Question: what are the densest stars known to exist
[{'answer': 'Neutron stars', 'provenance': [{'wikipedia_id': '21869', 'title': 'Neutron star', 'start_paragraph_id': 1, 'start_character': 127, 'end_paragraph_id': 1, 'end_character': 140, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]