When retired Villanova University astronomy professor Carol Ambruster was stabbed to death in her Germantown apartment on Dec. 9, 2013, what was expected to be a normal day turned into “a nightmare,” a prosecutor told a Philadelphia jury Tuesday.

Upon returning to her second-floor apartment at 5501 Wayne Ave. about 5:50 p.m., Ambruster, 69, encountered Jose Diaz, a handyman who worked in the building, Assistant District Attorney Jason Grenell said.

“Mr. Diaz got a knife from Ms. Ambruster’s kitchen, and he stabbed her again and again and again,” altogether 11 times with the 8-inch blade, Grenell said. “He would shove that blade into her throat.” The knife remained lodged there when she was found by her roommate.

The prosecutor alleged in his opening statement that Diaz — now 48 and charged with murder, robbery, and related offenses — killed Ambruster “all for a couple of bucks.”

“Irrefutable forensic evidence” — Ambruster’s DNA found on a pair of size 8 Timberland boots in Diaz’s Germantown home, and Diaz’s bloody fingerprint discovered on Ambruster’s belt — pointed to Diaz as her killer, Grenell said.

But defense attorney Gina Amoriello told jurors in her opening statement that Diaz did not kill Ambruster and pointed the blame elsewhere. The victim “had defensive wounds on her hands,” Amoriello told jurors. And “DNA evidence under her nails” belonged to her roommate, Daniel Sapon, the attorney said.

Sapon, 69, testified Tuesday that he was a close friend of Ambruster’s and had met her around 1982. They had never been romantically involved, he said. He moved into her four-bedroom apartment about six months before she was killed, and they had separate bedrooms and bathrooms.

Asked by Grenell, Sapon said he and Ambruster shared some things in the apartment, including items in the kitchen such as sponges and kitchen towels. He said both sat in the TV room, and both petted her three cats.

On that Dec. 9, after he returned from work by bus to the neighborhood about 5:30 p.m., he said, he didn’t go inside the apartment building but instead went to his car, parked on the street, because he wanted to drive to a store in Queen Village.

Later that night, when he returned to the apartment and put his key in the door, he thought it “very odd” that the door was unlocked. The lights were off.

After turning on the hallway light, he saw “strange brown stains smeared on the floor" and “boot prints,” he said. He then realized the stains might be blood and called out Ambruster’s name, but “it was silent."

After turning on another light, he saw her body on the floor and called 911 from the apartment phone, but the phone appeared to be dead, he said. “I was very agitated. I couldn’t get through to them,” he testified. “I was speaking out loud, ‘Carol, what happened to you?’”

He went into the second-floor hallway and knocked on doors for help, then saw another resident coming down the stairs.

Sapon didn’t realize it, but his 8:56 p.m. call to 911 had connected. When Grenell played a recording of it, jurors first heard heavy breathing, then a man’s voice saying, “Oh my God, Carol! Carol? Carol,” then more heavy breathing. Then a female dispatcher said: “Philadelphia police. Hello?”

As the 911 call was played, Sapon, sitting at the witness stand, winced.

Asked under cross-examination by Amoriello why he had not used his cell phone to call 911, Sapon said he had left it in his car.

Jesse Brandfass, the resident whom Sapon had seen descending the stairs, testified that when he got to the second floor, Sapon “was visibly shaken” and kept saying he needed a phone. Brandfass, then a medical student at Temple University, went into the apartment with Sapon and called 911. He said he checked her pulse and found none.

“There was blood everywhere,” Brandfass said. “In the entryway. A trail of blood down into the hall to the kitchen.”

During his 8:59 p.m. call to police, Brandfass provided the building’s address and said: “I think there’s been a murder.”

The trial continues Wednesday before Common Pleas Court Judge J. Scott O’Keefe.