Sky Survey Lab

In this lab, we are going to take a close look at some photographs which are part of the Palomar Observatory Sky Survey (POSS). They were taken with the 1.2-meter (mirror diameter) Schmidt telescope at Mount Palomar Observatory. Schmidt telescopes have very large fields of view and are therefore ideal for comprehensive surveys of the sky. The entire POSS covers all the sky visible from Mount Palomar (from the north celestial pole to 24 degrees south of the celestial equator). Each photographic plate covers 6.4 degrees x 6.4 degrees of the sky. The exposure times lie between 2400 s and 4200 s. Every field on the sky was photographed twice: once through a blue and once through a red filter. We are going to look at 17 different photographs, which represent a journey from one of the closest stars (Vega, 26 light-years away) to a distant cluster of galaxies (Hydra, 2,800,000,000 light-years away.)

Objects in the Milky Way Galaxy

The first set of photographs is not directly relevant for cosmology because it shows pictures of objects much to close to "home" to be usable for studies of the Universe as a whole. However, the distant galaxies that are studied in cosmology consist of many of the same components as our Milky Way galaxy. Many of the details that can't be resolved in distant galaxies can be studied in our own galaxy and the results can then be generalized to those galaxies.

Plate #	Description
1	Vega, one of the brightest and closest stars in the sky (Pretty boring, huh?).
2	The Orion Nebula, one of the closest star-forming regions. Where does the energy come from that the nebula gives off in the form of light?
3	The Pleiades, the most prominent galactic cluster in the sky. Where does the gas that is surrounding the stars come from?
4	M67, one of the oldest galactic clusters known.
5	M13, a globular cluster. What's the difference between galactic and globular clusters?
6	The Crab Nebula, the remnant of a supernova. Where does the energy come from that the nebula emits in the form of light?
7	The North America Nebula, a star forming region similar to the Orion Nebula. Why are some stars brighter on the blue print but the nebula is brighter on the red print?
8	The Galactic plane in Cygnus. Why are some of the regions almost white on the photographs?
9	Another photograph of the Galactic plane. What is a "planetary nebula"?
10	Field near the Galactic center. What causes the extremely high star densities in this field?

Extragalactic Objects

We start our tour of extragalactic objects with the Andromeda Galaxy, which is the closest large spiral galaxy. It is too close to have any significant recession velocity due to the expansion of the Universe. But it is nevertheless very important for cosmology because many of the things we know about distant spiral galaxies come from studies of the Andromeda Galaxy.

Plate #	Description
11	The Andromeda Galaxy, the closest large spiral galaxy. What are the bright knots in the spiral arms?
12	The Whirlpool Galaxy.
13	The Virgo Cluster of Galaxies, the nearest large cluster of galaxies. What makes this cluster so important for the determination of the distance scale of the Universe?
14	The Coma Cluster of Galaxies. Why would a precise measurement of the distance to this cluster be more valuable than the knowledge of the distance to the Virgo Cluster?
15	The Hercules Cluster of Galaxies.
16	The Corona Borealis Cluster of Galaxies.
17	The Hydra Cluster of Galaxies.

The Space Telescope Science Institute has produced a digital version of the POSS (and also added a survey of the southern sky done with a telescope in Australia). It is available on the web under the URL http://stdatu.stsci.edu/cgi-bin/dss_form.

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bbeck@astro.washington.edu