additionally, a few of these peculiar supernovae showed very strongly the presence of helium. Furthermore, these supernovae appeared to be occurring among populations of massive stars in galaxies. For these reasons, it was realized that Type I supernovae can be further subclassified into those with the silicon spectral feature, and these were called Type Ia supernovae, and those that do not show this feature; this latter group were called Type Ib supernovae.
Making affairs even more complicated (whew!), not all of the Type Ib supernovae since 1985 have showed the presence of helium in their spectra. These first cousins of Type Ib supernovae are today called Type Ic supernovae. More and more, supernova researchers have realized that the Type Ib/Type Ic distinction involves splitting hairs, and so, many such supernova pundits put both of these Type I subtypes into one main category: Type Ibc.

Where Supernovae Occur- Supernovae are seen to occur in galaxies all over the Universe. Galaxies are basically classified into three major groups: spirals, ellipticals, and irregulars. Now, Type II and Type Ibc supernovae are seen to occur only in spiral and irregular galaxies, and these supernovae also tend to be discovered in regions of these galaxies where star formation, particularly the formation of massive stars, most certainly has recently occurred in the last 10 million years or so. These supernovae have not been seen in elliptical galaxies. It is therefore thought that these supernovae arise from the explosions of massive stars in galaxies.
Type Ia supernovae are discovered in all three types of galaxies. But, Type Ia supernovae are generally not found near massive star formation. Since very little, if any, star formation occurs today in elliptical galaxies, it is thought that Type Ia supernovae arise from older, less massive stars.

Theories About Supernovae- In conjunction with this "environmental" evidence for the nature of supernovae, astronomers, who develop physical theories to explain celestial phenomena, and are therefore generally called theorists by their colleagues (as opposed to the other group of astronomers, who are usually, more purely, observers), develop theoretical models to explain supernova explosions. Today, these models involve sophisticated and complex computer simulations of the explosions. What the theorists tend to find is that stars more massive than about 8 solar masses, or, in other words, 8 times the mass of our Sun, become Type II and Type Ibc supernovae. These are young, relatively massive stars, which form in spiral and irregular galaxies. They also find that the Type Ia supernovae can best be explained by the explosion of somewhat exotic low-mass stars known as white dwarfs.
Stellar evolution is the study of how stars evolve and change, both internally and externally, throughout their lives. Stars generate their own energy during their lives by the process of nuclear fusion. The nuclei of lighter elements, such as hydrogen and helium, are forced to fuse, or combine, under the tremendous pressures and temperatures at or near the centers of stars, into the nuclei of heavier elements. (The nucleus of an atom is the central body which generally contains protons and neutrons; for atoms and ions, electrons orbit the central nucleus. At the temperatures and pressures within stars, electrons are totally ripped free from the nuclei.)
As Albert Einstein discovered, in his famous mass-energy equivalence principle that everyone knows (but not nearly as many understand), E=mc2, energy can be produced in large quantities from matter. When nuclear reactions occur inside stars, these reactions liberate huge amounts of energy, which inevitably trickles out from the star's interior to its surface, resulting in the light we see from the stars, their starshine.
In massive stars, those more massive than about 8 solar masses, the sequence of nuclear fusion progresses from the very simplest reaction of hydrogen nuclei to form helium nuclei, to more complicated reactions, involving the synthesis, as it is known, of silicon nuclei into iron nuclei. The iron nucleus is the most stable nucleus in Nature, and it resists fusing into any heavier nuclei, unless it is forced to do so with the input of truly formidable amounts of energy. As a result, when the central core, as it is known, of a star becomes pure iron nuclei, the core, which is generally the site of most of a star's energy production, is no longer able to produce energy and therefore support the star. The core can no longer support the crushing force of gravity, resulting from all of the matter above the core, and the core therefore collapses under its own weight.
Some really exotic physics takes place during this core collapse. But, basically, only neutrons can generally survive the collapse, and when the neutrons act together under truly unimaginable crushing pressure to resist the collapse, the core becomes what is known as a neutron star. The core then becomes stable, but the rest of the massive star is left in limbo. The core collapse suddenly stops, and the core, like a squeezed sponge, bounces back, releasing a huge amount of energy, which rips through the outer layers of the star. The original massive star dies in a fiery explosion, with only the newly-formed neutron star surviving this huge explosion.
The star has ended its life as a Type II or Type Ibc supernova. And the death throes of this star occur extremely rapidly, over only a time of several milliseconds! This, compared to a star that, up to that point, had existed for several million years!
If the star began its life with a really large amount of mass, the theorists say that not even the neutrons at the star's core can hold back the crushing force of gravity. At that point, as the star ends its life, the core becomes a black hole. Possibly, the result of the formation of the black hole is a supernova explosion, but some questions remain if this is really the chain of events for such very massive stars.
Now, this sort of evolution will not occur for the Sun. The Sun will continue to very quietly fuse its central hydrogen into helium for the next five billion years or so. The core will become pure helium, which will then fuse to carbon in a relatively short time. Finally, the carbon at the core cannot get hot enough to fuse into other type of nucleus. The carbon core can no longer sustain the Sun's energy and collapses under its own weight, much as the more complex cores of massive stars do. However, electrons in the core act to resist the collapse, and the core of the Sun will become what is known as a white dwarf. As the formation of the central white dwarf occurs, the outer layers of the Sun will be sloughed off into space to form a planetary nebula. As the nebula disperses over many thousands of years, the skeletal white dwarf remnant of the previous Sun will sit in the Galaxy and glow away its residual heat over many billions of years.
White dwarfs, as you may suspect, are not very massive, since one will form from the core of the Sun, which today contains, by definition, one solar mass. In 1938 the Indian astronomer, S. Chandrasekhar, determined that white dwarfs cannot be more massive in the Universe than 1.4 solar masses. If a white dwarf were to exceed this limit, called the Chandrasekhar limit (in his honor), the star would cease to exist. So, if a white dwarf finds itself in a binary star system, where the two stars are close enough that their mutual gravity results in their interaction, then the binary companion may dump matter onto the white dwarf. The white dwarf's mass slowly and steadily increases, to the point that it may exceed the Chandrasekhar limit. If this happens, then... poof! The white dwarf explodes in a Type Ia supernova and is completely destroyed. The matter that once was the white dwarf gets incinerated into radioactive elements, which decay over time, and continue to power the light curve of the supernova.

The Effects of Supernovae- When supernovae explode, they have profound effects on their surroundings in galaxies. The tremendous energy that is liberated affects the gas in its environment, pushing on it and compressing it. If the gas was originally fairly dense, then the compressed denser gas can actually go on to collapse and form new stars. The energy of the explosion also synthesizes new elements, particularly those heavier than iron. These fresh, new elements are then sprinkled into the surrounding gaseous medium, enriching it. Therefore, later generations of stars formed after the supernova contain more heavy elements than previous generations. In fact, the enrichment of the gas in our region of the Milky Way reached such a point that a sufficient quantity of heavy elements existed to give rise to life, as we know it, here on Earth. Supernovae are thought to be directly responsible for us all!
Supernovae also likely through small atomic and subatomic particles out into the galaxies, which we call cosmic rays. These particles, moving through the Milky Way Galaxy, pass through space and impinge on the Earth; it is thought that these high-speed, high-energy cosmic rays might be partially responsible for genetic mutation and, therefore, evolution of life here on Earth.

Supernovae Tells Us About the Fate of the Universe- Supernovae, particularly Type Ia supernovae, are intrinsically very bright, among the brightest objects in the Universe. As such, they can serve as beacons of light that can act as signposts indicating distances within space. Currently, astronomers are actively exploiting this fact about Type Ia supernovae, to measure the distances to very remote galaxies. It is thought that by determining these distances fairly accurately, and combining that information with the speeds at which the host galaxies are receding from us, due to the expansion of the Universe, originally studied most intently by Edwin Hubble, we can determine how much matter there is in the Universe, and, therefore, the Universe's ultimate fate. That's because, according to Einstein's theory of general relativity, the total amount of matter in the Universe determines what geometrical shape the Universe has. According to Einstein, matter curves the space and time around it. All of the matter in the Universe, of course, curves the entire Universe. The more matter, the more the curvature. The more curved the Universe, the more likely it is that the current expansion, resulting from the original Big Bang, will halt, due to the force of gravity, and the Universe will collapse back on itself in a Big Crunch. Alternatively, if there's not enough matter to cause a Big Crunch, then the Universe will expand forever, with essentially no end.
Astronomers are locating these supernovae by observing distant galaxies over and over. Quite often, they find bright, new objects appearing on their images. By taking the spectra of and producing light curves for enough distant supernovae, the astronomers can place constraints on the value of the mass of the Universe, and therefore determine whether it will collapse on itself or expand forever. Currently, new results seem to indicate that the amount of matter in the Universe is not enough to halt the expansion. But more results need to be obtained to verify these findings. The ultimate fate of the Universe is a profound question that humans have tried to answer. For creatures so used to beginnings and endings, having something last forever boggles the imagination. But, then, we're talking about the Universe.

SOME FACTS ABOUT SUPERNOVAE-

• A supernova (plural supernovae) is the final, gigantic explosion of a supergiant star at the end of its life.
• A supernova lasts for just a week or so, but shines as bright as a galaxy of 100 billion ordinary stars.
• Supernovae happen when a supergiant star uses up its hydrogen and helium fuel and shrinks, boosting pressure in its core enough to fuse heavy elements such as iron (see nuclear energy).
• When iron begins to fuse in its core, a star collapses instantly — then rebounds in a mighty explosion.
• Seen in 1987, supernova 1987A was the first viewed with the naked eye since Kepler's 1604 sighting.
• Supernova remnants (leftovers) are the gigantic, cloudy shells of material swelling out from supernovae.
• A supernova seen by Chinese astronomers in AD 184 was thought to be such a bad omen that it sparked off a palace revolution.
• A dramatic supernova was seen by Chinese astronomers in AD 1054 and left the Crab nebula.
• Elements heavier than iron were made in supernovae.
• Many of the elements that make up your body were forged in supernovae.

The Sun

THE SUN is the star of our solar system. All the planets rotates around it. It is small then any star. Sun is ending approximately in 100 years. Today's sun and 100 years later's sun is given in photo above. Let read the High Tech info about The Sun-

The Sun is the most prominent feature in our solar system. It is the largest object and contains approximately 98% of the total solar system mass. One hundred and nine Earths would be required to fit across the Sun's disk, and its interior could hold over 1.3 million Earths. The Sun's outer visible layer is called the photosphere and has a temperature of 6,000°C (11,000°F). This layer has a mottled appearance due to the turbulent eruptions of energy at the surface.
Solar energy is created deep within the core of the Sun. It is here that the temperature (15,000,000° C; 27,000,000° F) and pressure (340 billion times Earth's air pressure at sea level) is so intense that nuclear reactions take place. This reaction causes four protons or hydrogen nuclei to fuse together to form one alpha particle or helium nucleus. The alpha particle is about .7 percent less massive than the four protons. The difference in mass is expelled as energy and is carried to the surface of the Sun, through a process known as convection, where it is released as light and heat. Energy generated in the Sun's core takes a million years to reach its surface. Every second 700 million tons of hydrogen are converted into helium ashes. In the process 5 million tons of pure energy is released; therefore, as time goes on the Sun is becoming lighter.

The chromosphere is above the photosphere. Solar energy passes through this region on its way out from the center of the Sun. Faculae and flares arise in the chromosphere. Faculae are bright luminous hydrogen clouds which form above regions where sunspots are about to form. Flares are bright filaments of hot gas emerging from sunspot regions. Sunspots are dark depressions on the photosphere with a typical temperature of 4,000°C (7,000°F).
The corona is the outer part of the Sun's atmosphere. It is in this region that prominences appears. Prominences are immense clouds of glowing gas that erupt from the upper chromosphere. The outer region of the corona stretches far into space and consists of particles traveling slowly away from the Sun. The corona can only be seen during total solar eclipses.
The Sun appears to have been active for 4.6 billion years and has enough fuel to go on for another five billion years or so. At the end of its life, the Sun will start to fuse helium into heavier elements and begin to swell up, ultimately growing so large that it will swallow the Earth. After a billion years as a red giant, it will suddenly collapse into a white dwarf -- the final end product of a star like ours. It may take a trillion years to cool off completely.
Sun Statistics
Mass (kg)
1.989e+30
Mass (Earth = 1)
332,830
Equatorial radius (km)
695,000
Equatorial radius (Earth = 1)
108.97
Mean density (gm/cm^3)
1.410
Rotational period (days)
25-36*
Escape velocity (km/sec)
618.02
Luminosity (ergs/sec)
3.827e33
Magnitude (Vo)
-26.8
Mean surface temperature
6,000°C
Age (billion years)
4.5
Principal chemistry
HydrogenHeliumOxygen CarbonNitrogenNeonIronSiliconMagnesiumSulfurAll others
92.1%7.8%0.061%0.030%0.0084%0.0076%0.0037%0.0031%0.0024%0.0015%0.0015%
* The Sun's period of rotation at the surface varies from approximately 25 days at the equator to 36 days at the poles. Deep down, below the convective zone, everything appears to rotate with a period of 27 days.

Black Holes

Black Holes

Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a " singularity ". As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole.
But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun was suddenly replaced with a black hole of the same mass, the Earth's orbit around the Sun would be unchanged. (Of course the Earth's temperature would change, and there would be no solar wind or solar magnetic storms affecting us.) To be "sucked" into a black hole, one has to cross inside the Schwarzschild radius. At this radius, the escape speed is equal to the speed of light, and once light passes through, even it cannot escape.
The Schwarzschild radius can be calculated using the equation for escape speed:
vesc = (2GM/R)1/2For photons, or objects with no mass, we can substitute c (the speed of light) for Vesc and find the Schwarzschild radius, R, to be
R = 2GM/c2
If the Sun was replaced with a black hole that had the same mass as the Sun, the Schwarzschild radius would be 3 km (compared to the Sun's radius of nearly 700,000 km). Hence the Earth would have to get very close to get sucked into a black hole at the center of our Solar System.
If We Can't See Them, How Do We Know They're There?
Since stellar black holes are small (only a few to a few tens of kilometers in size), and light that would allow us to see them cannot escape, a black hole floating alone in space would be hard, if not impossible, to see. For instance, the photograph above shows the optical companion star to the (invisible) black hole candidate Cyg X-1.
However, if a black hole passes through a cloud of interstellar matter, or is close to another "normal" star, the black hole can accrete matter into itself. As the matter falls or is pulled towards the black hole, it gains kinetic energy, heats up and is squeezed by tidal forces. The heating ionizes the atoms, and when the atoms reach a few million Kelvin, they emit X-rays. The X-rays are sent off into space before the matter crosses the Schwarzschild radius and crashes into the singularity. Thus we can see this X-ray emission.
Binary X-ray sources are also places to find strong black hole candidates. A companion star is a perfect source of infalling material for a black hole. A binary system also allows the calculation of the black hole candidate's mass. Once the mass is found, it can be determined if the candidate is a neutron star or a black hole, since neutron stars always have masses of about 1.5 times the mass of the Sun. Another sign of the presence of a black hole is its random variation of emitted X-rays. The infalling matter that emits X-rays does not fall into the black hole at a steady rate, but rather more sporadically, which causes an observable variation in X-ray intensity. Additionally, if the X-ray source is in a binary system, and we see it from certain angles, the X-rays will be periodically cut off as the source is eclipsed by the companion star. When looking for black hole candidates, all these things are taken into account. Many X-ray satellites have scanned the skies for X-ray sources that might be black hole candidates.
Cygnus X-1 (Cyg X-1) is the longest known of the black hole candidates. It is a highly variable and irregular source, with X-ray emission that flickers in hundredths of a second. An object cannot flicker faster than the time required for light to travel across the object. In a hundredth of a second, light travels 3,000 kilometers. This is one fourth of Earth's diameter! So the region emitting the X-rays around Cyg X-1 is rather small. Its companion star, HDE 226868 is a B0 supergiant with a surface temperature of about 31,000 K. Spectroscopic observations show that the spectral lines of HDE 226868 shift back and forth with a period of 5.6 days. From the mass-luminosity relation, the mass of this supergiant is calculated as 30 times the mass of the Sun. Cyg X-1 must have a mass of about 7 solar masses, or else it would not exert enough gravitational pull to cause the wobble in the spectral lines of HDE 226868. Since 7 solar masses is too large to be a white dwarf or neutron star, it must be a black hole.
However, there are arguments against Cyg X-1 being a black hole. HDE 226868 might be undermassive for its spectral type, which would make Cyg X-1 less massive than previously calculated. In addition, uncertainties in the distance to the binary system would also influence mass calculations. All of these uncertainties can make a case for Cyg X-1 having only 3 solar masses, thus allowing for the possibility that it is a neutron star.
Nonetheless, there are now about 20 binaries (as of early 2009) for which the evidence for a black hole is much stronger than in Cyg X-1. The first of these, an X-ray transient called A0620-00, was discovered in 1975, and the mass of the compact object was determined in the mid-1980's to be greater than 3.5 solar masses. This very clearly excludes a neutron star, which has a mass near 1.5 solar masses, even allowing for all known theoretical uncertainties. The best case for a black hole is probably V404 Cygni, whose compact star is at least 10 solar masses. With improved instrumentation, the pace of discovery has accelerated, and the list of dynamically confirmed black hole binaries is growing rapidly.

What About All the Wormhole Stuff?
Unfortunately, wormholes are more science fiction than they are science fact. A wormhole is a theoretical opening in space-time that one could use to travel to far away places very quickly. The wormhole itself is two copies of the black hole geometry connected by a throat. The throat, or passageway, is called an Einstein-Rosen bridge. It has never been proven that wormholes exist, and there is no experimental evidence for them, but it is fun to think about the possibilities their existence might create.

Stars

What is a Star?
The Pleiades is a cluster of young stars. A star is a ball of gas held together by its own gravity. The force of gravity is continually trying to cause the star to collapse. This is counteracted by the pressure of hot gas and/or radiation in the star's interior. This is called hydrostatic support. During most of the lifetime of a star, the interior heat and radiation is provided by nuclear reactions near the center; this is phase of the star's life is called the main sequence. Before and after the main sequence, the heat sources differ slightly: before the main sequence the star is contracting, and is not yet hot nor dense enough in the interior for the nuclear reactions to begin. During this phase, hydrostatic support is provided by the heat generated during contraction; after the main sequence, most of the nuclear fuel in the center has been used up. The star now requires a series of less efficient nuclear reactions for internal heat, before finally collapsing when these no longer generate sufficient heat to support the star against its own gravity.
The Main SequenceThe properties of a main sequence star can be understood by considering the various physical processes acting in the interior. First is the hydrostatic balance, also called hydrostatic equilibrium. This determines the density structure of the star as the internal pressure gradient balances against the force of gravity. Another way of thinking about this is to imagine the star as a large number of nested thin spherical shells (sort of like an onion). The inward forces on each shell consist of the gravitational pull from all the shells inside it, and the gas and radiation pressure on the outside of the shell. The only outward force on each shell is the gas and radiation pressure on the inside of the shell; there is no gravitational force from material outside the shell (this is known as Gauss's theorem). In hydrostatic equilibrium, the inward and outward forces must balance. If they don't, the shell will either collapse or expand. The timescale for this to occur is called the 'free-fall timescale', and it is about 2000 seconds for a star like the Sun. Since we know the Sun has been more or less stable over the age of the Earth (several billion years), the hydrostatic balance must be maintained to a very high accuracy. A consequence of hydrostatic balance is that the pressure on each shell from material outside it must be less than the pressure from material inside it. This is because gravity acts only in the inward direction. Thus, the pressure in the star must decrease with increasing radius. This is an intuitively obvious result; the pressure at the center of the star is greater than it is at the surface.
Diagram of a Solar-type Star The second physical process to consider is the transport of energy from the interior of the star to the edge. The interior of the star (that is, near the center) is heated by nuclear reactions, while at the surface of the star electromagnetic radiation can escape essentially freely into space. This situation is analogous to a pot of water on a stove, in which heat is deposited at the bottom by the stove burner, and is transported upward through the water to the surface where it can escape. The rate at which the water on the stove can transport the heat determines the temperature; a lid on the pot will cause the temperature in the water to be higher than it would be with no lid, since heat is impeded from escaping the pot. In the case of a star, the temperature of the gas determines the density structure via the hydrostatic equilibrium condition, so understanding the transport is important. The transport can occur by either of two mechanisms: either the energy is carried by radiation, or it is carried by convection. Radiation is the mechanism by which the Earth receives heat from the Sun, and its efficiency depends on the opacity of the material that the radiation must traverse. Opacity is a measure of the transparency of a gas, and it depends on the gas temperature, density, and elemental composition in a complicated way. Convection is analogous to the turbulent motion in a pot of water as it boils. It involves motion of the fluid in the pot (or the interior of the star) which transports heat. The operation of convection depends on how easily the gas can move, i.e. its viscosity and any forces (such as gravity) which tend to resist the convective motion. In addition, convection can only operate if it transports more heat than radiation. This turns out to be important! When the opacity is high (and radiation is inefficient), convection takes over. The details of the efficiency of convection are not well understood, and they are probably the major source of uncertainty in the study of stellar structure and evolution. A third energy transport mechanism, conduction, is relatively unimportant in stellar interiors.
Main sequence stars have zones (in radius) which are convective, and zones which are radiative, and the location of these zones depends on the behavior of the opacity, in addition to the other properties of the star. Massive stars (i.e., greater than several solar masses) are convective deep in their cores, and are radiative in their outer layers. Low mass stars (i.e., mass comparable to the Sun and below) are convective in their outer layers and radiative in their cores. Intermediate mass stars (spectral type A) may be radiative throughout. Convection is likely to be important in determining other properties of the star. The existence of a hot corona may be associated with active convection in the outer layers, and the depth of the convective layer determines the extent to which material from the deep interior of the star is mixed into the outer layers. Since interior material is likely to have undergone nuclear reactions, which change the elemental abundances, this mixing affects the abundances in the star's atmosphere. These can be observed by studying stellar spectra. They may also be ejected from the star in a stellar wind, and so affect the composition of interstellar gas.
The final ingredient in determining the structure of a main sequence star is the source of heat in the interior, nuclear reactions. There are many of these, and the details are complicated and there is still some uncertainty about the exact rates for the reactions (for example, the solar neutrino problem). The basic reactions which operate on the main sequence are fusion reactions which convert hydrogen nuclei (protons) into helium nuclei. These reactions require very high temperatures (greater than 10 million degrees) and densities (greater than 10,000 gm per cubic centimeter), and the rates are very sensitive functions of temperature and density. This is the factor which ultimately determines the lifetime of a main sequence star. More massive stars have greater central temperatures and densities and so exhaust their nuclear fuel more rapidly (in spite of the fact that they have more of it) than do lower mass stars. It turns out that the main sequence lifetime is a sensitive function of mass. For a star like the Sun the main-sequence stage lasts about 10,000,000,000 years, whereas a star 10 times as massive will be 1,000 to 10,000 times as bright but will only last about 20,000,000 years. A star one tenth of the Sun's mass may only be 1/1,000th to 1/10,000th of its brightness, but will last about 1,000,000,000,000 years.
It is interesting to consider what would happen to the star if the nuclear reactions were to suddenly turn off. The timescale required for the energy from a photon released at the center of the star to make its way to the surface is approximately 1,000,000 years for the Sun. Along the way, the original gamma-ray photon interacts with the gas in the Sun and loses energy. Through multiple interactions like this, this energy "random walks" its way out of the Sun, ultimately being emitted at the surface as many UV and optical photons. Thus, if the nuclear reactions were to turn off today, the Sun's luminosity would stay approximately constant for a long time by human standards. We do have historical records which tell us that the Sun's output has been approximately constant over the course of written human history, so we feel fairly confident that the nuclear reactions are still operating. However, there is the possibility that nuclear energy generation in the center of the Sun is not perfectly constant in time.
The three physical processes discussed so far, hydrostatic equilibrium, radiation transport, and nuclear energy generation, serve to determine the structure of a star. As with most things, the devil is in the details, and the areas of greatest uncertainty are the behavior of opacity and convection. These are active areas of scientific research.
A convenient way to characterize a star from observations is by its luminosity and its color (or temperature). It is customary to plot these two quantities in an x-y plot, called a Hertzsprung-Russell diagram (after its inventors). It turns out that when this is done for main sequence stars with a range of masses, the points tend to occupy a narrow band in the diagram. The location of a main sequence star in the diagram depends only on its mass (see Figure below).
Stellar EvolutionThe mass of the star determines what happens after the main sequence phase. Stars similar in mass to the Sun burn hydrogen into helium in their centers during the main-sequence phase, but eventually there is not enough hydrogen left in the center to provide the necessary radiation pressure to balance gravity. The center of the star thus contracts until it is hot enough for helium to be converted into carbon. The hydrogen in a shell continues to burn into helium, but the outer layers of the star have to expand in order to conserve energy. This makes the star appear brighter and cooler, and it becomes a red giant. During the red giant phase, a star often loses a lot of its outer layers which are blown away by the radiation coming from below. Eventually, in the more massive stars of the group, the carbon may burn to even heavier elements, but eventually the energy generation will fizzle out and the star will collapse to a white dwarf. Astronomers think that white dwarfs ultimately cool to become black dwarfs.
Stars having masses between 0.08 and 0.4 times that of the Sun can have main sequence lifetimes greater than the age (so far) of the Universe. These are known as red dwarfs, and are quite plentiful in the Universe.
There are very few stars with masses greater than five times the mass of the Sun, but their evolution ends in a spectacular fashion. They finish their main sequence lifetime in a way similar to the lower mass stars, but become brighter and cooler on the outside and are called red supergiants. Carbon burning can develop at the star's center and a complex set of element-burning shells can develop towards the end of the star's life. During this stage, many different chemical elements will be produced in the star and the central temperature will approach temperatures between 100,000,000 K and about 600,000,000 K. During this stage, the structure can resemble an onion skin with progressive layers (going inward) dominated by elements with greater and greater atomic mass. This process ends when the core is composed primarily of iron. For all the elements up to iron, the addition of more nucleons to the nucleus produces energy and so yields a small contribution to the balance inside the star between gravity and radiation. To add more nucleons to the iron nucleus requires an input of energy, and so, once the center of the star consists of iron, no more energy can be extracted. The star's core then has no resistance to the force of gravity, and once it starts to contract a very rapid collapse will take place. The protons and electrons combine to give a core composed of neutrons and a vast amount of gravitational energy is released. This energy is sufficient to blow away all the outer parts of the star in a violent explosion and the star becomes a supernova. The light of this one star at its peak during the explosion is then about as bright as that from all the other 100,000,000,000 stars in the host galaxy. During this explosive phase, all the elements with atomic weights greater than iron are formed and, together with the rest of the outer regions of the star, are blown out into interstellar space. The central core of neutrons is left as a neutron star, which could be a pulsar. This is remarkable since in the early Universe there were no elements heavier than helium. The first stars were composed almost entirely of hydrogen and helium and there was no oxygen, nitrogen, iron, or any of the other elements that are necessary for life. These were all produced inside massive stars and were all spread throughout space by such supernovae events. We are made up of material that has been processed at least once inside stars.

Galaxy

Galaxies have planets, stars, comets, solar systems, meteors, asteroids and aliens also. Our Galaxy is named as MILKY WAY or 'Aakash Ganga'.

GALAXIES are many types, are as follow-
Galaxies are large systems of stars and interstellar matter, typically containing several million to some trillion stars, of masses between several million and several trillion times that of our Sun, of an extension of a few thousands to several 100,000s light years, typically separated by millions of light years distance. They come in a variety of flavors: Spiral, lenticular, elliptical and irregular. Besides simple stars, they typically contain various types of star clusters and nebulae.
We live in a giant spiral galaxy, the Milky Way Galaxy, of 100,000 light years diameter and a mass of roughly a trillion solar masses; our Sun is one of several 100 billions of stars of the Milky Way. The nearest dwarf galaxies, satellites of the Milky Way, are only a few 100,000 light years distant (and closer in case of some dwarfs which are currently merged with the Milky Way), while the nearest giant neighbor, the Andromeda Galaxy (M31), also a spiral, is about 2-3 million light years distant.
Spiral Spiral galaxies usually consist of two major components: A flat, large disk which often contains a lot of interstellar matter (visible sometimes as reddish diffuse emission nebulae, or as dark dust clouds) and young (open) star clusters and associations, which have emerged from them (recognizable from the blueish light of their hottest, short-living, most massive stars), often arranged in conspicuous and striking spiral patterns and/or bar structures, and an ellipsoidally formed bulge component, consisting of an old stellar population without interstellar matter, and often associated with globular clusters. The young stars in the disk are classified as stellar population I, the old bulge stars as population II. The luminosity and mass relation of these components seem to vary in a wide range, giving rise to a classification scheme. The pattern structures in the disk are most probably transient phenomena only, caused by gravitational interaction with neighboring galaxies.
Our sun is one of several 100 billion stars in a spiral galaxy, the Milky Way.
Lenticular (S0) These are, in short, "spiral galaxies without spiral structure", i.e. smooth disk galaxies, where stellar formation has stopped long ago, because the interstellar matter was used up. Therefore, they consist of old population II stars only, or at least chiefly. From their appearance and stellar contents, they can often hardly be distinguished from ellipticals observationally.
Elliptical Elliptical galaxies are actually of ellipsoidal shape, and it is now quite safe from observation that they are usually triaxial (cosmic footballs, as Paul Murdin, David Allen, and David Malin put it). They have little or no global angular momentum, i.e. do not rotate as a whole (of course, the stars still orbit the centers of these galaxies, but the orbits are statistically oriented so that only little net orbital angular momentum sums up). Normally, elliptical galaxies contain very little or no interstellar matter, and consist of old population II stars only: They appear like luminous bulges of spirals, without a disk component.
However, for some ellipticals, small disk components have been discovered, so that they may be representatives of one end of a common scheme of galaxy forms which includes the disk galaxies.
Irregular Often due to distortion by the gravitation of their intergalactic neighbors, these galaxies do not fit well into the scheme of disks and ellipsoids, but exhibit peculiar shapes. A subclass of distorted disks is however frequently occuring.
The first known galaxies were longly known before their nature as "island universes" came to light - this fact was finally proven only in 1923 by Edwin Powell Hubble, when he found Cepheid variable stars in the Andromeda Galaxy M31. Ancient observers have known the Milky Way and - on the Southern Hemisphere - the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) since prehistoric times. There are speculations that also the Andromeda Galaxy M31 may have been observed and recorded as a nebulous patch by anonymous Babylonian observers around 1,300 B.C.. This object was certainly known to medevial Persian astronomers before 905 A.D., and cataloged and described by Persian astronomer Al Sufi in 964 A.D, who also describes the LMC. Both LMC and SMC have become known by the reports of Vespucci and Magellan in the early 16th century. All other galaxies have been discovered only after the invention of the telescope: The Triangulum Galaxy M33 was first seen by Italian Priest astronomer G.B. Hodierna before 1654. Next, French astronomer Legentil discovered M32, a companion of the Andromeda Galaxy, in 1749, and his compatriot Nicholas Louis de Lacaille found M83 in 1752, the first galaxy beyond the Local Group to be discovered. These six were all external galaxies to be known, before Charles Messier started to survey the sky for comets and "nebulae." His first original discovery of a galaxy, M49, a giant elliptical member of the Virgo Cluster, occurred in 1771. The Messier Catalog in his modern form contains 40 galaxies, all but the two Magellanic Clouds that have been found up to 1782. Starting in 1783, William Herschel found and cataloged over 2,500 star clusters and "nebulae" up to 1802, 2,143 of them actually galaxies. J.L.E. Dreyer's NGC catalog contains 6,029 (about 75.9%), and his IC catalog another 3,971 galaxies (about 73.7%).
Today's modern catalogs contain far larger numbers; millions of galaxies have been cataloged, and it was estimated that the observable part of the universe contains probably hundreds of billions (10^11) galaxies. For example, at the time of this writing (2009), the Sloan Digital Sky Survey project has scanned more than about 1/4 of the sky, and determined properties of more than one million of galaxies.
From their appearance, galaxies are classified in types as given above, as spiral, lenticular, elliptical, and irregular galaxies, where spirals may be further classified for the presence of a bar (S: spirals, SAB: Intermediate, SB: Barred spirals). More precisely, ellipticals are sub-classified for ellipticity from E7 (strongly elongated) to E0 (circular), and spirals for prominence of bulge versus spiral arms from Sa (or SABa, SBa) to Sc or Sd. This so-called Hubble Classification Scheme can well be illustrated by Messier's galaxies:
Hubble Scheme with Messier galaxies for HTML browser supporting tables (e.g., Netscape, IBM Webexplorer)
Hubble Scheme for HTML browsers not supporting tables (Lynx, Mosaic) Historically, according to obsolete models of "nebula" evolution, the terms of "Early" and "Late" types of galaxies have been introduced and are still sometimes used; "Early" types include elliptical ("E") and lenticular ("S0") galaxies as well as Sa spirals, while "later" Sb spirals are called "intermediate," and "Sc" and "Sd" spirals as well as irregulars are called "late."
Galaxies of all types, though of a wide variety of shapes and appearances, have many basic common features. They are huge agglomerations of stars like our Sun, counting several millions to several trillions. Most of the stars are not lonely in space like our Sun, but occur in pairs (binaries) or multiple systems.
The most massive galaxies are giants which are a million times more massive than the lightest: Their mass range is from at most some million times that of our Sun in case of the smallest dwarfs, to several trillion solar masses in case of giants like M87 or M77. Accordingly, the number of stars in them varies in the same range.
The linear size of galaxies also scatters, ranging from small dwarfs of few thousands of light years diameter (like M32) to respectable several 100,000 light years. Among the biggest Messier galaxies are the Andromeda galaxy M31 and the bright active Seyfert II galaxy M77.
Our Milky Way Galaxy, a spiral galaxy, is among the massive and big galaxies with at least 250 billion solar masses (there are hints that the total mass may even be as large as 750 billion to 1 trillion times that of the Sun) and a disk diameter of 100,000 light years.
Besides very many individual stars, most galaxies contain the following typical objects:
Globular star clusters, large but quite compact agglomerations of some 100,000 to several million stars. These large clusters have about the same mass as the smallest galaxies, and are among the oldest objects in galaxies. Often, they form conspicuous systems, and occur at galaxies of every type and size. The globular cluster systems vary in a wide range in richness between the individual galaxies.
As the stars develop, many of them leave nebulous remnants (planetary nebulae or supernova remnants) which then populate the galaxies.
While the older stars, including the globular clusters, tend to form an ellipsoidal bulge, the interstellar gas and dust tends to accumulate in clouds near an equatorial disk, which is often conspicuous (i.e., in spiral and lenticular galaxies).
The interstellar clouds are the places of star formation. More acurately, huge diffuse star-forming nebulae are places where crowded (open) clusters and associations of stars are formed.
A rather dense galactic nucleus, which is somewhat similar to a "superlarge" globular cluster. In many cases, galactic nuclei contain supermassive central objects, which are often considered as Black Hole candidates. Some of the more massive and conspicuous globulars are suspected to be the remnants of former nuclei of small galaxies which have been disrupted and cannibalized by larger galaxies. Galaxies normally emit light of every wavelength, from the long radio and microwave end over the IR, visual and UV light to the short, high-enregy X- and gamma rays. Interstellar matter is coolest and therefore best visible in radio and IR, while supernova remnants are most conspicuous in the high-energy part of the electromagnetic spectrum. Galaxies with high star formation activity, like M82, are brightest in the infrared; at wide ranges of infrared wavelengths, considerably small M82 is the brightest galaxy in the sky. At radio wavelengths, where M82 is also considerably bright, giant galaxies M87 and M77 are among the most conspicuous.
Some galactic nuclei are remarkably distinguished from the average: These so-called Active Galactic Nuclei (AGNs) are intensive sources of light of all wavelengths from radio to X-rays. The activities seen in the AGNs are caused by gaseous matter falling into, and interacting with, the supermassive central objects mentioned above, according to the current consensus of most researchers. See Peterson (1997) for a semi-recent review and textbook on AGN. Sometimes, the spectra of these nuclei indicate enormous gaseous masses in rapid motion; galaxies with such a nucleus are called Seyfert galaxies (for their discoverer, Karl Seyfert; see Seyfert 1943). M77 is the brightest Seyfert galaxy in the sky. Some of the AGNs are faint or quiet, others bright or loud in the radio light; the latter are called radio galaxies; a famous radio galaxy is M87. Few galaxies have even more exotic nuclei, which are extremely compact and extremely bright, outshining their whole parent galaxy; these are called quasars (an acronym for QUAsi-StellAR objects). From their properties, quasars resemble extremely active Seyfert galaxy nuclei. However, quasars are so rare and the nearest is so remote that the brightest of them, 3C273, about 2 billion lightyears away in the constellation Virgo, is only of magnitude 13.7, and none of them is in Messier's or even in the NGC or IC catalog.
Studies have shown that about 1/3 of all galaxies show low nuclear-luminosity activity in their nucleus; this type of AGN was discovered by Heckman (1980) is called Low-Ionization Nuclear Emission-line Region (LINER); examples include the Andromeda Galaxy (M31) and M65.
Occasionally, at irregular intervals given by chance, in any type of galaxies, a supernova occurs: This is a star suddenly brightning to a high luminosity which may well outshine the whole galaxy; the maximal absolute magnitude of a supernova may well reach -19 to -20 magnitudes. This remarkable phenomenon has attracted the attention of many astronomers (equally both professionals and amateurs), who observe galaxies regularly as they "hunt" supernovae. Supernovae have been observed in several Messier catalog galaxies.
The formation and evolution of galaxies is a major issue of current research. For a long time, two different types of models of galaxy formation were common: First, "top-down" theories according to which galaxies have formed during a comparatively short period, at about the same time, within the first billion years after the universe started to expand, from an initial hot state, such as the Eggen - Lynden-Bell - Sandage (ELS) model (Eggen et.al. 1962). According to the second type of models, "bottom-up" theories, smaller structures of perheps the size of globular clusters formed first, and later coalesc or accrete to form larger galaxies, e.g. the Searle-Zinn (SZ) model (Searle, Zinn 1978).
During the last couple of years, new deep observations, in particular with the Hubble Space Telescope, have revealed evolutionary effects of galaxies on cosmological timescales: During the last 3-4 billion years, galaxies seem to be of similar types as they are observed in our neighborhood, with disk galaxies showing expressed spiral and bar structures. For times further back than about 5 billion years, barred spirals get less frequent, and spiral arms appear less developed. Back 6 billion years in time, many more interacting galaxies and mergers are observed, and the percentage of irregular systems increases rapidly. These results indicate that in the early universe, about 10-15 billion years ago, small building blocks were formed first, when primordial clouds of gaseous matter (hydrogen and helium),were singled out and started to collapse by their own gravity to form proto-galaxies. Halos of dark matter and with massive central nuclei, as well as interaction with neighboring systems, seem to play an important role in the formation and evolution of galaxies to their present state.
Messier's galaxies are not distributed equally across the sky, but can be grouped into a large group of Northern Spring/Southern Fall, and a smaller Northern Fall/Southern Spring group:
Northern Fall/Southern Spring galaxies (6):
Local Group members (4): M31 group (M31, M32, M110), and M33.
Other Northern Fall/Southern Spring galaxies (2): M74, M77.
Northern Spring/Southern Fall galaxies (34):
Virgo Cluster Galaxies (16): M49, M58, M59, M60, M61, M84, M85, M86, M87, M88, M89, M90, M91, M98, M99, M100.
Galaxies in Leo (5): M66 group (M65, M66), M96 group (M95, M96, M105).
Galaxies in and around Ursa Major (11): M81 group (M81, M82), M101, M102, M108, M109, M51 group (M51, M63), M94, M106, M64.
Southern galaxies (2): M83, M104. In the regions between, there are RA ranges without any Messier galaxies (3-8 and 16-23h in RA); these include the regions of the Milky Way band of stars and interstellar matter, which obscures the background galaxies.
Links
Radial velocities of the Messier galaxies
Nasa's Extragalactical Database (NED) (also available by telnet)
NED data of the Messier galaxies
Galaxy Informations from the University of Alabama
Galaxies and the Universe - WWW Course Notes by Bill Keel
Galaxies text and Milky Way Galaxy text (by Nick Strobel)
Galaxy Catalogs List
Arp's Catalog of Peculiar Galaxies; this catalog includes some of Messier's galaxies (see also the Arp Galaxies in Other Catalogs page of the Online Arp Catalog)
Zsolt Frei's Catalog of 113 Nearby Galaxies with images taken at different wavelengths; [also by ftp]
ARVAL Catalog of Bright Galaxies
Globular cluster systems in other galaxies: Catalog by W. E. Harris (we hold a possibly older copy); Globular Clusters in Other Galaxies (M. Kissler-Patig)
From An Atlas of the Universe:
The 200 Brightest Galaxies
Galaxies and Groups of Galaxies within 20 million Light Years
The Virgo Supercluster within 100 million Light Years
The Universe within 200 million Light Years
Look at Galaxies in Messier's Catalog
Also look at our collection of some significant non-Messier galaxies
References
O.J. Eggen, D. Lynden-Bell, and A.R. Sandage, 1962. Evidence from the motion of old stars that the Galaxy collapsed. Astrophysical Journal, Vol. 136, p. 748. [ADS: 1962ApJ...136..748E]
Timothy M. Heckman, 1980. An optical and radio survey of the nuclei of bright galaxies - Activity in normal galactic nuclei. Astronomy and Astrophysics, Vol. 87, pp. 152-164. [ADS: 1980A&A....87..152H]
Bradley M. Peterson, 1997. An introduction to active galactic nuclei. Cambridge University Press. 238+xvi pp.
L. Searle and R. Zinn, 1978. Composition of halo clusters and the formation of the galactic halo. Astrophysical Journal, Vol. 225, No. 1, pp. 357-379 [ADS: 1978ApJ...225..357S]
Carl K. Seyfert, 1943. Nuclear Emission in Spiral Nebulae. Astrophysical Journal, Vol. 97, pp. 28-40 (01/1943) [ADS: 1943ApJ....97...28S] Imagery and atlasses:
Allan Sandage. The Hubble Atlas of Galaxies. Carnegie Institution of Washington, 1961. 185 superb black & white photographs of galaxies of all types, obtained by the Mt. Palomar and Mt. Wilson Observatory telescopes, with captions and data, and a technical and scientific introduction.
James D. Wray. The Color Atlas of Galaxies. Cambridge University Press, 1988. 3-color (UBV) images of 616 galaxies (including all Messier galaxies but M89), taken with telescopes at the McDonald Observatory, Texas, and the Cerro Tololo Interamerican Observatory, Chile, with data and captions.
Timothy Ferris. Galaxies. Sierra Club Books, San Francisco, 1980. Superb book (look to get the more expensive full-size edition) with color and b/w photographs of galaxies and some other objects, from various observatories. Of course, fine galaxy photos can be found in many more general astronomy books also.
Special observing Guides:
Kenneth Glyn Jones (editor). Webb Society Deep-Sky Observer's Handbook, Volume 4, Galaxies, 1981; Volume 6, Anonymous Galaxies, 1987. Enslow Publishers, Hillside, NJ. Most general Deep Sky Observing Guides are good as well.
Textbooks:
Dimitri Mihalas and James Binney. Galactic Astronomy. W.H. Freeman, 1981 (probably out of print). Now replaced by: James Binney and Michael Merrifield. Galactic Astronomy. Princeton University Press, 1998. This is a good introduction and review especially for the observational properties of galaxies (as they were known at the time of publication).
James Binney and Scott Tremaine. Galactic Dynamics. Princeton Series in Astrophysics, Princeton University Press, 1987. In-depth treatment of the physics of galaxies. Some mathematical and physical background is required for this book.
Paul W. Hodge. Galaxies. Harvard University Press, 1986. Historical Review:
Richard Berendzen, Richard Hart, and Daniel Seeley. Man Discovers the Galaxies. Science History Publications, Neale Watson Academic Publications, New York 1976.
Galaxy ClustersSome galaxies are isolated "island universes" which float lonely through an otherwise empty region of the universe. But usually, space is too densely crowded with them, so that they form groups of some galaxies (or some dozens of galaxies), or even large clusters of up to several thousands of galaxies. The galaxies of these groups are in mutual gravitational interaction which may have significant influence on their appearance.