M81 Jan 6 through Feb 10 2021

C8 f7 1494 mm

L=44x60s R=43x120s G=43x120s B=52x120s Ha=39x600s

Total integration ~ 12 hours over many nights

A great big galaxy far far away! This lovely spiral is sometimes called Bode’s Nebula after the German astronomer Johann Bode who first described it in 1774. Charles Messier later added it as #81 on his list of not-comets. It lies in the constellation Ursa Major, but it outside the bowl of the familiar Big Dipper. Some people claim to be able to spot it with the naked eye from super-dark locations, but I’ve never been able to do that even from the high mountains of Wyoming. But it’s easy with binoculars.

You can see some subtle straight dark bands that cross the galaxy’s disk at upper right. They don’t seem to follow the spiral structure at all. These are streamers of foreground dust in our own galaxy that slightly obscure our view into the intergalactic gulf beyond. They are known as “galactic cirrus.”

As with all spiral galaxies, the arms are not material objects but rather waves of creation, sweeping gas and dust together into bands sufficiently dense for clumps to collapse under their own gravity. The collapsing clumps ignite by thermonuclear fusion to form jewel-like clusters of bright blue clusters of newborn stars. Giant blue stars emit copious amounts of ionizing radiation which causes their natal gas to fluoresce a lovely ruby-red. These hot blue behemoths will blow up as spectacular supernovae before the arms have a chance to move on, and it’s thought that the shock waves caused by these explosions are part of the process that propagates the waves of creation through the disk.

By contrast, the inner disk is tired and old and there is no star formation going on in there. Instead there are clouds of countless yellow and red stars shining there for billions of years as the youthful arms swing round and round.

In the center of M81 lies a supermassive black hole with the mass of 70 million Suns. Like all black holes, its density is essentially infinite so it occupies no volume, yet bends space and time into itself and profoundly affects the environment of the galaxy’s core.

Relative to their colossal size, galaxies are a whole lot closer together than stars. In our part of the cosmos, big galaxies are around 20 times as far apart as the are wide. By contrast the nearest star to our Sun is something like 26 million times as far away as the Sun is wide! So on interstellar scales space is ridiculously empty, but on intergalactic scales it’s positively teeming with ginormous systems of trillions of stars and their associated worlds.

We live in a big spiral galaxy that’s one of three in our Local Group. Each of the big spirals around here is surrounded by a dozen or more blobby little companions called “dwarf irregulars” which orbit uneasily around us and will slowly be eaten by their big neighbors. The irregulars are indeed galaxies in their own rights, comprising up to billions of solar systems each. There are also hundreds of globular clusters – spherical shells of spherical swarms of thousands of ancient stars – orbiting around each big spiral.

The very fabric of space and time is stretching apart in every direction, but The Local Group is gravitationally bound. That means that the mutual gravitation of all these trillions of solar systems and the ethereal dark matter surrounding them holds our little corner of the cosmos together. The universe is full of these groups and clusters of galaxies – clots of mass which resist the general expansion.

Beyond the Local Group are other groups. One of the closest galaxy groups to our own centers on this lovely spiral. It’s about 12 million light years away in the Ursa Major, just outside the bowl of the Big Dipper. It’s about the same size as our own Milky Way – about 90,000 light years across and contains hundreds of billions of solar systems.

This is 9 hours exposure over a bunch of nights in January and February 2021. Between the Moon and the clouds and the snow, it was tough to get each hour! The telescope is an 8-inch diameter combination of mirrors and lenses, with a focal length just under 1500 mm, and I used a special black-and-white camera with a refrigerated chip. Combination of black-and-white images shot through clear filters (44 x 1 min), red, green, and blue filters (about 45 x 2 min for each color). To these I added 39 x 10 minutes shot through a narrowband filter that only allows the light of fluorescent hydrogen which shows the big red nebulae in the galaxy’s spiral arms where clusters of new stars are bring born from gas and dust.