Supernovae: Seeding the Elements and Measuring the Universe

Supernovae are some of the most spectacular events we can observe in the Universe. These stunning stellar endings seed the cosmos with elements that give rise to stars, solar systems, and even life, while providing one of the best ways to measure distances in the Universe. They serve as standard candles at large distances revealing evidence for the accelerated expansion of the Universe. Observers have recorded about 1,000 supernovae with descriptions of these events found recorded in texts of many civilizations stretching back 1,000 years. Today astronomers study supernova explosions in distant galaxies and remnants in our own and nearby galaxies with sensitive telescopes. LSST’s data will dwarf the observations to date as it discovers over 10 million supernovae during its ten-year survey. And “LSST will find ten of each type of rare, once-in-a-million supernova,” according to Michael Wood-Vasey. Using this unprecedented compendium of stellar death throes, scientists will expand our understanding of stellar evolution, the large-scale structure of the Universe, and dark energy using observations of these cataclysmic events.

What is a supernova?

A supernova is a spectacular stellar explosion, which for a brief time can outshine its host galaxy. Our understanding of these catastrophic explosions comes from the observations of spectra and light curves. Generally, supernovae come in two varieties: Type I, supernovae with spectra lacking hydrogen lines, and Type II, with spectra displaying hydrogen lines. Differences in the spectra and light curves define subgroups within these categories. Type Ia supernovae are understood as white dwarf stars that accrete mass from a close companion until they reach a critical limit at 1.4 the mass of our Sun and collapse catastrophically. Type Ib, Type Ic and Type II supernovae are thought to result from the core collapse of stars at least eight times as massive as our Sun. LSST data will reveal even more variations in behavior and properties as its survey increases observations by 10,000 times current numbers. These results will enable scientists to explore how and why a star evolves into a supernova.

LSST will discover almost as many Type II supernovae as Type Ia supernovae. It will obtain finely sampled light curves for both types in many colors. The chemistry of these core collapse supernovae will provide understanding of the cosmic chemical evolution of iron-group elements. Supernovae in galaxies can be used to measure the star formation history of the Universe.

Although Type Ia supernovae are currently understood well enough to measure the accelerating expansion of the Universe, the details of variation in peak luminosity need to be understood to enable the exploitation of ever-growing supernova data sets. There is considerable debate about the physics of supernovae. With a large sample size, researchers will be able to test for the underlying causes of the dispersion of peak luminosity and determine if well-measured properties of the supernova and its host galaxy can be used to reduce this dispersion. Dependence on cosmic time or redshift would indicate evolution of the progenitor population. There is good evidence for a variation in Type Ia supernovae properties as a function of galaxy type – higher luminosity, slowly brightening and slowly declining Type Ia supernovae are associated with star-forming galaxies. LSST will allow correlations of supernovae properties with those of their host galaxies. Wood-Vasey points out: “With millions of supernovae, we can take only the best 1% to form a uniform set of 10,000 Type Ia supernovae that will be free from the most important systematic uncertainties limiting Type Ia supernova cosmology today.” The significantly larger sets of data will help in understanding the progenitors and explosion models of supernovae and improve their precision for cosmology.

What will LSST supernovae data reveal about the earliest times of stellar evolution?

Pair-production supernovae are incredibly massive stars, 140-260 solar masses, which astronomers expect only in primordial environments. In today’s Universe the prevalence of elements heavier than hydrogen and helium prevent such massive stars from forming by absorbing radiation from proto-stellar cores and pushing away additional infalling material. The centers of the progenitors of pair-production supernovae are so hot that they can produce gamma rays with sufficient energy to create electron-positron pairs as outer layers of the star collapse inward during the last phases of the star’s lifetime. LSST will be able to discover these distant rare events through its unique deep, wide and rapid coverage. The light-curves of these explosions will be identified by their extended rise and fall in brightness over hundreds of days that will be very well sampled by the LSST main survey cadence. Pair-production supernovae will provide unique probes into extreme events in stellar evolution in the earliest times of the Universe.

How will supernovae reveal the large-scale structure of the Universe?

Type Ia supernovae can be used as standard candles (a class of objects with same/similar brightness) to determine their distance using the inverse square law. Type II supernovae can serve as distance indicators and validate Type Ia distances measured in the same surveys. Furthermore, since Type Ia supernovae explode in galaxies, they can be used to trace the large-scale structure of the Universe.

What will supernovae tell us about dark energy cosmology?

“The underlying nature of dark energy is unknown, and it is the only fundamental interaction that cannot be studied in a terrestrial laboratory. Astronomical observations are the only tools available to study dark energy, and supernovae observations are among the most precise methods,” points out Rick Kessler.

Because type Ia supernovae are the best standard candles at large distances, they provided the first observational constraint for the dark-energy model of cosmology. The challenge for researchers in the next decade will be to understand the physics of supernovae, their relationship with their environments, and the nature of the redshift-luminosity relation for Type Ia supernovae. LSST will provide the massive numbers of samples at all redshifts needed to achieve this understanding. Systematic effects can be well studied by dividing the LSST sample into many large independent data sets, each reflecting different properties of the supernova and its host galaxy, and checking for consistency among these samples.

One of the most powerful properties of Type Ia supernovae as cosmological probes is that each single event provides useful constraints. Comparing the LSST Type Ia sample to other cosmological probes will reveal and minimize systematic errors. Kessler declares, “once this is done, researchers will be able to assess the spatial and temporal uniformity of the dark energy with vastly improved precision.”

Supernovae are spectacular events, which inspire awe for the energy they release, for the beauty of the remnants they leave behind and for the knowledge about stars, structure and cosmology that they can produce. “Each supernova is a probe of its place in the Universe and the cosmos in between it and us,” states Wood-Vasey. LSST’s discoveries will provide data to illuminate the nature and expand the scientific uses of supernovae for generations.

Article written by Anna H. Spitz and Michael Wood-Vasey