Characterizing Stellar Populations to Solve Puzzles about the Milky Way and Nearby Galaxies

By Anna H. Spitz and Kevin Covey

What do we know about the 3 septillion or so stars in the Universe? Although centuries of observations have revealed much about the stars that populate our Galaxy, many mysteries about individual populations remain. A population is a group of stars that shares consistent spatial, kinematic, chemical or age characteristics. Characterizing and mapping these populations provide powerful probes of a wide range of astrophysical phenomena and address fundamental scientific questions that extend more broadly than the details of a particular group: How do the properties of the individual stars within these populations inform our understanding of stellar evolution? What do the characteristics these populations have in common, i.e., that make them populations, tell us about the early history and evolution of the Universe?

LSST will acquire deep homogenous photometry for billions of stars in our Galaxy and the Local Group. These data will complement those obtained by other current or near-term surveys, such as PanSTARRS, SkyMapper, and the Gaia mission.LSST’s large aperture and red-sensitive CCDs will enable it to detect faint, red stars to a depth of r ~27, reaching out to the edge of the Galactic Halo .

With this unprecedented reach, LSST will help explicate the characteristics and processes within a gallery of stellar populations, providing the foundation to create more complete pictures of the Milky Way and nearby galaxies and to answer questions about galaxy formation, origins and even the nature of dark matter.

White Dwarfs

The most spectacular – and visually breathtaking – explosions in the Universe are the supernovae produced by dying stars. The vast majority (~97%) of all stars end their lives in a more passive manner, however, shedding their outer layer to form low mass white dwarfs as our Sun is expected to do in another 5 billion years. The quiet deaths of these dim stars can shed much light on a diverse range of astrophysical problems. White dwarfs provide information about stellar chemical evolution and the nature of dark matter (for example, are there the number of white dwarfs in the dark halo sufficient to account for an appreciable fraction of dark matter?). White dwarfs are also superb cosmic clocks–cooling predictably with time–and thus can give us important clues to the age and timescales of various Galactic components. LSST’s extensive catalog of Milky Way white dwarfs will permit refinement of theoretical models of star clusters, supernovae, and the production of exotic species of white dwarfs.

Metal Poor Stars

Most of the heavy elements within the Sun were produced by previous generations of stars; stars that lack those heavy elements are known as metal poor stars and are thought to have formed from material relatively unchanged since its origin in the early universe. The chemical composition of metal poor stars provides an estimate of the baryon-to-photon ratio in models of the Big Bang and insights into the elements produced in supernovae. The distribution of masses of metal poor stars can also give information about the nature and formation of the first stars. The composition and kinematics of metal-poor stars can shed light on the formation of the Galactic system.

Improving the Variable Star Distance Ladder

Cepheids and RR Lyrae stars have been indispensable to understand the scale of the Universe. Work remains, however, to fully understand these populations: LSST’s time-resolved observations will make a significant contribution to the study of pulsating variables, leading to improved distance estimates throughout the Universe.

Small and Cool Species

New surveys will expand the census of very low mass stars and brown dwarfs, objects too small to sustain hydrogen fusion in their cores, in the solar neighborhood over the next decade. LSST will produce the largest samples ever assembled of these intrinsically faint objects. LSST’s multiple images of these objects will also enable precise, direct distance estimates to these objects via parallax measurements. This large catalog will characterize the observable properties of cool stars and brown dwarfs with unprecedented precision, including their stability over time, and reveal how efficiently these ultra-low-mass objects form, now and in the Galactic past.

Eclipsing Binary Star Systems

Analysis of eclipsing binaries produces a host of critical information including calibration-free physical properties, such as masses, radii, surface temperature and luminosities, accurate stellar distances, precise stellar ages and tests of stellar evolution models. LSST’s time-resolved observations will enable the detection and characterization of a massive catalog of eclipsing binaries, useful for characterizing basic stellar properties, understanding stellar energy transfer and dynamics, and calibrating the cosmic distance scale.

Star Formation History of the Milky Way

LSST’s wide-field, high precision photometry and astrometry will allow measurement of proper motions, parallaxes, and time-variable age indicators for stars throughout the Milky Way. Using techniques such as gyrochronology (the technique that determines the age of stars using spin rates), age-activity relations and binary star isochronal ages, scientists will be able to decode the star formation history of the Milky Way and refine tools to understand galaxy formation.

Stars in Nearby Galaxies

The information about stellar populations doesn’t end in the confines of our Galaxy. The Small and Large Magellanic Clouds and other nearby galaxies are important laboratories for studying topics from stellar astrophysics to cosmology. LSST’s unique combination of depth and areal coverage will enable the detection of main sequence stars outside the Milky Way over entire stellar systems, such as the Large and Small Magellanic Clouds. These observations will enlighten scientists about important marker objects such as RR Lyrae stars and the nature of the Clouds themselves, LSST will be able to explicate star formation histories, look at how populations change with location in the galaxies, and reveal important clues about how the galaxies form.