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NGC 891 (Caldwell 23)

Bob Lanser



The NGC 891 galaxy is 27.3 (±1.8) Mega Light-years away from our solar system. Despite this immense distance, this galaxy is perceived to be one of the galaxies most similar to the Milky Way galaxy in size and luminosity. This Galaxy is interesting because from our perspective it is viewed directly edge-on. This galaxy is an unbarred spiral galaxy, which means it has a bulge in the center of the galaxy surrounded by a rotating disk of stars that are organized in "arms". The arms could be viewed in galaxy NGC 891 if it were viewed at a different angle, but because it is edge-on we only observe the flat disk from the side of the galaxy. NGC 891 can be seen from earth with fairly weak telescopes, which is why this galaxy was able to be discovered all the way back in 1784 by William Herschel.

The size of the galaxy was found using the small angle formula, which states θ= l/d. The distance was found on the NED site, this distance was 27.3 Mly. The image was then processed to see the amount of pixels were in the length and height of the galaxy. Pixels were converted into arcseconds which were then converted into radians. The small angle formula could then be applied. A length of 55 kly and a width of 5 kly was estimated for this galaxy. There is a sizeable uncertainty in the distance of the object (1.8 Mega light-years) which would lead to a large uncertainty in the size of the galaxy. This distance was achieved from the Tully-Fisher relationship on the distance ladder.

Infrared Image

Hubble Space Telescope image

NGC 891 has been viewed in Infrared wavelengths due to this reddening and extinction. The famous Hubble Space Telescope viewed this Galaxy in infrared in 1999, and has since been viewed by other infrared telescopes. After the infrared imaging was complete there was still a red hue to the arms of the galaxies, which was determined to be from red giant stars populating this galaxy. This infrared imaging can also give us more accurate sizing for the galaxy, the optical image cannot give us an accurate description of the size of the galaxy because the dust makes it look larger than the galaxy really is. With an infrared image a more accurate size can be obtained for the galaxy. The Hubble image is shown above. The red color is still apparent despite the lack of galactic dust. The Hubble image was obtained using a wavelength of 1.6 μm and 1.87μm.

Published Colored Image

Hubble Telescope Infrared/Clear Image

This image uses both infrared, and clear filters. This allows us to see through some dust, while getting an amazing optical picture of the galaxy. This galaxy has a lot of dust in the arms of the galaxy as seen in the colored picture above. This dust in the arms causes some extinction and reddening of light of the stars in the arms. Extinction is the complete blocking of visible light to the viewer, and reddening is when some dust gets in the way of the light of the stars making the galaxy appear more red to the viewer. NGC 891 appears to have a red hue in the colored image, and this can be a result of the reddening due to galactic dust. One way to get around this reddening or extinction is to view the light in different wave lengths.

Table of image details

Right Ascension (J2000) 02:22:33.4
Declination (J2000) +42:20:57
Filters used B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter B(55 min), V(25 min), R(20 min), and C (22 min)
Image dimension 556x48.33 pixels;421.6x36.6 arcseconds
Date/time observed March 17-19, 2017

Color image data reduction

The telescope data received from Rehoboth NM was analyzed to get the first colored picture above.

All of the data from the different filters was corrected with a bias, and a dark image. The bias and dark images negated some noise in the picture. All of the data points were combined into one picture for each filter. The filtered data (B, C, R, V) were compiled into one picture and adjusted for the filters.

The filters that were used are as follows:

Red: 1

Green: 1.4

Bleu: 4.2

The image was colored to match published data on this galaxy. The image was then cropped and shown above.


Wikipedia, "NGC 891".





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