ANN ARBOR, Mich. — The sounds from the bell tower resonate through the motherhouse of the Dominican Sisters of Mary Mother of the Eucharist.

They walk silently in procession as their white habits brighten an already well-lit chapel. The sisters genuflect before an ornate altar and then take a seat in their individual choir stalls. After a few minutes of prayer and chanting, a pipe organ's warm sound fills the room with the hymn “Angels We Have Heard on High.”

The sisters invite their fans around the world into their world this holiday season through their latest album, “Jesu, Joy of Man’s Desiring.” And surprisingly for a group that doesn’t tour, the album has bested the likes of Celine Dion and Josh Groban, positioning it at the top of Billboard’s classical music charts for weeks since its release late this fall.

“I have to say I like a lot of the ones on the tops of the charts,” says Sister Joseph Andrew Bogdanowicz, one of four original foundresses for the order and music director on the new album. “I do think one chord that we really do hit is that longing for the human heart for that contact with God, however they understand God to be.”

The sisters aren’t new by any account to the austere world of classical music recordings. They’ve previously released two other best-selling albums “Mater Eucharistae” and “The Rosary Mysteries, Meditations & Music.”

But there’s something about this latest album they believe is different.

“It isn't something that you hear every day on the street corners where you hear Christmas carols,” said Sister John Dominic Rasmussen. “We are giving them a glimpse of our life."

The very walls of their sacred chapel were designed with singing and chanting in mind.

“We're not making CDs for profit,” said Sister Mary Samuel Handwerker, who has overseen several construction projects for the religious community. “We happen to be able to share this music with others. But the profit that's gonna be made is to lift the minds and hearts of the people that are gonna hear it.”

The chapel at the Dominican Sisters of Mary Motherhouse decorated for Christmas. De Montfort Music

And as much as their singing is attracting new fans, they’re also winning hearts and minds: The Dominican Sisters of Mary now total 138 at their motherhouse in Ann Arbor, a new record. Officials within the religious community say they expect to begin work on a new convent in central Texas next year.

For an order that started out with four sisters living on a barn, that’s not a bad progress report some 20 years later after founding. The average number of nuns in the U.S. has dropped to approximately 45,000 from an all-time high of nearly 200,000 in the mid-1960s, according to Georgetown’s Center for Applied Research in the Apostolate.

America’s nuns are an aging group, too, with a majority of all religious sisters being in their upper 70s. But that’s not the case for the Dominican Sisters of Mary, where most sisters are in their late 20s to early 30s.

Sister Maria Veritas Marks joined the order in 2010 after a distinguished academic career at Harvard.

Sister Joseph Andrew Bogdanowicz accompanies a fellow violin playing sister (right) during vespers. NBC News

“I think when you pursue your vocation in life, of course you leave things behind,” she said. “That's part of choosing one thing that you have to leave everything else behind.”

Marks graduated as her class as salutatorian. Her salutatory address, which was a big YouTube hit, was delivered entirely in Latin.

Entering the order isn’t always so easy. It takes nearly eight years of study and preparation before a novice — a prospective member of a Catholic religious institute — takes their final vows and says “I do.”

“We’ve helped more realize their vocation is marriage because we need good families today,” said Bogdanowicz, who also consults with young women who believe they have a religious vocation.

Sister Mary Avila, in her third year with the community, says she felt her call through music the sisters put out.

"I really felt them saying 'Come, you can be a part of something beautiful and you can have heaven on earth,'" she said.