Friday, April 29, 2011

Milky Way: a Distance to the Galactic Center - 2

Why distance to the galactic center is so important?

The distance from Sun and the center of Milky Way is used as a reference stick for many other extragalactic distance calculations, making its accurate determination a matter of extreme importance.
The only direct method to determine distances to cosmic objects outside the Solar System is trigonometrical parallax. Ground-based telescopes allow to measure parallaxes up to ~0.01-arcsec, which allowed the distance estimates up to 100 pc. We can observe less than 1000 stars at these distances. The Hipparcos satellite measures parallaxes  up to ~0.001 arcsec, which provides good distances out to 1000 pc for about 100,000 stars. After 1000 pc we should employ indirect methods for distance estimates, such as spectroscopic parallaxes and properties of periodic variable stars.
No single method can provide accurate distances on all cosmic scales. Instead, we have to rely on a multi-step multi-method approach carefully choosing each method and calibrating at each step. This makes the Cosmic Distance Scale look like a ladder with a series of steps going from near to far. We should
Calibrate parallaxes based on the orbit of the Earth (the Astronomical Unit);
Calibrate H-R diagram methods based on distances to the stars with measured parallaxes;
Calibrate distances to Cepheid and RR Lyrae stars using H-R diagrams;
Calibrate extragalactic distances based on the known distance to galactic center.
So one must calibrate, calibrate and calibrate.
Because of the above reason, astronomers have tried to accurately measure the distance to the Galactic Center since it discovery in the early 20th century. Before that people still believed that our Solar System is the center of the Milky Way.
While the first estimates provided by Harlow Shapley and Jan Oort were despondently incorrect, recent technological advancements have enabled astronomers to estimate the distance to the galactic center with increasing accuracy.


References:


Carretta and Gratton 2000, Distances, Ages, and Epoch of Formation of Globular Clusters, The Astrophysical Journal, 533:215-235

Carretta, Gratton, Clementini, 2000, Mon. Not. R. Astron. Soc. 316, 721±728 (2000)



Popowski and Gould, 1998, "Mathematics of Statistical Parallax and the Local Distance Scale". arXiv, Ohio State University. Retrieved 2008-10-20.

Percival, Salaris and Kilkenny, 2003, The open cluster distance scale, DOI: 10.1051/0004-6361:20030092



Percival, S., Salaris, M., & Kilkenny, D. (2003). Why distance to the galactic center is so important? Astronomy and Astrophysics, 400 (2), 541-552 DOI: 10.1051/0004-6361:20030092

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