Type Ia Supernovae

Type Ia Supernovae are in many aspects still enigmatic objects. Their observational and theoretical exploration is in full swing, but we still have plenty to learn about these explosions. Recent years have already witnessed a bonanza of supernova observations. The increased samples from dedicated searches have allowed the statistical investigation of Type Ia Supernovae as a class. The observational data on Type Ia Supernovae are very rich, but the uniform picture of a decade ago has been replaced by several correlations which connect the maximum luminosity with light curve shape, color evolution, spectral appearance, and host galaxy morphology. These correlations hold across almost the complete spectrum of Type Ia Supernovae, with a number of notable exceptions. After 150 days past maximum, however, all observed objects show the same decline rate and spectrum. The observational constraints on explosion models are still rather sparse. Global parameters like synthesized nickel mass, total ejecta mass and explosion energetics are within reach in the next few years. These parameters bypass the complicated calculations of explosion models and radiation transport. The bolometric light curves are a handy tool to investigate the overall appearance of Type Ia Supernovae. The nickel masses derived this way show large variations, which combined with the dynamics from line widths, indicate that the brighter events are also coming from more massive objects. The lack of accurate distances and the uncertainty in the correction for absorption are hampering further progress. Improvements in these areas are vital for the detailed comparison of luminosities and the determination of nickel masses. Coverage at near-infrared wavelengths for a statistical sample of Type Ia Supernovae will at least decrease the dependence on the absorption. Some of the most intriguing features of Type Ia Supernovae are best observed at these wavelengths, like the second peak in the light curve, the depression in the J band, and the unblended [Fe ii] lines in the ashes. To appear in The Astronomy and Astrophysics Review