LOS ANGELES - As the events in Baltimore continue to grip the nation, a few celebrities are raising their voices for the city and the cause. But experts say many stars are reluctant to address such heated emotional issues publicly.

Artists such as Beyonce, Cher, Dwayne Johnson, Jada Pinkett Smith and "Grey's Anatomy" actor Jesse Williams have taken to social media to demand change and share their thoughts on the death of 25-year-old Freddie Gray while in police custody.

And some celebrities are expressing themselves in other ways.

Former Baltimore Ravens linebacker Ray Lewis and New York Knicks star Carmelo Anthony, who was raised in Baltimore, marched with the protesters this week.

Prince recorded a song about Baltimore and "the slew of killings of young black men," said a spokeswoman. The performer also planned a "Dance Rally 4 Peace" in Minneapolis on Saturday night, asking everyone to wear gray in memory of Freddie Gray.

But many other celebrities aren't speaking out in any form - and not just because it could tarnish their valuable brands.

"This is an issue that's difficult for a lot of people because people in this society often have a hard time criticizing police," said professor Todd Boyd, chair for the study of race and popular culture at the University of Southern California. "People in Hollywood are very aware of what the status quo is in terms of politics and they're very good at aligning themselves in relationship to the status quo."

Boyd suggested some stars' silence could also be because they agree with police treatment of Gray.

"I think a lot of people in this society feel that cops are justified in being suspicious of African-American males," he said.

"United States: A land where the National Guard is more quickly mobilized to beat black people than to rescue them from a hurricane." - Rage Against the Machine co-founder Tom Morello on Instagram.

Even when celebrities do speak out about hot-button issues - and they have some of the loudest voices on the planet - they may not have their usual influence on matters of the moment.

"The challenge is, when you are looking to influence a very small group of people's behavior, what matters most isn't what Hollywood thinks. It's what their peers think," said branding expert Dorie Clark. "A police officer in the middle of an altercation is not going to be thinking, when he's weighing whether or not to shoot someone, what such-and-such actor says."

But through it all, image remains the overriding concern for celebrities, who can damage their reputations by saying the wrong thing, says veteran Hollywood publicist Howard Bragman, especially if the artists are uninformed.

"It's really important that it's appropriate to their brand, number one," he said. "And, number two, that they know what they're talking about."

The more immediate and controversial the issue is, the trickier the territory is for stars, Clark said.

"It is far easier for a celebrity to talk about stopping hunger or helping kids or saving pets, because everyone will think it's great and no one will criticize them," she said, adding that most stars need a compelling reason, "usually a reason of conscience," to publicly address potentially polarizing issues.

"It becomes really important for a celebrity to have standing to speak out - meaning do they have some connection to the issue, some personal experience or some particular knowledge to share," she said. "Otherwise, it can look like news-jacking, where a celebrity comments on a timely topic to get their name in the headlines."

Some of the celebrity voices heard in the wake of the Baltimore unrest have been simple pleas for peace, and from stars with direct connections to the issue.

"Don't give the very system that has been using violence to terrorize and exterminate us any justification to use that very violence upon us to keep us in order," Baltimore native Pinkett Smith wrote on Facebook Wednesday. "This is a time of self-discipline so that the true offenders of justice can be revealed."