KNOW ABOUT GALACTIC NUCLEI

The nucleus of a galaxy is like a deep well: It is easy to fall in, but hard to get out. As a result, gas and stars accumulate there. In the 1960s, astronomers discovered that some galactic nuclei were truly remarkable: They could outshine an entire galaxy from a volume not much larger than

that of the solar system. These objects, termed quasars, are the most luminous type of active galactic nucleus. Theorists immediately conjectured that such prodigious power output could come only from the accretion of gas onto a supermassive black hole; later it was realized that energy could be extracted from the spin of the black hole as well. A consequence of these ideas is that many galaxies should harbor supermassive black holes in their nuclei.

Three decades later, this conjecture has been amply verified. Observations of both gas and stars have shown that even in our own “backyard,” the Milky Way Galaxy harbors a black hole 3 million times more massive than the Sun (Figure 2.14)—and that black hole masses in the nuclei of other galaxies can exceed a billion solar masses. Exquisitely precise measurements of the positions and three-dimensional velocities of water masers made with the Very Long Baseline Array (VLBA) toward the nucleus of the galaxy NGC 4258 provided incontrovertible evidence for the presence of a supermassive black hole (Figure 2.15). ARISE has the power to study the water emission in other galactic nuclei to search for black holes and determine their mass and the characteristics of the accreting gas.

15 Some of the data that provide strong evidence for the presence of a supermassive black hole in the center of the nearby spiral galaxy NCG 4258. The top panel is the actual image of the point-like maser clouds constructed from very long baseline interferometry (VLBI) data having a resolution of 200 microarcsec (with a wire grid depicting unseen parts of the disk). Also shown is the image of the continuum emission at 1.3-cm wavelength caused by synchrotron radiation from relativistic electrons emanating from the position of the dynamical center (black dot). The central mass required to gravitationally bind the system

is 39 million solar masses. Since all the mass must be within the inner boundary of the molecular disk of about 0.13 pc, this mass is probably in the form of a supermassive black hole. The bottom panel shows on a larger scale the synchrotron emission that arises from relativistic electrons ejected along the spin axis of the black hole. Observations with HST have confirmed that most nearby galaxies harbor supermassive black holes in their nuclei.

How do these supermassive black holes form and evolve? Do they grow from stellar “seeds” or do they originate at the very beginning of the formation of a galaxy? These key questions are ripe for a frontal attack now. Addressing them will require the observation of active galactic nuclei (AGNs) when they first turn on, over the entire electromagnetic spectrum. With its enormous sensitivity in the infrared, NGST will be able to detect AGNs out to redshifts beyond 10. Radiation emitted in the thermal infrared will be redshifted into the band detectable by SAFIR. The EVLA will detect much longer wavelength radio emission from AGNs to redshifts beyond 5. Constellation-X will be able to observe the first quasars even if they are heavily obscured by dust. EXIST will make a census of obscured, low-redshift AGNs over the whole sky; this sample can be compared with younger AGNs, seen at


high redshifts by Constellation-X, to study how the AGNs evolve. In this case the energies of the most penetrating hard x rays will be conveniently shifted by the expansion of the universe into the energy region of maximum sensitivity of the telescope. Furthermore, by observing the spectrum of hot gas as it disappears into supermassive black holes, Constellation-X will provide a laboratory for studying the physical processes occurring near the event horizons of black holes under conditions that differ substantially from those near stellar-mass black holes.

In a tremendously scaled-up version of the process of mass ejection from disks around protostars, massive black holes not only accrete material but also eject from their vicinity powerful jets at nearly the speed of light (Figure 2.16). This highly relativistic material is thought to generate extremely energetic photons, with frequencies more than 100 billion times that of visible light. VERITAS has the power to detect individual photons of this radiation interacting with Earth’s atmosphere, and can therefore probe the relativistic particle acceleration occurring near these massive black holes. Observing somewhat less energetic photons, GLAST will help determine how jets are powered and confined. ARISE has the spatial resolution to resolve the base of the jet and thereby provide a complementary probe of the acceleration region.




Galaxy mergers are inferred to be common, and it is quite possible that the massive black holes in their nuclei would merge as well. Such a cataclysmic event would produce powerful gravity waves that could be detected by LISA out to very large distances (redshifts up to at least 20). This gravitational radiation would be detectable for up to a year before the actual merger, enabling accurate prediction of the final event so that it could be observed by telescopes sensitive to the entire range of electromagnetic radiation. Observation of such a merger would provide a unique test of Einstein’s theory of general relativity in the case of strong gravitational fields. Further discussion of what scientists can learn about black holes can be found in the physics survey report Gravitational Physics: Exploring the Structure of Space and Time (NRC, 1999).

Galactic nuclei can become extremely luminous as a result of intense bursts of star formation or the presence of a supermassive black hole.

EVOLUTION OF THE INTERSTELLAR MEDIUM IN GALAXIES

The interstellar medium in a galaxy controls the rate of star formation and thus the evolution of the galaxy itself. It is the repository of the heavy elements produced in stars. If star formation becomes too violent, interstellar gas may be ejected from a galaxy into the surrounding intergalactic medium. An understanding of the interstellar medium is necessary if researchers are to address such key questions as the following: What are the physical processes that determine the rate at which stars form in a galaxy? What is the feedback between star formation and the interstellar medium? What is the effect of the extragalactic environment on star formation?

All these issues come into play when the formation of the first galaxies is considered. The first galaxies formed out of enormous clouds of neutral atomic hydrogen. Once the galaxies had formed, the interstellar media of these galaxies remained primarily atomic hydrogen, although with increasing amounts of heavier elements as massive, short-lived stars ejected new elements into the medium. The hydrogen gas should be observable at redshifts above 10 with LOFAR. When the SKA is built, it will be able to map the atomic hydrogen up to redshifts of about 10. Within galaxies, some of the atomic gas will be converted to molecular form on its way to being incorporated into stars.
If the earliest stars have ejected enough carbon and oxygen into the interstellar medium, the broad spectral capabilities of the EVLA will enable observation of carbon monoxide, the most abundant molecule after molecular hydrogen, out to redshifts beyond 10. Newly formed stars ionize some of the gas, producing emission lines detectable by NGST. Supernovae heat large volumes


This optical wavelength picture shows the large spiral galaxy M31 (also known as the Andromeda Galaxy) and its small companions M32, lower center, and M110, to the upper right. Andromeda is the Milky Way’s closest large neighbor at a distance of about 2.2 million light-years, and it is very similar in appearance to, and slightly larger than, the Milky Way. In fact, M31 is visible to the naked eye, although we can see only the bright inner bulge. This image comes from photographic plates taken with the 0.6-m Burrell Schmidt telescope of the Warner and Swasey Observatory of Case Western Reserve University. GSMT will be able to study individual stars near Andromeda’s center, which is a very tightly packed star cluster not visible in this saturated image.

of the interstellar gas to millions of degrees, and x rays from this hot gas will be measured by Constellation-X to determine the temperature, pressure, and elemental abundances in this hot plasma. These same instruments will also permit astronomers to trace the evolution of gas in galaxies through cosmic time, as the universe synthesizes the elements needed to form planets and eventually to enable life.

Structure in the interstellar medium of a galaxy spans a wide range of scales, from much less than 1 light-year for the molecular cores that produce individual stars to 100,000 light-years for the galaxy as a whole. The gaseous galactic halo extends farther; it comprises both gas blown

out of the disk and gas accreting from the intergalactic medium. Much of the mass of interstellar gas in disk galaxies is atomic and molecular gas that is quite cold, with a temperature that is less than 100 degrees above absolute zero. A substantial (but uncertain) fraction of the volume of such galaxies is filled by gas that has been heated to more than a million degrees by supernova explosions. There is also a significant amount of gas at intermediate temperatures that is heated by starlight. All this gas is permeated by cosmic rays, particles moving almost at the speed of light, and by magnetic fields. The primary hindrance to a greater understanding of how the interstellar medium mediates the evolution of galaxies is ignorance of the spatial distribution of these various components of the interstellar medium and how they are interrelated. Surveys of the interstellar medium in nearby galaxies with the recommended radio, infrared, x-ray, and gamma-ray facilities will provide valuable data on these issues. Understanding the complex structure of the interstellar medium and how it interacts with the process of star formation is a daunting theoretical problem for this decade.

Formation of Supernova Star and Neutron Star

A big explosion takes place here. Well the stars those are much heavier than the sun end up in a more catastrophic way resulting in huge discharge. This can also be explained –

When a very big star is in the red-giant phase, then being big, its core contains much more helium. This big core made up of helium continues to contract (shrink) under the action of gravity producing higher and higher temperatures. At this extremely high temperature, fusion of helium into carbon takes place in the core and a lot of energy is produced. Since the star was very big and contained enormous nuclear fuel helium, so a tremendous amount of nuclear energy is produced very rapidly which causes the outer shell (or envelope) of this red-giant star to explode with a brilliant flash like a nuclear bomb. This type of exploding star is called supernova.

The energy released in one second of a supernova explosion is equal to the energy released by the sun in about 100 years. This tremendous energy would light up the sky for many days. When a supernova explosion takes place, then clouds of gases in the envelope of red-giant star are liberated into the space and these gases act as raw material for the formation of new stars. The heavy core left behind after the supernova explosion continues to contract and ultimately becomes a neutron star.
This theory bring to a close that a supernova arises when the core of an extremely big star collapses under its own gravitational attraction, releasing tremendous amount of energy which causes the outer shell to explode. Thus, the inner part of the red-giant star undergoes an implosion, while the outer part undergoes an explosion (see Figure m).

The imploding core may form a neutron star. The stars, which are composed of matter mainly in the form of neutrons, are called neutron stars.

You must also know how a neutron star is formed. The neutron star arises from the collapsed core of a supernova. After the supernova explosion, the outer shell (or envelope) of the red-giant star explodes and only the helium core survives. This core continues to contract (shrink) under the tremendous force of gravitation and ultimately it forms an extremely dense lump of matter. This extremely dense lump of matter is called neutron star. The neutron stars have very high densities. The density of a neutron star is about a million tonnes per cubic centimeter or even higher

Evolution of a Star

Even stars have birth, life and death!! The raw material for the formation of a star is mainly hydrogen gas and some helium gas. The life cycle of a star begins with the gathering of hydrogen gas and helium gas present in the galaxies to form dense clouds of these gases. The stars are then formed by the gravitational collapse of these over-dense clouds of gases in the galaxy. The first step in the formation of a star from gases is the protostar.

Stars Do Not Collapse-

Why? Well, a star emitting heat and light energy, has two types of forces acting continuously: The gravitational attraction of the enormous gaseous matter which wants to compress the star further and the internal pressure developed in the star because of energy released during nuclear fusion reactions going inside it, tending to stop the gaseous matter from collapsing further.

As a consequence, the star is now in equilibrium under the action of two opposing forces-the gravitational force trying to compress the gases, and the internal pressure due to nuclear energy trying to stop the gaseous matter from collapsing. If internal pressures were not built up within the star, a star would have collapsed within less than half an hour.
In a mature star, the pressure forces from within the star balance the gravitational force from outside. This balance can continue for millions of years. Our sun is now in this balanced state of its development. At this stage, the temperature in the interior of the sun is just right to sustain the fusion reaction, and the rate of this fusion reaction produces just the right pressure to balance the gravitational compression.

During all this time, the nuclear fusion reactions continue to liberate energy and make the star shine. Our sun (which is also a star) was formed about 4600 million years ago and will continue to give out energy for an equal period of time. If, however, there were no internal pressure in the sun produced by the energy of nuclear fusion, then our sun would have contracted drastically (shrunk drastically) within half an hour, under the action of tremendous gravitational forces. Well, that could not have happened to the Sun!

AMAZING CONSTELLATIONS

Of star assemblies….
Thousands of stars on a clear moon-less night, some of these stars arraying in groups or patterns as though staging a celestial drama (!!) do make a poetic picture!

Since the ancient times, man has gazed at the stars and singled out identifiable shapes and patterns. One group of stars suggested the outline of a bear; another group of stars reminded them of a hunter, and so on and on. The stars, which appear in the form or gathered groups and in identifiable silhouettes and patterns, are known as constellations. Again our ancestors named these star groups or constellations after the objects, which they seemed to resemble.

The Indian name for the constellations is 'Nakshatras'. About 88 constellations (star groups) are known. Each constellation has been assigned a name signifying an animal, a human being or some other object, which it appears to bear a resemblance to. You may be interested to note some of the important constellations-

Ursa major (or Great bear) 'Saptarishi'
(ii) Ursa minor (or Little bear) 'Laghu Saptarishi' or 'Dhruva Matsaya'
(iii) Orion (or Hunter) 'Vyadha' or 'Mirga'
(iv) Scorpio 'Vrishchika'
(v) Pleides ' Kruttika'
(vi) Cassiopeia 'Sarmishtha'

And now for you it will be interesting to know about the star formation or gatherings-

Astrophysics

Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties (luminosity, density, temperature, chemical composition) of astronomical objects such as stars, galaxies, and the interstellar medium, as well as their interactions. The study of cosmology is theoretical astrophysics at its largest scale; conversely, since the energies involved in cosmology, especially the Big Bang, are the largest known, the observations of the cosmos also serve as the laboratory for physics at its smallest scales as well.



In practice, virtually all modern astronomical research involves a substantial amount of physics. The name of a school's doctoral program ("Astrophysics" or "Astronomy") in many places like the United States often has to do more with the department's history than with the contents of the programs.

History
Although astronomy is as old as recorded history, it was long separated from the study of physics. In the Aristotelian worldview, the celestial pertained to perfection—bodies in the sky being perfect spheres moving in perfectly circular orbits—while the earthly pertained to imperfection; these two realms were seen as unrelated.

For centuries, the apparently common-sense view that the Sun and other planets went round the Earth went unquestioned, until Nicolaus Copernicus suggested in the 16th century that the Earth and all the other planets in the Solar System orbited the Sun. Galileo Galilei made quantitative measurements central to physics, but in astronomy his observation didn't have astrophysical significance.

The availability of accurate observational data led to research into theoretical explanations for the observed behavior. At first, only ad-hoc rules were discovered, such as Kepler's laws of planetary motion, discovered at the start of the 17th century. Later that century, Isaac Newton, bridged the gap between Kepler's laws and Galileo's dynamics, discovering that the same laws that rule the dynamics of objects on earth rules the motion of planets and the moon. Celestial mechanics, the application of Newtonian gravity and Newton's laws to explain Kepler's laws of planetary motion, was the first unification of astronomy and physics.
After Isaac Newton published his Principia, maritime navigation was transformed. Starting around 1670, the entire world was measured using essentially modern latitude instruments and the best available clocks. The needs of navigation provided a drive for progressively more accurate astronomical observations and instruments, providing a background for ever more available data for scientists.

At the end of the 19th century it was discovered that, when decomposing the light from the Sun, a multitude of spectral lines were observed (regions where there was less or no light). Experiments with hot gases showed that the same lines could be observed in the spectra of gases, specific lines corresponding to unique chemical elements. In this way it was proved that the chemical elements found in the Sun (chiefly hydrogen) were also found on Earth. Indeed, the element helium was first discovered in the spectrum of the sun and only later on earth, hence its name. During the 20th century, spectrometry (the study of these spectral lines) advanced, particularly as a result of the advent of quantum physics that was necessary to understand the astronomical and experimental observations.
Observational astrophysics

Most astrophysical processes cannot be reproduced in laboratories on Earth. However, there is a huge variety of astronomical objects visible all over the electromagnetic spectrum. The study of these objects through passive collection of data is the goal of observational astrophysics.

The equipment and techniques required to study an astrophysical phenomenon can vary widely. Many astrophysical phenomena that are of current interest can only be studied by using very advanced technology and were simply not known until very recently.The majority of astrophysical observations are made using the electromagnetic spectrum.

Radio astronomy studies radiation with a wavelength greater than a few millimeters. Radio waves are usually emitted by cold objects, including interstellar gas and dust clouds. The cosmic microwave background radiation is the redshifted light from the Big Bang. Pulsars were first detected at microwave frequencies. The study of these waves requires very large radio telescopes. Infrared astronomy studies radiation with a wavelength that is too long to be visible but shorter than radio waves. Infrared observations are usually made with telescopes similar to the usual optical telescopes. Objects colder than stars (such as planets) are normally studied at infrared frequencies.

Optical astronomy is the oldest kind of astronomy. Telescopes and spectroscopes are the most common instruments used. The Earth's atmosphere interferes somewhat with optical observations, so adaptive optics and space telescopes are used to obtain the highest possible image quality. In this range, stars are highly visible, and many chemical spectra can be observed to study the chemical composition of stars, galaxies and nebulae.

Ultraviolet, X-ray and gamma ray astronomy study very energetic processes such as binary pulsars, black holes, magnetars, and many others. These kinds of radiation do not penetrate the Earth's atmosphere well, so they are studied with space-based telescopes such as RXTE, the Chandra X-ray Observatory and the Compton Gamma Ray Observatory.

Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances. A few gravitational wave observatories have been constructed, but gravitational waves are extremely difficult to detect. Neutrino observatories have also been built, primarily to study our Sun. Cosmic rays consisting of very high energy particles can be observed hitting the Earth's atmosphere.

Observations can also vary in their time scale. Most optical observations take minutes to hours, so phenomena that change faster than this cannot readily be observed. However, historical data on some objects is available spanning centuries or millennia. On the other hand, radio observations may look at events on a millisecond timescale (millisecond pulsars) or combine years of data (pulsar deceleration studies). The information obtained from these different timescales is very different.

The study of our own Sun has a special place in observational astrophysics. Due to the tremendous distance of all other stars, the Sun can be observed in a kind of detail unparalleled by any other star. Our understanding of our own sun serves as a guide to our understanding of other stars.The topic of how stars change, or stellar evolution, is often modelled by placing the varieties of star types in their respective positions on the Hertzsprung-Russell diagram, which can be viewed as representing the state of a stellar object, from birth to destruction. The material composition of the astronomical objects can often be examined using:

spectroscopy
radio astronomy
astronomical observatories

WONDERS OF NEBULA

The Triangulum Emission Nebula NGC 604 lies in a spiral arm of Galaxy M33, 2.7 million light-years from Earth. This nebula is a region in which stars are forming.
A nebula (Latin for "mist"; plur. nebulae) is an interstellar cloud of dust and gas. Originally nebula was a catch-all name for any extended astronomical object, including galaxies beyond the Milky Way (some examples of the older usage survive).
Solar nebula

In cosmogony, the solar nebula is a gaseous cloud (or accretion disc), from which, solar systems are formed. This nebular hypothesis was first proposed, in 1755, by Kant; who argued that nebulae slowly rotate, gradually condensing (due to gravity) and flattening; eventually forming stars and planets. A similar model was proposed, in 1796, by Laplace.
The Sol Nebula
The "life cycle" of the Sol Nebula is more or less similar to that of other solar nebulae.
Early solar nebulae (in the history of the universe) were formed of hydrogen, helium and lithium; while later stars were formed of heavier elements. As the Sol system is comparatively rich in these heavier elements, it can be argued that this system did not emerge directly from the "Big Bang".
This nebula had an initial diameter of 100AU and a mass of ~200-300% that of Sol's current mass. Over time, gravity caused the cloud to condense and, as density and pressure increased, a protostar emerged. The early system was heated, not by fusion, but by friction. Due to the conservation of angular momentum, the nebula did not fully collapse upon itself, and thus protoplanetary discs emerged, in orbit, around the protosun.Within this system, heavier elements tended to fall more towards the center (clumping into planetesimals and protoplanets). In addition, the outer part of the solar nebula cooled off (if it was ever hot to begin with) and, thus, ice and combustible gases were able to "survive". As a result, the inner planets are formed of minerals, while the outer planets are more gaseous/icy.
At some point, the heat within the protosun reached such a level that thermonuclear reactions began to occur. At this point, a "true" star was "born". The protostar lasted for ~100M years and the cycle was completed at about the same time the innermost planets had developed; this was ~4.6B years ago. Although the moons existed, they were not yet orbiting planets; this would occur over the next 800M years.
The Kant-Laplace and Near-Collision Theories
During the late-19th century the Kant-Laplace views were criticized by Maxwell, who showed that if matter of the known planets had once been distributed (around the Sun) in the form of a disk, forces of differential rotation would have prevented the condensation of individual planets. Another objection was that the Sun possesses less angular momentum than the Kant-Laplace theory indicated. For several decades most astronomers preferred the near-collision theory, in which the planets were considered to have been formed due to the approach of some other star to the Sun. This near-miss would have drawn large amounts of matter out of the Sun and the other star by their mutual tidal forces, which could have then condensed into planets.
Objections to the near-collision theory were also raised and, during the 1940s, the Kant-Laplace theory was modified such that it became accepted. In the modified version, the mass of the original protoplanet was assumed to be larger; and, the angular momentum discrepancy was attributed to magnetic forces