Police did not provide a motive for the killing or say whether Ambruster knew Diaz.
Detectives said there were no signs of forced entry at her apartment, suggesting that she knew her killer or was ambushed.
The roommate, Daniel Sapon, told police that when he returned home from running errands, he found the door unlocked.
Police had been trying to determine whether anything had been taken from Ambruster's blood-splattered apartment, which was filled with collections of American Indian art, books, antique bottles, stamps, and jewelry.
This week, police sources said, homicide detectives had interviewed a subcontractor who had done work in the apartment building and had keys to Ambruster's unit. The contractor had a criminal history, the sources said.
The sources said investigators were zeroing in on the subcontractor, whom they did not identify at the time. The sources also said the man had no wounds, which they expected Ambruster's killer to have because the struggle had been violent. He did, however, have blood on his boots and pants, and police were waiting for lab results before making their next move.
Ambruster had defensive wounds, including a deep to cut to one hand.
Police also said they believed the attack began at the door to her apartment and ended in the kitchen, where her body was found.
Ambruster, who was a professor of astronomy, was described by friends and colleagues as bright, warm, and kind, and devoted to her three cats.
She was dedicated to astronomy and continued to work on academic papers even in retirement. She made trips to New Mexico to study early astronomical uses of rock formations and buildings at Chaco Canyon.
Ambruster was a convert to Judaism and attended P'nai Or, a Renewal congregation in Mount Airy that incorporates Hasidic prayer and mystical teachings into its liturgy.
Elizabeth Jewell, Ambruster's closest friend at Villanova, welcomed the news of the arrest Friday night.
Jewell, who worked on the staff of the Villanova astronomy department, said she wanted to find out why someone would kill her friend of 25 years.
"We ask ourselves that," she said, "but we don't have the answer."
Inquirer staff writer Julie Zauzmer contributed to this article.