Observing Projects
While planetary nebulae are rather bright objects, their brightness is spread over a relatively large angular size, which can make seeing them a challenge for the beginning observer. For example, the Helix Nebula (shown in the left-hand image on the page that opens this chapter) has the largest angular size of any planetary nebula but is also one of the most difficult to see. To improve your view, make your observations on a dark, moonless night from a location well shielded from city lights. Another useful trick, mentioned in Chapters 17 and 18, is to use “averted vision.” Once you have the nebula centered in the telescope, you will get a brighter and clearer image if you look at the nebula out of the corner of your eye. The so-called Blinking Planetary in Cygnus affords an excellent demonstration of this effect; the nebula seems to disappear when you look straight at it, but it reappears as soon as you look toward the side of your field of view.
Another useful tip is to view the nebula through a green filter (a #58, or O III, filter available from telescope supply houses). Green light is emitted by excited, doubly ionized oxygen atoms, which are common in planetary nebulae but not in most other celestial objects. Using such a filter can make a planetary nebula stand out more distinctly against the sky. As a side benefit, it also helps to block out stray light from street lamps. The same tips also apply to observing supernova remnants.
Although they represent a fleeting stage at the end of a star’s life, planetary nebulae are found all across the sky. Some of the brightest are listed in the accompanying table. Note that the distances to most of these nebulae are quite uncertain. Observe as many of these planetary nebulae as you can on a clear, moonless night using the largest telescope at your disposal. Note and compare the various shapes of the different nebulae. In how many cases can you see the central star? The central star in the Eskimo Nebula is supposed to be the “nose” of an Eskimo wearing a parka. Can you see this pattern?
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| Planetary nebula | Distance (light-years) | Angular size | Constellation | Right ascension | Declination |
|---|---|---|---|---|---|
| Dumbbell (M27, NGC 6853) | 490–3500 | 8.0 × 5.7 | Vulpecula | 19h 59.6m | +22° 43°′ |
| Ring (M57, NGC 6720) | 1300–4100 | 1.4 × 1.0 | Lyra | 18h 53.6m | +33° 02′ |
| Little Dumbbell (M76, NGC 650) | 1700–15,000 | 2.7 × 1.8 | Perseus | 01h 42.4m | +51° 34′ |
| Owl (M97, NGC 3587) | 1300–12,000 | 3.4 × ′3.3 | Ursa Major | 11h 14.8m | +55° 01′ |
| Saturn (NGC 7009) | 1600–3900 | 0.4 × 1.6 | Aquarius | 21h 04.2m | –11° 22′ |
| Helix (NGC 7293) | 450 | 41 × 41 | Aquarius | 22h 29.6m | –20° 48′ |
| Eskimo (NGC 2392) | 1400–10,000 | 0.5 × 0.5 | Gemini | 7h 29.2m | +20° 55′ |
| Blinking Planetary (NGC 6826) | 3300(?) | 2.2 × 0.5 | Cygnus | 19h 44.8m | +50° 31′ |
Northern hemisphere observers with modest telescopes can see two supernova remnants, one in the winter sky and the other in the summer sky. Both are quite faint, however, so you should schedule your observations for a moonless night. The winter sky contains the Crab Nebula, which is discussed in detail in Chapter 23. The coordinates are R.A. = 5h 34.5m and Decl. = +22° 00′, which places the object near the star marking the eastern horn of Taurus (the Bull). Whereas the entire Crab Nebula easily fits in the field of view of an eyepiece, the Veil or Cirrus Nebula in the summer sky is so vast that you can see only a small fraction of it at a time. The easiest way to find the Veil Nebula is to aim the telescope at the star 52 Cygni (R.A. = 20h 45.7m and Decl. = +30° 43′), which lies on one of the brightest portions of the nebula. If you then move the telescope slightly north or south until 52 Cygni is just out of the field of view, you should see faint wisps of glowing gas.
Use a telescope to observe the remarkable triple star 40 Eridani, whose coordinates are R.A. = 4h 15.3m and Decl. = -7° 39′. The primary, a 4.4-magnitude yellowish star like the Sun, has a 9.6-magnitude white dwarf companion, the most easily seen white dwarf in the sky. On a clear, dark night with a moderately large telescope, you should also see that the white dwarf has an 11th-magnitude companion, which completes this most interesting trio.
Planetary nebulae represent the late stages of the evolution of stars whose masses are similar to that of the Sun and are found throughout our Galaxy. You can use Starry Night™ to explore the distribution of these objects in our sky and to view several of these spectacular nebulae. Set the view for your home location at some time in the evening with a field of view of about 100º. Open the Options pane and expand the Deep Space panel. Expand the NGC-IC Database list, click in its box to activate the display of the objects in this list, and click Off all entries in the list except Planetary Nebula. Use the hand tool to move around the sky. Note that these nebulae are mostly concentrated around the Milky Way in our sky. If you have access to a telescope, try to locate and observe several of these planetary nebulae, if possible on a clear, moonless night. Some of the more notable planetary nebulae include Little Dumbbell (M76), NGC 1535, Eskimo, Ghost of Jupiter, Owl (M97), Ring (M57), Blinking Planetary, Dumbbell (M27), Saturn Nebula, and NGC 7662.
If you do not have access to a telescope, use Starry Night™ to examine in detail two of these planetary nebulae, M57 (the Ring Nebula) and M27 (the Dumbbell Nebula), and compare their shapes and sizes. Select Favourites > Explorations > Atlas and use the Find pane to center upon and magnify these two nebulae in turn. You can compare a ground-based image of M57 with a high-resolution image taken by the Hubble Space Telescope. First, open the Options pane, expand the Deep Space layer and click on the Messier Objects to see an image taken with a ground-based telescope. Then replace this image by a space image by clicking in the Hubble Images box. Note that the Hubble image is displayed in a different alignment to that of the ground-based image. For each of these objects, note their distance from observer in the HUD, and then use the angular separation tool to measure the approximate angular radius of each of these nebulae. (a) How do you account for the difference in the shape of these two planetary nebulae? (b) What is the nature of the central star in each of these nebulae? (c) Calculate the physical size of these nebulae. (Hint: Translate angular size in arcseconds to radians and use the small-angle relationship: 1 radian = 206,265 arcseconds; 1 ly = 9.46 × 1012 km.) (d) Assuming that both of these nebulae have been expanding at the same rate (measured in km/s), which of the stars at the cores of these nebulae reached the end of its life cycle first (i.e., which of these nebulae is the oldest)? (e) Using an average rate of expansion of 20 km/s for the shell of gas that forms the Ring Nebula (M57), approximately how long ago, in years, did the star that formed this nebula initiate the expansion of its outer atmosphere?
The red supergiant Betelgeuse in the constellation Orion will explode as a supernova at some time in the future. Use the Starry Night™ program to investigate how the supernova might appear if the light from this explosion were to arrive at Earth tonight. Click the Home button in the toolbar to show the sky as seen from your location at the present time. Use the Find pane to locate Betelgeuse. If Betelgeuse is below the horizon, allow the program to reset the time to show this star. (a) At what time does Betelgeuse rise on today’s date? At what time does it set? (b) What is the apparent magnitude (mV) of Betelgeuse? (Hint: Use the HUD or the Info pane to find this information.) (c) If Betelgeuse became a supernova today, then at peak brightness it would be 11 magnitudes brighter than it is now. For comparison, mV = −4 for Venus at its brightest and mV = −12.6 for the full Moon. Would Betelgeuse be visible in the daytime? How would it appear at night? Do you think it would cast shadows? (d) Are Betelgeuse and the Moon both in the night sky tonight? (Use the Find pane to locate the Moon.) (e) If Betelgeuse were to become a supernova, how would the shadows cast by Betelgeuse differ from those cast by the Moon?
Use the Starry Night™ program to investigate the X–ray source and probable black hole, Cygnus X–1. This region of space is one of the brightest in the sky at X–ray wavelengths. Click the Home button in the toolbar and then use the Find pane to center the field of view on Cygnus X–1. If Cygnus X–1 is below the horizon, allow the program to reset the time to when it can best be seen. Click the checkbox to the left of the listing for Cygnus X–1 to apply a label to this object. Use the Zoom controls to set the field of view to 100 degrees. (a) Use the Time controls in the toolbar to determine when Cygnus X–1 rises and sets on today’s date from your location. (b) Zoom in until you can see an object at the location indicated by the label. What apparent magnitude and radius does Starry Night™ give for this object (you can obtain this information from the HUD or by using the Show Info command from the contextual menu for this object)? Keeping in mind that the object that gives rise to this X–ray source is a black hole, to what must this apparent magnitude and radius refer? Explain.
Use the Starry Night™ program to examine in some detail the central regions of two galaxies that contain supermassive black holes at their centers. (a) The Milky Way is one such galaxy. Select Favourites > Explorations > Milky Way Centre to view our Galaxy from the equivalent of the center of a transparent Earth. The star HIP86919 is very closely aligned with the direction of the central core of the Galaxy and can be used as a guide when viewing this region. You can brighten the appearance of the Milky Way by opening Options > Stars > Milky Way… and moving the Brightness slider to the far right in the Milky Way Options panel. The Time and Date, August 30, 2009, at 6:30 a.m., have been chosen when the Moon is crossing the Milky Way plane, thereby providing a convenient angular scale, about ½° in diameter, for comparison with Milky Way features. (Click on Options > Solar System > Planets-Moons… to display the Planets-Moons Options window and ensure that the Enlarge Moon Size at large FOVs box under Other is Off, and click OK.) You will notice that, at visible wavelengths, this central region of the Milky Way appears to be dark. What explanation can you give for this dark band across the galactic plane? Zoom in to a field of view of about 5° around the galactic center to examine the region surrounding the black hole using the highly penetrating X-rays detected by the Chandra X-ray Space Telescope. If necessary, open the Options pane, expand the Deep Space layer, click on the Chandra Images, and move the slider to the right to display the image mosaic showing numerous hot and intense X-ray sources very close to the supermassive black hole. (Note that this image is slightly offset from the galactic center to avoid confusion.) (b) M87 is an active galaxy for which evidence is strong for the presence of a supermassive black hole at its center. Click Home to return to your sky and stop the advancement of time. Use the Find tab to center your view on M87, allowing the program to adjust the time to ensure that this object is in your sky. Open the Options pane, expand the Deep Space layer and click Off the Chandra and Hubble Images, leaving the Messier Objects displayed, and then Zoom in to a field of view about 1° wide to show this giant elliptical galaxy as seen from ground-based telescopes. (You can click on the Info tab and click on Description to read about this somewhat featureless but surprising elliptical galaxy.) In the Options > Deep Space panel, move the slider to the right for the Chandra X-ray image to see the structure of hot gases around M87. Finally, move the Hubble Images slider to the right to display the high-resolution Hubble Space Telescope image of the gas jet emanating from the black hole. Again, this image is displaced, to the upper right, from the galactic center position by the program. Right-click over this square image, click Centre and then Zoom in to a field of view of about 1 arcminute to see this spectacular jet as it interacts violently with the interstellar medium above the black hole. Comment on the suggestion that supermassive black holes were discovered only after relatively recent advances were made in telescope and detector technology.
Use the Starry Night™ program to observe the sky in July 1054, when the supernova that spawned the Crab Nebula (M1) would have been visible, probably even in daylight, from North America and may have been recorded as a pictograph at this time by inhabitants of Chaco Canyon, New Mexico. Open Favourites > Explorations > Crab-Pictograph to position yourself in Chaco Canyon, at latitude 36°N and Longitude 108°W at 5 a.m. on July 5, 1054 a.d., looking toward the east, just before sunrise. Zoom in to display a field of view of about 10°, centered on the Crab Nebula, when you can see the position of the Moon near to the nebula. You may find it helpful to turn daylight on or off (select Show Daylight or Hide Daylight in the View menu). (a) What is the phase of the Moon? (b) Investigate how the relative positions of the Moon and the Crab Nebula change when you set the date to July 4, 1054, or July 6, 1054. On which date do the relative positions of the Moon and the Crab Nebula give the best match to the pictograph shown in the textbook? You can now investigate the nebula more closely by zooming in to see a ground-based view of this expanding gas cloud from the violent supernova explosion. You can also compare this image with those from spacecraft at both visible and X-ray wavelengths. Open the Options pane, expand the Deep Space layer, and click the checkbox beside the Hubble Images option to turn on this feature. Starry Night™ can also superimpose an X-ray image of this active region from the Chandra Space Telescope onto the visible light image of this Messier object. Turn on the Chandra Images and compare the X-ray and visible light images by using the slide controls for the Chandra Images and Messier Objects options in the Deep Space layer of the Options pane. (Unfortunately, both the Hubble telescope and the Chandra X-ray images are misaligned with respect to the visible images. Nevertheless, you can see the very active regions surrounding the central spinning neutron star, the pulsar, at the center of the Crab Nebula.) (c) Open the Info pane for the Crab Nebula and obtain its distance from Earth in light-years under the Position in Space layer and its angular diameter from the Other Data layer. Using this information and the year in which the supernova was seen to explode, calculate the speed of expansion of the supernova remnant in km/s. (d) Taking the travel time of light into account, in which year did the star actually explode?
Use the Starry Night™ program to examine the Veil Nebula, a large supernova remnant. Open Favourites > Explorations > Veil Nebula to see a view of the nebula high in the sky of Calgary, Canada, at midnight on August 1, 2013. (a) What significant feature do you notice about this supernova remnant in this 5° field of view? (b) Use the angular separation tool to measure the approximate angular distance between the components of this nebula. What angular distance separates these components? What form of optical aid is best suited to observing this object? (c) It is believed that the supernova that produced this nebula would have been visible to people on Earth about 7000 years ago and that its distance from Earth is about 2500 ly. From these data, what is the approximate speed of expansion of this nebula in km/s? Zoom in on each component of the nebula to examine the fine details of this expanding remnant of a violent stellar explosion.
Collaborative Exercises
Imagine that a supernova originating from a close binary star system, both of whose stars have less than 4 solar masses, began (as seen from Earth) on the most recent birthday of the youngest person in your group. Using the light curves in Figure 20-20, what would its new luminosity be today and how bright would it appear in the sky (apparent magnitude) if it were located 10 parsecs (32.6 ly) away? How would your answers change if you were to discover that the supernova actually originated from an isolated star with a mass 15 times greater than our Sun?
Consider the graph showing a recording of a pulsar in Figure 20-27. Sketch and label similar graphs that your group estimates for: (1) a rapidly spinning, professional ice skater holding a flashlight; and (2) a siren on an emergency ambulance.
As stars go, pulsars are tiny, only about 20 km across. Name three specific things or places that have a size or a separation of about 20 km.