We now turn our attention to galaxies, huge systems of containing billions and billions of stars.

We start with our own galaxy, the Milky Way.

Key points: What disguised the structure of the Milky Way; standard candles; how the structure was revealed

Discovery of the Milky Way

The Milky Way from Mount Graham (from Wiphu Rujopakarn)

A broad and ample road, whose dust is gold,         And pavement stars,—as stars to thee appear Seen in the galaxy, that milky way         Which nightly as a circling zone thou seest Powder’d with stars. - Milton, Paradise Lost. Book vii. Line 577. If it is so obvious, why do we talk about discovering it?? To the right, a section of the southern Milky Way from Arizona. The Milky Way has always been an inspiring sight, and figures prominently in religion, literature, and poetry However, partly because a lot of it is hidden by dust (seen prominently here) it was very hard to figure out how it is built. (From T. and D. Hallas, http://www.astrophoto.com/and http://www.ngcic.com/MilkyWay/default.htm)

The Herschels, in the late 1700s, attempted to observe the shape of the Milky Way by subdividing the sky into a series of square areas and counting the number of stars in each patch.

They thought that the Sun is located near the center of the Milky Way -- their counts showed a nearly symmetric drop-off in all directions from the Sun.

Below is the Herschels' Map of the Milky Way (the sun is the bright dot near the middle): (from Ned Wright, http://www.astro.ucla.edu/~wright/milkyway.html)

But, they were unaware of the existence of interstellar dust that was cutting off their view toward the true center of the system.

More star counts were available in ~1920 when Dutch astronomer, Kapteyn, made a modern version of Herschel's Milky Way model: (from http://www.d.umn.edu/~hmooers/astro/Galaxies.htm)

Kapteyn had the Sun slightly offset from the center of the Milky Way (circled dot), but still didn't appreciate the deception caused by dust!

Why is it important to understand the Milky Way

Harlow Shapley, around 1915, was interested in studying the distribution of globular clusters and what he might learn about the Milky Way from them. (from http://www.phys-astro.sonoma.edu/BruceMedalists/Shapley/)

Globular star clusters:

The Globular Cluster M9:     Definitely bound by gravity Contain large numbers of stars in a very small volume: 20,000-1,000,000 stars in a volume 20 pc in diameter very round and symmetrical in shape very old -- among the first stellar complexes formed in the galaxy (from http://www.astr.ua.edu/gifimages/m9r.html)

Shapley took advantage of 2 facts:

1) globular clusters can be seen at relatively great distances

2) globular clusters contain RR Lyrae stars, a type of pulsating variable star that can be used to measure distances

RR Lyrae stars are stars that have left the main sequence, but have not yet exhausted all of their nuclear fuels. They are in the mode of alternately expanding and contracting, which causes their light to vary periodically and repeatably.

Henrietta Leavitt had discovered the Period-Luminosity Relation while studying RR Lyrae stars that lie in the Large Magellanic Cloud and hence are all at the same distance from us. (From AAVSO, APOD, http://antwrp.gsfc.nasa.gov/apod/ap981027.html)	The brighter the star, the slower it changes.(From Nick Strobel at www.astronomynotes.com)

By measuring the period of the star's fluctuations, one could get its luminosity -- comparing the estimated luminosity with the observed brightness and using the inverse square law, the distance to an RR Lyrae variable can be calculated. Objects where we can estimate the luminosity in this way and use their apparent brightnesses to determine distances are called standard candles.         From Clem Pryke, http://find.uchicago.edu/~pryke/compton/flyer.html

The Period-Luminosity Relation gave Shapley a means of determining distances to globular clusters.

• - he repeatedly took photographs of a cluster
• - he measured the brightness as a function of time for the RR Lyraes he found
• - he extracted periods for the stars
• - he used the P-L relation to get the luminosity
• - he compared his apparent brightnesses from the photographs to the luminosities to get the distance, using the inverse square law

	Shapley found the globulars form a nearly spherical distribution around a point in the direction of the constellation Sagittarius. Here is his projection of their positions onto the plane of the sky, with the center of the plot (at coordinate 0, 0 and indicated with the yellow/green arrow) in Sagitarius. (figures from Astrophysical Journal)
	And here is his plot of their distances measured from the RR Lyrae stars and projected on a plane running from the sun (at 0,0, where there is an x -- and an added yellow/green circle) toward Sagitarius.

Shapley reasoned that the location at the center of the globular cluster distribution must indicate the location of the center of the galaxy which he placed at a distance of nearly 30,000 parsecs. Thus, he concluded that the sun lies toward the edge of a disk-shaped distribution of stars.

The modern distance is 8000 parsecs -- again, Shapley was tricked because he didn't know about dust and how it was dimming the RR Lyrae stars on top of the inverse r² law.

Painting of Crab supernova, Chaco Canyon, http://www.colorado.edu/Conferences/chaco/tour/blanco.htm		Mixtec personification of the Milky Way, from http://garnet.acns.fsu.edu/~dco2511/
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