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The phrase J-region asymptotic giant branch (JAGB) refers to a recently-uncovered, promising standard candle consisting of a particular type of AGB star. The term J-region refers to a region within a J-K color-magnitude diagram of AGB stars, which has the characteristic that the stars' J-magnitudes are roughly the same. The stars consist of AGB carbon stars within a particular range of carbon abundance in their atmosphere, which can be discerned in the diagram for having a J-K (J minus K color) magnitude between 1.3 and 2. This fact was first discerned in a 2MASS-based study of AGB stars within the Large Magellanic Cloud.
An advantage of the JAGB method is that these stars are both bright and relatively easy to find. Furthermore, less observation is necessary to collect the necessary data than for some other standard candles. Because the passbands are in the infrared, JWST is suitable for carrying out the method, and its aperture, which is larger than that of HST, gives it angular resolution and light-collection advantages.