My main contributions to astronomy (so far):

1. The Absolute Luminosities of Type Ia Supernovae
Pskovski and Phillips had previously proposed that the luminosities of Type Ia supernovae were correlated with their post maximum decline rates. Thanks to the high quality data of the survey that I carried out between 1990-1993 (the Calan/Tololo project) I confirmed these results and provided the most precesie tool to measure extragalactic distances. This method was later on adopted by Brian Schmidt and Saul Perlmutter to determine distances to very distant supernovae. Combining the distances to the distant and nearby supernovae discovered in the course of the Calan/Tololo survey, Schmidt and Perlmutter demonstrated in 1998 that the Universe is undergoing an accelerated expansion, an unexpected result which implies that we need to review the understanding of gravity formulated by Einstein in 1916 or that there is a new type of force in Nature.

2. Environmental Effects on Type Ia Supernovae on Galaxy Type
By comparing the luminosities of the Calan/Tololo supernovae to the properties of their host galaxies, in 1993 I discovered that the most luminous supernovae occur in spiral galaxies. This implies that either young progenitors or metal-poorer progenitors lead to brighter supernovae. Other people (eg. Garnavich) have shown that the role of metallicity is not the primary source of scatter, implying that younger progenitors lead to more luminous Type Ia supernovae.

3. Type II Supernovae as Standardized Candles
Toward the end of my PhD thesis in 2001, I discovered a correlation between the luminosities of Type II plateau supernovae and their expansion velocities, which established a new method to determine precise extragalactic distances. This technique will allow us to obtain an independent measurement of the acceleration of the Universe. Other people (Nugent, Olivares, Poznanski) have confirmed this finding from independent data sets.

4. First detection of Hydrogen in a Type Ia Supernova
In 2002 I took a spectrum of a Type Ia supernova. The object, SN 2002cx, was remarkable due to the presence of Hydrogen in the spectrum. This was the first long-awaited detection of hydrogen around a Type Ia supernova. The observations imply high density circumstellar gas around the supernova, possibly arising from a donor having undergone a high mass loss rate (eg. AGB star).

5. Last but not least: Spectrophotometric Standards
Between 1989-1992 I obtained CCD multiple-epoch spectrophotometry for 20+ stars which provided the most accurate and precise set of standard stars for flux measurements in the southern hemisphere. This set of stars still constitutes a fundamental reference for observers. With Stritzinger and Suntzeff we followed up on this work and extended the grid to a new set of 100+ stars.