Astronomers have recently identified a rare double-star system where one star appears to have orbited inside the other. This exotic system, named PSR J1928+1815, involves a rapidly spinning neutron star (a pulsar) and a helium star companion.
The discovery, detailed in the journal Science on May 22, 2025, suggests that the two stars went through a "common envelope" phase. In this scenario, the pulsar, which is the dense remnant of a supernova, spiraled into the outer layers of its companion star. This interaction caused the companion to shed most or all of its hydrogen-rich outer layers, leaving behind a stripped helium core.
Currently, the two stars are incredibly close, separated by only about 700,000 miles (1.12 million kilometers) and completing an orbit around each other in a mere 3.6 hours. The pulsar itself spins nearly 100 times per second.
Here's a breakdown of the discovery:
What is a double-star system? A double-star system, also known as a binary star system, consists of two stars gravitationally bound and orbiting a common center of mass. While some "double stars" might just appear close in the sky (optical doubles), true binary systems are physically linked. The majority of stars in our galaxy are actually part of binary or multiple-star systems.
The Unusual Discovery: In this newly identified system, one of the stars is a millisecond pulsar (PSR J1928+1815), which is a type of neutron star – the extremely dense remnant of a massive star that exploded in a supernova. The other star is a helium star, a star that has lost most or all of its outer hydrogen layers, leaving behind a core primarily composed of helium.
The "Common Envelope" Phase: The key to this unusual arrangement lies in a theoretical stage of stellar evolution called the "common envelope" phase. Scientists believe that in such a phase, the more massive star (the one that became the helium star) expanded, engulfing its companion (the star that would become the pulsar). The pulsar then spiraled inward within the outer layers of its larger partner.
How it Formed: Computer models suggest that the two stars initially formed much further apart. As the larger star evolved, it began to transfer mass to the smaller star (which would eventually become the pulsar). This mass transfer became unstable, leading to the formation of a common envelope around both stars. The pulsar then spiraled inward, getting closer to its companion's core. This process released immense energy, which likely ejected the remaining outer layers of the larger star, leaving behind the tightly bound binary system we observe today.
Current State of the System:
- The two stars are incredibly close, only about 700,000 miles (1.12 million kilometers) apart – roughly 50 times closer than Mercury is to our Sun.
- They complete an orbit around each other in just 3.6 hours.
- The pulsar spins extraordinarily rapidly, almost 100 times per second.
Rarity: This type of binary system is extremely rare. Researchers estimate that only about 16 to 84 such systems might exist in our entire Milky Way galaxy, which contains hundreds of billions of stars.
Significance: The detection of PSR J1928+1815 and its helium star companion provides the first strong observational evidence for this "common envelope" phase of binary star evolution, a process that was previously only theoretical. Understanding these systems helps astronomers piece together the complex life cycles of stars and how they interact in close binary environments.
Scientists believe that such systems are extremely rare, with estimates suggesting only 16 to 84 similar counterparts may exist in the entire Milky Way galaxy. This discovery provides crucial insights into the formation of these highly compact binary systems and the processes of stellar evolution.