Just delivered! Will you help me assembling it?
Artist’s impression of the Milky Way galaxy. The blue halo of material surrounding the galaxy indicates the expected distribution of dark matter. Credit: ESO/L. Calçada
Finally! My new galaxy building kit has arrived! I was so impatient that I kept looking through my window, waiting for the AstroAmazon delivery van to stop in front of my house. I am so happy to have it in my hands at last!
"Barred spiral galaxy. Model: Milky Way 2022". So the box reads. Believe me, it was really hard to choose. The galaxy catalogue is so big! (We can browse it together some day if you want) But I fell in love with the image of the elegant spiral arms swirling around a yellowish central part. Hope I won't be deceived! Would you like to help me building it?
Unboxing
Ok, let's first unpack everything and make sure that the kit is complete. Be careful, the box is heavy: three trillion times the mass of the Sun! This is what we find inside:
Several hundred billion stars of all colours (the blue ones standing particularly out).
A lot of gas and dust. (I cannot tell for sure, but this seems to be about five billion Sun's masses, around 10% of the mass of stars)
A yellowish, elongated bulge, 12,000 light years of diameter, made of about 10 million stars and a fair amount of gas and dust.
A supermassive black hole of about four million times the mass of the Sun (a bit crap, to tell the truth).
Around 150 globular clusters (spherical structures containing thousands of tightly packed stars, most of them yellowish or reddish in colour).
Twenty-something stellar streams (nicely stretched-out groups of stars).
A large amount of dark matter (about one trillion solar masses, in a first estimation)..
We can now begin with the building. Fortunately, the kit comes with detailed step-by-step instructions. I must admit that I am a bit clumsy; I don't think I would have managed with just a few text-free sketches like in the case of an IKEAâ„¢ piece of furniture!
Face-on and edge-on views of the Milky Way (artistic impression), with its main parts and the location of the Sun labeled. Credit: NASA/JPL-Caltech (left), ESA & ATG Datalabs (right)
Assembling instructions
Mix well the gas and dust and sculpt it to form a flattened disk-shaped structure of about 100,000 light years diameter, its density and height decreasing towards the edges and away from the disk's plane.
Stir the gas and dust in the disk until several spiral arms form. At least four major arms should appear. If desired, additional smaller arms or spurs may be added, at user's taste. (Note: It is by no means necessary, not even recommended, to obtain an homogeneous distribution within the arms; as a matter of fact, clumps –or clouds– are highly desirable in order to trigger eventual star formation)
Insert the supermassive black hole into the centre of the bulge. Make sure it is well wrapped within the bulge's gas and dust, hidden to human eyes, and that the bulge stars are well distributed around it.
Place the bulge at the centre of the galaxy's disk, so that the major spiral arms seem to emerge from it through the bars.
Distribute the stars in the disk. Blue stars should be located in areas with big amounts of gas and dust, and especially within the spiral arms, building a "thick disk" roughly 700 light years high. Red stars may be placed anywhere and reach higher heights, up to about 3000 light years (thus forming a "thin disk"). Form as many stellar clusters as you want, but overall, the density of stars must decrease radially –in other words, the number of stars per unit volume should be maximum at the centre and decrease as we move away from it. This decrease should be more abrupt from a distance of about 40,000 light years from the centre.
Place the globular clusters and stellar streams around the disk and bulge structure. They should be inside an imaginary sphere of not more than 200,000 light years as measured from the Galactic Centre, with the concentration decreasing with distance. Nicely warp the stellar streams, building arcs around the central structure.
Carefully wrap everything in dark matter, forming a spherical halo of at least 300,000 light years radius.
Stellar streams in the Milky Way. Credit: NASA
Activation
Set everything in motion around the central black hole. In the bulge and halo, the stars' orbits can be highly elongated and have as many different orientations as you wish. But in the disk, make sure that the stars all move roughly in the same plane, in nearly circular orbits, like the planets in the Solar System.
Warning! Use may create some warps in the disk. This is perfectly normal. Don't try to force the disk back to perfect flatness.
How objects orbit the Milky Way. Retrieved from Douglas College Astronomy 1105
Extra items
I decided to complement my galaxy with some satellites, small irregular galaxies orbiting around the big one. Placing them is trickier than it seems, because it is not unusual that a satellite is attracted towards the galaxy centre and gets disrupted. The good thing is that, in this way, I get yet more stellar streams. I even decided to keep one boldly crossing the galactic plane! I call this one "the Sagittarius Dwarf Galaxy".
Location of some of the satellite galaxies of the Milky Way. Credit: Richard Powell (via Wikimedia Commons)
I also purchased a "You are here" signpost to indicate the location of our Sun. I placed it in the Orion Arm, about 27 light years from the centre.
And this is my final artwork. Do you like it?
