= Microquasar
A <microquasar> is an accreting stellar binary whose compact object launches <relativistic jets>, providing a small-scale analogue of a jet-producing <active galactic nucleus>. The name refers to the analogy between accretion and outflow, not to a miniature galaxy. The accretor may be a <black hole> or a <neutron star>.
A revealing observation is a sequence of high-resolution radio images, showing emitting knots moving away from the compact central source. In <GRS 1915+105>, such observations demonstrated strikingly rapid apparent motions. If a knot moves at speed $\beta c$ at angle $\theta$ toward the observer, light-travel delays give
$$
\beta_{\mathrm{app}}=\frac{\beta\sin\theta}{1-\beta\cos\theta}.
$$
For the illustrative values $\beta=0.9$ and $\theta=30^{\circ}$, this is approximately $2.04$: <apparent superluminal motion> despite a locally subluminal speed. The receding jet has a different arrival-time factor. Comparing both sides and the source distance therefore constrains geometry, rather than measuring a speed directly from one image.<Microquasars in Our Own Galaxy by National Radio Astronomy Observatory>[Radio-image sequence and explanation of apparently faster-than-light ejecta]{ref}
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