Chapter 1: Discovery of Hyper-Velocity Stars

In an exciting revelation, a team of astronomers spearheaded by astrophysicist Kareem El-Badry from the Harvard-Smithsonian Center for Astrophysics has uncovered the fastest runaway stars in the Milky Way. Their groundbreaking findings have been submitted to the Open Journal of Astrophysics, with a detailed pre-print paper available on arXiv. In this overview, we will highlight the key aspects of their remarkable discovery and elucidate its significance.

The research team set out to identify hyper-velocity white dwarfs, which are stars that can travel at speeds surpassing the escape velocity of the Milky Way following a Type Ia supernova. Type Ia supernovae, or SNe Ia, are specific stellar explosions that occur within binary star systems, where two stars orbit a shared center of mass.

These supernovae are characterized by the explosion mechanism involving white dwarf stars—dense remnants that have exhausted their nuclear fuel. Through their gravitational influence, these white dwarfs accumulate mass from their companion stars until they reach a critical threshold of 1.4 solar masses, known as the Chandrasekhar limit. Upon reaching this mass, they undergo a thermonuclear explosion, resulting in a Type Ia supernova, which emits an immense amount of energy that can be detected from vast distances.

The energy released during the Type Ia supernova can propel the remaining white dwarf star into a hyper-velocity state, ejecting it from its original binary system at speeds that can reach several thousand kilometers per second. The astronomers found six stars believed to be remnants of double-degenerate binaries, where one white dwarf exploded in a Type Ia supernova. These stars, now termed hyper-velocity white dwarfs, have been launched away from their binary systems due to the explosive event.

An excerpt from their research paper states: "The high observed velocities suggest that a dominant fraction of the observed hyper-velocity WD population comes from double-degenerate binaries whose total mass significantly exceeds the Chandrasekhar limit."

Chapter 2: The Record-Breaking Stars

Now that we’ve covered the foundational concepts, let’s delve into the record-breaking stars. Four stars with extraordinary characteristics were identified, boasting radial velocities (RVs) that surpass 1000 km/s and total space velocities exceeding 1300 km/s. Radial velocity refers to the speed of an object along our line of sight, while total space velocity combines this with the star’s proper motion across the sky.

The two fastest stars were astonishingly found to be traveling at 1,694 kilometers (1,053 miles) per second (designated Star J1235) and the record-setting runaway star, J0927, which moves at an incredible speed of 2,285 kilometers (1,420 miles) per second—approximately 0.76% the speed of light. To put that into perspective, J0927 could cover the distance from Earth to the Moon in roughly 2.5 minutes.

Conversely, the two slowest stars out of the six observed were found to be moving at a more modest pace of 600 km/s, likely originating from white dwarfs following a Type Ia supernova.

In the video titled "The Fastest Runaway Star in the Galaxy," experts discuss the significance of these findings and the implications for our understanding of stellar evolution.

The video "A Runaway Star" provides a deeper look into the dynamics of runaway stars and their formation processes.

Final Thoughts

The six stars identified by the research team are likely survivors of double-degenerate binaries, with one of the white dwarfs having exploded in a Type Ia supernova. Among these, two stars have set new records for the fastest runaway stars observed. However, it's important to note that there may be even faster stars yet to be discovered, as our observations tend to favor brighter, larger stars. This discovery of hyper-velocity white dwarfs certainly enhances our understanding of binary systems and their evolutionary pathways.