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Stars and stellar populations

Understanding different stellar populations within our galaxy (and nearby galaxies) can tell us much about the history of our local Universe.  Here you find a few examples of the stellar populations we research here at Lund Observatory:


Galactic archaeology - mapping the stellar populations of the Milky Way

Researchers include:  Thomas Bensby, Sofia Feltzing, Nils Ryde

Understanding how spiral galaxies like our Milky Way formed and evolved to its current state is a major goal in galactic research, and requires two key ingredients:

  • A complete picture of the structure and composition of the galaxy
  • Theoretical models of how galaxies form and evolve.

There exists a flurry of models that can be tuned to fit the observational constraints. However, for the Milky Way, the galaxy that can be studied in greatest detail, large portions is extremely poorly mapped. It contains a plethora of exotic structures (rings, warps, streams), whose nature and origins we do not understand.

Our aim here is to unveil the nature of the Milky Way stellar structures and populations using detailed elemental abundance data from large spectroscopic surveys of the Milky Way (the Gaia-ESO survey and upcoming WEAVE and 4MOST surveys). Together with the astrometric data from the Gaia satellite, this forms a goldmine of unprecedented scale and quality for Milky Way studies, and will allow us to explore and map the Milky Way  in a multi-dimensional space consisting of distances, velocities, detailed elemental abundances, and stellar ages.


The Origin of the Galactic Center

Researchers include:  Nils Ryde, Ross Church, Florent Renaud, Oscar Agertz 

A coherent research theme at Lund Observatory is the investigation of the Galactic Center region. We study the Galactic Center structures from a wide range of perspectives: from chemical, stellar, and dynamical evolution, to the development of hydrodynamical and N-body dynamical models, to the next generation of zoom-in galaxy formation simulations. We observe the stellar populations with high¬-resolution spectrometers working in the infrared on the Gem¬ini, Keck, VLT, and Lowell telescopes.

Diagram showing the position of galactic center components. Logarithmic distance scale.
The anatomy of the Galactic Center region. The logarithmic distance axis indicates in distance from the central supermassive black hole in parsecs (pc).

At the very centre of the Milky Way is Sgr A*, a black hole 4 million times the mass of the Sun, surrounded by a small swarm of young, massive S-stars on orbits of separation <0.04 pc. At larger distances from Sgr A* is a nuclear stellar cluster of radius a few pc, containing about 26 million solar masses of stars, most of which are old. Within the nuclear stellar cluster, however, is a centrally concentrated spherical cluster of around 100 bright young stars, about 30% of which lie in a single disc. The nuclear star cluster is embedded in the much larger nuclear stellar disk, extending out to 200 pc and containing 15 billion solar masses of stars: 10% of the mass of the Galactic Bulge.  With the Milky Way being a benchmark for spiral galaxies, the study of its nuclear star cluster is important for understanding the nuclear star clusters that surround supermassive black holes at the centers of a majority of galaxies. This will in turn provide the needed constraints for cosmological zoom-in simulations and hydrodynamical models of nuclear star cluster formation. 

Annotated Spitzer Space Telescope image of the galaxy NGC5746.
Spitzer Space Telescope infrared images of the galaxy NGC5746, a Milky Way structural analogue. To enhance structures, the images are shown with different contrasts. At the center of the boxy bar (a.k.a. ‘bulge’) seen in the upper panel, lies the starforming inner bulge, or the ‘disky pseudobulge’, seen as the dark spot in the lower panel (adapted from Kormendy & Bender 2019).

Surfaces of planet-hosting stars

Researchers include:  Dainis Dravins

Spectra across spatially resolved stellar surfaces are obtained from exoplanet transits, revealing what successively becomes hidden behind the planet.  Data are simulated by hydrodynamic model atmospheres, with the aim to disentangle stellar variability from the small radial-velocity signal that could identify Earth-like exoplanets orbiting solar-type stars.

Illustration showing how stellar surfaces can be studied.
The Andromeda Galaxy. Astronomical image from S.E.O.