Quasar

A quasar is an extremely luminous active galactic nucleus whose compact central emission can outshine its host galaxy and be observed across cosmological distances. Its apparent resemblance to a star originally concealed its nature: a small angular image does not establish a small physical size or a nearby distance. In 1963, Maarten Schmidt identified familiar Balmer series emission lines in the unusual optical spectrum of 3C 273, shifted together to a redshift of 0.158. The optical spectra came from a spectrograph on the 200-inch telescope at Palomar. Interpreting the shift as cosmological implied a distant, extraordinarily luminous galactic nucleus. Modern black-hole accretion models provide the compact power source for that luminosity.^[ref]

3C 273 is a bright quasar whose star-like optical appearance initially concealed its extragalactic character. In 1963, Maarten Schmidt identified its emission lines as a familiar series shifted by a common redshift of about $z=0.158$.

The identification connected the spectrum to a large cosmological distance and hence an enormous luminosity. The lesson is methodological: an object's unresolved angular size does not establish that it is an ordinary nearby star. Interpreting spectral-line patterns can change the inferred physical scale.^[ref]

For a small bound system at cosmological redshift $z$, let $R_{\perp }$ be the projected source-to-cloud distance and $s$ the cloud displacement toward a distant observer. The extra source–cloud–observer path is $\sqrt{R_{\perp }^2+s^2}-s$, and cosmological time dilation multiplies its local delay by $1+z$. A projected separation alone does not determine the delay; $s=0$ is an explicitly transverse estimate.

A quasar light curve records its measured brightness as a function of time. Intrinsic accretion variability, measurement noise, and gravitational lensing can all contribute structure.