Written by Karsen Kitchen, Ishaan Balakrishnan, Jacob Blizzard, James Bowling, and Rianne Eccleston.
The Tarantula Nebula, one of the several star forming regions in space. It’s a large region, but due to its distance it gets easier to take pictures of. There’s several stars in this region, not to mention the clouds of dust, that hold characteristics of the types of stars that are formed here. This process has to start somewhere, and that’s with taking pictures of the nebula.
Given a choice of these nebulae, our group decided to do the tarantula one since there’s plenty of pictures of the Horsehead Nebula online, but taking these pictures are almost the same process as before with more exciting processes.
First comes the test pictures. These pictures have to be taken to set the durations for all of our filters later on, but for now it’s also how to see the nebula fits in the camera. Since these are only test observations and pictures, they can be taken with almost the minimum settings to work: allowing the sun to be closer to the target than normal and letting the target be seen close to the horizon. Test observations are taken with all filters with different test durations: red (r) with 40 seconds, green (v) with 60 seconds, blue (b) with 80, and luminance (lum) with 20. Luminance isn’t a color, and is really used to see the brightnesses of the stars and area around it. That isn’t all the filters, as we start using narrowband filters, which only captures a ‘narrow band’ of color from the nebula. Narrowband is different from lrgb, with the filters able to capture closer to the sun than the others, but still it needs a test. The narrowband filters we used are Halpha, and OIII (hydrogen and oxygen) originally, but due to initial picture defects, were retaken and included SII (sulfur), all of which with 300 second exposures. Narrowband filters are also different in the fact they’re extremely expensive, especially for 300 seconds, and later on 600 seconds.
Now that the initial test pictures are back, it’s time to calculate how long each duration for each filter needs to be. This depends on the amount of color in the area, the longer the duration the less of that color there is, the harder to capture. Narrowband images will almost always stay near what the telescopes’ max duration is, 300 or 600. Loading up the lrgb images into Afterglow is the best way to find the duration, plus a formula. The best way is to turn the image viewer to the brightest setting to find the brightest star for its “counts”. Using these, it’s possible to use the formula of ((old duration/counts) * 30,000) to find the new durations for each filter. Once done it’s back into Skynet with more restricted viewing windows to capture the nebula as how it should be; requiring the sun to be further away and the nebula needing to be higher in the sky. This observation is repeated, as more images in processing meaning more depth
It’s also possible to mosaic the region of the nebula, but that requires more calculation to figure out how offset the images need to be from each other. This method also loses depth compared to the single images above. Getting the depth of the single images with a mosaic is too costly, so is using narrowband for mosaics.
Once we have gathered all of our images from skynet, we transfer them over to Afterglow and follow the Image Stacking Guide to make our stacks. First, we need to make sure our images are in the right order. We put the narrowband images on top (in this case, I had Halpha, OIII, and SII), our Lum images in the middle, and our color images on the bottom. With the B,V, and R images, you want to color them their respective color, and set their blend modes to screen. For the luminance layer, we want to change the blend mode to “Luminosity”, which makes it act as the luminance layer. For Halpha, we color it Balmer, for OIII, we color it OIII, and for SII, we color it red. We also want to change the narrowband blend mode to “lighten”.
Next, we want to calibrate all of our images to fit each other, so we start by turning off the narrowband images and focusing on the B,V,R and Lum layers. In the display setting, we view the controls for the Luminance layer. In the top right, we want to link all layers (pixel value), so all the layers are tied to the luminosity layer. Next, we want to photometrically calibrate our images. We set our Green, Red, and Blue layers, and then afterglow will measure the zero points. In order to get rid of reddening, we need to calculate our extinction level. For Tarantula, it was .066, and then set our reference layer to red and calibrate our colors! Back in display settings, we want to set our stretch mode to midtone and choose “default preset”. I was pretty happy with my luminosity layer, so I did not change any of the controls, but changing the midtone level on the luminosity layer will either make the image brighter or darker. Before we start on our narrowband images, we want to neutralize sources. We want to neutralize the luminosity layer to the red layer, which adjusts the histogram so the levels more closely match eachother.
Turning on the Halpha layer, we select the controls for Halpha layer in display settings. First, we un-sync the brightness and contrast settings so we can manually adjust the colors. For the background levels, we want to make sure the background layer on the histogram is right behind the highest peak. We can now adjust the midtone levels to the color of your liking. For my Halpha layer, my midtone level was at 90, OIII at 94, and SIII at 99.9. For your other narrowband images, the process is the same. My SII level was horrible, so I ended up putting that layer underneath the luminosity layer so that the color is being added to the RVB layers, but the intensity is only coming from the luminance layer. With 3 narrowband layers, you can adjust the colors to make it anything you want. The original color of Tarantula was the orange-greenish color, so I ended up changing the colors to make it more exciting. I changed my Halpha to Red, OII to blue, and SII to blue-green, to add in a little pop of green. Once I was happy with my image, I started to gather data from the NASA Archives. We were instructed to use WISE 12 or 22 archival data, but none of the Wise data looked good in my image. I ended up gathering Spitzer archival data and adding it to my image, and coloring it Harmony to differentiate the colors. This was the final product I was left with!
Much of what is visible in this image is actually gas! This gas formation is driven by the most massive stars at the region’s center, namely the O and B stars. These stars are very powerful, and the photons that they emit carry enough energy to ionize the hydrogen gas around them (the most abundant gas not only in the region but in the universe overall!). This region of ionized gas is termed the HII region (HI refers to neutral Hydrogen, while HII refers to ionized Hydrogen). The HII region will not expand indefinitely, however, as the ionized protons and loose electrons will eventually refind each other, and combine back into neutral states. Therefore, the overall size of this region is set by when the rate of ionization is equal to the rate of recombination. This region can be estimated mathematically, and only requires some simplification! If we take N* to be the rate that the central stars are ionizing photons, the rate of recombination can be set to equal to the rate of protons available to recombine X the rate that ionized protons collide with electrons and recombine. To get a substantial estimate, it can be allowed to assume that the HII ionized region is spherical. This is most likely not the case, however it can yield an estimate that is relatively close to the true value. This allowance is called the Strömgren-sphere approximation. Using this approximation, and plugging it into and simplifying, we arrive at the equation:
Now, the only thing to do is to obtain values for N*, the rate that central stars are ionizing photons and RS, the Strömgren radius. The Strömgren radius is first determined by uploading our obtained image into Afterglow. Using the plotter tool in Afterglow, we plotted a distance from the center of the region to the edge of the nebula. This value is given in arcminutes, and is converted to degrees by dividing the answer by 60. The next step was to look up the distance from Earth to our star-forming region in parsecs. RS can finally be determined by multiplying: (the angle in degrees) * (Π/180°) * (distance in parsecs).
The value for N* is to be determined next, and is slightly more complicated to achieve. To get this value, it is necessary to sum the ionizing flux of all the central, driving stars of the region. For our star forming region, the brightest stars in our region had the spectral classes: O and B. These spectral classes are very large, hot, and luminous stars. Summing our region’s spectral class and subclasses O3, O4, and B0, we are able finally get our N* value of (1.161*1050). Plugging these values into the original equation, we are able to deduce an estimate for the density of Hydrogen in the HII region, as is presented in the image below.
The final piece of the puzzle was to determine if the star-forming region was emitting bremsstrahlung radiation. This occurs because of the hot stars that ionize the gas around them. This creates an abundance of free electrons and protons bouncing around in a very dense environment. Those electrons decelerate when passing close to the free protons, which causes them to emit light in the radio wavelength. Using the 20-meter radio telescope located in Greenbank Observatory, we would be able to determine if the tarantula nebula was emitting bremsstrahlung radiation.
Unfortunately, our initial optical observations of the region had been taken by the PROMPT telescopes located in Chile, while Green Bank observatory is in west Virginia. The target was too far south for us to observe it in the radio. To supplement our data, we chose to observe the horsehead nebula, since many star forming regions are relatively similar. We submitted observations for this region, detecting frequencies between 1335 and 1435 MHz, and mapping over the right ascension. Once the images came back, we could see some interference at 1400 and 1420 MHz so we used Skynet’s radio cartographer to clean up the file. We then uploaded it to Afterglow, where we could see for certain that we had detected radio emission within the horsehead nebula. At the same time, we had also submitted observations of Taurus A, to act as a standard candle. Since its flux density in janskies is known, we were able to calibrate the noise flux density of the horsehead nebula that was found from Afterglow’s aperture photometry. While there were a few different regions of emission, the brightest region was found to be emitting around 6 janskies of bremsstrahlung radiation. We can use this to infer that the tarantula nebula is also emitting a few janskies worth of radio waves, especially since this region is so hot and dense, creating the perfect environment for this to occur.
The Tarantula Nebula is one of the largest and most active star forming regions in the group of galaxies. Also known as 30 Doradus, it is located in a large magellanic cloud and is a satellite galaxy of the Milky Way.
A dense cloud of gas and dust gravitationally collapses to begin the Tarantula Nebula’s life cycle. The cloud warms up and starts to spin as it collapses, creating a protostar in its center. This protostar will eventually mature into a star by increasing in size and temperature. The huge stars in the nebula are about 200 times as massive as the sun, and they have a limited lifespan because they exhaust their fuel very quickly. This results in them emitting powerful radiation and star winds that heat and ionize the nearby gas, giving the nebula its distinctive pink and blue colors. Eventually, the nebula will experience a supernova explosion, scattering all of its remnants into space and starting the next cycle of star formation by compressing neighboring gas.
Nebulae are massive clouds of dust and gas and their circle of life is one of the most interesting ongoing processes in space. From the birth and evolution to the death of stars, internal processes within the nebula can cause it to collapse, ignite into a star, and eventually produce heavy elements that are dispersed into the surrounding space through supernova explosions. The elements from this process then enables the formation of new nebulae and stars and a whole new “Circle of Life”.