profc.jpg (13600 bytes)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


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 poetryribbon.wmf (2678 bytes) 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,

southern Milky Way photographed from Arizona

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.

herschel.jpg (613333 bytes) Caroline Herschel
William Herschel, in an idealized portrait showing the discovery of infrared light and one of his large telescopes in the background -- and his sister Caroline, who worked with him on his astronomical projects and deserves a fair share of the credit for his discoveries. (from

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,

The Herschels' map of the Milky Way

But, they were unaware of the existence of interstellar dust that was cutting off their view toward the true center of the systemlink to an extra topic.

More star counts were available in ~1920 when Dutch astronomer, Kapteyn, made a modern version of Herschel's Milky Way model: (from

Kapteyn's model of the Milky Way

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 Waylink to a key question

Harlow Shapley 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

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






globular cluster M9

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 which 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. brightness variations of an RR Lyrae star
Henrietta Leavitt animation of three stars obeying a period-luminosity relation
Henrietta Leavitt had discovered the Period-Luminosity Relation while studying RR Lyrae stars which lie in the Large Magellanic Cloud and hence all at the same distance from us. (From AAVSO, APOD, The brighter the star, the slower it changes.(From Nick Strobel Go to his site at for the updated and corrected version.)
Plot of period luminosity relation for RR Lyraes and Cepheids 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.standardcan.jpg (10559 bytes)





From Clem Pryke,

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

shap2.gif (86596 bytes)  

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)

Shapley's plot of the globulars toward the Galactic Center 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 kiloparsecs. Thus, he concluded that the sun lies toward the edge of a disk-shaped distribution of stars.

The modern distance is 8000 pcs -- 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.

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Anasazi painting of Crab supernova, Canyon de Chelly,

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John Flamsteed's constellation Andromeda, from Atlas Coelestis,

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hypertext copyright.jpg (1684 bytes) G. H. Rieke

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