Tuesday, February 02, 2021

The Wolf-Rayet bubble NGC 6888

 As a bit of amateur astronomy I signed up with iTelescope in July 2020, and took some images of NGC 6888, a favorite object of mine. It took several months to accumulate the set of images I wanted, and another several months to write the tools to process the raw data in the way I wanted, but finally here they are:

NGC 6888 SII, H-alpha, OIII
NGC 6888 taken in SII (red), H-alpha (green) and OIII (blue) filters.

NGC 6888 Red Green Blue filters
NGC 6888 using Astrodon Red, Green and Blue filters.

NGC 6888 is the wind blown bubble surrounding Wolf Rayet 136, at a distance of around 2,000 parsecs (6,500 light years) from us. WR 136 is an evolved massive star with a strong stellar wind (velocity approximately 1,500 km/s) that is blowing into the slower and denser ejecta from the star's previous Red Supergiant phase, creating a bubble approximately 10 pc x 7 pc in size (34 x 23 light years). WR 136 is the bright star at the center of the image. Although only about mV ~ 7.5, its intrinsic luminosity is approximately 500,000 times that of the Sun, and its effective temperature is 70 to 80 thousand degrees Kelvin,

The nebula emission seen in these images is almost exclusively swept-up and compressed RSG wind material, that has been photo-ionized by the WR star. The WR wind material inflating the bubble is shock-heated to million degree temperatures, and hence is not visible in these images. There are of course X-ray images of this hot gas, although the Chandra and XMM-Newton images only partly cover the bubble (given those observatories small field of view), and absorption from intervening interstellar gas and dust severely attenuates the soft X-ray emission. (I did a theoretical paper that modeled NGC 6888 as part of my PhD thesis work at the University of Birmingham back in the mid 1990's.)

The bubble itself is relatively young, at least compared to million-year-old main-sequence massive star wind bubbles and superbubbles: with an expansion velocity of approximately 80 km/s its dynamical age is only about 50,000 years. The relatively short lifespan of the RSG/WR wind interaction probably explains why WR bubbles are only clearly seen around a small fraction of galactic WR stars.

The hydro-dynamical instabilities caused by the interaction of the fast tenuous WR wind and the slow dense RSG wind are also the source of the beautiful substructure in the nebula, first demonstrated (I think) in some excellent papers by GarcĂ­a-Segura and Mac Low in the mid 1990's [e.g. this paper]. They are even more apparent in the Hubble Space Telescope images of a small region of the rim of the bubble.

Telescope and image information:
  • iTelescope T05 at New Mexico site (MPC H06)
  • Takahashi Epsilon 250 with Paramount PME mount and SBIG ST-10XME CCD.
  • Images taken on 2020-07-16, 2020-08-10, 2020-08-12, 2020-09-17 and 2020-10-16.
  • Resampled images have 1.8 arcsecond square pixels aligned North at the top, East at the left, and cover a 0.96 degree x 0.54 degree FOV.
  • Net exposure time:
    • SII/H-alpha/OIII image: 113 minutes
    • Red/Green/Blue image: 18 minutes
  • Individual raw frames:
    • SII/H-alpha/OIII image: SII 2x240s, 8x300s; H-alpha 9x180s; OIII 2x240s, 6x300s.
    • Red/Green/Blue image: Red 6x60s; Green 6x60s; Blue 6x60s.
  • Measured effective Point Spread Function FWHM = 5.8 arcseconds, which on paper sounds pretty poor. 
  • Processed from raw FITS images using my AstroPhotography python code (which uses astropy, photutils, ccdproc and Astrometry.net), some additional bash scripts, and Astromatic swarp (for resampling and co-addition) and stiff (for color image generation). No additional photoshop/gimp manipulation!


Saturday, July 11, 2020

Comet 2020 F3 (NEOWISE)

The long period comet C/2020 F3 (aka NEOWISE, named after the satellite that first detected it) is one of the most impressive comets in the last few decades.

Although my attempts to see it personally have so far been frustrated by cloud and lack of clear sight lines to the north-east, I have been following the images of it that have appeared in Astronomy Picture Of The Day (APOD). Most if not all of the images posted so far, or presented in pop sci articles online fail to tell you any quantitative details about what you're looking at, so I decided to work out some rough distances and sizes based on the JPL Small Body Database info on NEOWISE, and the image taken from the ISS on July 5th 2020, and the kstars desktop planetarium software.

This image is a cropped, resized and annotated version of the ISS image posted at APOD on July 10, 2020. I've labelled some of the prominent stars, although to a ground-based amateur observer the star Capella, which is hidden behind parts of the ISS off the top left of the image, would be the most most prominent star visible.

From the JPL SBDB we can find that NEOWISE was about 0.3 AU from the Sun on 7/5/2020, and about 1.1 AU (approx 165 million km) from Earth. At a distance of 1.1 AU an angular distance of 1 degree is about 3 million km.

Comet C/2020 F3 (NEOWISE) as seen from the ISS on 7/5/2020
My annotated version of APOD from 07/10/2020, Image Credit: NASA, ISS Expedition 63

The angular distance between Theta Aurigae and Elnath is approximately 10.54 degrees, and the angular distance between Elnath and Kabdhiliinan (Iota Aurigae) is approximately 7.76 degrees based on using a measuring tool in kstars. If we assume the ISS image is not significantly distorted then we can measure the number of pixels between the stars on the image and compare that to the length of the tail in the image to work out its angular size. In fact the image does seem to be be slightly distorted, but taking an average I get that the visible tail in this image is about 1.7 degrees long, so the visible tail is about 5 million km long. Its not every day you can go out and see something 5 million km in size!

Sunday, February 09, 2020

NASA Astrophoto Challenge for Winter 2019 is M82

Noted with approval: the NASA Astrophoto challenge for Winter 2019 is my old friend Messier 82. See if you can create a press-release worthy image of M82 using either existing NASA images for different orbiting observatories and/or ground-based optical images taken at your request by Harvard's MicroObservatory service.

Thursday, August 22, 2019

Mauna Kea

Looking toward the summit of Mauna Kea, from the small hill near the base of the access road. Taken in 2005.

A view from the summit, looking toward the Keck telescopes. Taken in 2005.