Binary Star Evolution#

Binary star systems evolve differently compared to single stars due to interactions between the two stars. Mass and angular momentum transfers between them strongly influence their evolution. Usually, the more massive star evolves faster and collapses first in a supernova (SN), forming either a neutron star (NS) or a black hole (BH). The remaining star emits stellar winds that are captured by the compact object, creating X-ray emissions:

  • If \(q = m_2 / m_1 < 1\): a low-mass X-ray binary forms.

  • If \(q \geq 1\): a high-mass X-ray binary forms.

The interaction and material flow between the stars is often described using the concept of the Roche lobe.

What are gravitational waves?

Gravitational-wave are ripples in the fabric of space-time caused by massive, accelerating objects, like orbiting NS or merging BH.

These waves were predicted by Einstein’s theory of general relativity and travel at the speed of light. Unlike electromagnetic waves, they pass through matter almost undisturbed, carrying pristine information about their cosmic origin.

What can we learn from gravitational waves?

  • Measure the mass, spin, and distance of compact objects

  • Study the properties of NS interiors (e.g. their equation of state)

  • Observe parts of the universe invisible to light

  • Combine GW data with electromagnetic signals in multi-messenger astronomy

What is a compact binary coalescence?

Compact binary coalescence explained

A CBC is the final stage of the life of a binary system composed of two compact objects such as a BNS, BBH, or NSBH. These systems lose energy and spiral inward due to gravitational wave emission until they eventually merge. As the two compact objects orbit closer and faster, they emit gravitational waves, losing orbital energy and eventually merging. This final collision emits a powerful burst of gravitational-waves that can be detected by observatories such as aLIGO, aVirgo, and KAGRA.

How do compact binary systems form?

  • Two massive stars are born as a binary system.

  • Each star may evolve and collapse into a compact object like a NS or BH after a supernova explosion.

  • Over time, the binary system loses energy via gravitational radiation.

  • If the separation is small enough, the two compact objects spiral in and merge within the age of the universe.

Gravitational wave sources

Wravitational-waves are emitted by several astrophysical sources:

  • Transient sources: like the merger of BH or NS systems, and core-collapse SN.

  • Continuous sources: such as rapidly rotating (fast-spinning) NS (pulsars) with asymmetries.

  • Stochastic background: a faint, persistent signal from many unresolved sources, possibly from the early universe.

  • Exotic sources/theoretical sources: like wormholes or other phenomena beyond general relativity.

Among these, the most powerful and well-understood signals come from CBC.