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A pseudobulge is a central stellar concentration in a disk galaxy whose structure and dynamics indicate growth through internal disk evolution rather than only rapid merger-driven assembly. A central brightness excess alone does not establish this origin.

Disk-like flattening, appreciable ordered rotation, nuclear spiral arms, galactic bars and ongoing star formation provide complementary diagnostics. A bar can torque gas and move it inward, feeding central star formation over a long timescale. Individual diagnostics are imperfect, so classifications combine imaging and kinematics instead of equating every low-index brightness profile with a unique history.^[ref]

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]

A galaxy with negligible present star formation can still gain stars through mergers. Stellar mass assembly and in-situ star formation therefore describe different histories; quenching the latter does not freeze the former.

An astrophysicist whose work connects models of gravity with astronomical observations. His lectures with Matthias Bartelmann develop gravitational lensing from the deflection of individual light rays to images made by galaxies and galaxy clusters. A useful example is the thin gravitational lens equation: an observed image position combines the deflection angle with the geometry between observer, lens and source. This turns a distorted image into information about the intervening mass distribution.^[ref]

Redshift-space distortions arise because observed galaxy redshifts include peculiar velocity as well as cosmological redshift. Distances inferred solely from expansion therefore distort clustering along the line of sight. Coherent large-scale motion and small-scale velocity dispersion produce different effects on the inferred matter power spectrum.

Robert C. Kennicutt Jr. studies how gas becomes stars in galaxies. His 1998 observational analysis combined normal spiral galaxies and starburst galaxies, finding an approximately $1.4$ power law between disk-averaged star formation rate per unit area and gas surface density. This Kennicutt–Schmidt law connects measurements across whole galaxies to prescriptions used in models of galaxy formation.^[ref]

With Neal J. Evans II, he also reviewed how different observations estimate star formation rates and how the gas–star formation relation changes between molecular clouds and whole galaxies.^[ref]

A rotation-anisotropy diagram compares ordered rotation divided by velocity dispersion with a galaxy's flattening. An oblate isotropic rotator provides a reference curve. Falling below it can indicate anisotropic random support, but inclination, measurement apertures, and the definition of rotation change the interpretation. Intrinsic axis ratios and projected ellipticities are different observables.

Sandra Faber contributed to the cold dark matter explanation of galaxy formation. With George R. Blumenthal, Joel Primack and Martin Rees, she developed the 1984 account in which small early particle velocity dispersion preserves galaxy-scale density fluctuations against free streaming.^[ref]

The central test connects particle behaviour to astronomical structure: the surviving fluctuation spectrum determines which scales can seed galaxies and galaxy clusters.

The scale factor $a(t)>0$ measures how spatial lengths in an FLRW metric change with cosmic time $t$. Choose $a$ to be a dimensionless quantity with $a(t_0)=1$ at a reference epoch, usually today. For two comoving observers, their fixed radial comoving distance $\chi$ and their proper distance in cosmology $d(t)$ satisfyThe proper distance in cosmology is measured along a spatial slice at one cosmic time; it is not the distance that a travelling photon covers between emission and reception. Only ratios of the scale factor have a normalization-independent meaning: replacing $a$ by $Ca$ and $\chi$ by $\chi /C$ leaves $d$ unchanged.

For example, take $\chi =100\mathrm{Mpc}$, where $\mathrm{Mpc}$ means a million parsecs. At $a=0.5,1,1.5$, the same pair of comoving observers has proper distance in cosmology $50,100,150\mathrm{Mpc}$ respectively. Every such separation changes by the same factor, although more distant pairs gain more distance. The illustration treats the marked objects as ideal comoving observers; internal motions in galaxies need not follow this background scaling.

The beauty of the scale factor is that one function describes the changing spatial ruler for every comoving observer. Spatial homogeneity gives no preferred centre of cosmic expansion: in a flat spatial patch, relative positions obey $\mathbf{r}-{\mathbf{r}}_O=a(\mathbf{x}-{\mathbf{x}}_O)$, so recentering on any comoving observer preserves Hubble's law. The evolution of the scale factor is determined by the Friedmann equations, while its ratio between emission and reception determines cosmological redshift.

A Schechter function models a luminosity function or a galaxy stellar mass function by combining a low-value power law with an exponential cutoff. In stellar-mass notation, define the number per stated volume and mass interval byHere $M_{\ast }$ is a characteristic mass, ${\Phi }_{\ast }$ has units of inverse volume, and $\alpha$ is the low-mass slope. The measured galaxy stellar mass function is often quoted per logarithmic interval instead: the change-of-variables formula for a probability density gives $dn/d{\log }_{10}M=(\ln 10)M\Phi (M)$. Comparing slopes without specifying the measure introduces a shift of one.

Writing $x=M/M_{\ast }$ gives the momentswhere $\Gamma$ is the gamma function. Thus $\alpha =-3/2$ has an infinite extrapolated number of arbitrarily small galaxies but finite cosmic stellar mass density. A positive low-mass cutoff makes both integrals finite. The model is an empirical parametrization over a specified range, rather than evidence that the extrapolation continues to zero mass.

Paul Schechter proposed the luminosity version in 1976 and compared it with nearby-galaxy and cluster samples. His paper explicitly distinguished a sample luminosity distribution from a volume-normalized luminosity function, a distinction that remains essential when correcting a flux-limited survey.^[ref]

Scott Tremaine coauthored a phase-space constraint on fermionic dark matter. The analysis connects the phase-space density needed in a galaxy with an upper bound arising from the microscopic statistics of a proposed particle.^[ref]

The result shows why matching a total gravitational mass is not sufficient for a viable matter model. The candidate must also be able to occupy the measured spatial and velocity distribution. Both the dynamical inference and assumptions about the particle distribution enter the test.

Simon White studies the formation of galaxies and cosmic structure. With Martin Rees, he proposed a two-stage account in which extended dark haloes assemble gravitationally and luminous galaxies form through cooling and concentration of gas within them.

The key distinction is dissipative: gas can radiate away energy, whereas collisionless halo material does not cool in the same way. The cooling requirement helps determine which dark structures can host concentrated luminous systems. Their 1978 model thereby connected the dark gravitational framework to the observed scale of galaxies.^[ref]

The two resolved quasar images of 0957+561 provide an observational question: are they separate sources, or different light paths from one source? In May 1979, spectrophotometry used the McDonald Observatory 2.7 m reflecting telescope, the UVITS spectrograph, and an intensified dissector scanner at the f/9 focus of the Cassegrain reflector. A 600-groove-per-millimetre diffraction grating produced 4100-7100 ångström scans sampled every 2 ångströms, with about 7 ångström resolution over most of that range. Two spectrograph slits, about 52 arcseconds apart, recorded object and sky background simultaneously for subtraction. Interleaving observations of the two images reduced instrumental and atmospheric gain changes.^[ref]

The readout was received light versus wavelength for each image. Closely matching absorption line redshifts supported a common source and absorber; differences in continua and line strengths motivated differential interstellar extinction. Spectral agreement supplied evidence for gravitational lensing, rather than proving the point-mass gravitational lens model. The authors discussed a physical double and discrepant optical and radio light ratios. Their note added in proof attributed the redder spectrum to foreground galaxy contamination and emphasized an extended mass distribution.^[ref]

A star-forming galaxy is a galaxy with ongoing star formation. Its hot young stars can supply hydrogen-ionizing radiation to the intergalactic medium, with the contribution controlled by the ionizing photon escape fraction.

An elongated nonaxisymmetric stellar structure in a galaxy, often approximated as a rotating perturbation of an axisymmetric or spherical gravitational potential. Its pattern angular velocity need not equal an individual star's orbital angular velocity. The Galactic bar is the particular example in the Milky Way.

For two approximately spherical stars approaching with speed $v_{\infty }$, contact at separation $R_1+R_2$ has gravitationally focused cross section $\pi (R_1+R_2{)}^2[1+2G(m_1+m_2)/((R_1+R_2)v_{\infty }^2)]$. This measures direct stellar encounters, not the probability of a collective encounter between their host galaxies.

A stellar halo is the extended, approximately spheroidal stellar population surrounding a galaxy. It differs from the total dark matter halo. If its outer luminosity density is $r^{-p}$, its projected light follows the projection of a spherical power-law emissivity and scales as $R^{1-p}$ away from breaks and outer truncation.

A generalized mass function with low-mass slope $-7/4$ and cutoff $e^{-\sqrt{M_{\ast }/M_{\ast ,\mathrm{ch}}}}$ differs from the ordinary exponential Schechter cutoff. Its total stellar-mass density is finite, even though its untruncated total galaxy count diverges. This form arises from a scale-free Press-Schechter mass function with a constant stellar-to-halo mass mapping.

A stellar orbit is a star's trajectory in the gravitational field of its surroundings. A close binary star can often be approximated by a Kepler orbit, while a star moving through a galaxy responds to an extended distribution of stars, gas and dark matter. Its path need not close into an ellipse.

Observed positions, proper motion and line-of-sight velocity constrain different components of the motion. Reconstructing an orbit also requires a distance and a model of the gravitational potential. Comparing many stellar orbits can therefore constrain an unseen mass distribution, but one projected track need not uniquely determine the full three-dimensional trajectory or potential.

A stellar stream is a correlated collection of stars commonly originating in the disruption of a bound progenitor. Phase mixing can erase spatial contrast while leaving coherence in orbital integrals. Such fossil debris constrains a galaxy's past accretion, although not every stream is from a galaxy: star clusters also produce streams.