Astronomy magazine – Nov. 2009: Watch as Galaxies Collide

This is the version submitted and doesn’t include their editing.

By Alan Goldstein

The night sky is riddled with galaxies – vast agglomerations of stars, dust and gas. While a few seem isolated, most are found in groups or clusters containing dozens to more than a thousand galaxies. The size of an individual galaxy varies from several million stars stretching a couple thousand light years in space to more than a trillion stars spanning several hundred thousand light years.

A lot of empty space seems to lie between galaxies, but this is deceiving. Two near equal-mass galaxies several million light years apart may be drawn together by their mutual gravitational attraction. The effect both builds and destroys galaxies. These galactic collisions can be spectacular hurling stars millions of light years into intergalactic space or they can go virtually unnoticed.

Images from the Hubble telescope’s deep survey’s show that the farther back in time (more distant) one sees, the more galactic crashes may be observed. Astronomers believe (in part from these observations) that the galaxies you can see in your telescope were made by the collision and absorption of others. Physical laws favor the bigger galaxies when they smash together. Unlike a freight train hitting a compact car where the pieces scatter from the impact, the end result between galaxies can be a celestial object of amazing beauty! M51, the Whirlpool galaxy, is a classic example (photo).

M51, Whirlpool Galaxy – photo by Ron Yates

The affects of fly-bys, grazes or mergers between galaxies can make for breath-taking photography and very interesting visual observations. Watching galaxies collide is akin to seeing nature’s finest drama with a single still frame image. It would take thousands of lifetimes to see any changes between interacting galaxies. Disappointed? Don’t be – there is plenty of action “out there” and almost every combination of interaction can be observed.

“Near Misses – Near Hits?”

The gravitational influence between passing galaxies can be incredible even when they don’t appear to be in direct contact. Studies show that most galaxies are surrounded by a halo of gas and dark matter that adds to their bulk. As a result, even a “near miss” can alter the shape of one or both galaxies.

Andromeda’s M31 and M32 are the easiest pair of interacting galaxies to observe. The large spiral seems to be little affected by the proximity of dwarf elliptical M32, but in time the small galaxy will suffer. The bright compact elliptical is visible within the overall glow of the disk of M31. It is a good target for telescopes of any aperture!

M31 with M32 (9 o’clock position and M110 below – Ron Yates photo.

NGC2207 and IC2163 is a pair of spiral galaxies in Canis Major that are passing by one another. The result is a dramatic photograph – one of the favorites from the Hubble Telescope. Visually NGC2207 appears evenly illuminated and somewhat oval, while IC2163 is an open spiral with a low surface brightness giving it a ghostly appearance in comparison. Both are bright and relatively easy in a modest scope under good skies.  In larger scopes a compact nucleus is visible on IC2163.

NGC4618 is an SBc barred spiral interacting with the spiral NGC4625 in Canes Venatici. The latter appears to be a compact spiral, but in reality it has a large disk that has a very low surface brightness. The smaller galaxy turns out to be is physically larger! In his 1966 publication on peculiar galaxies, H. C. Arp noted this duo as odd because the interaction created a pair of one-armed spirals! The interaction has caused both to undergo intense star formation. Both are visible with an 8-inch scope, with NGC4618 dominating the scene.

The widely spaced double system of NGC5963 and NGC5964 are Sb spiral and SO (lenticular-type) galaxies. The latter was originally described as an elliptical galaxy with unusual gas and dust clouds. Located in Virgo, they are separated by 14’ in declination. NGC5964 appears to be relatively unfazed by the proximity of NGC5963. However the latter has open spiral arms. Both are bright and are good targets for small telescopes. The SO-galaxy is an oval glow, while the spiral is smaller because the arms have a low surface brightness and require a larger scope to be well seen.


Galaxies that are in physical contact, but do not appear to be in head-on collisions are a category I call “fly-by-night.” These graze pass one another with the outer rims of both interacting, but the main bodies remain intact. The smallest of the pair is usually “much worse for wear.” Eventually they will merge, but at the moment both galaxies are largely intact.

NGC3226 and NGC3227 is an easy target in Leo located a degree east of Gamma Leonis. At 11.4 and 10.3 magnitude, respectively, this pair is an elliptical and spiral galaxy. Their proximity has not distorted either appreciably, though deep photos show their halo stars are affected. With NGC3227 you should notice a bright stellar nucleus. It is a Seyfert galaxy with an explosive core powered by a super-massive black hole. This pair is relatively easy in a 6” and more impressive with increased aperture.

Canes Venatici is a small constellation but is well endowed with colliding galaxies. M51 is not only the favorite of the spring sky, but probably among any observer in the northern hemisphere. Called the Whirlpool Galaxy because of its easy-to-see spiral arms, it rates as the most famous interacting galaxy pair in the sky. NGC5194 and its dusty peculiar companion NGC5195 are 8.4 and 9.6 magnitude, respectively. Visible in binoculars, a 2.4” cm optical system will show it well. In moderate scopes the sweeping spiral structure may be seen and in instruments 12” and above, it can resemble its photograph.

If not overshadowed by the Whirlpool, NGC4485-90 could be the best-known colliding in Canes Venatici. It is a pair of Magellanic-type galaxies showing the slightest hints of spiral structure in good photos. The larger system NGC4490 (recently named the “Cocoon galaxy”) has a condensed nuclear region. Like the pair of NGC4618 and 25 above, this pair also shows dramatic star forming regions – only this time it is on the facing sides of each galaxy. Both are easy targets for a small telescope and are among the easiest interacting galaxies to find – less than a degree northwest of Beta Canum Venaticorium.


When galaxies hit hard, the result can be a real galactic mess! Spiral galaxies can become so distorted and tangled that two become one. A number of galaxies at first glance do look like one weird galaxy. The advent of HST and the super-large telescopes have sufficient resolution to bring a murky mess into sharp focus.

NGC4038 and NGC4039 – the Ringtail or Antennae galaxies – are a pair of spiral galaxies with “full contact.” The “ringtail” name comes from the shape of NGC4039 with a bright ring of stars in the outer periphery and the tail is the far-flung arms of NGC4038. The “antennae” name comes from the fact that the interaction has flung stars in a pair of slightly curved arcs many times the diameter of both galaxies into deep space. The larger NGC4039 was originally type Sb. NGC4038 is a small two-armed Sc spiral. The collision has energized star-forming nebula on the facing sides and thrown dust arcing between them. In a telescope the unusual shape is easily seem in a small telescope. Look at it carefully – what shape do you see?

NGC4676A and B – the Mice – is a pair of galaxies undergoing collision and major deformation. This is a faint target at 13th magnitude, visible in telescopes larger than 8”. “A” is an SBb-type, while “B” could be an edge spiral, but is difficult to discern even with the best images because of debris from “A” crossing its middle.

NGC5128 is another very bright, very peculiar galaxy – the merging spiral and giant elliptical galaxy. The pair has become one, but the dust of the spiral bisects the globe-shaped elliptical. Also designated radio source Centaurus A, this is probably the nearest major collision at 30 million light years distant. The dust belt bisecting what would otherwise appear to be a circular elliptical type galaxy is very dramatic.

There are many other bright colliding galaxies to be observed. A second list of ten galaxies can be found on Get out and enjoy the universe’s demolition derby and enjoy the spectacle unfold in the slowest of slow motions.

Data for Selected Targets

Name                           Coordinates (for brightest)    Mag.Vis.     Size (minutes arc)

M31/M32                    00h 42.7m       +41o 16’           3.4/8.1          185’x75’ / 11’x7.3’

NGC2207-IC2163      06  16.4           -21  22’            10.8/10.9         4.8×2.3 / 3.0×1.2

NGC3226-7                10   23.5          +19  52            11.4/10.3         2.5×2.4 / 6.9×5.4

NGC4038-9                12  01.9           -18  53             10.5/10.3         5.4×3.9 / 5.4×2.5

NGC4618-25              12   41.3  &n犀利士 bsp;       +41  09            10.8/12.3         4.1×3.2 / 1.4×1.3

NGC4485-90              12  30.6           +41  38            9.8/11.9           6.4×3.3 / 2.7×2.3

NGC4676                   12  46.2           +30  44            13.0/13.2         2.0×0.3 / 1.7×0.7

M51 (NGC5194-5)     13  29.9           +47  12            8.4/9.6             8.2×6.9 / 6.4×4.6

NGC5128                   13  25.5           -43  01             6.7                   31.0×23.0

NGC5363-4                13  56.1           +05  15            10.1/10.5         5.0×3.2 / 6.6×5.1

Added for their consideration for the magazine’s website:

The “Second Ten”

Name               Coordinates (brightest)           Mag.Vis.          Size (minutes arc)    Type

NGC750-1      01h   57.5m     +33 o 13           11.9/12.8         1.7×1.3 / 1.4×1.4          Ep/Ep

NGC770-2      01  59.3           +19  01            10.3/12.9         7.3×4.6 / 1.0×0.7         Sb/dE3

NGC1510-12  04  03.9           -43   21            12.4/10.2         2.3×1.1 / 8.3×3.6     SO?/SBa

NGC1531-2    04 12.1            -32  52             12.1/9.9           11.2×3.2/                 Sbp/SOp

NGC3395-6    10  49.8           +32  59            12.1/12.1         1.6×0.9 / 3.4×1.3         S(B)cdp/IBmp

NGC4435-8    12  27.8           +13  01            10.8/10.2         3.2×2.0 / 8.9×3.6         SBO/SO/ap

NGC4567-8    12  36.6           +11  14            11.3/10.8         2.7×2.3 / 4.7×2.2         Sbc/Sbc

NGC4627-31  12  42.1           +32  32            9.2/12.7           2.1×1.6 / 15.5×3.3       dE4p/Sc

NGC5963-5    15  34.0           +56  42            12.5/11.7         3.7×2.8 / 5.5×0.9        Sp/Sb

NGC6962-4    20h 47.3          +00  19            12.1/13.0         2.7×2.1 / 1.6×1.1        S(B)a/SOp