«1. Introduction The aim of this project is to gain experience using the CCD camera by obtaining images of planetary nebulae or H II regions. ...»
CCD Imaging of Planetary Nebulae or H II Regions
The aim of this project is to gain experience using the CCD camera by obtaining
images of planetary nebulae or H II regions. Planetary nebulae are ionized clouds of gas
surrounding very hot stars. The nebula is formed when a star of moderate mass reaches
the end of its life as a red giant star, and makes the transition to its ﬁnal state as a white
dwarf. In this process the outer layers of the star are ejected to form the nebula, while the newly exposed core appears as a very hot central star, which emits most of its radiation in the far ultraviolet, thus ionizing and heating the nebula. Most of the radiation emitted by the nebular gas, which has a temperature of ∼10,000 K, is concentrated in the emission lines of a few elements such as oxygen and nitrogen, as well as the most abundant element, hydrogen. An H II region is an interstellar gas cloud with newly formed stars in or near it which are hot enough to ionize some of the gas. While the physical conditions in the gas and the emitted spectrum are similar to planetary nebulae, H II regions are much less regular in appearance and contain much more gas and dust.
We will take the images of the nebulae through two narrow-band interference ﬁlters which transmit the light of the strongest emission lines in the nebular spectrum. These are the Hα line of hydrogen at 656.3 µm and the line of doubly ionized oxygen (O++ or [O III]) at 500.7 µm. The great advantage of such narrow ﬁlters is that they only transmit light with wavelengths near that of the nebular emission line you are interested in observing.
Thus you get all the photons of the nebular line, but greatly reduce the photons from the bright College Park sky, with its reﬂected city lights, since these are either due to incandescent lights (which emit continuum emission) or mercury lamps (which emit line emission at wavelengths not transmitted by the ﬁlters). Once you have the images, you can then compare them with published pictures, observe the diﬀerences in the distribution of H+ compared to O++, and perhaps measure the ﬂux in the nebula compared to that in the star.
An alternative project would be to image a bright H II region. In this case, we would observe the nebula through the Hα ﬁlter and also through a ﬁlter centered on the Hβ line at 486.1 µm. These lines are always emitted with a constant intensity ratio, Hα/Hβ = 2.8.
However, the ratio we observe may be diﬀerent because the Hβ line suﬀers more absorption by interstellar dust than the Hα line. By studying the variations in this line ratio, we can map out the distribution of dust in the nebula. The problem with this project is that the Orion Nebula, which is by far the best H II region, can only be observed in the early morning hours at this time of year!
We want to image planetary nebulae of suﬃcient size to be able to study their shapes.
Since nebulae of this size are faint, all groups should start with the Ring Nebula (NGC 6720, M 57), which is probably the easiest object to ﬁnd. It is in the constellation of Lyra, and the nebula is located almost exactly between two naked-eye stars. The nebula itself looks like a faint “smoke ring” in the main telescope. Look up the RA and Dec and ﬁnd the object on a star chart before you go out to observe.
You will observe a second planetary nebula in addition to the Ring Nebula. You will want to pick a bright planetary nebula which has as large a size as possible without exceeding the ﬁeld of view covered by the CCD camera. You will ﬁnd the ﬁeld of view of the CCD in the hand-out on the 14” telescope. You also want to pick an object that is high in the sky sometime during the evening (or early morning) hours. Look at the illustrations in the catalog to see what sort of structure the nebula has and if the central star is visible.
It would be nice if you could observe the central star. (In principle, such observations would allow 