Investigation of the state of dense matter,especially at the supranuclear density,is closely linked with our understanding of the nature of quantum and gravity.It is still a hot topic in the new era of multi-messenger astronomy.The compact objects formed from this dense matter are the remains of massive stars left after the exhaustion of their nuclear energy,providing reliable targets for astronomical telescope observations.All nuclei within the core of a star compress together into a"gigantic nucleus",to be manifested in the form of a pulsar if the mass is not so large that its self-gravity can be strong enough to collapse into a black hole.Currently,no definitive opinion on the nature of dense matter exists because of the lack of understanding about the rich effects of nonperturbative strong interaction at low energy.However,clear observational evidence demonstrates that many extreme astrophysical events,such as gamma ray bursts and fast radio bursts,are closely related to gigantic nuclei.Solid components exist in pulsars,enabling starquakes to occur when stress exceeds a critical value.The starquakes are analogous to earthquakes on Earth and can further trigger explosive astrophysical activities.In this article,we first review the historical notes on dense matter and briefly discuss the physics of earthquakes,relevant theories,and the similarities between earthquakes and starquakes.We then introduce the observational aspects related to pulsars and present a theoretical model for starquakes and an outlook for future studies.We hope that our effort will lead to more interdisciplinary collaborations between seismological and astrophysical communities so that the nature of gigantic nuclei can be revealed using advanced multi-messenger observations.This knowledge will help further understand the fundamental strong force at low energy using gigantic nuclei.
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