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What distinguishes FUors from EXors?
The labels come from two prototype stars, FU Orionis and EX Lupi. Astronomers use them to describe related kinds of young-star eruptions, not two perfectly separate physical processes. A classification is strongest when several clues agree: how long the brightening lasts, whether it recurs, how much the star brightens, its accretion rate, and the spectrum at maximum.
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| Clue | FU Orionis-type (FUor), typical pattern | EX Lupi-type (EXor), typical pattern |
|---|---|---|
| Duration | More than a decade in classical descriptions; some eruptions last for decades | Usually months to about a year in common descriptions |
| Recurrence | Historically regarded as too infrequent to track easily; recurrence may take longer intervals | Recurrent eruptions are characteristic, with intervals that can span years or decades |
| Optical brightening | Often around five or more magnitudes | Often smaller, but large events occur |
| Accretion rate | Generally higher; literature describes a broad range, often around 10-5–10-4 solar masses per year during outburst | Generally lower; a historical comparison gives about 10-6–10-5 solar masses per year |
| Optical spectrum near maximum | Often absorption features resembling F- or G-type supergiants, associated with the hot inner disk | Often T Tauri-like emission lines, with a stronger contribution from the star and accretion shocks |
The duration and accretion figures summarize common descriptions rather than universal cutoffs. The European Southern Observatory’s 2008 comparison table gives FUor eruptions as longer than 10 years and EXor eruptions as about one year, with typical accretion values of 10-4 and 10-6–10-5 solar masses per year, respectively (ESO, 2008). A review by Hartmann and Kenyon describes FU Orionis systems across low and high states of roughly 10-7 to 10-4 solar masses per year (Hartmann & Kenyon, 1996).
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Why do the spectra look different?
FUors: a disk-dominated absorption spectrum
During a strong FUor outburst, the inner accretion disk becomes hot and optically thick. Its light can dominate the observed optical spectrum, producing absorption features that resemble those of an F- or G-type supergiant. This does not mean the young star itself has evolved into a supergiant; the resemblance reflects the bright disk emission.
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EXors: emission associated with the young star and accretion
EXors more often retain a T Tauri-like spectrum with emission lines. These features are associated with the young star and material accreting onto it, including emission from accretion shocks. In practice, spectra vary with the state of the object, so this contrast is a tendency rather than a rule that every observation must follow.
What EX Lupi’s 2008 eruption shows
EX Lupi’s 2008 eruption lasted about seven months and brightened by more than five magnitudes, according to Kóspál and colleagues (Kóspál et al., 2010). Its exceptional brightness did not by itself make it a FUor: its accretion strength remained below that associated with FU Orionis. The event is a useful reminder that amplitude alone cannot classify an eruption.
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Why the class boundary is not sharp
The distinction was built from a small set of prototypes. Continued observation has revealed candidates with intermediate durations, conflicting spectral clues, or a mixture of FUor- and EXor-like traits. Researchers therefore consider the light curve and spectrum together instead of applying one duration or accretion-rate threshold.
The labels describe observed behavior and do not establish a unique trigger. Proposed explanations for enhanced accretion include instabilities in the disk, material infalling onto the disk, and interactions with a companion. NASA’s 2024 discussion of new Hubble observations of FU Ori considers these as possibilities; none is established as the universal cause of every FUor or EXor eruption (NASA Science, 2024).
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