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The companion to the millisecond pulsar NGC 362D is a very low-mass helium white dwarf that has not yet entered its ordinary cooling phase. A 2026 study estimates, using updated binary-evolution models, that the mass transfer that spun up the pulsar ended about 600 million years ago. The star may also still be surrounded by material, although the observations do not confirm a direct detection of that material.
What is the dead star orbiting NGC 362D?
NGC 362D is a binary system in the globular cluster NGC 362. One member is a millisecond pulsar: a rapidly rotating neutron star whose radio pulses can act like a highly regular natural clock. Its companion is an optical white dwarf, identified in archival Hubble Space Telescope observations.
The pulsar was recently discovered with South Africa’s MeerKAT radio telescope. The Hubble observations used to identify its companion were collected from 2006 through 2016, according to Space.com’s 2026 report. The underlying study describes the data as deep, multi-band and multi-epoch observations.
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Why is the white dwarf considered unusually young?
It is still in a pre-cooling phase
The study classifies the companion as a very low-mass helium white dwarf, with a mass of approximately 0.18 times the Sun’s mass. It is still in the pre-cooling phase: an early stage in which a newly exposed stellar core has not yet settled into the more familiar, steadily cooling white-dwarf state.
The age estimate comes from evolutionary models
By comparing the system with updated binary-evolution models, the authors infer that mass transfer ended roughly 0.6 billion years ago. That is a model-based estimate of when the transfer stopped, not a timestamp directly measured by Hubble or MeerKAT.
How can a neutron star and white dwarf form this kind of pair?
The two stars did not begin their lives as a pair of dead stars. In the sequence described by the Space.com report, one star first became a neutron star in a supernova. Later, its companion transferred matter to it. That inflow added angular momentum and accelerated the neutron star’s rotation, a process called pulsar recycling. As the donor lost its outer layers, its remaining core became a white dwarf.
Could material still surround the white dwarf?
The study reports photometric variations that depend on wavelength and says the observed properties are consistent with residual circumstellar material. The ultraviolet signal’s attenuation may indicate such material, Greta Ettorre, the study team leader, told Space.com. This is a possible explanation of the photometric pattern, not a confirmed direct image or an unqualified detection of a surrounding envelope.
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The authors describe NGC 362D as the youngest white-dwarf companion to a millisecond pulsar identified to date. Its inferred recent end to mass transfer makes it a potential opportunity to study the immediate aftermath of pulsar recycling and the early evolution of a proto-white dwarf.
There is also a classification implication. If material around a young white dwarf produces radio or optical signals that resemble those associated with a non-degenerate companion, astronomers could mistake the nature of a young pulsar system. NGC 362D offers a case in which those signals and the companion’s evolutionary state can be examined together.
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