The hottest object in the universe is difficult to determine with certainty, as it depends on various factors such as the temperature achieved and the specific definition of "object." However, there are several astronomical phenomena and objects that are known to reach extremely high temperatures:
Supernovae: A supernova is the explosive death of a massive star. During this cataclysmic event, temperatures can reach billions of degrees Celsius (or Kelvin) for a brief period. The core of the collapsing star can become incredibly hot and release an immense amount of energy.
Quasars: Quasars are extremely luminous and distant objects powered by the accretion of mass onto supermassive black holes at the centers of galaxies. The matter falling into the black hole gets heated to extremely high temperatures, emitting enormous amounts of energy across a wide range of wavelengths.
Gamma-ray bursts (GRBs): GRBs are short-lived bursts of gamma-ray radiation associated with massive stellar explosions or the merging of neutron stars. These events release an enormous amount of energy, and during the initial phase, temperatures can reach trillions of degrees Celsius.
Accretion disks around black holes: When matter falls into a black hole or a compact object like a neutron star, it forms an accretion disk around it. The material in these disks can be heated to millions or even billions of degrees Celsius due to friction and compression as it spirals inward.
These are just a few examples of extremely hot objects in the universe. It's worth noting that temperatures in these phenomena are often measured indirectly by observing their radiation across various wavelengths, and the actual matter within them may not exist as a solid or liquid due to the extreme conditions involved.