New image- NGC 6888, the “Crescent”

The “Crescent” nebula. NGC 6888

The Crescent Nebula (also known as NGC 6888, Caldwell 27, Sharpless 105) is an emission nebula in the constellation Cygnus, about 5000 light-years away from Earth. It was discovered by William Herschel in 1792. It is formed by the fast stellar wind from the Wolf-Rayet star WR 136 (HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 250,000 to 400,000 years ago. The result of the collision is a shell and two shock waves, one moving outward and one moving inward. The inward moving shock wave heats the stellar wind to X-ray-emitting temperatures.
Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. The surface temperatures of known Wolf–Rayet stars range from 20,000 K to around 210,000 K, hotter than almost all other kinds of stars.
Classic (or population I) Wolf–Rayet stars are evolved, massive stars that have completely lost their outer hydrogen and are fusing helium or heavier elements in the core.

In this image 3 narrow band channels, hydrogen alpha, hydrogen beta and oxygen III were color mapped to their precise hue values so this is not really a “false” color image but shows the colors emitted by the nebula in these 3 wavelengths of light. Hydrogen alpha emits in the red. Hydrogen beta and Oxygen are in the blue/teal range. Hydrogen alpha dominates but the blending with the bluish hues produces areas of pink.

Why is it called the “Crescent”nebula when it looks more like a chestnut or perhaps a kiwi fruit or something?

This is because it has a pretty low surface brightness and in a telescope or in images with short exposure times all you can see is the outer crescent of emission as in the image above.

Imaging platform currrently being used in “Gamma” complex at SkyPi Remote Observatory in Pie Town, New Mexico US

This was actually a first light image for the platform shown above.

Capture info:
Location: SkyPi Remote Observatory, Pie Town, NM US
Telescope: Orion Optics UK AG14 (14” Newtonian f/3.8)
Mount: 10 Micron GM3000
Camera: SBIG STXL 16200
Data:
Ha, HB, OIII 10 hours each
Processing: Pixinsight

Crescent nebula location is the red square which is circled in blue

Where is it located? The Crescent can be found pretty much dead center of the Northern Cross or the constellation Cygnus the swan, shown above. This is mid-nothern declination region in the Northern Hemisphere and sits in the heart of the Milky Way during the Summer/Autumn months. For most telescopes a filter is required to bring out the nebulosity. Larger scopes are required to see any structure visually.

That’s all for now!

Thanks for reading!

DrDave

The “other” Supernova 2023ijd

Supernova 2023ijd in NGC 4568. This animation consists of just two raw 15 minute luminance frames taken with the 400mm telescope at SkyPi Remote Observatory in Pie Town NM. These were obtained over a 12 day period covering just before discovery and about 10 days after.

A lot of internet activity has surrounded the discovery on May 19 of Supernova 2023ixf in the “Pinwheel Galaxy” M101. And deservedly so, as this is the closest supernova to us that has occurred in the last 5 years. It is also bright enough at magnitude 11 to be visible in the Northern Hemisphere in small telescopes! For more on this discovery see this article

Now only about 5 days before SN 2023ixf blew up, another star went through it’s last moments of life and subsequently blew up in another galaxy about three times as far away from us as M101. Consequently this supernova is not as bright as the one in M101 but it occurred in the fascinating interacting galactic pair of NGC 4568 and 4567. The “host” galaxy is NGC 4568 which is the larger of the two as you can see. These galaxies are in the process of colliding and merging with each other, as studies of their distributions of neutral and molecular hydrogen show, with the highest star-formation activity in the part where they overlap. However, the system is still in an early phase of interaction (courtesy Wikipedia). These types of high energy galactic processes are known “hotbeds” of supernova activity.

On May 12, I started imaging this region not for the purpose of supernova discovery at all but mainly because it’s a fascinating example of a galactic merger that we can actually see happening! About two days later I received a bulletin from one of the online spectroscopy groups about this new supernova. Unfortunately my high res spectroscopy set up has a mag limit of about 9-10 so I was not going to be able to capture a spectra of it, but I was already imaging it serrendipitously! I checked my first images of the galaxy pair but unfortunately there was no evidence of a star there, so I cannot claim discovery. I had to skip a couple of nights due to weather and this is the period when the actual discovery happened. Close but no cigar!

The discovery is credited to the All Sky Automated Survey for Supernovae based out of Ohio State University

The short animation shown here was created with two uncalibrated raw 15 minute frames I took about 12 days apart, from two days prior to discovery until 10 days after. The actual day of the discovery I could not image due to weather!

Some Supernova FAQ’s:

What is a Supernova?

Supernovae are massive stellar explosions that result in a visible stellar object where none was seen before. This is probably the the biggest explosion in the universe we can observe visually. There are currently two types:

Type Ia supernovae occur in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf, which is the small remnant of a star that has exhausted it’s fuel. The other star can be anything from a giant star to an even smaller white dwarf. If a white dwarf gradually accretes mass from a binary companion, or merges with a second white dwarf, the general hypothesis is that a white dwarf’s core will reach the ignition temperature for carbon fusion as it approaches the Chandrasekhar mass. Within a few seconds of initiation of nuclear fusion, a substantial fraction of the matter in the white dwarf undergoes a runaway reaction, releasing enough energy to cause the supernova explosion. The Type Ia category of supernova produces a fairly consistent peak luminosity because of this fixed critical mass at which a white dwarf will explode. Their consistent peak luminosity allows these explosions to be used as standard candles to measure the distance to their host galaxies: the visual magnitude of a type Ia supernova, as observed from Earth, indicates its distance from Earth. (courtesy Wikipedia)

Type II supernovae result from the rapid collapse and violent explosion of a massive star. A star must have at least eight times, but no more than 40 to 50 times, the mass of the Sun (M☉) to undergo this type of explosion. Type II supernovae are distinguished from other types of supernovae by the presence of hydrogen in their spectra. They are usually observed in the spiral arms of galaxies. Depending on initial mass of the star, the remnants of the core form a neutron star or a black hole

Both SN 2023 ixf and ijd have been determined to be the Type II variety.

How common is a supernova?

In any one galaxy we can expect maybe a couple supernovae per century. But thankfully the universe is filled with billions of galaxies so from Earth we are able to detect a few hundred per year!

How are supernovae discovered?

Usually supernovae are discovered with telescopic surveys of the sky such as the one that detected SN 2023 ijd. Robotic telescopes are commonplace now and cover a huge amount of space in a short time. Multiple galaxies are targeted in a single night and short exposure images are obtained and compared to previous sky surveys to see if there is any change. The NuSTAR (Nuclear Spectroscopic Telescope Array) mission, uses X-ray to investigate supernovas and young nebulas to learn more about what happens leading up to, during, and after these spectacular blasts.

What is learned from studying supernovae?

As mentioned above the Type Ia supernova can be used to measure galactic distance. We have also learned that stars generate chemical elements needed to make pretty much everything including life as we know it. When stars explode these elements are distributed throughout space.

Hope you learned something about supernovae!

Thanks for reading!

DrDave

New Year Dawning

Sunrise over Las Cruces NM Dec 29, 2021

As 2021 comes to a close and we reflect on the year, a few events stand out:

The Mars Rover lands!

Road trip to Pie Town

RS Ophiuchi variable star blows up!

Live outreach returns!

High resolution spectroscopy for the astroimager

New Mexico Skies takes a major hit

A few image publications along the way

As the New Year dawns we already have a new space telescope, the James Webb, on it’s way out to the deployment point beyond the Moon, and what looks like another amazing naked eye comet . On the personal front 2022 will bring many changes and new adventures! Stay tuned.

Happy New Year and thanks for reading!

DrDave

High Resolution Spectroscopy for the Astro imager: Why , What and How

Hi folks! I know it’s been awhile since I’ve been on here. Back to work full time… for awhile anyway. However I had a chance to do a presentation on amateur spectroscopy for the Astro imaging channel (also known as TAIC) on Youtube this past Sunday.

You can see the presentation in its entirety right here: https://www.youtube.com/watch?v=e3wZos5CMC0

Hope you enjoy it!

As always, thanks for reading!

DrDave

Observatory news

Bad monsoon rising

Hopefully the worst monsoon season here in the Southwest US that I have seen since moving here 8 years ago is finally winding down. Intermittent rain and thunderstorms have shut down Orion’s Belt Remote Observatory in Mayhill NM for the past several weeks. Typically here the rainy months are July and August and it certainly has been that.

Sky Roof control board likely affected by a lightning strike had to be replaced

Most likely a lightning strike damaged the computer running Pier 2 at the remote observatory and also took down the Sky Roof control board. Both had to be replaced. The whole lightning thing is very strange as everything in the observatory is fully surge protected but apparently it’s not bullet proof. No other major equipment damage occurred.

Recurrent Nova “wakes up”

Recurrent nova RS Ophiuchi became suddenly 7 magnitudes brighter on August 8-9. Courtesy of skyandtelescope.com. Gif produced by E. Guido, M Rocchetto and A. Valsavari

One of the few recurrent novae, RS Ophiuchi blew up to around 5th magnitude on August 8-9. The recurrence period is around 15-20 years. While remote imaging was shut down because of weather, I did have an opportunity to observe the nova from the spectroscopy observatory in Las Cruces, NM

The Talavera Space Hut, an 8 foot manual roll-off, located in Las Cruces NM is where I carry out spectroscopic observations. The spectrograph is a high resolution instrument known as Lhires produced by Shelyak Instruments. I use a C14 with an Atik 460 EX camera and ASI 174 guide camera. Mount is a Paramount ME.

RS Ophiuchi as mentioned is one of the few observable recurrent symbiotic novae. The binary consists of a red giant and a white dwarf with an orbital period of 454 days. The red giant is the “donor” star and the white dwarf accretes material from it. The accretion produces a very hot source of radiation which ionizes the outer atmosphere of the red giant and produces a constant nebular emission. If the accreted material reaches a critical value then a phenomenon called “thermonuclear runaway” occurs on the surface of the white dwarf and then it blows up many magnitudes brighter. Anyway that’s as much astrophysics as I am able to breakdown.
Due to the clouds I was not able to capture spectra at the peak of the event but was actually able to get a couple nights at around day 10 and 12 and combined them into this short gif. My guess is the nova is around 7th mag by now? Basically there is a very broad emission zone from the nova ejecta from which the hydrogen alpha peak rises up and dominates early but then retreats. You can see the Ha spike (at the 6563 angstrom mark) by itself in the gif animation. At this point it isn’t nearly as prominent (Spikes going upward are emission and going downward are absorption). I also attached a spectrum taken by Olivier Garde from France on day 1 so you can see the evolution, sort of, from the peak and also a raw spectrum of the nova I captured so you can see the bright broad emission zone what that looks like.

Animated gif of 2 consecutive observations 2 days apart beginning about 10 days after the reported nova onset. Captured from the Talavera Space Hut as pictured above

Spectrum captured by Olivier Garde from France which shows the appearance on Day 1 of the outburst. You can see how at this stage the Hydrogen alpha emission spike is very dominant (at center)

Single raw spectrum of RS Oph from 8/21. The bright section of the spectrum is the region of emission

Location of RS Oph, just to the west of the Milky Way facing south after sunset

Illustration of the binary courtesy of skyandtelescope.com and M Kornmesser. The white dwarf is on the left accreting mass from the giant star companion on the right

Thanks for reading!

DrDave

New Image- M81 Group

The M81 galaxy group in the constellations Ursa Major and Camelopardalis contains Messier 81 (M81), the large spiral galaxy visible just to the left of center, Messier 82 (M82), the irregular galaxy to the right of M81, with the red glowing gas, also known as the “Cigar” galaxy for its shape, and many other galaxies, some of which are also visible here. NGC 3077 is the small but bright elliptical galaxy to the upper left. M82 is a prototypical “starburst” galaxy where an exceptionally high rate of star formation is taking place, and is thought to be triggered in this case by the gravitational interaction with M81.

This whole galaxy menagerie, about 12 million light years distant, is seen amidst a faint vast and complex network of interstellar gas and dust known as the Integrated Flux Nebula (IFN), a portion of which can be seen in the background of this image. Other significant visible features in this busy intergalactic field include the well known Arp’s loop (described by astronomer Halton Arp) which is an arcing faint loop of gas arising from the right side of M81 in this image, the top portion of which forms almost a heart shape. This structure is thought to be the result of gravitational interaction between M81 and M82. Finally there is the dwarf galaxy satellite of M81 called Holmberg IX, an irregular galaxy which is the small bluish patch of gas and small stars visible right above M81.

The full resolution version can be seen by clicking on the thumbnail under “My Astroimages” lower right on this page.

Capture info:
Location: Orion’s Belt Remote Observatory, Mayhill NM
Telescope: Takahashi 180ED
Camera: SBIG STXL 16200
Mount: Paramount MX+
Capture details: LRGB 4,3,3,4 hours respectively (5 min subframes)
Processing: Pixinsight 1.8.8-7

Key visual components of this image are illustrated here. A few tiny galaxies from the PGC catalog designations can also be spotted in the background, a couple of which are also gravitationally connected to the M81 Group. The astronomer Halton Arp was an American astronomer who is best known for his 1966 publication “Atlas of Peculiar Galaxies” where he catalogued many interacting and merging galaxies which did not fit into Edwin Hubble’s neat galaxy “tuning fork” diagram which described a much more orderly and predictable galaxy evolution. Arp discovered many inconsistencies with the Hubble model. The M81 tidal loop described by Arp is just one example of intergalactic tidal interaction occurring throughout the observable universe.

Edwin Hubble’s classification of galaxy morphology and evolution showing the progression of elliptical galaxies “E” toward more organized types of spiral galaxies “S”. They diverge into two arms as you see here depending on whether the spiral galaxies have a central “bar” or not. The result is sometimes referred to as Hubble’s “tuning fork” diagram which was challenged by Halton Arp. (Courtesy Wikipedia)

M81 and M82 are more closely connected with the constellation Ursa Major, The “Great Bear”, as you can see by the cartoon overlay in the image above. Note the Big Dipper (white circle), which is very near and dear to us northerners, is only a small part of Ursa Major. The bright Big Dipper stars make up just a portion of the Bear’s hindquarters. The galaxies M81 and M82 are bright enough to be detected in binoculars in the same field! A small telescope such as a 4″ refractor will definitely reveal their shapes. You can see here their location just to the left of the bright pointer stars in the front edge of Big Dipper’s pot (Merak and Dubhe). If you extend the line between those two stars and draw it straight like an arrow further to the left you will eventually arrive at Polaris, the North Star. This is why the two stars are referred to as the “pointer stars” because they “point” to Polaris. For Northern Hemisphere observers this is a good time of year going into the Spring to observe the galaxy pair. Now the Moon is basically full so it’s not a good time but go out in another week a couple of hours after sunset and look toward the north- northeast to find first the bright big dipper stars in the orientation above, then sweep over slightly to the left and up to find the galaxies.

Happy galaxy hunting!

Thanks for reading!

DrDave

News from Talavera

Equipment housed in the Talavera Space Hut: ‘S’ is the Lhires spectrograh, ‘C’ is the main imaging camera, ‘G’ is the guide camera. Telescope is to the left.

Yes my friends we will now have a regular posting for the Talavera Space Hut as we affectionately call it. This is where citizen science is happening! The small 8 foot square roll-off which is located right in my backyard houses a 14” Celestron Schmidt Cassegrain reflector and an Lhires (Littrow high resolution) spectrograph. The spectrograph is the instrument that analyzes light coming from deep space objects and is able to record in high resolution the spectral emission or absorption lines produced. You can do a search on this site for “spectroscopy” and I am sure a hundred posts will come up on this topic. The main imaging camera here is an Atik 460EX and the guider is an ASI 174.

In this case we are typically looking at stars and specifically “variable” stars. These are stars that vary in brightness due to physical properties happening in the star or in its local environment. They are probably the most extensively studied objects in space. Amateurs are actively involved in this process and an entire organization, the AAVSO (American Association of Variable Star Observers) was established in 1911 to enable anyone, anywhere, to participate in scientific discovery through variable star astronomy. You and I, with fairly modest backyard equipment, can play a crucial role in real discovery even today! No joke. Let’s face it. There are a lot of stars out there. Professional astronomers do not have access to telescopes every clear night like we do. They have to apply months in advance for telescope time on the world’s largest telescopes just to get a few nights of observing time. The good news for them is that many of these stars do not require large optics to observe and our equipment these days is more than adequate to acquire data that is accurate enough for scientific publication. That is amazing!

The star “Navi” or gamma Cassiopeia is the third brightest star in the constellation. The star was used as an easily identifiable navigational reference point during space missions and American astronaut Virgil Ivan “Gus” Grissom nicknamed the star Navi after his own middle name spelled backwards! (Courtesy Wikipedia)

The star gamma Cas or the third brightest star in the constellation Cassiopeia seems like a harmless pretty blue white twinkling star that is easy to spot. However it is hardly that. It is a specific type of variable star called a Be or B “emission” star belonging to the spectral class B being the second most luminous spectral type and these are typically stars 3-100 times as massive as the Sun and can be 5-10 times as large.

A Be star binary system showing a schematic of the circumstellar disc with a neutron star companion

The subtype of Be is the most extensively studied because these stars are frequently not just one star but part of a binary system where the companion star is much smaller and can in many cases be close enough to the parent B star that gravity causes a transfer of mass between the 2. The smaller star is typically a white dwarf but could possibly be even a neutron star or a black hole! This mass transfer often creates a rotating shell of gas. We are able to learn more about the physical properties of these systems by observing their spectra. The gas shell emits light and as it changes shape and position the spectrum produced can also change over time. Gamma Cassiopeia was the first Be star discovered, in the 1860’s, and it is still extensively studied. In August of this year a sudden outburst of emission and variability in the helium line of the Gamma Cas spectrum was detected and the call was made for additional observations to be done over the next several months. The star is well positioned now for observations in the Northern Hemisphere and it is a very easy target since it so bright.

Three observations of the star were made from the end of September through the end of October. The 100 Angstrom region containing the helium 6678 line was targeted and you can see the results here:

He region of gamma Cass (orange vertical line) showing twin helium emission peaks. This was on September 17

On October 22 one of the helium emission peaks is shrinking (black arrow)

This one taken the following week is a little too noisy to make much sense out of (probably due to the Moon nearing full!) but there appears to be further changes in the second peak

There is predominantly a double peak emission in the region of the Helium line 6678 Angstroms (orange line). While the physics is obviously beyond the scope of this post, it is clear there is dynamic change happening even over the course of a month! The second or red emission peak is decreasing in intensity (arrow) I was very excited to be able to capture for the first time a spectral profile beyond the H-alpha region. The significance of these changes is currently under investigation so I cannot tell you exactly what this means yet!

Amateur observations of the gamma Cass helium emission phenomenon during October. My data is the yellow line.

Several observations have been submitted by amateurs of this phenomenon (shown above). The spectrum I captured is in yellow. The black arrow at the bottom is pointing to a possible developing flare within the red emission peak.

This is a schematic of a Be star with circumstellar gas disc and you can see that depending on your line of sight you will see either emission spectrum (upward spike) or absorption (downward spike). The gaseous disc is what is causing emission peaks because it absorbs energy and re-emits it at a specific wavelength. When you are in position A you are seeing mostly the disc so emission predominates. When you are looking at the system edge on, or position C, the central star is mostly what you see so absorption predominates because the stars’ atmosphere absorbs the energy from the stars’ interior before it reaches us. Position B is in between so you will still observe mostly emission because the disc is still the major contributor.

Alright folks, stay tuned for more on the gamma Cass helium emission phenomenon. What is actually happening there? Have we discovered a new stellar physical process happening within these circumstellar discs?

Thanks for reading!

DrDave

Bar Codes of the Universe

During the Great Southwest American Smoke-out we had last month when the smoke from fires in Washington down through California blew over to Southern New Mexico, not much could be done in terms of deep space imaging. It was supposedly clear of clouds but only a few of the brightest stars could be seen through the haze.

Smoke filled skies precluded any imaging possibilities but did produce spectacular sunsets!

So with the main observatory out of operation I decided to continue testing equipment for possible usage during our public outreach events at the astronomy club’s observatory at the Leasburg State Park in Radium Springs NM. Last time we discussed a new camera for planetary imaging. This time it was the same color video camera but in addition to that was a small screw on filter which is not actually a filter but something called a “diffraction grating”.

This is an example of a diffraction grating you can screw on the end of your camera to record spectra from objects in space

The purpose of this is to observe spectra from objects in space. Spectroscopy is the analysis of light we see coming from stars, galaxies and other extended objects. This is actually how astronomers have figured out most of what we know about the physical properties of what’s out there. Remember in elementary school you held a prism up to a light source and saw a band of different colors projected on the wall. It was probably red-orange-yellow-green-blue-indigo-violet. That is a spectrum! The prism separates the white light into its individual components based on their wavelengths. Longer wavelength light is red and shorter is blue. Astronomers do exactly the same thing only they point a sophisticated instrument called a “spectrograph” at an object in space instead of a prism. The other key difference is that in a spectrum of for example a star, in addition to the colors you see a bunch of vertical dark lines. They can also be bright lines in certain cases. I call this the “bar codes of the universe”. Much like the bar codes on commercial goods tell the store what exactly the products are, the “bar codes” or absorption lines in spectra can tell astronomers many things including temperature of the object, what it is made of, whether it is moving toward or away from us, the mass of the object if it is part of a group of objects, and many other physical properties. Each dark line in the spectrum is the result of some element in the atmosphere of the star or other object absorbing the light at that specific wavelength. Every element in nature has characteristic absorption lines so you can see right away what elements are present in the object you are looking at!
A number of posts on this site are devoted to astronomical spectroscopy as I do spend a fair amount of time with it and my backyard observatory is devoted exclusively to it. However with this intended astro club set up the purpose really is to introduce people to how it works and hopefully more will get a feel for how “real” astronomy is done! In this set up we use a simple diffraction grating which is a piece of glass that has lines etched into it, in this case about 100 per millimeter. What you do is screw it onto the nosepiece of your camera and when the light from, in this case, a star passes through the grating the light is spread out into a spectrum!

Here is the basic spectroscopy set up we will have at the club’s observatory. We’re using the 110mm refractor and the same color video camera we used for planetary imaging (red object at the bottom)

Here is a spectrum of the star Vega (on the left) and its “bar code”, the absorption lines, shown by the white arrows. Vega is a blue-white star much hotter than our Sun. This is a raw unprocessed spectrum but the lines are created by hydrogen in the atmosphere of the star absorbing the energy at those specific wavelengths of light to create the “absorption lines”

Anyway that is an example of a stellar spectrum with its characteristic “bar code” or group of absorption lines, one of many we can observe in the universe! For more on spectroscopy you can search this site which should turn up quite a few posts on the subject.

Thanks for reading!

DrDave

Itsy bitsy spider

Alright, not so “itsy bitsy”. Located in the constellation Auriga, IC417, also known as the “spider nebula”is a hydrogen gas region which absorbs energy from massive hot stars in the vicinity, then re-emits that energy as light in the hydrogen alpha wavelength i.e. red light. The region measures about 100 light years across at a distance of 10,000 light years. This image from the 16” telescope was taken through hydrogen alpha filter only so it highlights the emission components of the gas cloud in a single monochrome channel. It is not as often imaged and has a very low surface brightness. Consequently the other commonly used “narrow band” filters, SII (sulphur) and OIII (oxygen) yielded hardly any signal at all when I tried them for this target, so I decided to pass on those additions.

The designation “IC” for the catalog number stands for “Index Catalog of Nebulae and Clusters of Stars”. Published in 2 parts in the late 19th and early 20th century, this was a supplement to the well known “NGC” or New General Catalog of extended objects (galaxies, nebulae etc). The IC catalog contains over 5000 objects in it!

What was interesting to me for this project was the name “spider nebula”. I was curious to discover where was the “spider” in the image. Some wider field views of this also contain a “fly” component supposedly. Amateur astronomers, perhaps like the ancients who named constellations or many of us who look at clouds and claim to see a cow or an elephant, have given many of these nebulae similar likenesses. These supercede any catalog designation: the jellyfish nebula (IC 443), the running chicken (IC 2944), the witch head nebula (IC 2118), the horsehead nebula (IC 434). You get the idea. So I looked at this image again after final processing and in this orientation I believe I see the spider! You can certainly let me know if I’m way off but I don’t see any other features that would resemble anything close to a spider or any other creature 🙂

IC 417, the “spider” nebula. Compare to the top image to see the unedited features!

Thanks for reading!

DrDave

 

News from Talavera

Let’s not forget about the “science arm” of our astronomical ventures! It’s not always about pretty pictures of galactic and extragalactic objects (although a lot of it is about that!) . The Talavera Space Hut as it is affectionally termed houses a C14 telescope and Lhires (which stands for Littrow, high resolution) spectrograph. I am doing a fair amount of spectroscopy and many earlier posts have been devoted to this so you can just do a search here and that will turn up a lot of info! You can see the equipment and set up here.

At any rate the coolest thing about amateur astronomical spectroscopy in my opinion is that it is very relevant to current astrophysics and astrophysical research. Most of our understanding of stars, how they work and the large scale structure of the universe comes from analysis of spectra of stars and other objects. High resolution spectroscopy which is what I am doing, focuses on stars and stellar systems. Professional astronomers need people like you and me to help them with observations! They do not have the daily access to equipment that we do and while it may be hard to believe that modest amateur equipment such as what we use can make such a contribution, all you have to do is get on a few internet spectroscopy groups and almost every week there will be a “news flash”, “bulletin” or some other announcement regarding a current observation being done by a professional astronomer somewhere requesting additional observations from you and me!

Recently a request for observations of Rigel, a blue supergiant star in Orion, came out of the Cote d’Azur Observatory in France. This is an internationally recognized center for research in astronomy.Their ongoing project involves luminous BA-type supergiants which can be observed in distant galaxies and are potential accurate distance indicators. However, the impact of the variability of the stellar winds on the distance determination remains poorly understood. By monitoring high resolution spectra of these stars over time they hope to understand this variability better. Now while I do have an undergraduate astronomy degree, I am not going to break down the quantum physical intricacies of subtle changes in the morphology of spectral absorption lines, at least not today. But what I can do is obtain a high resolution spectrum of Rigel to contribute to their ongoing research. That is what I did. Before showing that just a couple of definitions to clear up. Firstly is “high resolution”. What is that? Resolution refers to the smallest detail discernible in a spectrum obtained with the particular spectrograph. For my set up we are looking at a small region of the spectrum, on the order of about 100 angstroms (The entire visible spectrum spans about 3500 angstroms) Resolution is typically designated by the “R” value where R is the ratio of the wavelength considered divided by the smallest change visible. In the amateur community low resolution is typically in the R= 100 to 300 range. High resolution might be anything greater than 10,000. Professionals may be up to even 100,000 with some of the setups they are using. Lhires spectrograph which is what I am using is typically in the 16-19000 range. Most of the observations of stars and stellar systems require high resolution data mainly because the phenomena are reflected in very small wavelength changes, on the order of a few tenths of an angstrom. Low resolution data are ideal for analysis of distant objects such as galaxies, supernovae and the like.

Now on to Rigel. This is a familiar bright blue star in the constellation Orion. About zero magnitude, it is a spectral class B8Ia. Spectral classification is a fascinating subject in and of itself but in a “nutshell” from your high school astronomy class the letter classification sequence of O, B, A, F,G, K, M, with numeric sub-classifications from 0-9, that was established at the beginning of the 20th century still holds, of course with some additions and modifications. So generally speaking it goes from the brightest, most luminous “O” stars down to the dimmer “M” stars. Numbers increasing from 0-9 indicate decreasing surface temperatures. Our Sun is in the middle at G2. Type B stars are characterized spectroscopically by their strong absorption lines of neutral Helium. This is around 6678 angstroms. These tend to decrease in intensity from B0-B9. There is also a hydrogen alpha absorption line which is where the researchers were observing but this line is not as strong as in the stellar A class. Hydrogen alpha strength typically increases from B0-B9. However the H-alpha line for Rigel is very weird! It has variable emission components in it (see below). Based on observations of Rigel’s variable H-alpha line, the star is estimated to lose mass at a rate of (1.5 ± 0.4) × 10−7 solar masses per year, or 10 million times faster than the Sun!

My observation was done this month and consisted of about 1-2 hours total imaging time, including calibration frames and flats. 12 45 second spectra were obtained and processed.

Single raw 45 second spectrum of Rigel. Arrow points to the hydrogen alpha absorption line. It has a “washed out” appearance due to the fact that there is some component of emission occurring at this wavelength, not just absorption. At this resolution there are a lot of lines visible but many of the others are likely from water in our atmosphere.

The spectrum of Rigel is shown here. So as it turns out, Rigel is not “just” a type B star. It is a variable star and is actually part of a multistar system. The red arrow points to the H alpha line. The blue arrow points to what is an emission component at this wavelength. You can see how the relative intensity is increasing there!. This is characteristic of emission lines. Emission activity in the H-alpha line of the so-called BA supergiants such as Rigel (B8Ia)  is indicative of the presence of some type of mass ejection from the star.

The peculiar appearance of the H-alpha line with emission feature on the red side of the line and absorption on the blue side is called a “P Cygni” profile. This is named after the prototypical variable star in the constellation Cygnus. P Cygni gives its name to a type of spectroscopic feature , again, where the presence of both absorption and emission in the profile of the same spectral line indicates the existence of a gaseous envelope expanding away from the star. The emission line arises from a dense stellar wind near to the star, while the blue-shifted absorption lobe is created where the radiation passes through circumstellar material rapidly expanding in the direction of the observer. These profiles are useful in the study of stellar winds in many types of stars.

The second item on the agenda at Talavera was not directed at any particular research but was a task I had to do in order to be able to submit spectra to the database at the AAVSO, American Association of Variable Star Observers. The AAVSO is probably the largest repository for professional-amateur collaboration in the world. We regularly get bulletins and notices of ongoing research requiring amateur assistance. If you are considering spectroscopy, AAVSO membership is a must! At any rate to become a “legitimate observer” you have to submit a spectrum from a list of their “standard stars” and have it validated. I chose epsilon Canis Major, also known as Adhara, for the exercise. This is a more “normal” B type star, actually a B2. The interesting thing is that you can see the H alpha line is stronger here for this star compared to Rigel which is a later B type star (B8) and so the H alpha absorption strength should be less. This is because the line in Rigel has emission characteristics as described earlier.

This is a 1 minute raw exposure for the star Epsilon Canis Major. Even in this raw image you can see the hydrogen alpha line is way darker than the previous line seen for Rigel, when it should be the opposite!

This is a pretty “normal” looking hydrogen alpha absorption line for the star Adhara or Epsilon CMaj. Note it is pretty symmetric in both the “blue” arm on the left and the “red” arm on the right. Compare this to the emission feature on the red side of Rigel’s H-alpha line.

Ok folks. That’s the February news from the Talavera Space Hut. I guess it’s enough astrophysics for 1 day!

Thanks for reading!

Dr Dave