Quasar
A quasar (quasi-stellar object) is an extremely luminous active galactic nucleus (AGN) powered by a supermassive black hole (10⁶–10¹⁰ M☉) accreting material at the centre of a distant galaxy, producing energy output that can exceed the combined light of an entire galaxy by factors of 100–1,000. Quasars were among the first objects identified at cosmological redshifts (z > 0.1), appearing star-like in early optical surveys despite being billions of light-years away, and their spectra showed enormous redshifted emission lines confirming their cosmological distances. Because quasar light has travelled billions of years to reach us, they serve as luminous probes of the early universe, intergalactic medium, and the history of black hole growth throughout cosmic time.
Key Formula
L_Edd = 4π G M m_p c / σ_T ≈ 1.26 × 10³¹ × (M/M_sun) watts
LaTeX: L_{\rm Edd} = \frac{4\pi G M m_p c}{\sigma_T} \approx 1.26 \times 10^{31}\left(\frac{M}{M_\odot}\right)\text{ W}
| Symbol | Meaning | Unit |
|---|---|---|
| L_Edd | Eddington luminosity (maximum accretion luminosity) | watts (W) |
| G | Gravitational constant | N·m²/kg² |
| M | Black hole mass | kilograms (kg) or M☉ |
| m_p | Proton mass (1.67 × 10⁻²⁷ kg) | kg |
| c | Speed of light | m/s |
| σ_T | Thomson scattering cross-section (6.65 × 10⁻²⁹ m²) | m² |
Worked Example
Problem
The quasar 3C 273 has a luminosity of approximately 4 × 10³⁹ W. Assuming it radiates at the Eddington limit, estimate the minimum black hole mass required.
Solution
Step 1 — Use the Eddington luminosity relation: M = L_Edd × σ_T / (4π G m_p c). Step 2 — Rearranging the approximation: M/M☉ = L / (1.26 × 10³¹ W). Step 3 — M/M☉ = 4 × 10³⁹ / 1.26 × 10³¹ ≈ 3.17 × 10⁸. Step 4 — So M ≈ 3.2 × 10⁸ M☉ (320 million solar masses).
Answer
Minimum black hole mass ≈ 3.2 × 10⁸ M☉
Comparison of Quasars, AGN Types, and Normal Galaxies
| Object Type | Luminosity (W) | Central Engine | Redshift Range | Example |
|---|---|---|---|---|
| Normal Galaxy (Milky Way) | ~4 × 10³⁶ | Quiescent SMBH | z ≈ 0 | Milky Way |
| Seyfert Galaxy | 10³⁷ – 10³⁹ | Low-luminosity AGN | z < 0.1 | NGC 4151 |
| Radio Galaxy | 10³⁸ – 10⁴¹ | AGN with jets | z < 2 | Centaurus A |
| Quasar (radio-quiet) | 10³⁹ – 10⁴² | Accreting SMBH | 0.1 < z < 7 | 3C 273 |
| Blazar / OVV Quasar | 10⁴⁰ – 10⁴² | Relativistic jet pointed at Earth | z ∼ 0.3–5 | BL Lacertae |
| Brightest Known Quasar (J0529) | ~5 × 10⁴¹ | SMBH ~17 billion M☉ | z ≈ 3.96 | J0529-4351 (2024) |
Interactive Tools
SDSS SkyServer Quasar Spectra
Explore real quasar spectra and redshifts from the Sloan Digital Sky Survey.
Open ToolWolframAlpha Eddington Luminosity
Compute Eddington luminosities and black hole mass estimates.
Open ToolNASA HEASARC Quasar Catalogue
Access the Véron-Cetty catalogue of quasars and active nuclei.
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Variable Star
A variable star is any star whose observed brightness (apparent magnitude) changes over time, whether due to intrinsic physical changes in the star itself or due to geometric effects such as eclipses or rotation. Intrinsic variables include pulsating stars (Cepheids, RR Lyrae, Mira), eruptive variables (novae, flare stars), and cataclysmic variables (dwarf novae, Type Ia supernovae). Of particular cosmological importance are Cepheid variable stars, whose pulsation period is directly related to their intrinsic luminosity (the period–luminosity relation), making them crucial standard candles for measuring distances to nearby galaxies.
Stellar Nucleosynthesis
Stellar nucleosynthesis is the process by which nuclear fusion reactions inside stars create heavier atomic nuclei from lighter ones, releasing energy that sustains the star against gravitational collapse. Main-sequence stars primarily fuse hydrogen into helium via the proton–proton chain or CNO cycle, while more massive stars in later evolutionary stages fuse helium, carbon, oxygen, and silicon up to iron (Fe-56), the most tightly bound nucleus. Elements heavier than iron are synthesised through neutron-capture processes (s-process in AGB stars; r-process in neutron star mergers and supernovae), making stars the principal factories of the chemical elements in the universe.
Pulsar
A pulsar is a highly magnetised, rapidly rotating neutron star that emits beams of electromagnetic radiation from its magnetic poles; when these beams sweep across Earth like a cosmic lighthouse, observers detect precise periodic pulses ranging from milliseconds to seconds. Pulsars are the remnants of massive stars (>8 M☉) that exploded as core-collapse supernovae, leaving behind an object ~20 km in diameter but with a mass of 1.4–2 M☉ and a density (~10¹⁷ kg/m³) comparable to atomic nuclei. The extreme regularity of pulsar timing makes them natural clocks used to test general relativity, detect gravitational waves, and probe the interstellar medium.
Contraction of "quasi-stellar radio source," coined by Chinese-American astronomer Hong-Yee Chiu in 1964 to describe the puzzling star-like objects with large redshifts first discovered by Maarten Schmidt at Caltech in 1963.