"We're looking at her background and her associates, but as of right now, we don't have any definitive leads," Homicide Capt. James Clark said.
Ambruster's background, her family and colleagues said, wouldn't have made her an enemy to anyone.
"She was really a very private person," said Mark Ambruster of Westchester County, N.Y., who described himself as a distant cousin.
Although she was retired from Villanova, Ambruster was still active in astronomy - particularly archaeoastronomy, the study of how past civilizations integrated stars into their cultures. In the spring and summer, she often traveled to the pueblos of Chaco Canyon, a national historic park about 150 miles northwest of Albuquerque.
Her killing was "just a total shock," said Ed Guinan, a professor of astronomy at Villanova.
Ambruster's roommate, a Philadelphia School District employee, found her body and contacted a neighbor, who called 9-1-1, Clark said. The roommate was not romantically involved with Ambruster, friends and colleagues said, but Clark said he was questioned. The roommate, who was back at the apartment building yesterday afternoon taking clothes and other belongings out to his car, declined to comment.
Police said there were no signs of forced entry at Ambruster's apartment. Gates secured both entrances to the building and the parking lot. Ambruster's car, still covered in snow, sat there yesterday.
People coming in and out of the apartment building said they didn't know Ambruster. Officers with the Police Department's Crime Scene Unit exited the building about 2:45 p.m. with a brown bag and looked inside Ambruster's car before leaving.
Ariel Ambruster, who lives near San Francisco, said her half sister was divorced from an early marriage and that her ex-husband had died in an accident. She said her sister had taken in a roommate to help pay expenses and that they had an agreement that he would help around the house.
"She was just helping the guy out," Ariel Ambruster said.
Clark said nothing appeared to have been stolen from the home, although police were still checking through her belongings. Ariel Ambruster said her half sister's most cherished possessions were her cats, but said colleagues and acquaintances believe that someone may have thought there were other valuables inside.
"I think the hypothesis going around is that some junkie or something like that broke in and expected to find something," Ariel Ambruster said. Clark said it appears that Ambruster died a few hours before she was found.
Ambruster grew up in California and attended Northeastern University and the University of Pennsylvania, where she earned a doctorate in astronomy in 1984. A member of the International Astronomical Union, she held a number of teaching and research positions, including a predoctoral research position at the Naval Research Laboratory in Washington, D.C. She was a postdoctoral research associate at the Joint Institute for Laboratory Astrophysics at the University of Colorado.
Ambruster taught at Villanova from 1987 to 2011, university spokesman Jonathan Gust said, and her areas of expertise included cool stars, dwarf stars, archaeoastronomy and flare stars.
"The university was saddened to learn of the death of Carol Ambruster, and our thoughts and prayers go out to her family and friends," Gust said.
Ambruster's former students described her as sweet and caring.
"She was mostly a quiet woman who kept to herself, but if you had the opportunity [to] talk with her, it was very easy," said Trisha Mizusawa, a 2009 graduate who served as Ambruster's teaching assistant for five semesters. "I was very sorry to hear about her death, and in such a violent way. I don't know who would want to hurt such a sweet older woman."
Ambruster converted to Judaism in recent years and moved to Germantown from Havertown to be closer to P'nai Or, a Jewish Renewal congregation on Greene Street near Westview in West Mount Airy. Rabbi Marcia Prager said Ambruster was active in the congregation.
"Carol was a refined, thoughtful person, highly educated and insightful," Prager said. "This is grievous for us here."
On Twitter: @JasonNark