Once every 104–105 years, a star wanders too close to its galaxy's central black hole and is shredded. Half the stellar debris is flung away; half falls back and powers a luminous, evolving accretion disk that briefly turns a dormant SMBH into a transient.
Approach the SMBHTwo-body relaxation in the dense nuclear star cluster of a galaxy occasionally scatters a star onto an orbit whose periastron lies inside the tidal radius of the central SMBH (typically rt ≈ 10–100 rg).
The star arrives essentially unbound — a nearly parabolic orbit — and is about to be subjected to differential gravitational forces strong enough to overcome its self-gravity.
At periastron the tidal force exceeds the star's self-gravity. The star is stretched into a long, thin debris stream — the textbook "spaghettification" — within a fraction of an orbital period.
Roughly half the debris becomes unbound and is launched on hyperbolic trajectories; the other half remains bound to the SMBH, with a spread in orbital energies determined by the depth of penetration into the tidal radius.
The most-bound debris returns to periastron first. The fallback rate famously scales as t−5/3, the iconic light-curve signature TDEs were predicted to show before any were observed.
How this stream circularizes into a coherent accretion disk — via stream–stream collisions, magnetic stresses, or general-relativistic apsidal precession — is one of the central open questions in TDE physics today.
Once a hot, geometrically thin accretion disk forms, the SMBH briefly transitions from quiescent to bright. The emission spans UV through optical — and in many TDEs, soft X-rays from the inner disk regions.
The fact that optical and X-ray TDEs differ systematically in their light curves, spectra, and host-galaxy demographics is still being worked out — possibly viewing-angle effects through a reprocessing layer, possibly genuinely different physical channels.
A handful of TDEs (Swift J1644+57, AT2022cmc, …) have launched relativistic jets with luminosities exceeding 1047 erg/s — beamed emission visible to high redshift. They probe how rapidly an SMBH can be spun up and how robustly it can launch a Blandford–Znajek-style jet.
The full TDE population — including these outliers — is one of the best laboratories we have for measuring SMBH masses, spins, and demographics at the low-mass end where AGN surveys lose sensitivity.