Astronomers may have found a remarkable binary star system where one star appears to have orbited inside its partner, offering potential evidence for a crucial phase in stellar evolution. This rare pairing, located about 455 light-years away, consists of a rapidly spinning pulsar and a stripped-down helium star, orbiting incredibly close together.
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Unveiling a Strange Cosmic Duo
The star system, known as PSR J1928+1815, was studied using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in China. The discovery centers on a pulsar, which is a type of neutron star – the super-dense corpse of a massive star that exploded as a supernova. Imagine squeezing something heavier than our Sun into a city-sized ball – that’s a neutron star. Pulsars are neutron stars that spin furiously, sweeping beams of radio waves across space like a cosmic lighthouse.
The companion to this pulsar is a helium star, weighing in at about 1 to 1.6 times the mass of our sun. This star has shed most, if not all, of its outer hydrogen layers, leaving behind a core primarily made of helium.
What makes this pair so unusual is their extreme closeness. They are separated by only about 700,000 miles (1.12 million kilometers). To put that in perspective, that’s about 50 times closer than Mercury is to the sun. Because they’re so close, they complete an orbit around each other in a mere 3.6 hours.
The “Common Envelope” Mystery
PSR J1928+1815 is classified as a millisecond pulsar, spinning almost 100 times every second. Millisecond pulsars are thought to reach these dizzying speeds by ‘feeding’ on material pulled from a companion star. This ‘cannibalism’ also causes the stars to draw closer together.
Scientists have long theorized that as this process unfolds, the binary system might enter a dramatic phase called a “common envelope.” During this brief but violent period, the smaller star (in this case, the pulsar) would orbit within the expanded, puffy outer layers of its larger companion. While simulations predict this phase is critical for forming close binary systems, direct observational evidence has been scarce – until potentially now.
Illustration of a binary star system with a smaller star orbiting inside the expanded outer layers of a larger star, representing a common envelope.
Evidence from Cosmic History
Using computer models, the researchers reconstructed the likely past of this binary system. They suggest the stars began much further apart, about twice the distance between Earth and the sun. As the pulsar siphoned material, the larger star would have expanded, eventually engulfing the pulsar in a common envelope. The pulsar, orbiting within this envelope, would have spiraled inward towards the companion’s core over roughly 1,000 years, eventually causing the outer envelope to be ejected and leaving behind the tightly bound pair we see today.
Finding a system like PSR J1928+1815 is incredibly rare. Based on their characteristics, researchers estimate that only a small number of such systems, perhaps between 16 and 84, might exist in our entire Milky Way galaxy, which contains hundreds of billions of stars. This makes the discovery a significant clue in understanding the dramatic lives and deaths of stars in binary systems.
The findings were published in the journal Science.
A Window into Stellar Evolution
This discovery offers a tantalizing glimpse into the dynamic and sometimes violent processes that shape binary star systems. Finding a system that appears to be in the direct aftermath of the predicted common envelope phase provides strong support for this crucial model of stellar evolution. Future studies of this rare pair could help astronomers refine their understanding of how stars interact, evolve, and create the fascinating systems we observe across the cosmos. To learn more about stars, explore our articles on neutron stars and pulsars.
