Variable Star

Full definition

Until the 16th century, the sky was assumed immutable: the 'fixed stars' changed neither position nor brightness. Fabricius's 1596 discovery that a star in Cetus, Mira, disappeared and reappeared on a ~330-day cycle was a cosmological shock comparable to Galileo's sunspots. Gradually, it was discovered that many stars vary — and we now know more than two million.

A variable star is a star whose apparent brightness changes over time. Causes are extraordinarily diverse. Two main families.

INTRINSIC variables. The star's actual luminosity changes. Several sub-types: • Pulsating. The star physically oscillates — it swells and contracts, like a beating heart. Surface size and temperature change, and therefore brightness. Cepheids, RR Lyrae, Miras, delta Scuti, semi-regulars. • Eruptive. Sudden surface eruptions, often on active red dwarfs (UV Ceti) or young stars (T Tauri). • Cataclysmic. Close binary systems where a white dwarf accretes matter from a companion, producing dramatic flashes: dwarf novae, classical novae, type Ia supernovae.

EXTRINSIC variables. Actual luminosity does not change; what we see does: • Eclipsing. Two stars of a binary mutually obscure each other periodically (Algol, Beta Lyrae, W UMa). • Rotating. Dark or bright spots, like sunspots, rotate with the star (BY Draconis, RS CVn, optical pulsars).

The scientific reach is immense. Pulsating variables such as Cepheids and RR Lyrae are standard candles — their pulsation period is directly related to intrinsic luminosity, so distance follows. On this mechanism Hubble built the first extragalactic distance ladder in 1925. Eclipsing variables give the most precise stellar mass and radius measurements. Type Ia supernovae revealed the accelerating expansion of the Universe in 1998 (Nobel Prize 2011).

Historical line: Fabricius (1596, Mira) → Goodricke (1783-86, Algol, Delta Cephei) → Henrietta Leavitt (1912, period-luminosity relation on SMC Cepheids) → precision amateur astronomy (AAVSO, founded 1911) → Gaia (2013-) continuously cataloging the whole sky.

Classification and amplitudes

AAVSO and the Variable Star Index (VSX) list more than 2 million variables, sorted into ~80 distinct types by mechanism, period and amplitude.

Classical pulsators: • Cepheids (DCEP). 1-100 days, amplitude ~0.1-2 mag. Evolved supergiants. Extragalactic standard candles. • RR Lyrae (RR). 0.2-1 day, amplitude 0.3-2 mag. Old metal-poor stars in globular clusters. Galactic standard candles. • Miras (M). 80-1,000 days, amplitude 2.5-11 mag — huge changes. Pulsating red giants at life's end. • Delta Scuti (DSCT). 30 minutes to 6 hours, small amplitude (< 0.1 mag). F-type main-sequence stars. • ZZ Ceti (ZZ). Pulsating white dwarfs, very short periods (100-1,000 s). • Semi-regulars (SR), irregulars (L): less strictly periodic.

Eruptive: • UV Ceti (UV). Intense flares on red dwarfs, amplitude up to 5 mag in minutes. • T Tauri (TT). Young stars still accreting, very erratic. • Wolf-Rayet. Massive stars with chaotic mass ejections.

Cataclysmic: • Dwarf novae (UG). Brisk rises of 2-5 mag every month or few years. • Classical novae (N). Rises of 6-20 mag, return to quiescence over years/decades. • Supernovae (SN). Most extreme event, ~15-20 mag increase then months-long decline.

Extrinsic: • Eclipsing binaries (EA Algol, EB Beta Lyr, EW W UMa). Periodic sawtooth curves. • Rotating (BY Dra, RS CVn, ACV). Small amplitudes over a rotation period.

Famous examples

Mira (Omicron Ceti). The historical archetype. Mira-type variable, period ~332 days, magnitude 2 (at maximum, easy) to 10 (invisible to binoculars). In Cetus. First variable identified (Fabricius 1596).

Algol (Beta Persei). Archetype of eclipsing binaries. Period 2.867 days, magnitude 2.1 → 3.4. Identified as variable by Montanari (1667) and mechanically explained by Goodricke (1783).

Delta Cephei. The star that gives Cepheids their name. Magnitude 3.5 → 4.4, period 5.366 days. Found variable by Goodricke in 1784. Naked-eye in Cepheus, easy to track for amateurs.

Betelgeuse (Alpha Orionis). Semi-regular variable, red supergiant. Surprised the world in 2019-2020 with a dramatic dip to magnitude 1.6 (from 0.4 typical), caused by an ejected dust plume. Future supernova candidate.

RR Lyrae. Prototype of the eponymous class, magnitude 7.1 → 8.1, period 0.567 day (13 h 36 min). Widely observed by amateurs.

SS Cygni. Prototype of dwarf novae. Rises to magnitude 8 every ~50 days, falls back to 12. Cygnus, reachable in binoculars at maximum.

SN 1987A. Type II supernova in the Large Magellanic Cloud, observed on 23 February 1987. First naked-eye supernova since Kepler (1604). Still under study today.

T Coronae Borealis (Blaze Star). Recurrent nova, outbursts in 1866 and 1946. Another outburst has been expected since 2024 — if it occurs, naked-eye spectacle in Corona Borealis.

Eta Carinae. Luminous blue variable hypergiant (LBV). Peaked at magnitude -0.8 in 1843. Certain future supernova.

How do we observe them?

Naked eye. Several variables are trackable without instruments: Mira at maximum, Betelgeuse (compare with neighbor Rigel and Orion's Belt), Algol during its eclipses, Delta Cephei. Just estimate magnitude by comparison with reference stars ('Argelander's method').

Binoculars. Open up the field: dozens of accessible variables, methodical monitoring possible with an AAVSO comparison chart. A great exercise for beginners.

Amateur CCD/CMOS photometry. The big revolution. A cooled camera at ~€1,000-2,000, free software (AstroImageJ, MaxIm DL, C-Munipack), and you can produce measurements at 0.01 mag precision. AAVSO accepts these and integrates them into databases used by professional researchers. Some amateurs have co-authored publications (supernovae discovered, exoplanet transits confirmed).

Amateur spectroscopy. For bright variables, an Alpy 600 or UVEX spectrograph reveals the underlying physics. Follow-up of nova eruptions, nearby supernovae, Be-star outbursts, RS Oph 2021 eruption.

Professional telescopes and networks. TESS (NASA, 2018-) samples millions of stars every 2 min, revealing faint variables. Gaia (ESA) published a ~10-million-star variable catalog in DR3 (2022). ASAS-SN watches the whole sky each night for transients. ZTF (Zwicky Transient Facility) lists supernovae in near real time.

To plan tonight's variable-star session, our sky map tool lets you locate targets and our 'What to see tonight' planner proposes highlight stars visible from your location.

Not to be confused with

Strict binary star. Eclipsing binaries ARE variables, but not all binaries are variables (orbit must be near-edge-on), and not all variables are binaries (Cepheids pulsate intrinsically, most Miras too). Smooth sinusoidal light curve → pulsation likely; sharp, flat-bottomed eclipses → eclipsing binary.

Transiting exoplanet. A planet crossing its star causes a tiny dip (~0.01 to 1 %) of very short duration. This is technically eclipse variability, but infinitely more subtle than a stellar companion's. Space missions TESS / Kepler have detected thousands of exoplanets this way. On the ground, amateurs can confirm transits of hot Jupiters (HD 189733b, WASP-12b).

Nova vs supernova. Classic confusion. A nova is a thermonuclear eruption on the surface of a white dwarf accreting gas from a companion — bright but survivable, recurrent. A supernova is the explosive death of a massive star (type II) or the complete detonation of a white dwarf (type Ia) — one-off, irreversible, 10,000 to 100,000 times brighter than a nova.

Cepheid vs RR Lyrae. Two classes of pulsating standard candles, but distinct. Cepheids are young supergiants, bright (L ~ 1,000-30,000 L☉), period 1-100 days, useful out to ~30 Mpc. RR Lyrae are old horizontal-branch stars, less luminous (L ~ 50 L☉), period < 1 day, useful out to ~1-2 Mpc.

Flickering candle vs variable star. Many beginners confuse atmospheric scintillation (the star 'twinkling' from turbulence) with intrinsic variability. Scintillation is instantaneous and depends on the star's altitude above the horizon. Real variability is measured over hours to years.