Digital Universe Guide - Hayden Planetarium
Digital Universe Guide - Hayden Planetarium
Digital Universe Guide - Hayden Planetarium
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3.3. MILKY WAY DATA GROUPS 89<br />
brightness scale and was then able to find the distances to the clusters. Jan Oort, a Dutch astronomer,<br />
confirmed this result by studying the motions of stars, showing that they are all in orbit about a<br />
distant center.<br />
We know today that Shapley overestimated their distances by about a factor of three, making the<br />
Galaxy about 300,000 light-years in diameter. Shapley was a proponent of the Milky Way <strong>Universe</strong><br />
cosmology, believing that all that we see is part of our Galaxy and that our Galaxy is the entire <strong>Universe</strong>.<br />
In April 1920, Shapley and the astronomer H. D. Curtis met at the National Academy of Sciences to<br />
debate this cosmology in what is now called The Great Debate. This question would be answered within<br />
five years by Edwin Hubble (1889–1953) when, in 1923, he discovered Cepheid variables in the<br />
Andromeda Nebula. He found the distance to the Andromeda and M33 to be about 300 kiloparsecs (just<br />
less than 1 million light-years). An underestimation, but these results set the scale. Now the Andromeda<br />
Nebula could be considered a galaxy in its own right and the <strong>Universe</strong> was now known to be far larger<br />
than the Milky Way.<br />
Data Variables for Globular Star Clusters<br />
Number Name Description Units<br />
0 metal Log of the metallicity suns<br />
1 rcore Core radius arcmin<br />
2 distly Distance light-years<br />
Exploring the Catalog The globular clusters form one of the most complete data sets in the Atlas.<br />
Data for the 145 clusters in the catalog were compiled by William Harris (McMaster University, Canada)<br />
and represent almost all the clusters in our Galaxy (a few on the opposite side of Galactic center may be<br />
invisible to us).<br />
Selection Expressions for the Globular Clusters<br />
Alias Partiview Command Description<br />
oldgc thresh metal -2.0 -1.5 Show the metal-deficient, older clusters<br />
younggc thresh metal -1.0 1.0 Display the metal-rich, younger clusters<br />
In Partiview, it’s easy to see what Shapley observed 80 years ago: that most of the clusters are<br />
located in Sagittarius near the Galactic center as seen from Earth. The closer the marker, the closer the<br />
cluster. One of the nearest clusters to us is Messier 4 (M4), in the constellation Scorpius. It is about