A Ring Galaxy

A Cosmic Hit and RunThe odd galaxy next to the bright foreground star in this picture is the Vela ring galaxy, visible as a bright core surrounded by a blue halo. As the name suggests, this ring galaxy is located in the southern constellation of Vela (The Sails). It is notable for its compact core and large circular belt of gas and stars.

It is thought that a ring galaxy like this is created when a small galaxy passes through the center of a larger one, triggering a shock wave that spreads outward. That forces gas to the galaxy’s periphery where it begins to collapse and form new stars. The Vela ring galaxy is unusual in that it actually exhibits at least two rings, suggesting that its collision was not a recent one.

Image Credit: ESO

One Ring to Rule Them All

Zw II 28 is a ring galaxy. Ring galaxies are thought to form when one galaxy passes through the disc of another, larger one. Because galaxies are mostly empty space, such “collisions” are not as destructive as one might suppose. The probability of two stars physically colliding is nearly zero. It’s the gravitational effects of the two galaxies that causes disruption, upsetting the balance both galaxies and causing the pair to redistribute to form a dense central core surrounded by bright stars. In the process, clouds of gas and dust collapse, triggering intense star formation in the outer ring and filling it with hot, young, blue stars.

Zw II 28’s sparkling pink and purple loop is not typical of a ring galaxy. It lacks a visible central companion, but recent observations using Hubble have shown that there may be a possible companion lurking just inside the ring, where the loop seems to double back on itself.

Image Credit: ESA / NASA

Hoag’s Object

hoag_hubble_960Is this one galaxy or two? That question came up back in 1950 when astronomer Art Hoag (a very distant relative) found this unusual extragalactic object. The ring full of bright blue stars surrounds a center of older, redder stars. The gap appears almost completely dark. How Hoag’s Object formed is unknown, but similar objects have now been identified and collectively labeled as a form of ring galaxy. Best guesses include a galaxy collision billions of years ago and the gravitational effect of a central bar that has since vanished. This photo was taken by the Hubble Space Telescope in 2001 and shows unprecedented details of Hoag’s Object. It spans about 100,000 light years and is around 600 million light years away toward the constellation of the Serpens (the Snake). Another ring galaxy is visible in the gap (at about one o’clock).

Image Credit: NASA

A Double Ring Galaxy

NGC 1512 is a barred spiral galaxy with an unusual double ring structure, one ring around the galactic nucleus and another further out in the main disk. It’s about 38 million light-years away in the constellation Horologium. It’s on a collision course to merge with its smaller neighbor NGC 1510 (on the far right in the picture).

Image Credit: ESA / NASA

The Vela Ring Galaxy

A Cosmic Hit and RunThe odd galaxy next to the bright foreground star in this picture is the Vela ring galaxy, visible as a bright core surrounded by a blue halo. As the name suggests, this ring galaxy is located in the southern constellation of Vela (The Sails). It is notable for its compact core and large circular belt of gas and stars.

It is thought that a ring galaxy like this is created when a small galaxy passes through the center of a larger one, triggering a shock wave that spreads outward. That forces gas to the galaxy’s periphery where it begins to collapse and form new stars. The Vela ring galaxy is unusual in that it actually exhibits at least two rings, suggesting that its collision was not a recent one.

Image Credit: ESO

A Ring Galaxy

AM 0644-741The blue ring of young stars around the nucleus of AM 0644-741 is 150,000 light-years in diameter, making it larger than our entire galaxy, the Milky Way. Ring galaxies are one example of how collisions between galaxies can produce an significant change in one galaxies structure. One may result from a collision in which an intruder galaxy plunges directly through the disk of a target galaxy. In the case of AM 0644-741, the galaxy that pierced through the ring galaxy is out of the frame of this Hubble image but can be seen in larger-field images. The resulting gravitational shock caused by the collision drastically alters the orbits of stars and gas in the target galaxy’s disk, causing them to rush outward. As that wave spreads out, gas clouds collide and are compressed. The dense clouds contract under their own gravity, collapse, and new stars form.

Image Credit: NASA