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Approximately 6.6 billion humans now inhabit the Earth. By comparison, there might be 20 million mallard ducks and, among a multitude of threatened and endangered species, perhaps 100,000 gorillas, 50,000 polar bears, and less than 10,000 tigers, 2,000 giant pandas and 200 California condors. Notably, the human population has grown nearly ten-fold over the past three centuries and has increased by a factor of four in the last century. This monumental historical development has profoundly changed the relationship of our species to its natural support systems and has greatly intensified our environmental impact. Equally amazing are the signs that, in our generation, the human population explosion has begun to abate (Figure 1; note that, here and below, many of the values given are estimates and, after the year 2005, projections). Our numbers are expected to rise by another 50% before reaching a peak late in this century; a decline is likely to follow. What caused this population surge; what caused its reversal; where are we headed; and how might the proliferation of our species affect its future well-being?

Until recently, the growth of our numbers was slow and variable. A pronounced expansion began with the advent of the Industrial Revolution, about two centuries ago. Whereas tens of thousands of years passed before our species reached the one billion mark, around 1800 C.E., it took only 130, 33, 15, 13 and 12 years to add each succeeding billion. This accelerating rate of increase is what is meant by the term population explosion. Around year 1970, population growth reached a maximal rate of about 2% per year—perhaps a thousand times faster than growth in prehistoric times. The annual increment has since dropped from 2.0 to 1.1% (or, as demographers prefer, to 11 per thousand), and it is still going down. The greatest annual increment in population, about 90 million individuals, occurred in 1995, while our numbers grew by only around 76 million in 2004 (Figure 1). Nevertheless, this cohort is comparable to adding the population of Germany to the planet each year.



Until recently, the growth of our numbers was slow and variable. A pronounced expansion began with the advent of the Industrial Revolution, about two centuries ago. Whereas tens of thousands of years passed before our species reached the one billion mark, around 1800 C.E., it took only 130, 33, 15, 13 and 12 years to add each succeeding billion. This accelerating rate of increase is what is meant by the term population explosion. Around year 1970, population growth reached a maximal rate of about 2% per year—perhaps a thousand times faster than growth in prehistoric times. The annual increment has since dropped from 2.0 to 1.1% (or, as demographers prefer, to 11 per thousand), and it is still going down. The greatest annual increment in population, about 90 million individuals, occurred in 1995, while our numbers grew by only around 76 million in 2004 (Figure 1). Nevertheless, this cohort is comparable to adding the population of Germany to the planet each year. At the other extreme, the average woman in Japan and in much of Europe bears approximately 1.3 live babies. Correspondingly, population growth rates vary with locale, from more than 3% per year in some African nations to a slightly negative rate (i.e., population loss) in some Eastern European states. Among industrialized nations, the U.S. has the highest rates of both procreation and immigration, giving it the greatest overall population growth rate of any industrialized nation—roughly 1% per year. he average human life-span has risen from 30-40 years in pre-industrial times to about 65 years today (Figure 3). Longevity is still not much greater than 40 years in Angola but it is more than double that in Sweden and Japan. In developing nations, longevity has sometimes increased by more than half a year in a calendar year. At the same time, the average life-span has been deflected downward in parts of Africa by infectious diseases such as AIDS and by the sociopolitical upheaval that followed the end of communist rule in Eastern Europe.

The demographic transition
The aforementioned historic trends are well understood. Excluding migration, the rate of change of the number of individuals in a population is the difference between birth rate and death rate. The explosion in human population thus reflects the excess of births over deaths fostered by the Industrial Revolution. Until about two centuries ago, birth rates and death rates were both high. Because these two rates were about equal in magnitude, the population grew slowly and unevenly. For example, human numbers grew at roughly 0.25% per year in 1700 C.E. Soon thereafter, as discussed below, institutional and technical advances caused death rates to fall in one nation after another around the globe. But because birth rates remained high, population growth rates soared, an unintended consequence of the alleviation of human hardship in the modern era. Then, after a few decades of declining death rates, families in those nations developed the inclination and found the means to dramatically limit procreation. As a result, fertility rates fell, often rapidly, to approach the low death rates, and population growth slowed.
A theoretical model called the demographic transition explains the pattern of population growth in these stages. Figure 4 illustrates its four stages in an idealized fashion. In early times, birth and death rates are high (perhaps both near 5% per year), and there is little sustained growth in the population. In the second stage, death rates decline but birth rates remain high; consequently, the rate of net population growth increases, as indicated in the figure by the shaded area labeled natural increase. Third, birth rates decline to approach the low death rate, causing population growth to subside. Finally, low birth and death rates ensue (each perhaps 1% per year) and growth abates or even becomes negative. The outcome is not a return to the pre-transition state, since the size of the population will have expanded and longevity increased during the demographic transition.
The demographic transition paradigm can be applied both to individual nations and the world population as a whole. The historical prototype is 19th century England. In short order, the transition spread, along with industrialization, to Western Europe and then to the United States. As they modernized in the 20th century, Japan and then certain other Asian nations replicated this transformation. The decades following World War II and the end of the colonial era saw most developing nations embark on this path; now, their death rates are typically at low levels and their birth rates are on the way down. Thus, most nations are currently somewhere in the third stage of their demographic transition and some are in stage four. In fact, the growth rate of the world population in year 2006 was 1.1%, the difference between a birth rate of 2.0% and death rate of 0.9% per year.

In some [[developing nation]s], in which custom may outweigh modern alternatives, the demographic transition has stalled midstream. That is, low death rates (say, 1-2% per year or less) may have been achieved but birth rates linger at 3 to 5% per year. Thus, as in Niger, Mali and Uganda, population growth can exceed 3% per year, making the corresponding doubling time of the population less than 25 years. (Doubling time in years is approximately equal to 70%/growth rate in percent per year. In this example, the doubling time would be 70/3 or 23 years.) This situation can lead to a demographic trap where rapid growth undercuts the very technical, social and economic progress that might otherwise resolve it. The developing nations as a group now have 80% of the world’s population and generate 96% of its growth. The on-going increase of world population can therefore be understood to represent unfinished demographic transitions in diverse pre-industrial societies.

Why death rates have declined
Infectious disease has always been a major cause of human mortality. Over the years, these diseases have included malaria, influenza, tuberculosis, cholera, and a variety of parasitic infections. In particular, childhood diarrhea and respiratory diseases of bacterial or viral origin ravage the young in poor nations; infant mortality can amount to more than 10% of the live births in those settings, more than 20-times that in many industrialized states. The battle against infectious diseases gained force early in the modern era through the development of public health regimes. Thus, long before the era of twentieth-century patient-directed medicine, we learned how to avoid the perils of contaminated drinking water, to drain swamps where mosquitoes harbor the malaria parasite, to immunize the young, to quarantine the infected and to teach public hygiene. (A classical example of an early public health intervention was the introduction of vaccination against smallpox by Edward Jenner more than two centuries ago; this scourge has now been entirely eradicated.) In addition, improved nutrition not only saved lives by itself but also strengthened resistance to infection. These simple preventive strategies were inexpensive and colonists brought them along to protect themselves and their workers. Even today, the transfer of readily-available technology and know-how from more developed countries (MDCs) continues to reduce mortality rates in less developed countries (LDCs).
Why birth rates have declined
Children are naturally loved and valued for themselves. But, especially in traditional (i.e., pre-modern) settings, children are also economic assets: a ready source of capital and security when alternatives are out of reach. Sons are of particular value, since it is they who typically inherit both the family plot and the responsibility for caring for aging parents. For practical reasons, daughters are often less desired: they may be regarded as not as productive and as likely to marry and move on, often with a costly dowry payment. Thus, time-honored wisdom might suggest an investment strategy of having, say, eight offspring. A parent can then expect four sons, one or two of whom will hopefully survive childhood and be there to serve with devotion in the distant future. Such views become institutionalized in cultural norms and shared practices.
While it is possible for a woman to bear as many as 15 children in her lifetime, this is rare. Rather, parents universally chose to limit family size because too many children present costs in excess of benefits. Thus, many traditional values and practices foster procreative restraint. Pregnancy can be avoided by celibacy, late marriage and sexual abstinence; various other precautions such as the rhythm method also reduce the risk of conception. Of particular importance is prolongation of the nursing of children. (This is because lactation inhibits ovulation through the mother’s endocrine system, thereby thwarting pregnancy.) In addition, desperate measures to control family size are frequently taken by those who lack better options. For example, perhaps 20 million pregnancies a year, more than one-tenth of the total, are ended by septic (criminal) abortion despite serious risks to the mothers. Infanticide is another long-standing expedient. Especially in hard times, the girls are selected against. For example, ultrasound previews, while typically illegal, are widely used in Asia these days to pick out female fetuses for abortion. The various practices favoring male heirs is said to account for “100 million missing women” world-wide.
Just as the Industrial revolution precipitated a fall in death rates with a consequent surge in population, it has also driven the subsequent fall in birth rates and the resolution of that explosion. This is because industrial societies have substituted alternative sources of economic security for large family size. This is not just a wealth effect. Rather, modern countries have elaborated civil institutions that provide a social safety net that makes possible smaller families with greater investment in each individual. The safety net promotes health and education; property rights (ownership) and civil rights (e.g., the vote, equality before the law and public safety); some measure of financial security (e.g., insurance, loans, retirement plans, unemployment benefits, job creation and retraining programs); and income redistribution (e.g., public welfare programs and graduated taxation). Individual aspirations then become reoriented from security to self-realization, and from subsistence to productivity, as desperate choices are replaced by good options.

Women in MDCs typically expect to have two children and generally have slightly fewer. In these nations, women tend to marry late—or not at all. Contraception is widespread and the choices are diverse; for example, condoms, cervical loops and caps, vaginal spermicides and surgical sterilizations for both of the sexes. There are also pills to prevent ovulation (i.e., oral contraceptives), to induce the early miscarriage of a pregnancy and to thwart the implantation of a fertilized egg “the morning after.” Surgical abortions are common and (quite the opposite of criminal or septic abortion) safer than live births.
The empowerment of women (e.g., their literacy and employment) has been of special importance, as have international efforts to provide access to family planning assistance and contraception where needed. The steady decline of LDC fertility and population growth documents the impact of these processes (Figures 2 and 5). Nevertheless, more than 300 million women in LDC presently express the unfulfilled desire for fewer children and for family planning assistance such as information and contraceptive devices. On the other hand, many traditional mothers still aspire to large families (Figure 6). Thus, the population explosion is far from over.
The overall human death rate is not likely to change significantly in the foreseeable future. It will presumably decline in nations like Russia which have recently suffered sociopolitical upheavals. The opposite trend can be expected for developing nations as their youthful populations age. Globally, birth rates will probably continue to decline in the coming decades since, nowadays, couples are increasingly prone to limit their family size, whatever their wealth. Coercion by national governments, such as China’s one-child policy, appears to be unnecessary. If and when the global birth rate again matches death rate, we will hit zero population growth. This could occur by the year 2070 when the population might be 9.5 or 10 billion. Negative population growth may then ensue, as is now approaching in an increasing number of MDCs.
Although this is an era of decreasing birth rates, diverse constituencies nevertheless regard procreation as either a good in itself or as a practical imperative. Among these pro-natal voices are the following:
Measuring population
An existing population fluctuates because of births, deaths and migrations. When determining the size of any population demographers consider the number born, or the crude fertility rate of an existing population and they subtract the number dying, or the crude mortality rate, to arrive at what is called the natural increase per year of any existing population. In addition to the rate of natural increase, net migration must be calculated to accurately reflect the population. The net migrants are then accounted for by subtracting the number of emigrants--or people leaving an area-- from the number of immigrants--or the people coming into an area. That net migration figure is then added to or subtracted from the resident population to estimate the aggregate number of inhabitants in any place at a given time.
Fertility or natality
The number of births per one thousand people per year is called the annual crude birth rate and it is one of several measures of fertility in any population. Fertility rates in nations vary but crude birth rates of 48 in Chad or 49 per thousand in Angola are considered high rates, while 8 per thousand in Germany or Hong Kong are considered low. The world average for the crude fertility rate is 11 per one thousand, but when less developed nations are averaged the figure is 27 per one thousand.
Mortality
The number of deaths per one thousand people per year is called the annual crude death rate, or mortality. The relation between the age structure and the mortality rates in most populations coincide; the older the population, the higher the crude death rates. For example, 16 per thousand in Russia coincides with an aging population, as do 17 in the Ukraine. There are exceptions to this correlation of the crude death rates in an aging population. In the cases of Afghanistan and nations in Africa both war and infant mortality contribute to higher crude death rates among relatively younger populations. In Afghanistan, war has raised the death rate to a high of 22 and 23 in Sierra Leone.
In other parts of Africa the HIV infection rate has had an effect on the mortality rates. In Swaziland, Zimbabwe, and South Africa where infection rates are high with one third to one fifth of the population estimated to have HIV, crude death rates are as follows: for Zimbabwe it is 23 per one thousand, South Africa 18 and Swaziland 28 per one thousand.
Experts and aid agency specialists have noted that poverty exacerbates the access to health care, adequate nutrition and clean water. In those poor nations the crude death rate is also higher than the world average which is 10 per one thousand of the population. For instance, in Africa where 66 percent of the population lives on less than $2 per day (equivalent in American dollars), the crude death rate averages 15 per one thousand. In Canada it is less that half that rate or 7 per thousand while in the United States rate rises to 8 and in Western Europe 9 per one thousand of the population.
Migration


Projected average annual rate of natural increase per 1,000 population: 1995 to 2025. (Source: US Census Bureau)
The movement of people from one place to another is a fundamental characteristic of human and many herd animal populations. Those people leaving an area are called emigrants from the nation or place they leave and referred to as immigrants in the places they come to or in which they arrive. Net migration rates annually per one thousand vary widely from a high of 13 in Ireland to negative numbers in Poland, South Korea, Lithuania and the Netherlands. Lithuania has the fewest net migrant rate at a negative 2.6 in 2005. International migrants, it is estimated, comprise three percent of the world's population in any given period.
Population and carrying capacity


A rice terrace in Ubud, Bali. (Credit: Joseph Siry)
The number of living things in any area or place may vary but ecologists have suggested that there are several factors that limit the size of natural and even human numbers in any given situation. That limit, derived from the study of grazing animals in an acre or hectare of a field is called the carrying capacity. This capacity is derived from the number of individuals that the grazing land can nourish from one generation to the next without appreciable loss of food, water, and nutrients that sustain a herd of animals. For ecologists the population of organisms in any place or habitat cannot long exceed the carrying capacity of the arable land. The excess of the population above the carrying
capacity triggers migration, or can lead to famine in human or starvation in animal populations.
Critics charge that ecologists cannot apply the findings from the study of herd animal populations to humans. Some economists and demographer's argue that rising populations improve economic conditions while declining populations, such as in the Great Plains communities of the United States and Canada foster economic decline. The debate remains contentious as ecologists insist that the capacity of the Earth to sustain more people and more affluent behavioral patterns is at, near or beyond its limits with often only trade, technology or other social policies able to extend the carrying capacity of wealthy regions.
Historical and constitutional contexts of population


Regional distribution of global population: 1950, 2002, and 2050. (Source: US Census Bureau)
This increase in the survival rates of young and old are a principle contributor to the growth of world population since World War One. The fall in the death rates and the subsequent decline in fertility rates have both lead to a surge in population. Since the eighteenth century, when the number of people in the United Kingdom's industrial areas began to grow, the causes and consequences of population growth have interested economists, policy makers, religious organizations and governments. Many governments, such as the United States, are constitutionally required to take a periodic census (or enumeration of the population) of the number of people in the nation to apportion the number of votes in the House of Representatives to reflect the loss or gain of people from one state or district within a state to another. Population growth and decline are viewed correctly as engines of both economic and environmental aspects of social change.
87,236,532-phil
Human Population
Author and Page information
• by Anup Shah
• This Page Last Updated Thursday, June 13, 2002
• This page: http://www.globalissues.org/issue/198/human-population.
• To print all information e.g. expanded side notes, shows alternative links, use the print version:
o http://www.globalissues.org/print/issue/198
"Go Forth And Multiply!" That's what the human population has successfully been doing for thousands and thousands of years, expanding, exploring, migrating, conquering, utilizing, evolving, civilizing, industrializing, and now, destroying the very land upon which we live.
The food scarcity part of the argument in the population debate is an interesting one -- people are hungry not because the population is growing so fast that food is becoming scarce, but because people cannot afford it. Food may be scarce, but it is international trade, economic policies and the control of land that have lead to immense poverty and hunger and therefore less access to food, not food scarcity due to over population.
In previous lectures, we have described how human cultural development was closely tied to changes in the natural environment. Successive cultural revolutions, such as the agricultural revolution, have led to surges in population. Figure 1 summarizes again the historical record, typical of a "J-shaped" growth, with humans filling new niches and (perhaps) not yet reaching a limiting carrying capacity. One feature to note in this plot is the lack of huge fluctuations associated with famines or wars. In fact, the nature of J-shaped (exponential) growth is such that episodic reductions due to such catastrophes usually do not affect the inexorable and overpowering upward acceleration in population size. An exception is the period of the "black death" in Europe, which produced a noticeable but small downward spike in the curve. The wholesale loss of life due to world wars of the 20th century produced only small perturbations to the upward trend.
In previous lectures, we have described how human cultural development was closely tied to changes in the natural environment. Successive cultural revolutions, such as the agricultural revolution, have led to surges in population. Figure 1

The human population growth of the last century has been truly phenomenal. It required only 40 years after 1950 for the population to double from 2.5 billion to 5 billion. This doubling time is less than the average human lifetime. The world population passed 6 billion just before the end of the 20th century. Present estimates are for the population to reach 8-12 billion before the end of the 21st century. During each lecture hour, more than 10,000 new people enter the world, a rate of ~3 per second!
Of the 6 billion people, about half live in poverty and at least one fifth are severely undernourished. The rest live out their lives in comparative comfort and health.
The factors affecting global human population are very simple. They are fertility, mortality, initial population, and time. The current growth rate of ~1.3% per year is smaller than the peak which occurred a few decades ago (~2.1% per year in 1965-1970), but since this rate acts on a much larger population base, the absolute number of new people per year (~90 million) is at an all time high. The stabilization of population will require a reduction in fertility globally. In the most optimistic view, this will take some time.
Fertility
The current growth of population is driven by fertility. Figure 2 shows how total fertility rate is a strong function of region. It can be readily seen that the more developed countries ("the North") have lower fertility rates than the less developed countries ("the South"). The fertility rates in the developed world are close to replacement levels (i.e., the population is roughly stable), while the rates in the developing world are much higher. Thus, population growth and level of development are clearly linked.


Astronomy
Asteroids, sometimes called minor planets or planetoids, are bodies not showing cometary activity, and historically refer primarily to the inner Solar System since this term came into common usage when the outer solar system was poorly known. Asteroids are smaller than planets but larger than meteoroids. The distinction between asteroids and comets is made on visual appearance: Comets show a perceptible coma while asteroids do not.
Asteroids within the Asteroid Belt are presumed to be the remnants of matter that did not clump during the formation of the solar system. Even if the materials did collide, the gravity from Jupiter pulled them apart from each other. Composed of rock, dust, and metal, the early asteroids were formed when the heavy metal within them sunk to the center of the rock, forming a metal core. Over time, the lighter rocks formed layers around the core. The rock would then cool steadily, eventually becoming a solid.[11]
Asteroids are commonly classified according to two criteria: the characteristics of their orbits, and features of their reflectance spectrum.
In 1975, an asteroid taxonomic system based on colour, albedo, and spectral shape was developed by Clark R. Chapman, David Morrison, and Ben Zellner.[22] These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-types for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include a number of other asteroid types. The number of types continues to grow as more asteroids are studied.
Traditionally, small bodies orbiting the Sun were classified as asteroids, comets or meteoroids, with anything smaller than ten metres across being called a meteoroid.[1] The term "asteroid" is somewhat ill-defined. It never had a formal definition, with the broader term minor planet being preferred by the International Astronomical Union until 2006, when the term "small Solar System body" was introduced to cover both minor planets and comets. Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English for the larger asteroids. The word "planetesimal" has a similar meaning, but refers specifically to the small building blocks of the planets that existed at the time the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets,[2] but is not in common use.
When discovered, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatile ices and become asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets.
hese inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or TNOs were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets rather than asteroids.
The eighth planet from the Sun, Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. (Galileo had recorded it as a fixed star during observations with his small telescope in 1612 and 1613.) When Uranus didn't travel exactly as astronomers expected it to, a French mathematician, Urbain Joseph Le Verrier, proposed the position and mass of another as yet unknown planet that could cause the observed changes to Uranus' orbit. After being ignored by French astronomers, Le Verrier sent his predictions to Johann Gottfried Galle at the Berlin Observatory, who found Neptune on his first night of searching in 1846. Seventeen days later, its largest moon, Triton, was also discovered.
Nearly 4.5 billion kilometers (2.8 billion miles) from the Sun, Neptune orbits the Sun once every 165 years. It is invisible to the naked eye because of its extreme distance from Earth. Interestingly, due to Pluto's unusual elliptical orbit, Neptune is actually the farthest planet (including dwarf planets) from the Sun for a 20-year period out of every 248 Earth years.

A comet is a minor planet made up of rock, dust and ice. It originates from a cloud of debris remaining from the condensation of the solar nebula. Comets are unique because they are created in the outer solar system, and are greatly affected by the planets they pass. While a comet is orbiting, its path is constantly being altered as it nears surrounding planets. These changes in orbit can send it on a path approaching the sun, where it will burn up, or can be cast completely out of the solar system.

The tail of a comet is actually called the coma, which is composed of gas and dust streams. When a comet passes through the inner solar system, the sun lights up these streams so that we are able to see it. This is how we have been able to see Halley’s Comet from Earth.

The orbital periods of comets vary, but have been divided into three categories: Short period comets; long period comets; and Single-apparition comets. While Short period comets orbit for 200 years or less, long period comets are bound by gravity to the sun, and remain much longer. Single-apparition comets have unusual orbits and are thrown out of the solar system forever.

Below, you can find additional information on Halley’s Comet, pictures of comets, or search for anything in the cosmos.


Comets:




Comet Borrelly as seen by Deep??Space??1.


Throughout history, people have been both awed and alarmed by comets, stars with "long hair" that appeared in the sky unannounced and unpredictably. We now know that comets are dirty-ice leftovers from the formation of our solar system around 4.6 billion years ago. They are among the least-changed objects in our solar system and, as such, may yield important clues about the formation of our solar system. We can predict the orbits of many of them, but not all.
Around a dozen "new" comets are discovered each year. Short-period comets are more predictable because they take less than 200 years to orbit the Sun. Most come from a region of icy bodies beyond the orbit of Neptune. These icy bodies are variously called Kuiper Belt Objects, Edgeworth-Kuiper Belt Objects, or trans-Neptunian objects. Less predictable are long-period comets, many of which arrive from a distant region called the Oort cloud about 100,000 astronomical units (that is, 100,000 times the mean distance between Earth and the Sun) from the Sun. These comets can take as long as 30 million years to complete one trip around the Sun. (It takes Earth only 1 year to orbit the Sun.) As many as a trillion comets may reside in the Oort cloud, orbiting the Sun near the edge of the Sun's gravitational influence.
transit
Sid Villa: venuz
Sid Villa: almost earth
Sid Villa: hottest brightest
Sid Villa: moon
Sid Villa: 7o
Sid Villa: space crafts
Sid Villa: 12 astraunauts
Sid Villa: affect earth wooble
Sid Villa: 4.5 billion of years
Sid Villa: mars sized body hit earth and resutin debis
Sid Villa: mars moon deimos and phobos
Sid Villa: nasa's maniner 9
Sid Villa: 4 largest moon....jupiter...galilean...galileo galilei
Sid Villa: simon marius...claimed he 1st saw d moon...
Sid Villa: 61 moon

Pioneers 10 and 11 (1973 to 1974) and Voyager 1 and Voyager 2 (1979) offered striking color views and global perspectives from their flybys of the Jupiter system. From 1995 to 2003, the Galileo spacecraft made observations from repeated elliptical orbits around Jupiter, passing as low as 261 kilometers (162 miles) over the surfaces of the Galilean moons. These close approaches resulted in images with unprecedented detail of selected portions of the surfaces.

Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.
Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.



As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.

When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the Sun, fluorescence usually becomes more intense than reflected light.

As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.

The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun. The following view of Comet West shows two distinct tails. The thin blue plasma tail is made up of gases and the broad white tail is made up of microscopic dust particles.

most distant to the ancient...saturn...ring

Like Jupiter, Saturn is made mostly of hydrogen and helium. Its volume is 755 times greater than that of Earth. Winds in the upper atmosphere reach 500 meters (1,600 feet) per second in the equatorial region. (In contrast, the strongest hurricane-force winds on Earth top out at about 110 meters, or 360 feet, per second.) These super-fast winds, combined with heat rising from within the planet's interior, cause the yellow and gold bands visible in the atmosphere.

Once considered one of the blander-looking planets, Uranus (pronounced YOOR un nus) has been revealed as a dynamic world with some of the brightest clouds in the outer solar system and 11 rings. The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel. The seventh planet from the Sun is so distant that it takes 84 years to complete one orbit. Uranus, with no solid surface, is one of the gas giant planets (the others are Jupiter, Saturn, and Neptune).
The atmosphere of Uranus is composed primarily of hydrogen and helium, with a small amount of methane and traces of water and ammonia. Uranus gets its blue-green color from methane gas. Sunlight is reflected from Uranus' cloud tops, which lie beneath a layer of methane gas. As the reflected sunlight passes back through this layer, the methane gas absorbs the red portion of the light, allowing the blue portion to pass through, resulting in the blue-green color that we see. The planet's atmospheric details are very difficult to see in visible light. The bulk (80 per-cent or more) of the mass of Uranus is contained in an extended liquid core consisting primarily of 'icy' materials (water, methane, and ammonia), with higher-density material at depth.

Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples is Barringer Meteor Crater near Winslow, Arizona.


Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers (932 miles) across -- less than half the diameter of our Moon.

Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.

Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel; the rest are a mixture of the three materials. Stony meteorites are the hardest to identify since they look very much like terrestrial rocks.

Because asteroids are material from the very early solar system, scientists are interested in their composition. Spacecraft that have flown through the asteroid belt have found that the belt is really quite empty and that asteroids are separated by very large distances. Before 1991 the only information obtained on asteroids was though Earth based observations. Then on October 1991 asteroid 951 Gaspra was visited by the Galileo spacecraft and became the first asteroid to have hi-resolution images taken of it. Again on August 1993 Galileo made a close encounter with asteroid 243 Ida. This was the second asteroid to be visited by spacecraft. Both Gaspra and Ida are classified as S-type asteroids composed of metal-rich silicates.

On June 27, 1997 the spacecraft NEAR made a high-speed close encounter with asteroid 253 Mathilde. This encounter gave scientists the first close-up look of a carbon rich C-type asteroid. This visit was unique because NEAR was not designed for flyby encounters. NEAR is an orbiter destined for asteroid Eros in January of 1999.

Astronomers have studied a number of asteroids through Earth-based observations. Several notable asteroids are Toutatis, Castalia, Geographos and Vesta. Astronomers studied Toutatis, Geographos and Castalia using Earth-based radar observations during close approaches to the Earth. Vesta was observed by the Hubble Space Telescope.

The term meteor comes from the Greek meteoron, meaning phenomenon in the sky. It is used to describe the streak of light produced as matter in the solar system falls into Earth's atmosphere creating temporary incandescence resulting from atmospheric friction. This typically occurs at heights of 80 to 110 kilometers (50 to 68 miles) above Earth's surface. The term is also used loosely with the word meteroid referring to the particle itself without relation to the phenomena it produces when entering the Earth's atmosphere. A meteoroid is matter revolving around the sun or any object in interplanetary space that is too small to be called an asteroid or a comet. Even smaller particles are called micrometeoroids or cosmic dust grains, which includes any interstellar material that should happen to enter our solar system. A meteorite is a meteoroid that reaches the surface of the Earth without being completely vaporized.
One of the primary goals of studying meteorites is to determine the history and origin of their parent bodies. Several achondrites sampled from Antarctica since 1981 have conclusively been shown to have originated from the moon based on compositional matches of lunar rocks obtained by the Apollo missions of 1969-1972. Sources of other specific metorites remain unproven, although another set of eight achondrites are suspected to have come from Mars. These meteorites contain atmospheric gases trapped in shock melted minerals which match the composition of the Martian atmosphere as measured by the Viking landers in 1976. All other groups are presumed to have originated on asteroids or comets; the majority of meteorites are believed to be fragments of asteroids.
On the first day of January 1801, Giuseppe Piazzi discovered an object which he first thought was a new comet. Asteroid discoverer
11 comets and asteroids have been explored by spacecraft so far
A constellation is a group of stars that appear to have a physical proximity in the sky. The stars in a constellation are often vastly distant from each other, but they appear close to each other from the perspective of Earth. The word is used colloquially to refer to asterisms: groups of stars that appear to form patterns in the sky; different world cultures have divided the stars into different constellations. However, in modern astronomy the word refers instead to a method of dividing the sky into 88 areas with exact boundaries.
In common usage, a constellation is what astronomers call an 'asterism': a group of celestial bodies (usually stars) that appear to form a pattern in the sky or appear visibly related to each other. Examples are Orion (which appears like a human figure with a belt, often referred to as "The Hunter"), Leo (which contains bright stars that outline the form of a lion), Scorpius (which can seem reminiscent of a scorpion), and Crux (a cross).
In astronomy, however, a constellation is an area of the sky, and contains all the stars and other celestial objects within that area. The International Astronomical Union (IAU) divides the sky into 88 official constellations[1] with exact boundaries, so that every direction or place in the sky belongs within one constellation. Most of these constellations are centred on the traditional constellations of Western culture.
The 88 official constellations defined by the IAU(International Astronomical Union) are mostly based upon those of the ancient Greek tradition, passed down through the Middle Ages, which includes the 'signs of the zodiac,' twelve constellations through which the sun passes and which thus have had special cultural significance. The rest consist of constellations which were defined in the early modern era by astronomers who studied the southern hemisphere's skies, which were invisible to the Greeks.
Andromeda Andromeda Alpheratz (Sirrah)
Ant Antlia Air Pumpe
Aps Apus Bird of Paradise
Aqr Aquarius Water Carrier
Aql Aquila Eagle Altair
Ara Ara Altar
Ari Aries Ram Hamal
Aur Auriga Charioteer Capella
Boo Bootes Bear Driver Arcturus
Cae Caelum Graving Tool
Cam Camelopardalis Giraffe
Cnc Cancer Crab
Birth of Stars
Stars are born in cold interstellar clouds like the Orion Nebula and the Eagle Nebula. In these stellar nurseries, dense regions undergo gravitational collapse to form a rotating gas globule. As the globule collapses, the temperature and pressure increase and it spins faster. This causes the globule to have a central core and a surrounding flattened disk of dust. The central core becomes a star, while the disk may coalesce into planets and asteroids. The process of collapse takes between 10,000 and 1,000,000 years.
Lives in the Balance
A star's life is an extended battle between two opposing forces: gravity and pressure. A star can maintain its internal pressure only if it continually generates energy to replace the energy that it radiates into space. This energy comes primarily from nuclear fusion of light elements into heavier elements, through which a star shines for millions or billions of years.
All stars spend a significant amount of their lifetime fusing hydrogen to helium. This phase of the star's life is called the main sequence. Examples of main sequence stars are the Sun, Vega, Sirius and Spica. When the hydrogen in the core of the star is depleted, the envelope of the star expands tremendously and the star becomes a red giant. Examples of red giants are Betelguese, Arcturus, Aldebaran and Antares.
As a star has a limited amount of material in its core, it cannot rely on thermal energy to resist gravity forever and its ultimate fate depends on whether something other than thremal pressure manages to halt the unceasing crush of gravity.
Death of Stars
The outcome of a star's struggle between gravity and pressure depends entirely on its birth mass. Stars with masses below about 5 solar masses swell into red giants near the ends of their lives, after which the envelope is ejected as a planetary nebula, while the core becomes a white dwarf. Examples of planetary nebulae are the Ring Nebula, Eskimo Nebula, Helix Nebula and the Cat's Eye Nebula.
High mass stars with masses above 5 solar masses end their lives much more violently. The stronger gravity of high mass stars compress their cores to higher temperatures, and consequently, these stars are much brighter than low mass stars. In the final stages of their lives, they proceed to fuse increasingly heavier elements until they have exhausted all possible fusion sources. When fusion ceases, gravity drives the core to implode, resulting in a titanic supernova explosion, leaving behind a neutron star or a black hole. Examples of supernova remnants are Vela, Crab Nebula, Veil Nebula and Supernova 1987A.
Star Clusters
Most stars are believed to have their origin in clusters. There are two kinds of star clusters: open clusters and globular clusters.
Open Clusters
Open clusters are physically related groups of stars held together by mutual gravitational attraction. Open clusters populate about the same regions of the Milky Way and other galaxies as diffuse nebulae, and are found along the band of the Milky Way in the sky. Most open clusters have only a short life as stellar swarms. As they drift along their orbits, some of their members escape the cluster due to velocity changes from tidal interactions with other objects. Examples of open clusters are M37 and M52.
Globular Clusters
Globular clusters are gravitational bound concentrations of approximately ten thousand to one million stars, spread over a volume of several tens to 200 light years in diameter. They are believed to be very old, estimates of their ages being 12 to 20 billion years. Examples of globular clusters are M13 and M28.
Stars are not boring objects with the same physical features. They come in a wide variety of sizes and colors. To fully understand why, you need to know the physics behind these. Let me try to put this in a simple way:
Astronomers can detect colors of stars by taking their spectra (very similar to splitting of light using a prism, which was done by Newton). The spectrum of a star resembles that of a black body (though there are some differences), which is characterized by a single parameter, the temperature. Hence, one can tell the temperature of a star by looking at the shape of the spectrum of a star.
We see the temperatures of stars go all the way from around 3000 K to 50,000 K. Blue stars are hotter and red stars are cooler. So the next time you see stars, keep in mind that the reason why stars like Betelguese, Arcturus and Antares look red is because they are "cool" (cool here means about 4000 K), and stars like Vega are white because they are much hotter. Based on the temperature, stars are classified into categories as O, B, A, F, G, K and M. O stars are the hottest and M stars are the coolest. For example Vega is an A star, while our Sun is a G star. So the Sun is much cooler than Vega.
The brightness of a star depends on its luminosity and its distance from us. Thus, a star may be faint because it is intrinsically faint, or because it may be very far away. So to know which star is more luminous than the other, we need to know the distance to the star. There are techniques to determine the distance to a star like parallax, Cepheid variables, etc. But once the distance is known, we can determine the actual luminosity of the star. The luminosity of a star may be less than the Sun by a factor of 10,000, or may be as high as more than a million times that of the Sun.
Once we know the luminosity of the star, we can determine the radius of the star. We see that many stars have very large radii with respect to that of the sun; such stars are called Giant or Supergiant stars (an example is Betelguese which has about 500 times the radius of the Sun).
Depending of the temperature of the star, its surface features may vary. Cool stars have molecules like Titanium oxide on the surface, while hot stars have ionized atoms. So you can see that stars come in a HUGE variety which can boggle our minds.
Star classification
(Stellar classification)

In astrophysics, stars are classified by their surface temperature, that is associated to specific spectral patterns. An early schema from the 19th century ranked stars from A to P, which is the origin of the currently used spectral classes. After several transformations, today the spectral classification includes 7 main types: O, B, A, F, G, K, M.
A popular mnemonic for remembering this order is "Oh, Be A Fine Girl, Kiss Me".

This is called “Morgan-Keenan spectral classification”, even though its form was already by Annie Cannon, also based on the work of other astronomers from the Harvard College Observatory. The classes, listed from hottest to coldest, are:
Class
Temperature
Star Color
O
30,000 - 60,000 °K
Blue
B
10,000 - 30,000 °K
Blue
A
7,500 - 10,000 °K
White
F
6,000 - 7,500 °K
White (yellowish)
G
5,000 - 6,000 °K
Yellow (like the Sun)
K
3,500 - 5,000 °K
Orange
M
2,000 - 3,500 °K
Red

binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star or secondary. Research between the early 1800s and today suggests that many stars are part of either binary star systems or star systems with more than two stars, called multiple star systems. The term double star may be used synonymously with binary star, but more generally, a double star may be either a binary star or an optical double star which consists of two stars with no physical connection but which appear close together in the sky as seen from the Earth. A double star may be determined to be optical if its components have sufficiently different proper motions or radial velocities, or if parallax measurements reveal its two components to be at sufficiently different distances from the Earth. Most known double stars have not yet been determined to be either bound binary star systems or optical doubles.
Binary star systems are very important in astrophysics because calculations of their orbits allow the masses of their component stars to be directly determined, which in turn allows other stellar parameters, such as radius and density, to be indirectly estimated. This also determines an empirical mass-luminosity relationship (MLR) from which the masses of single stars can be estimated.
Binary stars are often detected optically, in which case they are called visual binaries. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known. They may also be detected by indirect techniques, such as spectroscopy (spectroscopic binaries) or astrometry (astrometric binaries). If a binary star happens to orbit in a plane along our line of sight, its components will mutually eclipse and transit each other; these pairs are called eclipsing binaries, or, as they are detected by their changes in brightness during eclipses and transits, photometric binaries.
If the orbits of components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these close binary systems can exchange mass, which may bring their evolution to stages that single stars cannot attain. Examples of binaries are Algol (an eclipsing binary), Sirius, and Cygnus X-1 (of which one member is probably a black hole). Binary stars are also common as the nuclei of many planetary nebulae, and are the progenitors of both novae and type Ia supernovae. Barycenter…center of elliptic orbit
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. The nearest dwarf galaxies, satellites of the Milky Way, are only a few 100,000 light years distant, while the nearest giant neighbor, the Andromeda Galaxy, also a spiral, is about 2-3 million light years distant.
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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.
In general relativity, a black hole is a region of space in which the gravitational field is so powerful that nothing, including electromagnetic radiation (e.g. visible light), can escape its pull after having fallen past its event horizon. The term derives from the fact that absorption of visible light renders the hole's interior invisible, and indistinguishable from the black space around it.
Despite its invisible interior, a black hole may reveal its presence through interaction with matter orbiting the event horizon. For example, a black hole may be perceived by tracking the movement of a group of stars that orbit its center. Alternatively, one may observe gas (from a nearby star, for instance) that has been drawn into the black hole. The gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected from earthbound and earth-orbiting telescopes.[2][3] Such observations have resulted in the general scientific consensus that—barring a breakdown in our understanding of nature—black holes do exist in our universe.[4]
The idea of an object with gravity strong enough to prevent light from escaping was proposed in 1783 by John Michell,[5] an amateur British astronomer. In 1795, Pierre-Simon Laplace, a French physicist independently came to the same conclusion.[6][7] However, such "Newtonian black holes" are very different from black holes in general relativity. They prevent only light from escaping (not, for example, a rocket ship) and only in certain Newtonian models of light (such as an emission theory).
Black holes, as currently understood, are described by the general theory of relativity. This theory predicts that when a large enough amount of mass is present in a sufficiently small region of space, all paths through space are warped inwards towards the center of the volume, preventing all matter and radiation within it from escaping.
While general relativity describes a black hole as a region of empty space with a point-like singularity at the center and an event horizon at the outer edge, the description changes when the effects of quantum mechanics are taken into account. Research indicates that, rather than holding captured matter forever, black holes may slowly leak a form of thermal energy called Hawking radiation and may well have a finite life.[8][9][10] The as yet unknown theory of quantum gravity is believed to give the fully correct description of black holes.
he term black hole to describe this phenomenon dates from the mid-1960s, though its precise origins are unclear. Physicist John Wheeler is widely credited with coining it in his 1967 public lecture Our Universe: the Known and Unknown, as an alternative to the more cumbersome "gravitationally completely collapsed star". However, Wheeler insisted that someone else at the conference had coined the term and he had merely adopted it as useful shorthand. The term was also cited in a 1964 letter by Anne Ewing to the AAAS:[11]
According to Einstein’s general theory of relativity, as mass is added to a degenerate star a sudden collapse will take place and the intense gravitational field of the star will close in on itself. Such a star then forms a "black hole" in the universe.
The phrase had already entered the language years earlier as the Black Hole of Calcutta incident of 1756 in which 146 Europeans were locked up overnight in punishment cell of barracks at Fort William by Siraj ud-Daulah, and all but 23 perished.[12]
The phenomenon appeared in science fiction in a radio episode of Space Patrol which aired October 25, 1952, in which it was called a "cycloplex" or a "hole in space".
What makes it impossible to escape from black holes?

This section is missing citations or needs footnotes. Please help add inline citations to guard against copyright violations and factual inaccuracies. (July 2008)


Far away from the black hole a particle can move in any direction. It is only restricted by the speed of light.

Closer to the black hole spacetime starts to deform. There are more paths going towards the black hole than paths moving away.

Inside of the event horizon all paths bring the particle closer to the center of the black hole. It is no longer possible for the particle to escape.[13]

Popular accounts commonly try to explain the black hole phenomenon by using the concept of escape velocity, the speed needed for a vessel starting at the surface of a massive object to completely clear the object's gravitational field. It follows from Newton's law of gravity that a sufficiently dense object's escape velocity will equal or even exceed the speed of light. Citing that nothing can exceed the speed of light they then infer that nothing would be able to escape such a dense object.[14] However, the argument can only be seen as an incomplete analogy. It explains neither why light should be affected by gravity in the first place, why it cannot travel beyond the horizon, nor why a rocket-powered spaceship would not be able to break free.
Two concepts introduced by Albert Einstein are needed to explain the phenomenon. The first is that time and space are not two independent concepts, but are interrelated forming a single continuum, spacetime. This continuum has some special properties. An object is not free to move around spacetime at will; it must always move forward in time and cannot change its position in space faster than the speed of light. This is the main result of the theory of special relativity.
The second concept is the base of general relativity; mass deforms the structure of this spacetime. The effect of a mass on spacetime can informally be described as tilting the direction of time towards the mass. As a result, objects tend to move towards masses. This is experienced as gravity. This tilting effect becomes more pronounced as the distance to the mass becomes smaller. At some point close to the mass, the tilting becomes so strong that all the possible paths an object can take lead towards the mass.[15] This implies that any object that crosses this point can no longer get further away from the mass, not even using powered flight. This point is called the event horizon.
Properties: mass, charge, and angular momentum
According to the "No Hair" theorem a black hole has only three independent physical properties: mass, charge and angular momentum.[16] Any two black holes that share the same values for these properties are indistinguishable. This contrasts with other astrophysical objects such as stars, which have very many parameters. Consequently, a great deal of information is lost when a star collapses to form a black hole. Since in most physical theories information is preserved (in some sense), this loss of information in black holes is puzzling. Physicists refer to this as the black hole information paradox.