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Curriculum: [ Sample Labs | Astronomy Courses @UI ] |
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Best
Observing Season: Winter, Spring Level:
Introductory Learning Goals: The student will study the properties of the Orion Nebula and the Trapezium, including the extent of spectral line emission in Ha and OIII. Terminology: emission nebula, ionization, O-type star, reflection nebula Software:
Maxim, Netscape Archive
Image Directory: orion Archive
Image List:
orion[b,v,w,x].fts References: Pasachoff,
Jay. 1991, Astronomy ( Background and Theory
Nebulae are clouds of dust and gas, and come in nearby stars. Reflection nebulae depend heavily on the presence of dust in the cloud. Gas alone does not reflect light efficiently, and without the presence of dust particles, most nebulae would be essentially invisible. In all reflection nebulae it is the dust which is most reflective. Dust can also obscure our view of a nebula. A cold, dusty region in front of a reflection nebula will absorb the light from that nebula, and make that region appear dark. This effect is sometimes called a ‘dark nebula’. An example of this phenomenon is the recent Hubble spacecraft image of Eagle Nebula (M16) in Serpentis, shown in Figure 2. An emission nebula is formed of gas and shines because it is hot. Vast amounts of ultra-violet radiation are required to make the gas hot enough to shine. Hot stars (O and B types), can provide this energy. An example of a nebula in which the gas itself glows is M42, The Orion nebula, located in the sword of Orion. This nebula is located 1,500 light years away, along our spiral arm in the Milky Way galaxy. The stars have formed in the last million years from clouds of interstellar dust- the most massive clouds are responsible for the very bright stars at the center of the nebula. Many faint stars are also present, which have not yet finished gravitationally collapsing, yet still have become hot enough to be self-luminous. The gas in the Orion nebula contains large amounts of ionized hydrogen and oxygen. Atoms are ordinarily neutral in charge. That is, they contain equal numbers of electrons (negatively charged) and protons (positively charged). When an atom heats up, it begins to lose electrons and becomes positively charged, or ionized. Different atoms lose electrons at different temperatures. Electrons which are close to the nucleus (as in atoms with few electrons such as hydrogen, or many times ionized oxygen) are bound more tightly than electrons far from the nucleus. The oxygen electrons in the Orion nebula are bound more tightly than the hydrogen electrons. Therefore, we expect to see much more HII than OIII. We can check this by observing the region with blue, green (visual), and red filters. There are four extremely luminous O-type stars which contribute the bulk of the illumination of the Orion nebula. This cluster is known as the Trapezium, because of its trapezoidal shape. ProcedureObserving1. Schedule observations of M42 (the Orion Nebula). You will want to schedule observations using the B, R, and V filters. Make observations for approximately 30 seconds in these filters. In addition, observe using B and V filters with very short exposure times (about 4 sec in B; 2 sec in V) to determine the apparent magnitudes and colors of the Trapezium stars. The short exposure times are necessary to see the stars, not the nebula.
Image AnalysisNebula 1. Run Maxim. Load your long B, V, and R filter images of the nebula. 2. Measure the extent of the Ha emission and the OIII emission in pixels. Recall that Hα emission is red, and that OIII emission is blue/green. Place your cursor approximately in the middle of the nebula, and record the x and y coordinates ( in the top of the image window) of this point. These coordinates are given in pixels. Next choose a point at the edge of the nebula, and record the x and y coordinates of this point. Use the Pythagorean Theorem: to calculate the extent of the emission in pixels. Do this for 10 points in each image. For each image, take the average of the measurement of the extent. A data table will help you to stay organized here.
3.
Convert the radii of these regions to km using the small angle formula
and the image scale (1.23 arcseconds/pixel
for the IRO). You will also need the distance to M42. Which is larger-
Ha or
OIII? Why? (Hint: review Background section). Trapezium 1. Load the image Orionmos.jpg into Maxim. This image of the brightest portion of M42 is a composite of 15 images taken by Hubble Space Telescope, and shows an area 2.5 light years across. Near the center the four stars of the Trapezium are visible. The brightest star is known as q1 Ori C. The others are q1A, q1B and q1D, with A, B, and D in order counterclockwise from C. Notice the bright star down and to the left of Trapezium. This is q2 Ori. Make a diagram showing the location of Trapezium with respect to this and other bright stars in the region. 2. Load the file m42-ref.jpg into Maxim or another image inspection program. This image is a finding chart containing the B and V magnitudes of the calibration star. Print out this finding chart, or keep this window open for the rest of the exercise. 1. You will now perform differential photometry on the images of the variable star. Differential photometry is done by first making a circle around a star (or asteroid or other (small) bright object) and adding up all of the ADU counts within that circle. The sky background brightness is subtracted from this total, and the resulting ADU counts are set equal to a magnitude (in this case zero). The magnitudes of other stars in the field are determined relative to this star by comparing ADU counts. One problem with this method is that it is possible to choose a variable star as your comparison star. If this is the case, then the calibrator star’s variability will contaminate the results. To eliminate this possibility, at least two check stars are chosen, and analyzed in the same way as the variable star. These check stars should show zero variability with time. If these stars show variability, a new set of calibrator and check stars are chosen, and the photometry repeated. Fortunately, programs such as Maxim remove most of the tedium of photometry by doing the calculations for you. 2. Run Maxim. Load the B image of Trapezium. 3. Perform differential photometry on the four stars in Trapezium (q1A-D). If you have forgotten how, consult with your instructor. Move the cursor over the stars in the Trapezium, one at a time. This is much more difficult than it sounds, because the stars are so close together, but if your image is not overexposed, it can be done. Remember to make your target circle as large as necessary but small enough to get only one star at a time. 4. Repeat step e for the V image. 5. Use Table 1 to determine the absolute visual magnitude Mv from your (B-V) values. 6. Determine the luminosity of the four stars in Trapezium in solar luminosities using the following relationship: where L is in solar luminosities and Mv sun = 4.79.
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Contact: web@phobos.physics.uiowa.edu
Last updated January 21, 2004