Showpieces of the September sky by Lauren Herrington


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Swan Nebula
M17

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The Swan Nebula, also known as the Omega Nebula, is an emission nebula which appears embedded within the upper end of the “steam” from the spout of the “Teapot”, Sagittarius. The Swan is located fairly close to the Sun within the Milky Way, lying a ‘mere’ 5,000 – 6,000 light years away inside the next arm over, the Sagittarius Arm.[1]

Long ago, the cloud which now forms the Swan began to collapse under its own gravity and form stars, which together make up the star cluster Collinder 377*. Today, extreme ultraviolet light from the young, hot, blue stars of Collinder 377 ionizes (knocks the electrons off) the atoms of gas within the remaining cloud. When the electrons eventually recombine with nuclei, they fluoresce brightly, and cause the nebula to glow.

Most of the Swan’s newborn stars and starforming regions are hidden from view completely in visible light by thick clouds of dust. An entire new branch of the nebula nearly as large as the Swan itself becomes visible in infrared images (which can see through dust)! The nebula also contains several notable sources of infrared and X-ray radiation, all related to the chaotic starbirth taking place within. One source in particular has been the subject of much debate: The “Kleinmann-Wright Object”, or KWO for short.[2] Read the citation to find out more.

The Swan is one of the brightest emission nebulae in the sky, visible to the naked eye under dark skies, and easy to see in a telescope. In medium-sized telescopes under dark skies, it’s clear where it gets its name: the nebula really does look just like a swan, with a graceful neck, windswept feathers, and even a short little swan tail. The mag 9.7 star SAO 161357 serves as the bright eye of the bird. Though not as bright as the Orion Nebula, in large telescopes it shows a similar structure to the bright Trapezium area; with many contrasting triangular facets, like crushed velvet. Under good conditions, nebulosity far outside the traditional shape of the Swan can be detected. Nebula filters such as “UHC” or OIII filters help greatly to pull out additional detail by blocking the non-nebula wavelengths of light from the background sky.

*The commonly-used designation NGC 6618 is shared by both the nebula and the embedded cluster. The designations Collinder 377 or OCL 44 refer to just the cluster.

 


 

M15
NGC 7078

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M15 is a globular cluster, among the densest and brightest of its kind. Like almost all other globular clusters, M15 was formed early in the life of the Universe about 12 billion years ago, in processes that we don’t yet understand. Today, it exists as a massive congregation of very old, red, low-mass stars. Globulars used to be full of all kinds of stars, including massive blue O and B types, but those burn hot and die young, leaving behind only neutron stars and black holes as their exotic remains.

M15 has undergone a process called “core collapse”, a vicious cycle where fast-moving stars flung away from the core deprive the core of kinetic energy, causing the orbits of the remaining stars to shrink, which in turn makes them move faster, thus providing more fast stars to fling away and remove kinetic energy… The end result of this process in the case of M15 is that a full half of the cluster’s mass is now contained in the central 1.4ly, or 0.0000005% by volume. This can be clearly seen at the eyepiece, with the cluster appearing not like a smooth “globe” of light, but like a steeply peaked “cone” of brightness poking out of the sky at you.

Another peculiarity of M15 is that it is one of only 4 globular clusters known to contain a planetary nebula: Pease 1. At magnitude 15.5, Pease 1 is detectable by larger amateur instruments, and would make a good challenge object. I encourage anyone who observes Pease 1 to contact me with your observation.

M15 is the 10th brightest globular cluster from the perspective of Earth, making it visible to the naked eye under dark skies and obvious in binoculars. In binoculars, the bright “peaked” core and faint fuzzy halo are easily visible, even from the city; in an 8″ dob in the city, the core remained visibly unusually bright, but no individual stars were yet resolvable.

If you go looking for M15 with the naked eye, be aware of the magnitude 6.1 star HR 8231 which lies less than a third of a degree away from M15. The star is of similar brightness or slightly brighter than the cluster, so if you only see one object and it looks stellar, it’s probably not M15: but keep trying, you’re close!

 


 

Blue Snowball
NGC 7662

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The Blue Snowball is a small, bright planetary nebula, one of the brightest deep-sky objects in the sky. Its distance and thus size are hard to measure, but it’s close to the plane of the Milky Way, probably embedded just on the other side of our own spiral arm, the Orion Spur.

The Blue Snowball has such a high surface brightness that color is visible even in small telescopes, hence the name. As with many objects, this high surface brightness comes with the trade-off that the object itself is very small; nonetheless, structure can be detected at high powers. The nebula can be clearly seen as nonstellar at 115x, with subtle but significant structural detail visible at higher powers. For a fun challenge, try to detect and record as much detail as you can before looking at any photos. Record everything you see, no matter how subtle. Then, look at photos of the nebula the next day and match up what you saw.

The Blue Snowball used to be a star somewhat less massive than the sun; around 0.6 solar masses.[3] As the star aged, the core gradually used up its hydrogen fuel reserves, switching over to helium instead. The byproducts of helium burning– carbon and oxygen– built up in the core, but the star was too low-mass to switch to burning those. Without being able to sustain nuclear fusion in its core, the star became unstable, and a complex chain of processes caused it to first shrink, then grow dramatically brighter and expand into a red giant, like the stars Aldebaran or Mira. The outer layers of the star were then so far away from the core– even farther than Earth is from the Sun– that they were hardly bound to the star at all; and so as the unstable star continued expanding and contracting, the outer layers were blown off, forming concentric shells of gas still energized by the remnant of the core left behind. This is what we see when we look at the Blue Snowball today.

 


 

Epsilon Lyrae
4 Lyr

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Epsilon Lyrae is a multiple star system, appearing very close to the bright star Vega from Earth’s perspective. In actuality, Epsilon Lyrae is six and a half times further away, so despite the systems being of similar absolute brightnesses, the apparent brightnesses as viewed from Earth are skewed.

If Vega were placed at the same distance as Epsilon Lyrae, it would appear to be the same brightness… but if instead of moving Vega farther, Epsilon Lyrae were moved closer, Epsilon Lyrae would still appear fainter! This is because each of the individual stars in the system are fainter than Vega, and only when combined together do they produce enough light to rival Vega. If we moved Epsilon Lyrae closer to Earth, the different stars would appear to separate, and we would no longer see their light as combined.

Despite being so much further than Vega, Epsilon Lyrae is still quite close to the Sun as far as stars go; it’s a member of the “local neighborhood” and thus well within the Sun’s spiral arm of the Milky Way.

What makes Epsilon Lyrae so special is that the star system is not a single star, nor is it a double; rather, Epsilon Lyrae is a set of two double stars, called ε¹ and ε²; four stars orbiting around each other in a complex gravitational dance. Actually, there is a fifth star in orbit around the ε² pair, but it is too close to be separately visible in regular telescopes. Several other stars are close enough that it’s possible they’re also a part of the star system, but they have not yet been proven to be physically bound.

The two main pairs of Epsilon Lyrae can be seen as a tiny double star with the naked eye by those with good eyesight. (If you can’t split them, look into ‘night myopia’. You might be able to get reading glasses that let you split them.)[4] In binoculars, the two pairs are obviously separate, but it still looks like a regular double star- just two points of light. It’s in a telescope when the magic happens. With apertures of 2″ or greater, starting at around 50x, it becomes possible to split each of the stars into a further double, so that three or four stars are visible instead of just two. While it’s technically possible to split the tighter pairs at 50x, powers significantly in excess of 100x will provide a much easier view. Nights with “good seeing” (steady air) offer the best chance of splitting the individual stars in small instruments.

 


 

Veil Nebula
NGC 6960, 6992, 6995, and more

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The Veil Nebula is a gigantic supernova remnant; cosmic debris generated by the detonation of a massive star which exploded roughly 8,000 years ago. Before going supernova, the star which formed the Veil was likely a red supergiant about 20 times the mass of the Sun. What we see of the Veil today is a gargantuan shockwave: light emitted where gas from the supernova, traveling at thousands of kilometers per second, is slamming into dense clouds of dark gas and dust floating freely in space.

Visually, the Veil is one of the most spectacular objects in the sky, and yet it is often regarded as a extremely challenging object. While it has a fairly low surface brightness, and thus is easily washed out by light pollution, the Veil emits most of its visible light in the very particular wavelengths (colors) of 496 and 500nm—the telltale blue-green of double-ionized oxygen. And it is thus that the Veil is saved. With the use of a nebula filter which isolates these “OIII” emissions, the Veil transforms from a few ghostly smudges to an intricate, knotted lacework of filaments. In large scopes, under dark skies, and with an OIII filter, the Veil is one of the rare objects that looks exactly like its photograph.

The Veil is easily visible in medium-sized telescopes without a filter under dark skies; in city skies, it may be invisible until a filter is used. Under very dark skies, it can be seen in binoculars, and there is one report[6] of it being seen with no magnification under very dark skies, using an OIII filter at the end of a paper towel tube held up to the naked eye. (The paper towel tube serves to block out extra light from the sky and allow deeper dark adaptation than usually possible. 8th magnitude stars can be glimpsed using this tube method.[7])

 

There are three main components to the Veil: the Western Filament, the Eastern Filament, and a fainter central portion (“Pickering’s Triangular Wisp”). The Western Filament is a thin, smooth rope pinned by the naked-eye star 52 Cyg, and splitting towards the south. The Eastern Filament is slightly brighter, and comparatively much more detailed, with indescribable branches, crosshatchings, and a complex “E” shaped region to the south. The Triangular Wisp (actually discovered by Williamina Fleming, not Pickering[5]) lies between the northern tips of the two bright filaments, and is much fainter. Many other small patches and filaments lie scattered in the space between the East and West filaments, and can be observed in large telescopes.

The entire loop is 2.5 degrees across, making it too big to capture all at once in a large telescope, but nicely sized for small telescopes and binoculars. Conversely, large telescopes can focus on the individual filaments, providing jaw-dropping views of the intricate details present.


Text ©2018 Lauren Herrington
Charts generated with SkyTools 3 by Skyhound

 

Further reading

Swan Nebula

Observing:
  • M17: The Nebula With Too Many Names – In this delightful article for Sky & Telescope, Howard Banich describes various portions of the Swan as they appear visually. He also explains the origin of its many names and includes a basic explanation of the science behind the nebula.

Science:
  • [1] Messier 17 – SEDS – An informative article with all the basic astrophysical data about the Swan Nebula.
  • Multiwavelength M17 – Cool Cosmos – This page has a very neat side-by-side comparison of the Swan at different wavelengths, but most importantly, walks the reader through each wavelength image, explaining the differences seen along the way.
  • [2] The nature of the KW object – Chini et. al – This facinating paper is the current authority on the Kleinmann-Wright Object. In summary, the KWO is currently thought to be a binary star deeply embedded within thick dust clouds, where one star is a Herbig Be star shining brightly in infrared, and the other emits x-rays. The KWO lies at the center of a dense cluster of large B, A, and F class stars, and a “reflection nebula” of infrared light reflecting off the dust clouds.

Media:
  • Swan Widefield – Space.com – This fantastic amateur astrophoto shows a wide view of the M17 Giant Molecular Cloud, with the central Swan shape more clearly discernable than usual for most deep images.
  • Swan Nebula – Cosmetography – Another excellent (semi-)amateur astrophoto of the Swan. Most of the faint outer nebulosity in this photo can be seen visually under dark skies, though not in such sharp detail.
  • M17: A Hubble Close-Up – APOD – An image of the interior of the Swan Nebula from the Hubble Space Telescope.
  • Aladin Lite interactive view – Click for an interactive view of the Swan with imagery in many wavelengths. (Make sure to check out the XMM/PN imagery for a view of the incredibly massive, X-ray emitting young OB type stars hidden behind the crook of the neck of the Swan! Were the cluster visible to the human eye instead of only X-rays and infrared, it would surely be known as a famous asterism, due to the picture-perfect ring surrounding a bright star.)

 


 

M15

Observing:
Science:
  • The Dynamic Lives of Globular Clusters – Sky & Telescope – In this facinating article, S. George Djorgovski provides an accessible introduction to the astrophysical mysteries of globular clusters, including an excellent explanation of core collapse.
  • Messier 15: Great Pegasus Cluster – Messier-Objects.com – This webpage aggregates much of the basic astrophysical data of the cluster, and includes some interesting historical observation logs from before 1840.
  • Star Clusters and Stellar Dynamics – Caltech – This slide deck must have been a boring lecture to sit through, but writing everything out paid off for us eager learners on the internet. While very high-level, this is an enjoyable read covering stellar dynamics, mass segregation, core collapse, and tidal disruption, mostly focusing on globular clusters. Math and physics heavy.

Media:

 


 

Blue Snowball

Science:
  • Blue Snowball – StarDate – This short episode of McDonald Observatory’s StarDate podcast is punchy and entertaining.
  • AGB Stars – NOAO – This “sidebar” gives a quick explanation of AGB stars (such as the Blue Snowball pre-nebula) and how an AGB star forms a planetary nebula.
  • Evolution of Low Mass Stars – Astr221 – The online course materials for Chris Mihos’ Astr221 course at Case Western Reserve University are a priceless gift to those of us who are trying to learn more in-depth than typical web summaries allow. Follow the links on the “Evolution of Low Mass Stars” page for a delightfully advanced but easy read about planetary nebulae and the stars that form them. (Feel free to skim the first link.)
  • Planetary Nebulae and How to Observe Them – Martin Griffiths – The sample pages from this book, available for download here, give an in-depth lesson on the complex physical processes that lead to the creation of a planetary nebula, without using any math.
  • [3] Physical Structure of Planetary Nebulae. II. NGC 7662 – Guerrero et. al. – A paper studying the physcial structure of the Blue Snowball.

Media:

 


 

Epsilon Lyrae

Observing and Science:
Media:

 


 

Veil Nebula

Observing:
Science:
  • FUSE – Piercing the Veil – A short article about the observation of a star behind the Veil Nebula, and what that tells us about the Veil itself.
  • NatSci102 – Supernova Remnants – A page about the basic astrophysics of supernovae and supernova remnants, from the University of Arizona.
  • Evolution of Stars More Massive than the Sun – A slide deck containg basic information about stellar evolution, including massive stars and supernovae, from the University of Texas.
  • UMD Astr480 – Supernova Remnants – The resources for the Astronomy 480 class at University of Maryland contain several documents with tons of information about supernovae and supernova remnants. The files are slide decks not optimized for web viewing, with heavy math and physics, and generally extremely dense to read… but if you want to learn about the nitty-gritty of supernova remnants, this might be a good resource. Hit Ctrl+F and type “Supernova” to highlight the relevant documents.

Media:

Clear Skies!
Lauren Herrington

4 Responses to

  1. EdF September 21, 2018 at 8:00 am #

    Thanks for your efforts, all will be on my list for September

  2. Craig Lamison September 21, 2018 at 2:18 pm #

    Thanks. Several of these will be first time observations for me. I will even revisit the double double when I do my Lyra star count. (I did split them into 4 but it was with my 4″ Mak-Cas at ~160X — so all you other old fogies don’t count on duplicating all Lauren’s feats 😉 Lauren, someday I expect we will all be able to say “I knew you when …”.

    • Lauren September 22, 2018 at 3:02 pm #

      Hi Craig!

      Splitting them in your 4″ at ~160x was good work! And thank you for your compliments. 🙂 I should state a disclaimer: I’ve never observed Epsilon Lyrae with a good scope smaller than 8″, so my statement about it being splittable in 50mm+ apertures at 50x is based on theoretical calculations, and reading others’ observations on Cloudy Nights.

      When I first started out in astronomy, I borrowed an “Edu-Science” 50mm refractor from a family friend- a department store telescope. Actually, “Edu-Science” is literally Toys-R-Us’ brand! Fortunately, I knew about department store telescopes, and not to expect much. I figured it would be impossible to see anything deep sky, so I thought, “what can this telescope see…. stars? Double stars!” So I Googled “best double stars” and many variations thereof, and one that came up was Epsilon Lyrae. Epsilon Lyrae became one of my favorite objects in that telescope. One night my mom drove me out a little ways from the city to try to find darker skies- it wasn’t far enough to see the milky way, and where we wound up setting up there were streetlamps, but I could still find good ol’ Epsilon Lyrae. That night, and perhaps some other too, I thought I could start to split one of the tighter components; it looked like a teeny comma. Unfortunately I have no idea what the magnification would have been, and it’s possible it was a lens aberration. One of these days I need to try again in another department store telescope, and see if it’s really possible! 🙂

      Dawes’ Limit for an unobstructed 2″ aperture telescope is 4.56/2 = 2.28 arcseconds. That means that both ε¹, with a separation of 2.46″, and ε², with a separation of 2.4″, should be resolvable, albeit barely. Let’s take a value of 2 arcminutes, or 120 arcseconds, as the closest separation that an observer with good vision can perceive. (In reality, I believe some may have slightly better, especially at small exit pupils.) Then the magnification needed to *perceive* them as resolved, assuming an observer with good vision and a small exit pupil to avoid off-axis aberrations within the human eye, is: 120″/2.4″=50x.

      That’s how I arrived at my conclusion that, if everything goes right, a good observer should be able to split Epsilon Lyrae at 50x in a 50mm. Now, all I need is to try! 🙂

      Clear Skies!
      Lauren Herrington

  3. Kenneth (drako) Drake September 22, 2018 at 12:15 pm #

    Excellent expose of these “must see” small scope perinials. Objects that even the seasoned veteran needs to revisit. Thanks, Zannadragon, for all you add to our community.

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