Galaxies and Universe

The 21-centimeter hydrogen line reveals atomic gas and its motion. A Doppler example explains what radio maps measure and why distance remains a separate problem.

Quasar absorption lines sample the gas around foreground galaxies. Redshift, ion columns and careful modeling reveal what the spectra can—and cannot—tell us.

A Lyman-alpha halo maps escaping radiation after resonant scattering. Its extent alone cannot locate every photon's birthplace or identify one power source.

A galaxy’s total magnitude does not describe how widely its light is spread. A worked surface-brightness example explains the difference and its limits.

Stars rarely collide directly during galaxy mergers. Compare stellar sizes with their separation, then distinguish gravity, gas interactions, and planetary risks.

Galaxy colors offer clues about stars and dust, but red does not automatically mean old. Wavelength shifts and display choices change how the same evidence looks.

Cosmic voids in the universe are enormous low-density regions that reveal how gravity, dark matter, and galaxies shape the cosmic web.

Learn how astronomers infer invisible dark matter by measuring galaxy motions and the way gravity bends light, creating maps of mass across the universe.

Galaxy rotation curves show that stars and gas orbit faster than visible matter alone can explain, providing key evidence for unseen dark matter.