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Messier 81 (NGC 3031)
Alex VanKooten

Messier 81

Discovered by German astronomer Johann Bode on New Year's Eve, 1774, Messier 81 is a large, bright spiral galaxy located a mere 12 million light years away. While this is a staggeringly large distance, M81 is actually one of the closest galaxies to the Milky Way. It's relative proximity and brightness mean that M81, otherwise known as Bode's galaxy or NGC 3031, is one of the most studied galaxies by professional and amateur astronomers. M81 is located within the constellation Ursa Major, the constellation best known for containing the Big Dipper; however, Messier 81 isn't quite bright enough to be seen with the naked eye.

As with all spiral galaxies, M81 is brightest in the center and dimmer towards the edges. The central galactic bulge consists of millions of stars whirling around a supermassive black hole with the mass of seventy million suns. This black hole is active, meaning that it is currently devouring dust and gas stolen from nearby stars. The yellow color of the stars surrounding the black hole suggests that they are very old, born billions of years ago. In the middle of the disk, on the right-hand side, is a band of dust that blocks the light from stars behind it. This band was probably caused by ancient gravitational interactions between Messier 81 and its neighbor galaxy, Messier 82. Around the edges of the galactic disk, almost 25 thousand light years from the center, are blue regions. These regions indicate young stellar populations as well as regions where new stars are being formed.

Several independent sources estimate that Bode's Galaxy is about 12 million light years away. Based on my own observations, the galaxy has an angular size of about fourteen arcminutes. These two measurements together can be used to calculate the galaxy's linear size, about fifty thousand light years across.

Bode's Galaxy is also tilted at an angle relative to us, and this tilt can be calculated. To find this, we first take the ratio of the apparent major and minor axis lengths:

Axis Lengths

The inverse cosine of this ratio gives the angle at which the galaxy is tilted, about 44 degrees. As I mentioned before, M81 contains an active black hole in its center. As it consumes gas and dust, this black hole spews gas and radiation in jets perpendicular to the galactic disk. If Bode's Galaxy were facing us directly, we would be looking straight down that jet of material; however, because the galaxy is at an angle, we can view those jets from the side. Keep in mind that these jets are so hot that the glow in wavelengths of light too short for us to see with the naked eye, meaning they won't be visible in the image above.

I also calculated the scale radius of M81. The scale radius, or half-light radius, is the distance from the center of the galaxy in which half of the galaxy's light is emitted. To do this, I graphed the surface brightness profile of M81, which shows how bright the galaxy is as a function of distance from the galactic center. I then used a line of best fit to calculate the scale radius, using the equation y=I0exp(-x/Rs)+C, where I0=2.773*104, Rs is the scale factor 7.502, and C=927.1:

Surface Brightness Profile

The scale factor was calculated in pixels. Converting the scale factor into parsecs, half of the galaxy's brightness is contained within 172 parsecs from the center, with an uncertainty of 3 parsecs. For comparison, our galaxy, the Milky Way, has a scale radius of about 3,000 parsecs, seventeen times larger than M81's scale factor.



Devereux, Nick et al. "STIS Spectroscopy of the Central 10 Parsecs of M81: Evidence for a Massive Black Hole." The SAO/NASA Astrophysics Data System. <>

"Messier 81." NASA/IPAC Extragalactic Database. <>

Nemiroff, Robert and Jerry Bonnell. "Bright Spiral Galaxy M81." Astronomy Picture of the Day. <>

Wikipedia, "Messier 81".

Right Ascension (J2000) 09:55:33.50
Declination (J2000) 69:04:02.0
Filters used B (Blue), C (Clear), R (Red), V (Green)
Exposure time per filter B (300s x 13), C (90s x 14), R (120s x 7), V(210s, x 6)
Image dimension 1092x736 pixels; 24.0x16.2 arcminutes
Date/time observed March 2, 2017, 04:30 UT

In order to get a clean, crisp image, several steps need to be taken to go from the initial images taken by a telescope to the final image, including the subtraction of a bias frame and a dark frame, and dividing out a flat field. For the bias frame, several bias images are taken which are then stacked together to create a single master bias. This image is subtracted from all other images, including the other calibration images. This bias frame eliminates any noise intrinsic to the camera chip used to take the pictures.

Another frame, known as the dark frame, undergoes a similar procedure. The bias frame is subtracted out from each dark frame. These dark frames are compiled into a single image, which is then subtracted from both the flat field images and the final color image. This dark frame gets rid of any thermal noise that might be present in the camera chip.

Finally, the dark and bias frames are subtracted out of each flat field image. The flat field images are then stacked according to the filter with which they were taken. This will result in final flat field images, one for each filter. These flat field images are then divided out from each color image according to the filter used for each image. Flat field images are used to get rid of any variations due to dust on the telescope's mirror or filter. They also account for some pixels being more sensitive than others and the fact that pixels on the outer edges of the camera chip aren't exposed to as much light as pixels at the center of the chip.

Finally, all the color images are combined to make a final image in each filter. These color images are combined to make the final color image. For the color combination, I used a 1.2 multiplier for red, 1.05 for green, and 2.4 for blue. I also used a luminosity weight of 75%, a color saturation level of 135%, and a gamma value of 0.8.


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