WASHINGTON - He walked into a room named for Theodore and Franklin D. Roosevelt, just behind President Obama and Vice President Biden, as members of the president's inner circle and national defense elites stood and applauded.

Ashton Carter, born in Philadelphia and raised in Abington, had a bright moment in the spotlight Friday morning as he was nominated to become secretary of defense, which would put him in charge of the world's most powerful military and a host of vexing challenges.

More moments in the spotlight await, many considerably more difficult than Friday's upbeat announcement.

For a moment, Carter, 60, could laugh at his new boss' jokes, thank cherished mentors, and exchange hugs with Treasury Secretary Jack Lew, CIA Director John Brennan, and others. The president greeted Carter's wife, Stephanie, with a kiss on the cheek.

"With a record of service that has spanned more than 30 years - as a public servant, as an adviser, as a scholar - 'Ash' is rightly regarded as one of our nation's foremost national security leaders," Obama told an audience that included Carter's mother-in-law, members of the Cabinet, and international policy leaders such as Brent Scowcroft, a Carter mentor who was national security adviser to Presidents Gerald R. Ford and George H.W. Bush.

Carter has served under 11 defense secretaries, Obama said. He has decades of experience inside the Pentagon as an aide, and was the Defense Department's second-highest-ranking official until leaving late last year.

"In your one-year attempt at retirement from public service," Obama deadpanned, "you've failed miserably."

Carter studied physics and medieval history at Yale - he wrote senior theses on the use of Latin by monastic writers to describe 12th-century Flanders and on the "charmed quark," according to an autobiographical sketch he wrote for Harvard's Kennedy School of Government. He later earned a doctorate in theoretical physics as a Rhodes scholar at Oxford.

Obama said Carter also loves Motown music.

His physics background, Obama half-joked, "means that he's one of the few people who actually understands how many of our defense systems work."

That background is what pushed Carter into defense work. He never served in the military, but during the Cold War was recruited to join a team trying to devise ways to protect U.S. missiles from Russian nuclear attack. (One idea: Move the 200,000-pound missiles to the skies on constantly shifting balloons.)

Over decades moving between the Pentagon and academia, Carter rose to become undersecretary of defense for acquisition, technology, and logistics, and then deputy defense secretary.

The technocratic job of buying weapons and overseeing budgets is unglamorous but important, said Matthew Kroenig, an associate professor at Georgetown University and former special adviser at the Pentagon.

"It has a huge budget, and because the acquisition timelines are so long, basically, what you're doing is building the future of the U.S. military," Kroenig said. "Budgeting decisions [Carter] was making a couple years ago are going to affect the military we have in 2025."

Obama praised Carter as "an innovator" and "reformer who's never been afraid to cancel old or inefficient weapons programs."

He added, "He knows the Department of Defense inside and out - all of which means that on day one, he's going to hit the ground running."

Scowcroft issued a statement Friday lauding Carter's "heartfelt devotion" to servicemen and women, and saying he "understands that national security must be bipartisan."

Carter will run into challenges on both policy and politics.

If confirmed by the Senate, he'll enter the job as the nation tries to end its military role in Afghanistan, escalates its conflict with the Islamic State in Syria and Iraq, warily eyes a more aggressive Russia, and delicately deals with a rising China.

Domestically, the Defense Department is facing budget cuts due to "sequestration," and Obama has urged the military to get leaner and prepare for modern threats such as cyber warfare.

Some struggles may come internally. Former defense secretaries have blasted Obama as relying on a close circle of White House advisers rather than his national security team.

Numerous reports have said that Obama's outgoing defense secretary, Chuck Hagel, could never pierce that inner circle, hurting his ability to shape policy. At least two top candidates to replace him took themselves out of the running, a sign of the job's diminished appeal.

Hagel, critics said, struggled as a public voice of the administration. He did not attend Friday's announcement. He released a supportive statement.

Carter could "have limited influence" over policy and "be subject to incessant micromanagement by the White House," Sen. John McCain (R., Ariz.), who is in line to lead the armed services committee, said in a news release.

McCain praised Carter's qualifications, but added that he looked forward to hearings at which he could "fully ventilate" the administration's "feckless foreign policy."

The prospect of those hearings shows the job Carter confronts as the new face of the administration's defense policy - though he is expected to receive relatively easy confirmation, even from a Republican-controlled Senate. Many Republicans have praised him.

Carter is known for being blunt.

"When you sit down with Ash Carter, you get a very direct answer," said Sen. Robert P. Casey (D., Pa.). "That will help him, obviously, in the confirmation process."

Carter, speaking Friday from notes on a folded piece of paper, promised Obama he would offer the president his "most candid advice."

While he has long had a role as an adviser, more people than ever before will now be watching.

"At a very different level," Kroenig said, "the spotlight will be on him."