Extragalactic Distance Measurements with Planetary Nebulae_

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It is common to think of using the brightest stars to determine extragalactic distances. Over 50 years ago Hubble first attempted to resolve stars in other galaxies to quantify the expansion of the universe, and since then, the use of blue and red supergiants for extragalactic distance determinations has been explored several times. However, only recently has it been appreciated that young planetary nebulae also fall into the "brightest stars" category and are therefore potentially useful as distance indicators. The central stars of these objects are almost as luminous as bright red supergiants, and, as a result, present telescopes and current technology allow planetary nebulae to be detected at distances as large as 25 megaparsecs. Planetary nebulae have several advantages over most other extragalactic distance indicators. Because they are not associated with any one stellar population they can be found in all types of galaxies, and are particularly valuable for probing elliptical galaxies. It is elliptical galaxies which define the cores of large groups and clusters. In this grant surveys of planetary in elliptical galaxies will be made out to a distance of 25 megaparsecs. These measurements will add to our understanding of the large scale motions of galaxies and in other studies. The Principal Investigator is a recognized expert in this research area.