Atlas · Galaxies
Our galaxy and the others
Four steps outward: the Milky Way, the small galaxies that keep it company, the Local Group, and the web of galaxies beyond. The first drawing is a model, because nobody has seen our galaxy from outside. The other three are measured.
Drawing I
The Milky Way
This drawing is a model, not a map. We live inside the Milky Way’s disc, and dust hides most of it from us, so nobody knows just where every arm runs.
What is measured: the Sun is 27,000 light years from the centre, the middle of the galaxy is a bar of old stars, and the arms wind outward at a shallow slant. The model is built from those figures: four arms slanted at 12.5°, a bar 33,000 light years long turned 30° from our line of sight, and the short Local arm that the Sun sits in.
The small ring round the Sun is 2,000 light years in radius. Nearly every star you can see by eye is inside it.
The drawing is 130,000 light years across.
Drawing II
The Milky Way’s companions
The 27 small galaxies in this catalogue that lie within about a million light years of the centre of the Milky Way. Most are held by the Milky Way’s gravity and go round it. The two Magellanic Clouds are the only ones easy to see by eye, from the southern half of the Earth.
The grey disc in the middle is the Milky Way of drawing I, to scale. Each dot is where a galaxy really is. A bigger dot is a brighter galaxy, but the dots are not to scale: most of these would be smaller than a full stop.
The drawing is 2 million light years across.
Drawing III
The Local Group
Step back five times. The Milky Way and Andromeda are the two big galaxies of a group about ten million light years wide. Each has its own swarm of small ones, and Triangulum is the third in size.
Andromeda is about 2.5 million light years away (783 kiloparsecs in the catalogue), the furthest thing most people can see without a telescope. It and the Milky Way are falling towards each other.
A ring instead of a dot means the catalogue gives no brightness for that galaxy.
The drawing is 9.8 million light years across. The catalogue’s other 23 galaxies are further off than that, just beyond the group. They are in the 3D view.
Drawing IV
The cosmic web
Step back two hundred times more. Each dot is now a whole galaxy: 11,356 of them, in a slice of space 130 million light years thick with us in the middle. Galaxies are not scattered evenly. They gather into clusters and threads, with great empty voids between.
Three things here are not real. The dots thin out towards the edge only because faint galaxies far away were too dim for the survey. Along a narrow line to the left and right of the middle there are fewer dots, because the Milky Way’s own disc hides what lies behind it. And a rich cluster is smeared into a streak pointing at us, because distance is worked out from speed and the galaxies in a cluster are also flying round each other.
The drawing is 2 billion light years across.
How to read it
Drawings I to III look straight down on the disc of the Milky Way from its north side, the same way up as the chart of the stars. A flat drawing cannot show height, so galaxies that look close together on the page may be far apart above and below it.
Drawing IV is turned a different way, to lie along the sheet in which most of the nearby galaxies are found. Astronomers call it the supergalactic plane. We are the ring in the middle.
The distances in drawing IV come from redshift: the faster a galaxy is moving away, the further it is, at 70 km a second for every megaparsec (3.26 million light years). That is good to about a tenth far away, and poor close in.
The larger galaxies near us
Andromeda and the 23 galaxies in the catalogue that give out more visible light than about 30 million Suns, nearest first. All 101 are in the 3D view.
| Galaxy | Distance from us | Light it gives out |
|---|---|---|
| Canis MajorAstronomers disagree on whether this is a galaxy at all. | 23,000 light years (7 kpc) | 49 million Suns |
| Large Magellanic Cloud | 170,000 light years (51 kpc) | 1.5 billion Suns |
| Small Magellanic Cloud | 210,000 light years (64 kpc) | 450 million Suns |
| NGC 6822 | 1.50 million light years (459 kpc) | 100 million Suns |
| NGC 185 | 2.01 million light years (617 kpc) | 71 million Suns |
| NGC 147 | 2.20 million light years (676 kpc) | 59 million Suns |
| IC 1613 | 2.46 million light years (755 kpc) | 100 million Suns |
| Andromeda | 2.55 million light years (783 kpc) | not given |
| IC 10 | 2.59 million light years (794 kpc) | 86 million Suns |
| M32 | 2.63 million light years (805 kpc) | 310 million Suns |
| Triangulum | 2.64 million light years (809 kpc) | 2.8 billion Suns |
| NGC 205 | 2.69 million light years (824 kpc) | 340 million Suns |
| WLM | 3.04 million light years (933 kpc) | 41 million Suns |
| NGC 3109 | 4.24 million light years (1,300 kpc) | 78 million Suns |
| Sextans B | 4.65 million light years (1,426 kpc) | 54 million Suns |
| Sextans A | 4.67 million light years (1,432 kpc) | 45 million Suns |
| NGC 55 | 6.30 million light years (1,932 kpc) | 2.1 billion Suns |
| IC 5152 | 6.36 million light years (1,950 kpc) | 150 million Suns |
| NGC 300 | 6.78 million light years (2,080 kpc) | 2.1 billion Suns |
| IC 3104 | 7.40 million light years (2,270 kpc) | 34 million Suns |
| IC 4662 | 7.97 million light years (2,443 kpc) | 180 million Suns |
| DDO 125 | 8.42 million light years (2,582 kpc) | 49 million Suns |
| DDO 190 | 9.11 million light years (2,793 kpc) | 49 million Suns |
| NGC 4163 | 9.32 million light years (2,858 kpc) | 37 million Suns |
Where this comes from
- Drawings II and III: McConnachie 2012, Astronomical Journal 144, 4, “The observed properties of dwarf galaxies in and around the Local Group”.
- Drawing IV: Huchra et al. 2012, Astrophysical Journal Supplement 199, 26 (the 2MASS Redshift Survey). The survey measured 43,448 galaxies over the whole sky, and the slice drawn here holds about a quarter of them.
- The Sun’s distance from the centre of the Milky Way: GRAVITY Collaboration 2019, Astronomy & Astrophysics 625, L10.
The Milky Way in drawing I is the model used by the 3D view. Its arms follow the usual picture of four arms winding out from a central bar, but their exact places are our own choice and should not be read as measurements.
The light a galaxy gives out is its visible light, compared with the Sun’s, worked out here from the brightness given in the catalogue.