Telescope Exercise for Astronomy 250Worth 10 points Due March 30, 2001
Finding Examples of Stellar Evolution with the 21-inch Telescope.
Posted outside Prof. Rieke's office (Rm 262), you will find sign-up sheets for observing on the nights of March 23,24 and 25. Sign up for one night and a one backup night in case of bad weather. You may find it convenient to review Lecture 1 which discussed astronomical coordinates.
The examples of stellar evolution that you will observe are planetary nebulae and globular clusters whose stars provide nice tests of how stellar works. Recall that a planetary nebula results from a red giant blowing off its outer layers which form the nebula. The core of the red giant is visible at the center of the nebular as a white dwarf stars.
You need to find in the library or on the Web coordinates for two planetary nebulae and two globular clusters which are visible at this time of the year and which are visible from Tucson. How can you figure out what is visible from Tucson now? Any object which has a declination>-15 can be observed with the 21-inch (if we were outside of the city and away from buildings, we could observe to closer to the horizon limit which is set by our latitude (32) and can be compute from -90+latitude = most southerly declination that rises above the horizon). In practice, the 21-inch can point to about 15 above the horizon. We can figure out what range of right ascensions are visible at night in late March from the definition of right ascension. Recall that 0 hr right ascension = position of sunrise on the first day of spring = March 21. These mean that at sunrise a position with a right ascension 90 different will be overhead on the meridian at sunrise. Right ascension increase towards the east so the location with have a right ascension which is 90/15/hr = 6 hr less or 18 hrs. We would like to observe before midnight rather than at sunrise so we want to observe objects whose right ascension are even smaller - 6 to 10 hrs less so we can observe from ~8pm to midnight. This means that you want to find sources with right ascensions in the range of 7 to 14 hrs and declinations greater than -15.
Two good catalogs to use in selecting objects are the Messier Catalog (see and the NGC or New General Catalog. Another possible source of information is the "Observer's Handbook" printed each year by the Royal Astronomical Society of Canada. When you select your sources, note down the names and coordinates for your objects. You also need to look at what epoch the coordinates refer to -- precession changes right ascension and declination for a source continuously. If no epoch is stated (you should seem something like RA(1950) meaning the right ascension refers to epoch 1950), it is likely that the catalog is assuming epoch 2000 but you might want to find the same object listed in another catalog that does give the epoch. Because you will not be observing at exactly the epoch for which the coordinates are given, you will need to compute the precession between the catalog epoch and the night of your observation. Here's a short recipe of how to do this:
For example: Compute the precession for the galaxy M31 which lies at 00h 42m 44.32s and +41 16' 08.5" from 2000.0 to Sept 1, 2009.
To finish the calculation, you would add these differences to the original coordinates to get the position for Sept 1, 2009/
To turn in:
Names and catalog positions for 2 planetary nebulae and 2 globular clusters.
Coordinates for all four objects precessed to the day on which you observed.
The coordinates reported by the 21-inch when the object was actually centered in the eyepiece.
A brief written description or sketch of the appearance of the planetary nebulae and of the globular clusters.