This year, Oregon Star Party saw a number of contributors to this website gathered together.
Hi, I’m Rob Brown, the newest of those contributors, a retired Optical Engineer, amateur telescope maker and visual observer. Starting 2026, I am also co-chair of the First Light Telescope Conference (formerly known as the Portland Alt/Az Workshop) a gathering of amateur telescope makers and visual observers from around the world, in a hybrid format. I also make some novel tools for visual observers, which you can buy on my website quinsightspectre.com. One of those is the Spectre, a high-performance visual spectroscope for amateur astronomers.
Akarsh Simha has a Spectre spectroscope but had not used it, so I took the time to train him. Of course, doing that at the Oregon Star Party meant that there would be an audience, one very interested in seeing the spectra of various stars. So we put together a brief observing program in which we looked at three very different stars and a planetary nebula. We looked at Altair, Mirach, and Rasalgethi. Howard Banich gave us a very brief history of William Huggins and his first observation of the Cat’s Eye Nebula (
The Stars: Altair, Mirach, and Rasalgethi
I’ll talk about the stars first. They are types A7 (Altair), M0 (Mirach), and M5 (Rasalgethi).
Altair (A7)
Altair showed its two or three absorption lines due to hydrogen. Note that human vision is quite limited and the hydrogen alpha (\(\text{H}\alpha\)) and delta (\(\text{H}\delta\)) lines are near the ends of the visible spectrum, so they are quite difficult to see, while the beta (\(\text{H}\beta\)) and gamma (\(\text{H}\gamma\)) lines are closer to the peak of the dark-adapted eye and show up quite well. So, some of us could see three lines, beta through delta, and I think one of the group was able to see the hydrogen alpha line because of the very large aperture being used.
“Altair is a type-A main-sequence star with about 1.8 times the mass of the Sun and 11 times its luminosity. It is thought to be a young star close to the zero age main sequence at about 100 million years old, although previous estimates gave an age closer to one billion years old.” — Wikipedia
Mirach (M0)
We then observed Mirach. This star produced a spectrum with myriad fine lines across the entire range, not unlike the Sun, but this is a much different spectral class. Again, the large aperture gave a very impressive view compared to what I am used to with my 12-inch. We talked about how and why hydrogen dominated the spectrum of Altair while many other elements could be seen in Mirach.
“Mirach is a single, aging red giant with a stellar classification of M0 III. It is currently on the asymptotic giant branch of its evolution. The star has an estimated 2.49 times the mass of the Sun. Having exhausted the supply of hydrogen at its core, the outer envelope of the star has expanded to around 86 times the size of the Sun. It is radiating 1,675 times the luminosity of the Sun at an effective temperature of 3,762 K. Mirach is suspected of being a semiregular variable star, with an apparent visual magnitude varies from +2.01 to +2.10. Since 1943 the spectrum of this star has been one of the stable anchor points by which other stars are classified.” — Wikipedia
Rasalgethi (M5)
Then we looked at the coolest star of the three, Rasalgethi. Physically cool at about 3300 Kelvin, it is visibly cool because at its temperature simple molecules can form in the atmosphere—in this case, titanium oxide (TiO)—producing very complicated spectral bands that are reminiscent of the aurora borealis. I learned from Akarsh that the broad bands are induced by molecular vibrations coupled with the electronic transitions. One can think of the titanium and oxygen atoms being coupled on a spring, with that spring exchanging energy with the quantum leaps of the electrons to produce broad energy bands. These dark curtains are superimposed on a background of fine spectral lines, and all of them are spread across the entire visible spectrum. Rasalgethi has a very similar spectrum to the much brighter Betelgeuse, and in the 28-inch the view was comparable to what I see in my 12-inch on the brighter star. In other words, spectacular!
“Alpha Herculis A is an asymptotic giant branch (AGB) star, a luminous red giant that has both hydrogen and helium shells around a degenerate carbon-oxygen core. It is the second nearest AGB star to the Sun. Its radius pulsates between 264 and 303 solar radii. At its minimum, the effective temperature is of 3,155 K (2,882 °C) and the luminosity is of 7,200 solar luminosities, while at its maximum the temperature is of 3,365 K (3,092 °C) and the luminosity is of 9,330 solar luminosities. If Alpha Herculis were at the center of the Solar System its radius would extend past the orbit of Earth at 1.23 – 1.4 AU but not quite as far as the orbit of Mars or the asteroid belt. The red giant is estimated to have started its life with about 2.175-3.250 M☉.” — Wikipedia
Astrophysical Spotlight: Understanding the Stellar Spectra
For more information about the spectra of stars, look in here and here for the original articles by Jim Shields.
(Note: This section was authored by Google’s Gemini AI, and proofread and edited by Akarsh)
Altair vs. Mirach: Temperature and Ionization
Why does hydrogen dominate the spectrum of Altair (an A7 star) while many other elements are prominent in Mirach (an M0 giant)? This is a classic question in stellar astrophysics, explained by the relationship between a star’s temperature and the excitation states of its atoms.
For hydrogen lines to appear in the visible spectrum (the Balmer series), neutral hydrogen atoms must have their electrons in the first excited energy level (\(n=2\)). In the relatively hot atmosphere of Altair (\(\approx 7,500\text{ K}\)), the thermal energy is just right to populate this \(n=2\) state in a large number of neutral hydrogen atoms, making Balmer absorption lines highly prominent. In the much cooler atmosphere of Mirach (\(\approx 3,760\text{ K}\)), the thermal energy is too low to excite hydrogen electrons to the \(n=2\) level; almost all hydrogen atoms remain in their ground state (\(n=1\)), where they can only absorb ultraviolet light (the Lyman series). Thus, hydrogen Balmer lines are very weak. However, this cool environment is perfect for neutral and singly ionized metals (like iron, calcium, sodium, and titanium). These heavier atoms have many low-lying electronic states that can absorb visible light at various wavelengths, producing a rich forest of thousands of fine absorption lines across the visible spectrum.
Rasalgethi: Molecular Bands
In even cooler stars like Rasalgethi (\(\approx 3,300\text{ K}\)), the temperature is low enough for simple molecules (like titanium oxide, TiO) to form and survive in the outer atmosphere.
Molecules possess rotational and vibrational energy levels in addition to electronic ones. When these levels couple, a single electronic transition is split into thousands of closely spaced transitions.
Going back to the spring analogy I had earlier, think of this as follows: in an atom, the electron needs the exact right amount of energy to jump between the quantum levels of the atom, so it can only absorb one frequency of photon (well, the truth is a little more complex because there is a really narrow line-width even for atomic lines). So we get sharp lines in atomic spectra. On the other hand, in a molecule, there is also the bond between the two (or more) atoms, which one can think of like a spring attaching them. Now the electron has another option: it can “steal” some energy from the spring and use that to boost its jump up even when the photon’s energy isn’t exactly enough to make the transition. Of course, this is an oversimplification of the quantum-mechanical nature of the system, but this should give you a sense of what we mean by “coupling of electronic and vibrational modes”. A similar thing can happen with rotational modes of energy as well, think of the molecule as a rotating dumbbell. It could shovel some energy from/to the rotation to accommodate a different frequency of photon as well.
Visually, instead of sharp lines, these possible coupled transitions merge into broad absorption bands that gradually darken toward one edge and terminate abruptly at the other. These “dark curtains” dominate the spectra of M-type stars, giving Rasalgethi its spectacular, banded appearance.
The Planetary Nebula: Cat’s Eye Nebula (NGC 6543)
Then it was time to go find the Cat’s Eye Nebula (NGC 6543), and this gave Howard some time to give us the background of William Huggins’s discovery. Adding to the anticipation of the view was that Howard did not detail what Huggins saw, only that what he saw answered the prevailing question of the day and ushered in a new era of astronomy.
Spoiler alert: I’m going to explain what we saw and what Huggins saw.
I saw three vertical emission lines in the blue-green. The two brightest lines were adjacent and at times could not be resolved from each other; the third line was much fainter and well separated. Akarsh wanted to know the source of these lines. We were pretty sure that we were seeing the \(\text{[O III]}\) (doublet) and hydrogen beta (\(\text{H}\beta\)), so the obvious thing to do was grab \(\text{[O III]}\) and hydrogen beta filters to see what lines were passed and blocked by each.
At first, this was a bit confusing because some of us did not realize the wavelength shift that results if the filter is tilted, and they were not careful to place the filter over the eyepiece without tilt. So some of us saw all the lines disappear—I won’t say who. But once that got sorted out, we started to get sensible results. The \(\text{[O III]}\) filter decisively eliminated the shorter-wavelength dim line that was hydrogen beta, while the hydrogen beta filter dimmed or blocked one of the two brighter lines—the longer-wavelength one—while leaving the dimmer hydrogen beta line unaffected.
Forbidden Lines and the Physics of Low Density
(This section too is a contribution of Google’s Gemini AI. Proofread and edited by Akarsh)
Why do we see emission from doubly ionized oxygen (\(\text{[O III]}\)) in a planetary nebula, but we don’t see any corresponding absorption lines in the Solar spectrum?
A transition is called “forbidden” in quantum mechanics when it has a very low probability of occurring spontaneously. The excited state of the atom is metastable, meaning the electron remains in that state for a relatively long time—ranging from milliseconds to hours—before decaying and emitting a photon (compared to nanoseconds for permitted transitions).
- In dense environments (like the Solar atmosphere, or laboratories on Earth), the gas density is high. An oxygen atom excited to a metastable state will almost certainly collide with another particle (like a free electron) and lose its excitation energy collisionaly (collisional de-excitation) long before it has a chance to spontaneously emit a photon. Therefore, no forbidden lines are produced.
- In low-density environments (like planetary nebulae, where densities are typically only \(10^2\) to \(10^4\text{ particles/cm}^3\)), the time between collisions for any given atom is hours. Under these near-vacuum conditions, the metastable oxygen ion is left undisturbed, giving it plenty of time to decay radiatively and emit the characteristic green \(\text{[O III]}\) photons.
In fact, for a long time, the \(\text{[O III]}\) lines were attributed to a hypothetical new element called nebulium before it was understood that they were in fact due to a forbidden transition in doubly-ionized oxygen.
By the way, the notation \(\text{[O III]}\) is not the same as \(\text{O}_3\) (Ozone), but instead refers to the \(\text{O}^{2+}\) ion. This is a system called spectroscopic notation that can be somewhat confusing. In this system \(\text{O I}\) refers to the neutral oxygen atom, \(\text{O II}\) to oxygen with one electron removed (singly ionized) and \(\text{O III}\) to oxygen with two electrons removed (doubly ionized). The brackets indicate a forbidden line. Similarly, the emission nebulae that glow pink in the sky are called \(\text{H II}\) regions because they are dominated by (singly) ionized hydrogen, whereas the 21cm radio emission from neutral hydrogen is associated with \(\text{H I}\) clouds.
Solar Spectroscopy
So we had now done some quantitative astronomical observations and felt a little bit like real astronomers. But then we started asking ourselves about the filters themselves and, in fact, we had multiple filters from multiple vendors each with slightly different behaviors. Knowing this, we made a commitment to observing the solar spectrum during the daytime and checking our filters against the known lines seen in the solar spectrum. For this we used my solar spectroscope setup, described as part of the Oregon Star Party 2026 Telescope Walkabout.
Sure enough, we could verify that the hydrogen beta filters seemed to be centered on the hydrogen beta line, while the \(\text{[O III]}\) filters had some variability in their bandpass widths. That’s when we noticed that we couldn’t tell which absorption line was \(\text{[O III]}\) in the Solar spectrum. A little research revealed that there isn’t an \(\text{[O III]}\) absorption line in the Solar spectrum and this is because it is forbidden at the temperatures and pressures in the Solar atmosphere. Forbidden lines are well known, the only can be present under special circumstances, particularly the extremely low pressure environments of interstellar space such as in bright diffuse nebulae and planetary nebulae. (We noted this above.) Instead, there is a strong iron (Fe) absorption line at about \(495\text{ nm}\) not far from the \(\text{[O III]}\) position at \(500\text{ nm}\).
Here is a video showing our experiments with filters with the solar spectroscope:
All this was quite fun with the lively company. Then it dawned on me that the presence of iron in the Solar spectrum was yet more evidence that the Sun and the Solar System formed from a nebula that was seeded with materials from previous stars that died in supernova explosions. Well, of course. Here we were covered in the reddish dust of the central Oregon High desert—almost the spitting image of the surface of Mars but for the “dammit bushes” (attend OSP and you’ll know!)—its color coming from iron oxide. The gold in my wedding ring gave me a little nudge too, reminding me that all of the naturally occurring elements on the periodic table are here because of those ancient supernovae.
Anyway, a simple spectroscope that drops into the focuser of a Dobsonian scope turns out to be a powerful tool! Don’t pass up a chance to observe with one if the opportunity presents itself.