The court's action gives the justices the chance to say by late June whether gay Americans have the same constitutional right to marry as heterosexuals. Several narrower paths also are open to the justices as they consider both California's voter-approved Proposition 8 and the provision of the federal Defense of Marriage Act that denies to legally married gay Americans the favorable federal tax treatment and a range of federal health and pension benefits given to heterosexual couples.

The court is embarked on what could be its most significant term involving civil rights in decades. In the area of racial discrimination, the justices already have agreed to decide cases on affirmative action in admission to college and a key part of the Voting Rights Act. The gay marriage cases probably will be argued in March and decisions in all the court's cases are likely by the end of June.

The order from the court extends a dizzying pace of change regarding gay marriage that includes rapid shifts in public opinion, President Obama's endorsement in May and votes in Maine, Maryland and Washington in November to allow gay couples to marry. Same-sex couples in Washington began picking up marriage licenses on Thursday.

Yet, even as gay marriage is legal, or soon will be, in nine states -- Connecticut, Iowa, Massachusetts, New Hampshire, New York and Vermont are the others -- and the District of Columbia, it is banned by the state constitutions of 31 others. Federal courts in California have struck down the state's constitutional ban on same-sex marriage, but that ruling and, thus, gay unions remain on hold while the issue is being appealed.

The high court's decision to hear the federal benefit question was a virtual certainty because several lower courts struck down the provision of the 1996 law and the justices almost always step in when lower courts invalidate a federal law.

There is nothing that compelled a similar response from the court in the case over California's Proposition 8, the state constitutional ban on gay marriage that voters adopted in 2008 after the state Supreme Court ruled that gay Californians could marry. Indeed, the gay-marriage supporters who prevailed in the lower courts urged the Supreme Court to stay out of the case and allow same-sex unions to resume in the nation's largest state.

Even some gay-rights activists worried that it was too soon in the evolution of views toward same-sex marriage to ask the justices to intervene and declare that same-sex couples have the same right to marry as heterosexuals. But Theodore Olson, the Washington lawyer who represents Californians who sued over Proposition 8, said he will argue that there is a "fundamental constitutional right to marry for all citizens."

Opponents of gay marriage said Friday they are heartened by the Supreme Court's action.

"We believe that it is significant that the Supreme Court has taken the Prop 8 case," said John Eastman, chairman of the National Organization for Marriage and a law professor at Chapman University in Orange, Calif. "We believe it is a strong signal that the court will reverse the lower courts and uphold Proposition 8. That is the right outcome based on the law and based on the principle that voters hold the ultimate power over basic policy judgments and their decisions are entitled to respect."

On the other side of the issue, advocates for same-sex unions said the court could easily decide in favor of gay marriage in California without issuing a sweeping national ruling to overturn every state prohibition on marriage.

In striking down Proposition 8, the 9th U.S. Circuit Court of Appeals crafted a narrow ruling that said, because gay Californians already had been given the right to marry, the state could not later take it away. The ruling studiously avoided overarching pronouncements.

"I think the court can easily affirm the 9th Circuit's decision and leave for a later day whether broader bans on marriage are unconstitutional as well," said James Esseks of the American Civil Liberties Union.

The other issue the high court will take on involves a provision of the Defense of Marriage Act, known by its acronym DOMA, which defines marriage as between a man and a woman for the purpose of deciding who can receive a range of federal benefits.

Four federal district courts and two appeals courts struck down the provision. Last year, the Obama administration abandoned its defense of the law, but continues to enforce it. House Republicans are defending DOMA in the courts.

The justices chose for their review the case of 83-year-old Edith Windsor, who sued to challenge a $363,000 federal estate-tax bill after her partner of 44 years died in 2009.

Windsor, who goes by Edie, married Thea Spyer in 2007 after doctors told them that Spyer would not live much longer. She suffered from multiple sclerosis for many years. Spyer left everything she had to Windsor.

There is no dispute that if Windsor had been married to a man, her estate-tax bill would have been zero.

The U.S. 2nd Circuit Court of Appeals in New York agreed with a district judge that the provision of DOMA deprived Windsor of the constitutional guarantee of equal protection of the law.

In both cases, the justices have given themselves a technical way out, involving the legal issue of whether the parties have the required legal standing to bring their challenges, which would allow them to duck all the significant issues raised by opponents and supporters of gay marriage.

The cases are Hollingsworth v. Perry, 12-144, and U.S. v. Windsor, 12-307.