Closing-in on NGC 300

In the above image, the top left corner shows a ground-based Digitized Sky Survey image of the full field of NGC 300. An outline of the Hubble Heritage ACS image is marked and shown in the image at top right. A detailed blow-up of this image (at bottom) shows individual stars in the galaxy. A background spiral galaxy is visible in the lower right corner. Illustration Credit: NASA and Z. Levay (STScI) Image Credits: NASA, The Hubble Heritage Project (STScI/AURA) and Digitized Sky Survey

Zoom Animation MPEG 5.5MB Zoom from the Digital Sky Survey NGC 300 and dissolve into the Hubble's Advanced Camera for Surveys image of a section of NGC 300.
Animation Credit: NASA, Z. Levay and G. Bacon (STScI)	Double ASC pointing of NGC 300 (Click to Enlarge)

Star forming regions in NGC 300

By Fabio Bresolin

Place cursor over image for a comparison of visible light and Halpha. Click to enlarge the Halpha image. Image courtesy of F. Bresolin (IfA/U. Hawaii)

NGC 300 is a very photogenic galaxy. Here is a narrow-band Halpha image taken by F. Bresolin at the ESO 2.2m telescope on La Silla, Chile. Clusters of young (few million years old) and massive (up to 100 solar masses) stars form along the spiral arms, and with their enormous output of radiation they ionize the surrounding clouds of gas, mostly made up of hydrogen. The recombination of hydrogen atoms back to the neutral state produces photons of light at specific wavelengths (emission lines). The Halpha line, that is the emission by hydrogen atoms at 6563 Angstroms (1 Angstrom = 0.00000001 cm), is particularly intense, and allows us to trace the location of recent star formation in spiral galaxies. Astronomers call these sites HII regions.

Besides being tracers of massive stars, the analysis of the emission line spectra of these regions of ionized gas provides us with crucial information on the chemical composition of spiral galaxies.

Most of the stars visible in this image do not belong to NGC 300, but are instead foreground Milky Way objects. The HST image shown in this release covers the very central part of the galaxy, devoid of large HII regions.

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Blue supergiant stars as distance indicators

Quantitative stellar astronomy can be useful for the determination of extragalactic distances. A small team of astronomers, composed by Rolf Peter Kudritzki, Fabio Bresolin (Institute for Astronomy, University of Hawaii) and Norbert Przybilla (now at Bamberg University in Germany), have recently discovered a simple relation linking the intrinsic luminosity of blue supergiants to their fundamental parameters: surface gravity and temperature. (Graph courtesy of F. Bresolin (IfA/U. Hawaii))

This plot shows such a relationship, based on observations of stars in NGC 300, NGC 3621 (another HST target galaxy; for an image follow this link to the Araucaria Project homepage) and a number of galaxies in the Local Group, including our own Milky Way. Each star analyzed is represented by a dot, with different symbols used for the different galaxies. The y-axis represents the bolometric magnitude, a measure of the stellar intrinsic luminosity (smaller numbers corresponding to brighter objects). The objects at the top of the diagram, with bolometric magnitudes between -9 and -10, are among the most luminous normal stars known. Only supernovae and certain members of a class of stars known as Luminous Blue Variables exceed these values of luminosity for a single star. The x-axis is a simple combination of the stellar gravity and effective temperature, which are measured from spectra of the stars. More extended (lower gravity) and/or hotter objects are found to the right in this plot.

This diagram shows a tight correlation between luminosity and the flux-weighted gravity, which can be used to measure extragalactic distances. For a given star it is relatively simple to determine gravity and temperature (from spectra), together with its apparent brightness or magnitude (from images). The vertical shift necessary to match the position of the star in the plot with the calibrating line (seen as dashed in the diagram) would provide the distance modulus (a measure of the distance). In practice, in order to reduce the statistical uncertainties, one needs data for a number of stars in a given galaxy.This technique is currently being tested as part of the Araucaria Project.