I review the observed properties of supernovae (SNe), concentrating on recent results obtained from high-quality optical spectra. Although most SNe Ia have similar characteristics at comparable phases, there are some notable variations, particularly in the velocities of the ejecta. Moreover, there is very preliminary evidence for significant differences between the spectra of SNe Ia in H II regions and in elliptical galaxies. SNe II exhibit a wide range of spectral properties, as expected if they are produced by massive stars. At late times strong lines of [O I] and [Ca II] appear in most SNe II, including SN 1987A. In some objects, however, these are absent; the spectrum of SN 1987F, for instance, closely resembled that of a Seyfert 1 nucleus, with prominent H I and Fe II emission lines. Type Ib SNe, which lack the 6150Â absorption band typical of SNe la near maximum brightness, develop strong emission lines of [O I] and [Ca II] many months past maximum, but hydrogen is absent. Hα emission with a P-Cygni profile has, on the other hand, been detected in the early-time spectrum of SN 1987K, an object which later became spectroscopically indistinguishable from an old SN Ib. There is evidence for very weak Hα in the early spectra of several additional SNe Ib. Moreover, 7 – 8 months past maximum the [O I]/Hα and [Ca II]/Hα intensity ratios in SN 1987A were higher than in a few other well-observed SNe II at corresponding times. Hence, Type II and Type Ib SNe may form a continuous sequence in which the mass of the hydrogen envelope is the main variable.

We are analysing late-time (older than about 150 d past explosion) optical spectra of Type II-Plateau (IIP) supernovae (SNe), which are H-rich SNe that come from red supergiant (RSG) progenitors. The dataset includes nearly 100 spectra of about 40 objects, making this the largest sample of SN IIP nebular spectra ever investigated. Quantitative criteria from within the spectra themselves are employed to determine if an observation is truly nebular, and thus should be included in the study. We present the temporal evolution of the fluxes, shapes, and velocities of various emission lines (see, for example, Fig. 1). These measured values are also compared to photometric data in order to search for correlations that can allow us to gain insight into the diversity of RSG progenitors and learn more about the details of the explosion itself.

The Supernova Working Group was re-established at the IAU XXV General Assembly in Sydney, 21 July 2003, sponsored by Commissions 28 (Galaxies) and 47 (Cosmology). Here we report on some of its activities since 2005.

We present results of optical follow-up observations of candidate ultra-luminous X-ray sources (ULXs). Using Keck optical spectroscopy, 17 of the candidates from the Colbert & Ptak (2002) catalog have been identified; this is one of the largest sets of optical identifications of such objects thus far. 15 are background active galactic nuclei (AGN); 2 are foreground stars in our Galaxy. These findings are compared with background and foreground object expectations, as derived from log $N$-log $S$ relations. Also, the results are briefly discussed in terms of the spiral-galaxy/ULX connection.

Large numbers of supernovae (SNe) have been discovered in recent years, and many more will be found in the near future. Once discovered, further study of a SN and its possible use as an astronomical tool (e.g., as a distance estimator) require knowledge of the SN type. Current classification methods rely almost solely on the analysis of SN spectra to determine their type. However, spectroscopy may not be possible or practical. We present a classification method for SNe based on the comparison of their observed colors with synthetic ones, calculated from a large database of multi-epoch optical spectra of nearby events. Broadband photometry at optical wavelengths allows classification of SNe up to z = 0.75, and the use of infrared bands extends it further to z = 2.5. We demonstrate the applicability of this method, outline the observational data required to further improve the usefulness of the method, and discuss prospects for its use on future SN samples. Community access to the tools developed is provided by a dedicated website.

Observations of Type Ia supernovae (SNe Ia) reveal correlations between their luminosities and light-curve shapes, and between their spectral sequence and photometric sequence. Assuming SNe Ia do not evolve at different redshifts, the Hubble diagram of SNe Ia may indicate an accelerating Universe, the signature of a cosmological constant or other forms of dark energy. Several studies raise concerns about the evolution of SNe Ia (e.g., the peculiarity rate, the rise time, and the color of SNe Ia at different redshifts), but all these studies suffer from the difficulties of obtaining high-quality spectroscopy and photometry for SNe Ia at high redshifts. There are also some troubling cases of SNe Ia that provide counter examples to the observed correlations, suggesting that a secondary parameter is necessary to describe the whole SN Ia family. Understanding SNe Ia both observationally and theoretically will be the key to boosting confidence in the SN Ia cosmological results.

We review the existing set of optical/UV/IR observations of Supernova 1993J, concentrating heavily on optical data because these are by far the most plentiful. Some results from theoretical modeling of the observations are also discussed. SN 1993J has provided the best observational evidence for the transformation of a SN from one spectral type to another, thereby providing a link between Type II and Type Ib supernovae (SNe). This has strengthened the argument that SNe Ib (and, by extension, SNe Ic) are core-collapse events. SN 1993J has remained relatively bright for 10 years; its late-time emission comes from the collision of supernova ejecta with circumstellar gas that was released by the progenitor prior to the explosion. The circumstellar material shows strong evidence of CNO processing.

The properties of the best-observed peculiar, SN 1998bw-like Type Ic supernovae (sometimes called “hypernovae” ) are reviewed, starting from SN 1998bw itself and including SNe 1997ef and 2002ap. Analysis of the light curves and the spectra shows that, while these SNe display a range of properties (kinetic energy, mass of the ejecta, mass of 56Ni synthesized in the explosion), they have in common a larger-than-normal explosion kinetic energy, giving rise to the characteristic broad-lined spectra. Also, they all come from the collapse of bare CO cores of massive ( ≳ 20M⊙) progenitor stars. Some of the properties of these SNe, such as kinetic energy and mass of 56Ni, are probably correlated with the mass of the progenitor. Evidence that these powerful events are intrinsically asymmetric, suggesting that a correlation with at least some gamma-ray bursts can be expected, is also discussed.

We present and analyze spectra of the Type IIn supernova 1994W obtained between 18 and 202 days after explosion. During the first 100 days the line profiles are composed of three major components: (i) narrow P Cygni lines with absorption minima at −700 km s−1; (ii) broad emission lines with blue velocity at zero intensity ~ 4000 km s−1; (iii) broad, smooth, extended wings most apparent in Hα. These components are identified with the expanding circumstellar (CS) envelope [5], shocked cool gas in the forward postshock region, and multiple Thomson scattering in the CS envelope, respectively. The absence of broad P Cygni lines from the supernova (SN) is the result of the formation of an optically thick, cool, dense shell at the interface of the ejecta and the CS envelope. Models of the SN deceleration and Thomson scattering wings are used to recover the Thomson optical depth of the CS envelope, τT ≥ 2.5 during first month, its density (n ~ 109 cm-3) and radial extent, ~ (4 — 5) × 1015 cm. The plateau-like SN light curve, which we reproduce by a hydrodynamical model, is powered by a combination of internal energy leakage after the explosion of an extended presupernova (~ 1015 cm) and subsequent luminosity from circumstellar interaction. We recover the pre-explosion kinematics of the CS envelope and find it to be close to homologous expansion with outmost velocity ≈ 1100 km s-1 and a kinematic age of ~ 1.5 yr. The high mass (≈ 0.4 M⊙) and kinetic energy (≈ 2 × 1048 erg) of the CS envelope combined with small age strongly suggest that the CS envelope was explosively ejected only a few years before the SN explosion.

Identifying the massive progenitor stars that give rise to core-collapse supernovae (SNe) is one of the main pursuits of supernova and stellar evolution studies, and is essential for understanding the birth of pulsars. Using ground-based images of recent, nearby SNe obtained primarily with the Katzman Automatic Imaging Telescope (KAIT), astrometry from 2MASS, and archival images from the Hubble Space Telescope (HST), we have attempted the direct identification of the progenitors of 16 Type II and Type Ib/c SNe. We may have identified the progenitors of the Type II SNe 1999br and 1999ev, the Type Ib SNe 2001B and 2001is, and the Type Ic SN 1999bu, possibly doubling the number of known SN progenitors. For the remaining SNe, limits placed on the absolute magnitude and color (when available) of the progenitor allows us to place limits on the progenitor's mass. Specifically, we have been able to place a relatively stringent limit on the progenitor of the Type II-P SN 2001du in NGC 1365, consistent with the limits placed on the masses of other Type II-P SNe. We have also recently identified the progenitor of the Type II-P SN 2003gd in Messier 74.

The Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory is a fully robotic 0.76-m reflector equipped with a CCD imaging camera. Its telescope control system checks the weather, opens the dome, points to the desired objects, finds and acquires guide stars, exposes, stores the data, and manipulates the data without human intervention. There is a 20-slot filter wheel, including UBV RI. Five-minute guided exposures yield detections of stars at R ≈ 20 mag when the seeing is good (≤2″).

One of our main goals is to discover nearby supernovae (SNe; redshifts generally less than 5000 km s−1), to be used for a variety of studies. Special emphasis is placed on finding them well before maximum brightness. A limit of ~ 19 mag is reached in the 25-sec unfiltered, unguided exposures of our Lick Observatory Supernova Search (LOSS). We can observe over 1200 galaxies in a long night, and we try to cycle back to the same galaxies after 3 to 4 nights. Our software automatically subtracts template images from new observations and identifies supernova candidates that are subsequently examined by student research assistants. LOSS found 20 SNe in 1998, 40 in 1999, and 36 in 2000, making KAIT the world’s most successful search engine for nearby SNe. We also find novae in the Local Group, comets, asteroids, and cataclysmic variables. Multifilter follow-up photometry is conducted of the most important SNe, and all objects are monitored in unfiltered mode. A Web page describing LOSS is at http://astro.berkeley.edu/~bait/kait.html.

Between 1984 and 1990 we conducted a survey at Palomar Observatory (Filippenko and Sargent 1985) to quantify the luminosity function of nearby active galactic nuclei (AGNs). The Hale 5 m telescope was used to obtain high-quality, moderate-resolution optical spectra of a nearly statistically complete sample of about 500 bright (BT ≤ 12.5 mag), northern (δ > 0°) galaxies (see Ho et al. 1995 for details). The survey has now been completed, and the first scientific results are reported in a series of papers by Ho et al. (1997a, b, c).

We describe a new sample of Seyfert nuclei discovered during the course of an optical spectroscopic survey of nearby galaxies. The majority of the objects, many recognized as AGNs for the first time, have luminosities much lower than those of classical Seyferts and populate the faint end of the AGN luminosity function. A significant fraction of the nuclei emit broad Hα emission qualitatively similar to the broad lines seen in classical Seyfert 1 nuclei and QSOs.

We present new and archival Hubble Space Telescope (HST) images of circumnuclear star-forming rings in barred spiral galaxies. We find that nuclear rings in barred galaxies are composed of large numbers of super star clusters similar to those found recently in other types of starburst systems. In NGC 1097 and NGC 6951, the young clusters have absolute magnitudes of up to Mv = −14 or −15, depending on highly uncertain extinction corrections, and effective radii of 2–3 pc. The images (especially that of NGC 6951) also show intricate spiral dust lane structure interior to the rings.

At its peak, SN 1993J was one of the brightest supernovae in this century, and it is being studied more thoroughly than any supernova except SN 1987A. It is proving to be similar to the transition object SN 1987K, which metamorphosed from being a hydrogen-rich Type II near peak to having a hydrogen-deficient nebular phase. SN 1993J has been observed throughout the electromagnetic spectrum and with optical spectropolarimetry. It is interacting with a dense circumstellar nebula and is generating radio and X-ray flux, but it has probably not been detected in gamma rays. The photometric and spectral evolution are consistent with a star of original mass ∼ 15 Mʘ that lost appreciable mass to a binary companion leaving an extended, helium-rich hydrogen envelope of ≲ 0.5 Mʘ and a helium core of ∼ 4 Mʘ. The spectral evolution will put strong constraints on the mixing of 66Ni and other species.

Theoretical studies suggest that large-scale stellar bars can be highly effective in delivering gas to the central few hundred parsecs of a spiral galaxy, which may then initiate rapid star formation. Further instabilities may lead to additional inflow to physical scales relevant for active galactic nuclei. We test these predictions in light of recent observations. Compared to unbarred spirals, barred galaxies of type S0-Sbc have a higher probability of exhibiting nuclear star formation, as well as a higher formation rate of massive stars; neither effect is present in spirals of later morphological type. Bars, on the other hand, do not have an obvious influence on active nuclei. We discuss the implications of these findings for the fueling of central star formation and active nuclei.

We describe an optical, spectroscopic survey of the nuclei of the 500 brightest galaxies in the northern sky. The primary goal is to search for low-luminosity active galactic nuclei (LLAGNs) in the centers of nearby galaxies. The results of this survey will have many astrophysical applications, including quantifying the faint end of the local AGN luminosity function and estimating the contribution of LLAGNs to the X-ray background. We summarize the statistical properties of the survey, describe our methods of analysis, and present some preliminary results based on ∼ 60% of the sample.

I discuss evidence for and against the hypothesis that Type Ib and Type Ic supernovae (SNe) are produced by core collapse in massive, evolved progenitors. A key object is SN 1987K, whose spectroscopic classification changed from Type II to Type Ib/Ic as it aged. The progenitor of SN 1987K may well have been a massive star which experienced incomplete mass loss, leaving a thin outer envelope of hydrogen. However, several arguments are used to conclude that in most SNe Ib/Ic, the pre-supernova mass loss cannot be caused entirely by strong winds as in Wolf-Rayet stars. Mass transfer in close binary systems is probably important, but in such cases the supernova progenitor is not necessarily a Wolf-Rayet star; instead, it may be a relatively quiescent, hot, low-mass helium star that explodes via core collapse. For example, the rapid decline of the light curve of the Type Ic SN 1987M, and its seemingly low ejected mass, are consistent with this idea. It is also possible that some, but not all, SNe Ib/Ic arise from deflagrations or detonations of white dwarfs.