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| M81 and M82 |
Objects: Messier 81 (NGC 3031, Bode's Galaxy) and Messier 82 (NGC 3034, the Cigar Galaxy)
Type: Galaxies (
classifications M81: SA(s)sb, M82: I0 )
Constellation: Ursa Major
Distance: 12 million light years
Dates: 20th and 27th. February 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD
Subframes: 12 x Red, 12 x Green, 12 x Blue, 12 x H-alpha (all 300s, 2x2 binned), 12 x 600s luminance, plus flats for each channel but no darks (hot pixel removal in Astroart).
The M81 Group is a
galaxy
group in the constellations Ursa Major and Camelopardalis that includes the
galaxies Messier 81 and Messier 82, as well as several other galaxies with high
apparent brightnesses. The approximate centre of the group is located at a
distance of 12 light years, making it one of the nearest groups to the
Local
Group. The M81 Group, the Local Group, and other nearby groups all lie
within the
Virgo Supercluster (i.e. the Local
Supercluster).
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| Location of M81/82 field |
The location of the M81/M82 group in the night sky is indicated
in the Stellarium sky map opposite.
Discovered by the German astronomer
Johann Elert Bode in
1774, M81 and M82 are two of the brightest galaxies in the night sky. Through a
pair of binoculars, the galaxies appear as faint patches of light in the same
field of view.
M81’s galaxy’s outer spiral arms are made up of young,
bluish, hot stars formed in the past few million years. They also host a
population of stars formed in an episode of star formation that started about
600 million years ago. Ultraviolet light from hot, young stars is the
surrounding clouds of hydrogen gas to fluoresce, giving rise to the
characteristic red emission nebulae that show in the above image as tiny red
speckles visible strewn along the spiral arms of the galaxy. A number of dust
lanes can also be seen tracing the arms into the nucleus of M81.
The galaxy’s central bulge contains much older, redder
stars. It is significantly larger than the Milky Way’s bulge. A black hole of
70 million solar masses resides at the centre of M81 and is about 15 times the
mass of the Milky Way’s central black hole.
M82 (nicknamed the “Cigar galaxy”) shines brightly at
infrared wavelengths and is remarkable for its star formation activity. The
galaxy experiences gravitational interactions with its galactic neighbour, M81,
which creates an extraordinarily high rate of star formation: for this reason
M82 is known as a “starburst” galaxy.
It is about five times more luminous than our Milky Way galaxy
and has a centre one hundred times more luminous. As the closest starburst
galaxy to Earth, M82 is the prototypical example of this galaxy type.
Around the M81’s centre, young stars are being born at a
rate 10 times faster than they are inside our entire Milky Way galaxy.
Radiation and energetic particles from these newborn stars carve into the
surrounding gas, and the resulting galactic wind compresses enough gas to make
millions of more stars. The rapid rate of star formation in this galaxy
eventually will be self-limiting. When star formation becomes too vigorous, it
will consume or destroy the material needed to make more stars. The starburst
will then subside, probably in a few tens of millions of years.
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| Background galaxies in the M81/M82 field |
Other distant background galaxies can be seen in this image,
all of which are hundreds of millions of light years away, with the exception
of Holmberg XI, which is a small satellite galaxy of M81. Some of these chillingly distant objects are
indicated in the luminance image opposite.
The region of M81 and M82 is permeated by faint trails of
dust that are part of our galaxy rather than associated with the M81
group. These
Integrated
Flux Nebulae are high galactic latitude nebulae that are illuminated not by
a single star (as most nebulae in the plane of the Galaxy are) but by the
energy from the combined light (“integrated flux”) of all the stars in our
Milky Way. These IFN are very faint and were only discovered in 2005 by amateur
astronomer
Steve Mandel.
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| Stretched luminance frame showing IFN |
The IFN shows only very faintly on the 600 second luminance
frames taken for this image: indeed, I first thought the traces on my stacked luminance frames were actually just poorly corrected gradients or other artefacts. To get the IFN to show up, I used the stretch, followed by the equalise function in PSP, which
displays every available detail in the image (see left). This revealed traces of the IFN, albeit
rather crudely. I then applied a large
Gaussian blur (4 pixels) to smooth up the noise, and then layered the smoothed
frame over the LRGB in luminance mode at about 10%. The burnt-out galaxy areas were erased to
give the final image, which shows the M81/M82 pair shining through the faint
dusty debris of our own galaxy. The faint arcs of dust evidenced around M82 in my image appear to correspond to similar features shown in much
deeper exposures.
Flats were essential in order to eliminate image artefacts
from the LRGBHa stacks, which would mask the faint IFN traces. The Ha and red frames were combined as a
30/70 blend, although it seems the H-alpha exposures were not really deep
enough to fully show up the starburst tendrils of H-alpha emissions that are
being ejected from the poles of M82.
