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NAKED EYE PHOTOMETRY

Many a night I saw the Pleiads, rising thro' the mellow shade,
Glitter like a swarm of fireflies tangled in a silver braid.

- Alfred, Lord Tennyson, 1837-8, Locksley Hall

Photometry is the astronomer's method of determining the brightness of stars and other objects at various wavelengths. You will have the opportunity to do two kinds of photometry this semester, one with the naked eye and the other using instrumentation. Having met Albert Jones, a venerable old astronomer from Richmond, New Zealand who has made countless professional observations of variable stars, asteroids, comets and other objects, all with his naked eye, I am reassured of the value a naked eye has in astronomy. Albert's observations have been repeatedly crosschecked with electronic instruments to find that his values have consistently been within a tenth of a magnitude of the measured one. It points to the extreme sensitivity of the naked eye. When comparing to a similar star close by, the naked eye can indeed see which object is brighter, dimmer, etc. This activity focuses (pardon the pun) on the naked eye technique. Later, when you use electronic instrumentation you'll see that the technique is remarkably similar. Sometimes too, a discovery is made and estimate are needed immediately. So the importance of the technique is to be emphasized.

The basic technique is one of comparison. Electronic and CCD Photometry both require comparing values of the the measured object's brightness be compared with that of known "standards."

Objectives:

  1. You will use your naked eye and the image of the Pleiades included in this lab or your telescope to visually compare the brightness of stars in at least one Open Star Cluster, eg. the Pleaides or Beehive Clusters. Attached is a chart with information regrading standard stars in the clusters visible to you this evening and the rest of the fall.

  2. You will, by the process of comparison, determine the Visual Magnitude of unknown stars. Standard and comparison star information is attached.

Procedure:

  1. The Brightness of stars is measured in the Magnitude. This is a comparative scale of brightness, originated by the Greek astronomer Hipparchus. He identified the brightest stars as being first magnitude, the next brightest at 2nd magnitude and so on. The naked eye can generally see to about the 5th or 6th magnitude, depending upon the brightness of the night sky. Really bright stars are zero or first magnitude while dim ones are higher. Really bright objects may be zero or even have negative values.

  2. Check the star chart and accompanying Table for the clusters for the cluster of stars you will be onserving. For Naked Eye photometry we use stars that are a bit separated but we'd like them to be as close to our varaible as possible. Our goal here is to develop the procedure. Once that is down the rest is straighforward.

  3. To make comparisons of visual magnitudes, compare the unknown star with star A and star B. If it is brighter than A and dimmer than B we know that the visual magnitude is somewhere between the two known values. You'll basically use these values as guides. By continuously comparing values of stars we can ultimately "home in on the true value. remember that the higher the magnitude the dimmer the ojject. Let the letters A, B, C, D and E represent "standard" stars. That means we already know the actual magnitude at visual wavelength, or mv, or visual wavelength.
SAMPLE MAGNITUDE DETERMINATION:
  1. The real values of comparison stars are listed in the Astronomical Almanac. This means we already "know" the values of them at certain wavelengths such as "m" or visual magnitude. The Almanacs are located in the Observatory. You do not have to use the Astronomical Almanac to do this lab. For this laboratory session you merely have to use the attached information. The Pleaides Star Cluster is represented below by the NASA Astronomy Picture of the Day, Sep 3, 1996.

    Figure 1: Pleiades galactic star cluster. The "fuzziness" is due to particles of gas and dust off of which scatter light. This is called a reflection nebula.

    Figure 2: The more obvious stars in the naked-eye cluster are named.

    (here is another map of the Pleiades with more info, but your browser must be able to do frames)

    Star NameVisual Magnitude
    Merope 4.18
    Electra 3.70
    Pleiore 5.09
    Atlae 3.62
    Asterope 1 5.80
    Asterope 2 6.43
    Alcyone 2.90
    Taygeta For you to determine
    Celaeno For you to determine
    Table 1: Visual Magnitudes of stars in the Pleiades star cluster.
  2. It is for you to determine the visual magnitudes of the two unknown stars in the cluster as shown in Figure 1. These stars are named in Figure 2 but this is not an actual picture of the cluster as in the former. For the two unknown stars, find one brighter and one dimmer (remember, the dimmer star has a higher visual magnitude while the brighter one has a lower visual magnitude.) Do this at least two, perhaps three or four successive times, narrowing in on the correct values. This is called iteration.

  3. You can do this in several ways. One, if the weather is cruddy you can do it indoors by observing the Pleiades Cluster on your PC Monitor using the above image. Better yet, if the weather is cooperative, you can use your telescope to do this first hand. You will do best by working with a partner to record values as you go back and forth between the unknown and comparison stars. One thing to keep in mind is that a telescope inverts the image so the arrangement of stars may look very different than the image above shows us. You basically can do this lab twice (if you want to) once with your naked eye using the electronic image provided and secondly, using a telescope. A grade will be given for each of these if you do both. If you choose to do only one, do at least the one with the image above, simulating what we'd see in a telescope.

Report:

  1. Show the comparisons and how you arrived at the value you think is correct for both Taygeta and Celaeno. i.e., show your work. Don't just estimate an answer. Life is not that simple.

  2. Report the results and estimate how close you are to the correct value. i.e., report your answer in the magnitude + a range that is reasonable with the information you have to work with. Be sure to show your work that led you to your answers. If you want to be really good, use your search engine and find the actual values (which are well known) and compare your estimates with them.

  3. Most importantly, while you are at it, really take a moment to savor the beauty within the Pleiades star cluster. It is one of the most beautiful such clusters in the sky and here it is, right on the shoulder of Taurus the Bull. This is what we call an Open or Galactic Cluster. These kind of clusters are found in the plane of the galaxy.