About Sky Alerts
Every clear night, the Vera C. Rubin Observatory (LSST) in Chile and the Zwicky Transient Facility (ZTF) in California scan the sky and subtract a deep reference map from each fresh exposure. Whenever something brightens, dims, or moves, the telescope emits an alert.
The ALeRCE broker classifies those alerts with machine-learning models, and Sky Alerts refreshes its database once a day so you can browse notable cosmic events without querying the live broker on page load. Rubin alerts arrive as a nightly batch roughly a day after observation.
Want the full architecture and pipeline walkthrough? Read How It Works.
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Class & confidence
Every class badge and confidence percentage is a machine-learning prediction, not a confirmed identification. A stamp classifier makes an initial guess from the first image cutout; once several nights of measurements accumulate, a light-curve classifier refines the subtype. Confirming a supernova for certain still requires a spectrum from a follow-up telescope.
Light curve
A light curve plots brightness over time, coloured by optical filter band (u, g, r, i, z, y). For ZTF, brightness is plotted in astronomical magnitudes, where smaller numbers are brighter—so brighter is up (↑). For Rubin (LSST), the chart plots difference flux in microjanskys (µJy) compared with the reference image: positive flux means brighter than the reference, and negative flux is a real measurement when a variable source has dimmed below its reference level.
Sky position
Coordinates are given as right ascension (RA) and declination (Dec, J2000). The all-sky map places the object on the celestial sphere alongside the dashed curve of the Milky Way’s galactic plane. Distant supernovae and active galaxies appear mostly away from the dusty plane, while variable stars cluster along it.
What the class groups mean
Click any class to browse recent objects of that type across the last 30 days.
Active galaxies
Supermassive black holes and young stars whose brightness wanders without a fixed rhythm.
- Active galactic nucleus
- A feeding supermassive black hole flickering at the heart of a galaxy.
- Quasar
- A very distant, very luminous active galactic nucleus.
- Blazar
- An active galaxy with its jet pointed almost straight at us.
- Young stellar object
- A newborn star still gathering gas and dust.
- Irregular variable
- Brightness that wanders without a fixed rhythm.
Explosions & outbursts
One-off cataclysms and sudden flares: exploding stars, black-hole tidal disruptions, and novae.
- Supernova
- An exploding star, brightening over days and fading over weeks.
- Type Ia supernova
- A white dwarf blown apart. Their even brightness makes them cosmic yardsticks.
- Type Ib/c supernova
- A massive star that lost its outer layers before collapsing.
- Type IIn supernova
- A supernova slamming into gas the star shed before it died.
- Type IIb supernova
- A massive star that kept only a thin shell of hydrogen before collapsing.
- Type II supernova
- A massive star whose core collapsed while still wrapped in hydrogen.
- Superluminous supernova
- A rare explosion ten or more times brighter than a normal supernova.
- Tidal disruption event
- A star torn apart by a supermassive black hole.
- Kilonova
- The glow left behind when two neutron stars merge.
- Cataclysmic variable
- A white dwarf flaring as it pulls gas off a companion star.
- Transient
- Something that appears or flares once, rather than repeating.
Variable stars
Stars that pulse or eclipse on a repeating schedule.
- Variable star
- A star whose brightness changes over time.
- RR Lyrae star
- An old star pulsing on a steady rhythm of about half a day.
- Cepheid
- A pulsing giant whose period reveals its true brightness.
- Delta Scuti star
- A star that pulses every few hours.
- Eclipsing binary
- Two stars taking turns blocking each other's light.
- Long-period variable
- A swollen red giant that brightens and dims over months.
- RR Lyrae star (ab)
- An RR Lyrae star pulsing in its fundamental mode.
- RR Lyrae star (c)
- An RR Lyrae star pulsing in its first overtone.
- Periodic variable
- A star that repeats on a regular schedule.
Solar system
Asteroids and other nearby bodies caught moving across the field between exposures.
- Asteroid
- A solar-system body caught moving across the field.
Other & unclassified
Host-galaxy position tags, unclassified alerts, and image-subtraction artifacts.
- At a galaxy core
- Sits at the centre of its host galaxy.
- Away from a galaxy core
- Sits away from the centre of its host galaxy.
- Artifact
- Not a real sky source: a glitch in the image subtraction.
Credits & data sources
- ALeRCE (Automatic Learning for the Rapid Classification of Events) ↗ — community astronomical alert broker that ingests, cross-matches, and classifies the survey streams, and hosts the ALeRCE Explorer ↗.
- Vera C. Rubin Observatory (Legacy Survey of Space and Time / LSST) ↗ — 8.4-metre Simonyi Survey Telescope and 3,200-megapixel LSST Camera on Cerro Pachón, Chile.
- Zwicky Transient Facility (ZTF) ↗ — wide-field optical sky survey on the Samuel Oschin 48-inch Schmidt telescope at Palomar Observatory.
For the technical design of Sky Alerts—difference imaging, database schema, least-privilege roles, and daily enrichment—see How It Works.