Can U.S. citizens maintain dual citizenship with another country?

A person who is considered a citizen by more than one nation has "dual citizenship". It is possible for a United States citizen to have dual citizenship; this can be achieved in various ways, such as by birth in the United States to a parent who is a citizen of a foreign country (or in certain circumstances the foreign nationality may be transmitted even by a grandparent) by birth in another country to a parent(s) who is/are a United States citizen/s, or by having parents who are citizens of different countries. Anyone who becomes a naturalized U.S. citizen is required to renounce any prior "allegiance" to other countries during the naturalization ceremony; however, this renunciation of allegiance renunciation of citizenship to those countries.

Citizenship of the United States

Citizenship of the United States is a status that entails specific rights, duties and benefits. Citizenship is understood as a "right to have rights" since it serves as a foundation of fundamental rights derived from and protected by the Constitution and laws of the United States, such as the right to freedom of expression, vote, due process, live and work in the United States, and to receive federal assistance. Certain rights are so fundamental that they are guaranteed to all persons, not just citizens. These include those rights guaranteed by the first 8 Amendments that pertain to individuals. However, not all U.S. citizens, such as those living in Puerto Rico, have the right to vote in federal elections.

There are two primary sources of citizenship: birthright citizenship, in which a person is presumed to be a citizen if he or she was born within the territorial limits of the United States, or—providing certain other requirements are met—born abroad to a U.S. citizen parent, and naturalization, a process in which an eligible legal immigrant applies for citizenship and is accepted. These two pathways to citizenship are specified in the Citizenship Clause of the Constitution's 1868 Fourteenth Amendment which reads:

"National citizenship" signifies membership in the country as a whole; "state citizenship", in contrast, signifies a relation between a person and a particular state and has application generally limited to domestic matters. State citizenship may affect (1) tax decisions and (2) eligibility for some state-provided benefits such as higher education and (3) eligibility for state political posts such as U.S. Senator.

In Article One of the Constitution, the power to establish a "uniform rule of naturalization" is granted explicitly to Congress.

U.S. law permits multiple citizenship. A citizen of another country naturalized as a U.S. citizen may retain their previous citizenship, though they must renounce allegiance to the other country. A U.S. citizen retains U.S. citizenship when becoming the citizen of another country, should that country's laws allow it. U.S. citizenship can be renounced by Americans who also hold another citizenship "via" a formal procedure at a U.S. Embassy, and it can also be restored.

Civic participation is not required in the United States. There is no requirement to attend town meetings, belong to a political party, or vote in elections. However, a benefit of naturalization is the ability to "participate fully in the civic life of the country". There is disagreement about whether popular lack of involvement in politics is helpful or harmful.

Vanderbilt professor Dana D. Nelson suggests that most Americans merely vote for president every four years, and sees this pattern as undemocratic. In her book "Bad for Democracy", Nelson argues that declining citizen participation in politics is unhealthy for long term prospects for democracy.

However, writers such as Robert D. Kaplan in "The Atlantic" see benefits to non-involvement; he wrote "the very indifference of most people allows for a calm and healthy political climate". Kaplan elaborated: "Apathy, after all, often means that the political situation is healthy enough to be ignored. The last thing America needs is more voters—particularly badly educated and alienated ones—with a passion for politics." He argued that civic participation, in itself, is not always a sufficient condition to bring good outcomes, and pointed to authoritarian societies such as Singapore which prospered because it had "relative safety from corruption, from breach of contract, from property expropriation, and from bureaucratic inefficiency".

The earliest recorded instances of dual citizenship began before the French Revolution when the British captured American ships and forced them back to Europe. The British Crown considered subjects from the United States as British by birth and forced them to fight in the Napoleonic wars.

Under certain circumstances there are relevant distinctions between dual citizens who hold a "substantial contact" with a country, for example by holding a passport or by residing in the country for a certain period of time, and those who do not. For example, under the Heroes Earnings Assistance and Relief Tax (HEART) Act of 2008, U.S. citizens in general are subject to an expatriation tax if they give up U.S. citizenship, but there are exceptions (specifically ) for those who are either under age 18½ upon giving up U.S. citizenship and have lived in the U.S. for less than ten years in their lives, or who are dual citizens by birth residing in their other country of citizenship at the time of giving up U.S. citizenship and have lived in the U.S. for less than ten out of the past fifteen years. Similarly, the United States considers holders of a foreign passport to have a substantial contact with the country that issued the passport, which may preclude security clearance.

U.S. citizens are required by federal law to identify themselves with a U.S. passport, not with any other foreign passport, when entering or leaving the United States. The Supreme Court case of "Afroyim v. Rusk" declared that a U.S. citizen did not lose his citizenship by voting in an election in a foreign country, or by acquiring foreign citizenship, if they did not intend to lose U.S. citizenship. U.S. citizens who have dual citizenship do not lose their United States citizenship unless they renounce it officially.

Citizenship began in colonial times as an active relation between men working cooperatively to solve municipal problems and participating actively in democratic decision-making, such as in New England town hall meetings. Men met regularly to discuss local affairs and make decisions. These town meetings were described as the "earliest form of American democracy" which was vital since citizen participation in public affairs helped keep democracy "sturdy", according to Alexis de Tocqueville in 1835. A variety of forces changed this relation during the nation's history. Citizenship became less defined by participation in politics and more defined as a legal relation with accompanying rights and privileges. While the realm of civic participation in the public sphere has shrunk, the citizenship franchise has been expanded to include not just propertied white adult men but black men and adult women.

Earlier on, U.S. citizenship was not given to people of Indian or East Asian descent. A. K. Mozumdar was the first person born in the Indian sub-continent to attain U.S. citizenship. A few years earlier, as a result of the 1898 United States v. Wong Kim Ark Supreme Court decision, ethnic Chinese born in the United States became citizens. During World War II, due to Japan's heavy involvement as an aggressor, it was decided to restrict many Japanese citizens from applying for U.S. citizenship, while Chinese citizens encountered no trouble, because of China's alliance with the United States.

The Equal Nationality Act of 1934 was an American law which allowed foreign-born children of American mothers and alien fathers who had entered America before age 18 and lived in America for five years to apply for American citizenship for the first time. It also made the naturalization process quicker for American women's alien husbands. This law equalized expatriation, immigration, naturalization, and repatriation between women and men. However, it was not applied retroactively, and was modified by later laws, such as the Nationality Act of 1940.

U.S. citizenship is usually acquired by birth when a child is born in the territory of the United States. In addition to U.S. states, this includes the District of Columbia, Guam, Puerto Rico, the Northern Mariana Islands and the U.S. Virgin Islands. Citizenship, however, was not specified in the original Constitution. In 1868, the Fourteenth Amendment specifically defined persons who were either born or naturalized in the United States and subject to its jurisdiction as citizens. All babies born in the United States—except those born to enemy aliens in wartime or the children of foreign diplomats—enjoy U.S. citizenship under the Supreme Court's long-standing interpretation of the Fourteenth Amendment. The amendment states: "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside." There remains dispute as to who is "subject to the jurisdiction" of the United States at birth.

By acts of Congress, every person born in Puerto Rico, the U.S. Virgin Islands, Guam, and the Northern Mariana Islands is a United States citizen by birth. Also, every person born in the former Panama Canal Zone whose father or mother (or both) are or were a citizen is a United States citizen by birth.

Regardless of where they are born, children of U.S. citizens are U.S. citizens in most cases. Children born outside the United States with at least one U.S. citizen parent usually have birthright citizenship by parentage.

A child of unknown parentage found in the US while under the age of 5 is considered a US citizen until proven, before reaching the age of 22, to have not been born in the US.

While persons born in the United States are considered to be citizens and can have passports, children under age eighteen are legally considered to be minors and cannot vote or hold office. Upon the event of their eighteenth birthday, they are considered full citizens but there is no ceremony acknowledging this relation or any correspondence between the new citizen and the government to this effect. Citizenship is assumed to exist, and the relation is assumed to remain viable until death or until it is renounced or dissolved by some other legal process. Secondary schools teach the basics of citizenship and create "informed and responsible citizens" who are "skilled in the arts of effective deliberation and action."

Americans who live in foreign countries and become members of other governments have, in some instances, been stripped of citizenship, although there have been court cases where decisions regarding citizenship have been reversed.

Acts of Congress provide for acquisition of citizenship by persons born abroad.

The agency in charge of admitting new citizens is the United States Citizenship and Immigration Services, commonly abbreviated as USCIS. It is a bureau of the Department of Homeland Security. It offers web-based services. The agency depends on application fees for revenue; in 2009, with a struggling economy, applications were down sharply, and consequently there was much less revenue to upgrade and streamline services. There was speculation that if the administration of president Barack Obama passed immigration reform measures, then the agency could face a "welcome but overwhelming surge of Americans-in-waiting" and longer processing times for citizenship applications. The USCIS has made efforts to digitize records. A USCIS website says the "U.S. Citizenship and Immigration Services (USCIS) is committed to offering the best possible service to you, our customer" and which says "With our focus on customer service, we offer you a variety of services both before and after you file your case." The website allowed applicants to estimate the length of time required to process specific types of cases, to check application status, and to access a customer guide. The USCIS processes cases in the order they're received.

People applying to become citizens must satisfy certain requirements. For example, there have been requirements that applicants have been permanent residents for five years (three if married to a U.S. citizen), be of "good moral character" (meaning no felony convictions), be of "sound mind" in the judgment of immigration officials, have knowledge of the Constitution, and be able to speak and understand English unless they are elderly or disabled. Applicants must also pass a simple citizenship test. Up until recently, a test published by the Immigration and Naturalization Service asked questions such as "How many stars are there in our flag?" and "What is the Constitution?" and "Who is the president of the United States today?" At one point, the Government Printing Office sold flashcards for $8.50 to help test takers prepare for the test. In 2006, the government replaced the former trivia test with a ten-question oral test designed to "shun simple historical facts about America that can be recounted in a few words, for more explanation about the principles of American democracy, such as freedom". One reviewer described the new citizenship test as "thoughtful". While some have criticized the new version of the test, officials counter that the new test is a "teachable moment" without making it conceptually more difficult, since the list of possible questions and answers, as before, will be publicly available. Six correct answers constitutes a passing grade. The new test probes for signs that immigrants "understand and share American values". A unique way to become a permanent resident is to apply to the US government DV lottery. This program is a drawing for foreigners to apply for a drawing to become a permanent resident.According to a senior fellow at the Migration Policy Institute, "citizenship is a very, very valuable commodity". However, one study suggested legal residents eligible for citizenship, but who don't apply, tend to have low incomes (41 percent), do not speak English well (60 percent), or have low levels of education (25 percent). There is strong demand for citizenship based on the number of applications filed. From 1920 to 1940, the number of immigrants to the United States who became citizens numbered about 200,000 each year; there was a spike after World War II, and then the level reduced to about 150,000 per year until resuming to the 200,000 level beginning about 1980. In the mid-1990s to 2009, the levels rose to about 500,000 per year with considerable variation. In 1996, more than one million people became citizens through naturalization. In 1997, there were 1.41 million applications filed; in 2006, 1.38 million. The number of naturalized citizens in the United States rose from 6.5 million in the mid-1990s to 11 million in 2002. By 2003, the pool of immigrants eligible to become naturalized citizens was 8 million, and of these, 2.7 million lived in California. In 2003, the number of new citizens from naturalization was 463,204. In 2007, the number was 702,589. In 2007, 1.38 million people applied for citizenship creating a backlog. In 2008, applications decreased to 525,786.

Naturalization fees were $60 in 1989; $90 in 1991; $95 in 1994; $225 in 1999; $260 in 2002; $320 in 2003; $330 in 2005. In 2007 application fees were increased from $330 to $595 and an additional $80 computerized fingerprinting fee was added. The biometrics fee was increased to $85 in 2010. On December 23, 2016, the application fees were increased again from $595 to $640. The high fees have been criticized as putting up one more wall to citizenship. Increases in fees for citizenship have drawn criticism. Doris Meissner, a senior fellow at the Migration Policy Institute and former Immigration and Naturalization Service Commissioner, doubted that fee increases deter citizenship-seekers. In 2009, the number of immigrants applying for citizenship plunged 62 percent; reasons cited were the slowing economy and the cost of naturalization.

The citizenship process has been described as a ritual that is meaningful for many immigrants. Many new citizens are sworn in during Independence Day ceremonies. Most citizenship ceremonies take place at offices of the U.S. Citizenship and Immigration Services. However, one swearing-in ceremony was held at Arlington National Cemetery in Virginia in 2008. The judge who chose this venue explained: "I did it to honor our country's warriors and to give the new citizens a sense for what makes this country great." According to federal law, citizenship applicants who are also changing their names must appear before a federal judge.

The title of "Honorary Citizen of the United States" has been granted eight times by an act of Congress or by a proclamation issued by the President pursuant to authorization granted by Congress. The eight individuals are Sir Winston Churchill, Raoul Wallenberg, William Penn, Hannah Callowhill Penn, Mother Teresa, the Marquis de Lafayette, Casimir Pulaski, and Bernardo de Gálvez y Madrid, Viscount of Galveston and Count of Gálvez.

Sometimes, the government awarded non-citizen immigrants who died fighting for American forces with the posthumous title of U.S. citizen, but this is not considered honorary citizenship. In June 2003, Congress approved legislation to help families of fallen non-citizen soldiers.

There is a sense in which corporations can be considered "citizens". Since "corporations" are considered persons in the eyes of the law, it is possible to think of corporations as being like citizens. For example, the airline Virgin America asked the United States Department of Transportation to be treated as an American air carrier. The advantage of "citizenship" is having the protection and support of the United States government when jockeying with foreign governments for access to air routes and overseas airports. Alaska Airlines, a competitor of Virgin America, asked for a review of the situation; according to U.S. law, "foreign ownership in a U.S. air carrier is limited to 25% of the voting interest in the carrier," but executives at Virgin America insisted the airline met this requirement.

For the purposes of diversity jurisdiction in the United States civil procedure, corporate citizenship is determined by the principal place of business of the corporation. There is some degree of disagreement among legal authorities as to how exactly this may be determined.

Another sense of "corporate citizenship" is a way to show support for causes such as social issues and the environment and, indirectly, gain a kind of "reputational advantage".

The Immigration and Nationality Act of 1952 (INA) made a minor distinction between U.S. citizenship and U.S. nationality. Citizenship comprises a larger set of privileges and rights for those persons that are U.S. citizens which is not afforded to individuals that are only U.S. nationals by virtue of their rights under the INA. It is well-established that all U.S. citizens are U.S. nationals but not all U.S. nationals are U.S. citizens.

The Naturalization Act of 1790 () provided the first rules to be followed by the United States in the granting of national citizenship after the ratification of the Constitution. A number of other Acts and statutes followed the Act of 1790 that expanded or addressed specific situations but it was not until the Immigration and Nationality Act of 1952 (), codified under Title 8 of the United States Code (), that the variety of statutes governing citizenship law were organized within one single body of text. The Immigration and Nationality Act of 1952 set forth the legal requirements for the acquisition of American nationality. The Fourteenth Amendment (1868) addressed citizenship rights. The United States nationality law, despite its "nationality" title, comprises the statues that embody the law regarding both American citizenship and American nationality.

For example, as specified in, a person whose only connection to the U.S. is through birth in an outlying possession (which, as of March 2015, was defined under as American Samoa and Swains Island), or through descent from a person so born, acquires only U.S. nationality but not U.S. citizenship. Such person is said to be a non-citizen national of the United States.

American Samoans continue to be U.S. nationals but not U.S. citizens. People born in American Samoa receive passports declaring the holder is only a U.S. national, not a U.S. citizen. For an America Samoan to become a U.S. citizen, he or she must relocate to another part of the United States, initiate the naturalization process, pay the $680 fee (as of February 11, 2014), pass a good moral character assessment, be fingerprinted and pass an English/civics examination.

In addition, residents of the Northern Mariana Islands who automatically gained U.S. citizenship in 1986 as a result of the Covenant between the Northern Mariana Islands and the U.S. could elect to become U.S. noncitizen nationals within 6 months of the implementation of the Covenant or within 6 months of turning 18.

The nationality status of a person born in an unincorporated U.S. Minor Outlying Island is not specifically mentioned by law, but under international law and Supreme Court dicta, they are also regarded as non-citizen nationals of the United States.

Non-citizen nationals of the United States may reside and work in the United States without restrictions, and may apply for U.S. citizenship under the same rules as permanent U.S. residents. Both of these groups are not allowed to vote in federal or state elections, although there is no constitutional prohibition against their doing so. Every national of the United States statutorily transmits nationality to children born outside the United States.

The U.S. passport issued to noncitizen nationals of the United States contains the endorsement code 9 which states: "The bearer is a United States national and not a United States citizen" on the annotations page.

The issue of citizenship naturalization is a highly contentious matter in US politics, particularly regarding illegal immigrants. Candidates in the 2008 presidential election, such as Rudolph Giuliani, tried to "carve out a middle ground" on the issue of illegal immigration, but rivals such as John McCain advocated legislation requiring illegal immigrants to first leave the country before being eligible to apply as citizens. Some measures to require proof of citizenship upon registering to vote have met with controversy.

Issues such as whether to include questions about current citizenship status in census questions have been debated in the Senate. Generally, there tends to be controversy when citizenship affects political issues. For example, issues such as asking questions about citizenship on the United States Census tend to cause controversy. Census data affects state electoral clout; it also affects budgetary allocations. Including non-citizens in Census counts also shifts political power to states that have large numbers of non-citizens due to the fact that reapportionment of congressional seats is based on Census data.

There have been controversies based on speculation about which way newly naturalized citizens are likely to vote. Since immigrants from many countries have been presumed to vote Democratic if naturalized, there have been efforts by Democratic administrations to streamline citizenship applications before elections to increase turnout; Republicans, in contrast, have exerted pressure to slow down the process. In 1997, there were efforts to strip the citizenship of 5,000 newly approved immigrants who, it was thought, had been "wrongly naturalized"; a legal effort to do this presented enormous challenges. An examination by the Immigration and Naturalization Service of 1.1 million people who were granted citizenship from September 1995 to September 1996 found 4,946 cases in which a criminal arrest should have disqualified an applicant or in which an applicant lied about his or her criminal history. Before the 2008 election, there was controversy about the speed of the USCIS in processing applications; one report suggested that the agency would complete 930,000 applications in time for the newly processed citizens to vote in the November 2008 election. Foreign-born naturalized citizens tend to vote at the same rates as natives. For example, in the state of New Jersey in the 2008 election, the foreign born represented 20.1% of the state's population of 8,754,560; of these, 636,000 were eighteen or older and hence eligible to vote; of eligible voters, 396,000 actually voted, which was about 62%. So foreign-born citizens vote in roughly the same proportion (62%) as native citizens (67%).

There has been controversy about the agency in charge of citizenship. The USCIS has been criticized as being a "notoriously surly, inattentive bureaucracy" with long backlogs in which "would-be citizens spent years waiting for paperwork". Rules made by Congress and the federal government regarding citizenship are highly technical and often confusing, and the agency is forced to cope with enforcement within a complex regulatory milieu. There have been instances in which applicants for citizenship have been deported on technicalities. One Pennsylvania doctor and his wife, both from the Philippines, who applied for citizenship, and one Mr. Darnell from Canada who was married to an American with two children from this marriage, ran afoul of legal technicalities and faced deportation. "The New York Times" reported that "Mr. Darnell discovered that a 10-year-old conviction for domestic violence involving a former girlfriend, even though it had been reduced to a misdemeanor and erased from his public record, made him ineligible to become a citizen—or even to continue living in the United States." Overworked federal examiners under pressure to make "quick decisions" as well as "weed out security risks" have been described as preferring "to err on the side of rejection". In 2000, 399,670 applications were denied (about of all applications); in 2007, 89,683 applications for naturalization were denied, about 12% of those presented.

Generally, eligibility for citizenship is denied for the millions of people living in the United States illegally, although from time to time, there have been amnesties. In 2006, there were mass protests numbering hundreds of thousands of people throughout the US demanding U.S. citizenship for illegal immigrants. Many carried banners which read "We Have A Dream Too". One estimate is that there are 12 million illegal immigrants in the U.S. in 2006. There are many American high school students with citizenship issues. In 2008, it was estimated that there were 65,000 illegal immigrant students. The number was less clear for post-secondary education. A 1982 Supreme Court decision entitled illegal immigrants to free education from kindergarten through high school. Undocumented immigrants who get arrested face difficulties in the courtroom as they have no constitutional right to challenge the outcome of their deportation hearings. In 2009, writer Tom Barry of the "Boston Review" criticized the crackdown against illegal immigrants since it "flooded the federal courts with nonviolent offenders, besieged poor communities, and dramatically increased the U.S. prison population, while doing little to solve the problem itself". Barry criticized the United States' high incarceration rate as being "fives times greater than the average rate in the rest of the world". Virginia Senator Jim Webb agreed that "we are doing something dramatically wrong in our criminal justice system".

U.S. citizens can relinquish their citizenship, which involves abandoning the right to reside in the United States and all the other rights and responsibilities of citizenship. "Relinquishment" is the legal term covering all seven different potentially-expatriating acts (ways of giving up citizenship) under. "Renunciation" refers to two of those acts: swearing an oath of renunciation before a U.S. diplomatic or consular officer abroad, or before an official designated by the Attorney General within the United States during a state of war. Out of an estimated three to six million U.S. citizens residing abroad, between five and six thousand relinquished citizenship each year in 2015 and 2016. U.S. nationality law treats people who performs potentially-expatriating acts with intent to give up U.S. citizenship as ceasing to be U.S. citizens from the moment of the act, but U.S. tax law since 2004 treats such individuals as though they remain U.S. citizens until they notify the State Department and apply for a Certificate of Loss of Nationality (CLN).

Renunciation requires an oath to be sworn before a State Department officer and thus involves in-person attendance at an embassy or consulate, but applicants for CLNs on the basis of other potentially-expatriating acts must attend an in-person interview as well. During the interview, a State Department official assesses whether the person acted voluntarily, intended to abandon all rights of U.S. citizenship, and understands the consequences of their actions. The State Department strongly recommends that Americans intending to relinquish citizenship have another citizenship, but will permit Americans to make themselves stateless if they understand the consequences. There is a $2,350 administrative fee for the process. In addition, an expatriation tax is imposed on some individuals relinquishing citizenship, but payment of the tax is not a legal prerequisite for relinquishing citizenship; rather, the tax and its associated forms are due on the normal tax due date of the year following relinquishment of citizenship. State Department officials do not seek to obtain any tax information from the interviewee, and instruct the interviewee to contact the IRS directly with any questions about taxes.


When did Pope Gregory XIII hold the papacy?

Gregory died in Pisa on 17 December 1187 of a fever after holding the papacy for only 57 days. He was buried in the Duomo in Pisa. He was succeeded by Pope Clement III. According to Joseph S. Brusher, "His pontificate though brief was glorious."

Pope Gregory VIII

Pope Gregory VIII (; c. 1100/1105 – 17 December 1187), born Alberto di Morra, reigned from 21 October to his death in 1187.

Alberto di Morra was born about 1105 in Benevento, Italy. His father was the nobleman Sartorius di Morra. He became a monk early in life, either as a Cistercian in Laon, or a Benedictine at Monte Cassino. Alberto later joined a new religious order, the Premonstratensian or Norbertine order, probably between the ages of 20-30. He was a canon at St. Martin's Abbey in Laon. He later became a professor of canon law in Bologna. Before the first Crusade, he believed that disobeying the Pope was to disobey God. With good reason, this attitude alienated many emperors and Byzantines.

In 1156, Pope Adrian IV made him cardinal-deacon of Sant'Adriano, and on 14 March 1158 he became cardinal-priest of San Lorenzo in Lucina. As a papal legate of Pope Alexander III, he was sent to teach canon law throughout Europe in the 1160s, and was sent to Portugal to crown Afonso II. He also brought an offer of reconciliation in 1163 to Holy Roman Emperor Frederick I Barbarossa, whom Pope Alexander III had excommunicated in 1160. Alexander also sent him to England to investigate the murder of Thomas Becket, and he absolved King Henry II of England of the murder during the Council of Avranches. From 1177-1179, Alberto also served as a legate in Italy and in February 1178 was named Chancellor of the Holy Roman Church. As Chancellor he generally pursued a conciliatory line toward the Emperor; in the controversy over the disputed succession of the Archbishopric of Trier he argued strongly in favor of setting aside both the pro-papal candidate Folmar of Karden and the pro-imperial Rudolf of Wied, and allowing the canons of Trier to hold a new election, but was overruled by Pope Urban III. It was in this position that di Morra "...compiled a "Forma Dicendi", a collection of official papal acts, and also completed a codification of the cursus, a compilation of the very stringent rules governing the euphonious arrangements of sentence endings and phrasing in papal acts. In his honor, the cursus was called "stylus gregorianus"." These two documents were very influential in shaping the rhetoric used in papal documents. Shortly before his election to the papacy, Alberto founded a monastery in his hometown of Benevento.

On 21 October 1187, the day after the death of Pope Urban III, Alberto di Morra was elected pope and took the name Gregory VIII in honor of Pope Gregory VII. He was consecrated on 25 October. His previous dealings with Frederick Barbarossa put the church back in a friendly relationship with the Holy Roman Emperor. In response to the defeat of the crusader Kingdom of Jerusalem at the Battle of Hattin, Gregory issued the papal bull "Audita tremendi" calling for the Third Crusade. Gregory travelled to Pisa in order to end Pisan hostilities with Genoa so that both seaports and naval fleets could join together for the crusade. On the way to Pisa, he stopped at Lucca and ordered Antipope Victor IV's body to be removed from his tomb and his remains thrown out of the church.


What is the most common type of star?

Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot be easily observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf (Type M5, apparent magnitude 11.05), as are fifty of the sixty nearest stars.
According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way.

Red dwarf

A red dwarf (or M dwarf) is a small and cool star on the main sequence, of M spectral type. Red dwarfs range in mass from about 0.075 to about 0.50 solar mass and have a surface temperature of less than 4,000 K. Sometimes K-type main-sequence stars, with masses between 0.50-0.8 solar mass, are also included.

Stellar models indicate that red dwarfs less than are fully convective. Hence the helium produced by the thermonuclear fusion of hydrogen is constantly remixed throughout the star, avoiding helium buildup at the core, thereby prolonging the period of fusion. Red dwarfs therefore develop very slowly, maintaining a constant luminosity and spectral type for trillions of years, until their fuel is depleted. Because of the comparatively short age of the universe, no red dwarfs exist at advanced stages of evolution.

The term "red dwarf" when used to refer to a star does not have a strict definition. One of the earliest uses of the term was in 1915, used simply to contrast "red" dwarf stars from hotter "blue" dwarf stars. It became established use, although the definition remained vague. In terms of which spectral types qualify as red dwarfs, different researchers picked different limits, for example K8–M5 or "later than K5". "Dwarf M star", abbreviated dM, was also used, but sometimes it also included stars of spectral type K.

In modern usage, the definition of a "red dwarf" still varies. When explicitly defined, it typically includes late K- and early to mid-M-class stars, but in many cases it is restricted just to M-class stars. In some cases all K stars are included as red dwarfs, and occasionally even earlier stars.

The coolest true main-sequence stars are thought to have spectral types around L2 or L3, but many objects cooler than about M6 or M7 are brown dwarfs, insufficiently massive to sustain hydrogen-1 fusion.

Red dwarfs are very-low-mass stars. As a result, they have relatively low pressures, a low fusion rate, and hence, a low temperature. The energy generated is the product of nuclear fusion of hydrogen into helium by way of the proton–proton (PP) chain mechanism. Hence, these stars emit little light, sometimes as little as that of the Sun. Even the largest red dwarfs (for example HD 179930, HIP 12961 and Lacaille 8760) have only about 10% of the Sun's luminosity. In general, red dwarfs less than transport energy from the core to the surface by convection. Convection occurs because of opacity of the interior, which has a high density compared to the temperature. As a result, energy transfer by radiation is decreased, and instead convection is the main form of energy transport to the surface of the star. Above this mass, a red dwarf will have a region around its core where convection does not occur.
Because low-mass red dwarfs are fully convective, helium does not accumulate at the core, and compared to larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence. As a result, red dwarfs have estimated lifespans far longer than the present age of the universe, and stars less than have not had time to leave the main sequence. The lower the mass of a red dwarf, the longer the lifespan. It is believed that the lifespan of these stars exceeds the expected 10-billion-year lifespan of our Sun by the third or fourth power of the ratio of the solar mass to their masses; thus, a red dwarf may continue burning for 10 trillion years. As the proportion of hydrogen in a red dwarf is consumed, the rate of fusion declines and the core starts to contract. The gravitational energy released by this size reduction is converted into heat, which is carried throughout the star by convection.

According to computer simulations, the minimum mass a red dwarf must have in order to eventually evolve into a red giant is ; less massive objects, as they age, would increase their surface temperatures and luminosities becoming blue dwarfs and finally white dwarfs.

The less massive the star, the longer this evolutionary process takes. It has been calculated that a red dwarf (approximately the mass of the nearby Barnard's Star) would stay on the main sequence for 2.5 trillion years, followed by five billion years as a blue dwarf, during which the star would have one third of the Sun's luminosity () and a surface temperature of 6,500–8,500 kelvins.

The fact that red dwarfs and other low-mass stars still remain on the main sequence when more massive stars have moved off the main sequence allows the age of star clusters to be estimated by finding the mass at which the stars move off the main sequence. This provides a lower limit to the age of the Universe and also allows formation timescales to be placed upon the structures within the Milky Way, such as the Galactic halo and Galactic disk.

All observed red dwarfs contain "metals", which in astronomy are elements heavier than hydrogen and helium. The Big Bang model predicts that the first generation of stars should have only hydrogen, helium, and trace amounts of lithium, and hence would be of low metallicity. With their extreme lifespans, any red dwarfs that were a part of that first generation (population III stars) should still exist today. Low metallicity red dwarfs, however, are rare. There are several explanations for the missing population of metal-poor red dwarfs. The preferred explanation is that, without heavy elements, only large stars can form. Large stars rapidly burn out and explode as supernova, spewing heavy elements that then allow higher metallicity stars population II stars, including red dwarfs to form. Alternative explanations of the scarcity of metal-poor red dwarfs, such as their dimness and scarcity, are considered less likely because they appear to conflict with stellar-evolution models.

The spectral standards for M-type stars have changed slightly over the years, but settled down somewhat since the early 1990s. Part of this is due to the fact that even the nearest red dwarfs are fairly faint, and the study of mid- to late-M dwarfs has progressed only in the past few decades due to evolution of astronomical techniques, from photographic plates to charged-couple devices (CCDs) to infrared-sensitive arrays.

The revised Yerkes Atlas system (Johnson & Morgan 1953) listed only 2 M-type spectral standard stars: HD 147379 (M0 V)
and HD 95735/Lalande 21185 (M2 V). While HD 147379 was not considered a standard by expert classifiers in later compendia of standards, Lalande 21185 is still a primary standard for M2 V. Robert Garrison does not list any "anchor" standards among the red dwarfs, but Lalande 21185 has survived as a M2 V standard through many compendia. The review on MK classification by Morgan & Keenan (1973) did not contain red dwarf standards. In the mid-1970s, red dwarf standard stars were published by Keenan & McNeil (1976) and Boeshaar (1976), but unfortunately there was little agreement among the standards. As later cooler stars were identified through the 1980s, it was clear that an overhaul of the red dwarf standards was needed. Building primarily upon the Boeshaar standards, a group at Steward Observatory (Kirkpatrick, Henry, & McCarthy 1991) filled in the spectral sequence from K5 V to M9 V. It is these M-type dwarf standard stars which have largely survived as the main standards to the modern day. There have been negligible changes in the red dwarf spectral sequence since 1991. Additional red dwarf standards were compiled by Henry et al. (2002), and D. Kirkpatrick has recently
reviewed the classification of red dwarfs and standard stars in Gray & Corbally's 2009 monograph. The M-dwarf primary spectral standards are: GJ 270 (M0 V), GJ 229A (M1 V), Lalande 21185 (M2 V), Gliese 581 (M3 V), Gliese 402 (M4 V), GJ 51 (M5 V), Wolf 359 (M6 V), Van Biesbroeck 8 (M7 V), VB 10 (M8 V), LHS 2924 (M9 V).

Many red dwarfs are orbited by exoplanets, but large Jupiter-sized planets are comparatively rare. Doppler surveys of a wide variety of stars indicate about 1 in 6 stars with twice the mass of the Sun are orbited by one or more Jupiter-sized planets, versus 1 in 16 for Sun-like stars and only 1 in 50 for red dwarfs. On the other hand, microlensing surveys indicate that long-orbital-period Neptune-mass planets are found around one in three red dwarfs.

Observations with HARPS further indicate 40% of red dwarfs have a "super-Earth" class planet orbiting in the habitable zone where liquid water can exist on the surface. Computer simulations of the formation of planets around low mass stars predict that Earth-sized planets are most abundant, but more than 90% of the simulated planets are at least 10% water by mass, suggesting that many Earth-sized planets orbiting red dwarf stars are covered in deep oceans.
At least four and possibly up to six exoplanets were discovered orbiting within the Gliese 581 planetary system between 2005 and 2010. One planet has about the mass of Neptune, or 16 Earth masses (). It orbits just 6 million kilometers (0.04 AU) from its star, and is estimated to have a surface temperature of 150 °C, despite the dimness of its star. In 2006, an even smaller exoplanet (only ) was found orbiting the red dwarf OGLE-2005-BLG-390L; it lies 390 million km (2.6 AU) from the star and its surface temperature is −220 °C (53 K).

In 2007, a new, potentially habitable exoplanet, Gliese 581c, was found, orbiting Gliese 581. The minimum mass estimated by its discoverers (a team led by Stephane Udry) is. The discoverers estimate its radius to be 1.5 times that of Earth (). Since then Gliese 581d, which is also potentially habitable, was discovered.

Gliese 581c and d are within the habitable zone of the host star, and are two of the most likely candidates for habitability of any exoplanets discovered so far. Gliese 581g, detected September 2010, has a near-circular orbit in the middle of the star's habitable zone. However, the planet's existence is contested.

On 23 February 2017 NASA announced the discovery of seven Earth-sized planets orbiting the red dwarf star TRAPPIST-1 approximately 39 light-years away in the constellation Aquarius. The planets were discovered through the transit method, meaning we have mass and radius information for all of them. TRAPPIST-1e, f and g appear to be within the habitable zone and may have liquid water on the surface.

Planetary habitability of red dwarf systems is subject to some debate. In spite of their great numbers and long lifespans, there are several factors which may make life difficult on planets around a red dwarf. First, planets in the habitable zone of a red dwarf would be so close to the parent star that they would likely be tidally locked. This would mean that one side would be in perpetual daylight and the other in eternal night. This could create enormous temperature variations from one side of the planet to the other. Such conditions would appear to make it difficult for forms of life similar to those on Earth to evolve. And it appears there is a great problem with the atmosphere of such tidally locked planets: the perpetual night zone would be cold enough to freeze the main gases of their atmospheres, leaving the daylight zone bare and dry. On the other hand, recent theories propose that either a thick atmosphere or planetary ocean could potentially circulate heat around such a planet.

Variability in stellar energy output may also have negative impacts on the development of life. Red dwarfs are often flare stars, which can emit gigantic flares, doubling their brightness in minutes. This variability may also make it difficult for life to develop and persist near a red dwarf. It may be possible for a planet orbiting close to a red dwarf to keep its atmosphere even if the star flares. However, more-recent research suggests that these stars may be the source of constant high-energy flares and very large magnetic fields, diminishing the possibility of life as we know it. Whether this is a peculiarity of the star under examination or a feature of the entire class remains to be determined.

Red dwarf stars are the smallest, coolest, and most common type of star. Estimates of their abundance range from 70% of stars in spiral galaxies to more than 90% of all stars in elliptical galaxies, an often quoted median figure being 73% of the stars in the Milky Way (known since the 1990s from radio telescopic observation to be a barred spiral). Red dwarfs are either late K or M spectral type. Given their low energy output, red dwarfs are never visible by the unaided eye from Earth; neither the closest red dwarf to the Sun when viewed individually, Proxima Centauri (which is also the closest star to the Sun), nor the closest solitary red dwarf, Barnard's star, is anywhere near visual magnitude.

Habitability of red dwarf systems

The habitability of red dwarf systems is determined by a large number of factors from a variety of sources. Although the low stellar flux, high probability of tidal locking, small circumstellar habitable zones, and high stellar variation experienced by planets of red dwarf stars are impediments to their planetary habitability, the ubiquity and longevity of red dwarfs are positive factors. Determining how the interactions between these factors affect habitability may help to reveal the frequency of extraterrestrial life and intelligence.

Intense tidal heating caused by the proximity of planets to their host red dwarfs is a major impediment to life developing in these systems. Other tidal effects, such as the extreme temperature differences created by one side of habitable-zone planets permanently facing the star and the other perpetually turned away and lack of planetary axial tilts, reduce the probability of life around red dwarfs. Non-tidal factors, such as extreme stellar variation, spectral energy distributions shifted to the infrared relative to the Sun, and small circumstellar habitable zones due to low light output, further reduce the prospects for life in red-dwarf systems.

There are, however, several effects that increase the likelihood of life on red dwarf planets. Intense cloud formation on the star-facing side of a tidally locked planet may reduce overall thermal flux and drastically reduce equilibrium temperature differences between the two sides of the planet. In addition, the sheer number of red dwarfs, which account for about 85% of at least 100 billion stars in the Milky Way, statistically increases the probability that there might exist habitable planets orbiting some of them. As of 2013, there are expected to be tens of billions of super-Earth planets in the habitable zones of red dwarf stars in the Milky Way.

For years, astronomers ruled out red dwarfs, with masses ranging from roughly 0.08 to 0.45 solar masses (), as potential abodes for life. The low masses of the stars cause the nuclear fusion reactions at their cores to proceed exceedingly slowly, giving them luminosities ranging from a maximum of roughly 3 percent that of the Sun to a minimum of just 0.01 percent. Consequently, any planet orbiting a red dwarf would have to have a low semimajor axis in order to maintain Earth-like surface temperature, from 0.268 astronomical units (AU) for a relatively luminous red dwarf like Lacaille 8760 to 0.032 AU for a smaller star like Proxima Centauri, the nearest star to the Solar System. Such a world would have a year lasting just six days.

Much of the low luminosity of a red dwarf falls in the infrared part of the electromagnetic spectrum, with lower energy than the visible light in which the Sun peaks. As a result, photosynthesis on a red dwarf planet would require additional photons to achieve excitation potentials comparable to those needed in Earth photosynthesis for electron transfers, due to the lower average energy level of near-infrared photons compared to visible. Having to adapt to a far wider spectrum to gain the maximum amount of energy, foliage on a habitable red dwarf planet would probably appear black if viewed in visible light.

In addition, because water strongly absorbs red and infrared light, less energy would be available for aquatic life on red dwarf planets. However, a similar effect of preferential absorption by water ice would increase its temperature relative to an equivalent amount of radiation from a Sun-like star, thereby extending the habitable zone of red dwarfs outward.

Another fact that would inhibit habitability is the evolution of the red dwarf stars; as such stars have an extended pre-main sequence phase, their eventual habitable zones would be for around 1 billion years a zone where water wasn't liquid but in its gaseous state. Thus, terrestrial planets in the actual habitable zones, if provided with abundant surface water in their formation, would have been subject to a runaway greenhouse effect for several hundred million years. During such an early runaway phase, photolysis of water vapor would allow hydrogen escape to space and the loss of several Earth oceans of water, leaving a thick abiotic oxygen atmosphere.

At the close orbital distances planets around red dwarf stars would have to maintain for liquid water to exist at their surfaces, tidal locking to the host star is likely, causing the planet to rotate around its axis once for every revolution around the star; as a result, one side of the planet would eternally face the star and another side would perpetually face away, creating great extremes of temperature. For many years, it was believed that life on such planets would be limited to a ring-like region known as the terminator, where the star would always appear on the horizon.

It was also believed that efficient heat transfer between the sides of the planet necessitates atmospheric circulation of an atmosphere so thick as to disallow photosynthesis. Due to differential heating, it was argued, a tidally locked planet would experience fierce winds with permanent torrential rain at the point directly facing the local star, the subsolar point. In the opinion of one author this makes complex life improbable. Plant life would have to adapt to the constant gale, for example by anchoring securely into the soil and sprouting long flexible leaves that do not snap. Animals would rely on infrared vision, as signaling by calls or scents would be difficult over the din of the planet-wide gale. Underwater life would, however, be protected from fierce winds and flares, and vast blooms of black photosynthetic plankton and algae could support the sea life.

In contrast to the previously bleak picture for life, 1997 studies by Robert Haberle and Manoj Joshi of NASA's Ames Research Center in California have shown that a planet's atmosphere (assuming it included greenhouse gases CO and HO) need only be 100 millibar, or 10% of Earth's atmosphere, for the star's heat to be effectively carried to the night side, a figure well within the bounds of photosynthesis. Research two years later by Martin Heath of Greenwich Community College has shown that seawater, too, could effectively circulate without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap. Additionally, a 2010 study concluded that Earth-like water worlds tidally locked to their stars would still have temperatures above on the night side. Climate models constructed in 2013 indicate that cloud formation on tidally locked planets would minimize the temperature difference between the day and the night side, greatly improving habitability prospects for red dwarf planets. Further research, including a consideration of the amount of photosynthetically active radiation, has suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.

The existence of a permanent day side and night side is not the only potential setback for life around red dwarfs. Tidal heating experienced by planets in the habitable zone of red dwarfs less than 30% of the mass of the Sun may cause them to be "baked out" and become "tidal Venuses." Combined with the other impediments to red dwarf habitability, this may make the probability of many red dwarfs hosting life as we know it very low compared to other star types. There may not even be enough water for habitable planets around many red dwarfs; what little water found on these planets, in particular Earth-sized ones, may be located on the cold night side of the planet. In contrast to the predictions of earlier studies on tidal Venuses, though, this "trapped water" may help to stave off runaway greenhouse effects and improve the habitability of red dwarf systems.

Moons of gas giants within a habitable zone could overcome this problem since they would become tidally locked to their primary and not their star, and thus would experience a day-night cycle. The same principle would apply to double planets, which would likely be tidally locked to each other.

Note however that how quickly tidal locking occurs can depend upon a planet's oceans and even atmosphere, and may mean that tidal locking fails to happen even after many gigayears. Additionally, tidal locking is not the only possible end state of tidal dampening. Mercury, for example, has had sufficient time to tidally lock, but is in a 3:2 spin orbit resonance.

Red dwarfs are far more variable and violent than their more stable, larger cousins. Often they are covered in starspots that can dim their emitted light by up to 40% for months at a time. On Earth life has adapted in many ways to the similarly reduced temperatures of the winter. Life may survive by hibernating and/or by diving into deep water where temperatures could be more constant. Oceans would potentially freeze over during extreme cold periods. If so, once the dim period ends, the planet’s albedo would be higher than it was prior to the dimming. This means more light from the red dwarf would be reflected, which would impede temperatures from recovering, or possibly further reduce planetary temperatures.

At other times, red dwarfs emit gigantic flares that can double their brightness in a matter of minutes. Indeed, as more and more red dwarfs have been scrutinized for variability, more of them have been classified as flare stars to some degree or other. Such variation in brightness could be very damaging for life. Flares might also produce torrents of charged particles that could strip off sizable portions of the planet's atmosphere. Scientists who subscribe to the Rare Earth hypothesis doubt that red dwarfs could support life amid strong flaring. Tidal-locking would probably result in a relatively low planetary magnetic moment. Active red dwarfs that emit coronal mass ejections would bow back the magnetosphere until it contacted the planetary atmosphere. As a result, the atmosphere would undergo strong erosion, possibly leaving the planet uninhabitable.
Otherwise, it is suggested that if the planet had a magnetic field, it would deflect the particles from the atmosphere (even the slow rotation of a tidally locked M-dwarf planet—it spins once for every time it orbits its star—would be enough to generate a magnetic field as long as part of the planet's interior remained molten). But actual mathematical models conclude that, even under the highest attainable dynamo-generated magnetic field strengths, exoplanets with masses like that of Earth lose a significant fraction of their atmospheres by the erosion of the exobase's atmosphere by CME bursts and XUV emissions (even those Earth-like planets closer than 0.8 AU—affecting also GK stars— probably lose their atmospheres). Atmospheric erosion even could trigger the depletion of water oceans. Planets shrouded by a thick haze of hydrocarbons like the one on primordial Earth or Saturn's moon Titan might still survive the flares as floating droplets of hydrocarbon are really efficient at absorbing ultraviolet radiation.

Another way that life could initially protect itself from radiation, would be remaining underwater until the star had passed through its early flare stage, assuming the planet could retain enough of an atmosphere to sustain liquid oceans. The scientists who wrote Aurelia believed that life could survive on land despite a red dwarf flaring. Once life reached onto land, the low amount of UV produced by a quiescent red dwarf means that life could thrive without an ozone layer, and thus never need to produce oxygen.

It is worth noting that the violent flaring period of a red dwarf's life cycle is estimated to only last roughly the first 1.2 billion years of its existence. If a planet forms far away from a red dwarf so as to avoid tidelock, and then migrates into the star's habitable zone after this turbulent initial period, it is possible for life to have a chance to develop.

The major advantage that red dwarfs have over other stars as abodes for life: they produce light energy for a very long time. It took 4.5 billion years before humans appeared on Earth, and life as we know it will see suitable conditions for as little as half a billion years more. Red dwarfs, by contrast, could exist for trillions of years, because their nuclear reactions are far slower than those of larger stars, meaning that life both would have longer to evolve and longer to survive. Furthermore, although the odds of finding a planet in the habitable zone around any specific red dwarf are unknown, the total amount of habitable zone around all red dwarfs combined is equal to the total amount around Sun-like stars given their ubiquity. The first super-Earth with a mass of a 3 to 4 times that of Earth's found in the potentially habitable zone of its star is Gliese 581g, and its star, Gliese 581, is indeed a red dwarf. Although tidally locked, it is thought possible that at its terminator liquid water may well exist. The planet is thought to have existed for approximately 7 billion years and has a large enough mass to support an atmosphere.

Another possibility could come in the far future, when according to computer simulations a red dwarf becomes a blue dwarf as it is exhausting its hydrogen supply. As this kind of star is more luminous than the previous red dwarf, planets orbiting it that were frozen during the former stage could be "thawed" during the several billions of years this evolutionary stage lasts (5 billion years, for example, for a star), giving life an opportunity to appear and evolve.

Planets can retain significant amounts of water in the habitable zone of ultracool dwarfs, with a sweet spot in the 0.04-0.06 M range, despite FUV-photolysis of water and the XUV -driven escape of hydrogen.

Water worlds exoplanets orbiting M-dwarfs, could have their oceans depleted over the Gyr timescale due to the more intense particle and radiation environments that exoplanets experience in close-in habitable zones. If the atmosphere were to be depleted over the timescale less than Gyr, this could prove to be problematic for the origin of life (abiogenesis) on the planet.

If methane-based life is possible (similar to the hypothetical life on Titan), there would be a second habitable zone further out from the star corresponding to the region where methane is liquid. Titan's atmosphere is transparent to red and infrared light, so more of the light from red dwarfs would be expected to reach the surface of a Titan-like planet.

A study of archival Spitzer data gives the first idea and estimate of how frequent Earth-sized worlds are around ultra-cool dwarf stars: 30-45%. A computer simulation finds that planets that form around stars with similar mass to TRAPPIST-1 (c. 0.08 M), most likely have sizes similar to the Earth.

The following examples of fictional "aliens" existing within Red Dwarf star systems exist: