Skip to main content

How did this beautiful ring galaxy form? Astronomers remain puzzled

Hoag’s Object: A Nearly Perfect Ring Galaxy Image Credit: NASA, ESA, Hubble; Processing: Benoit Blanco

NASA and the European Space Agency have shared this beautiful and striking image of a most unusual galaxy. Hoag’s Object, as it is known, is an almost perfect ring galaxy with an elegant circular form, meaning it is classified as a “peculiar galaxy.” The image was captured using the Hubble Space Telescope.

One question astronomers had when observing this object was whether it was one galaxy or two. You can see the outer ring of younger blue stars is very distinct from the inner sphere of older, redder stars. And there seems to be a gap of nearly star-less space between the two. These facts make it seem like these are two separate galaxies.

However, the outer ring is around 120,000 light-years wide, which is around the same width as our galaxy, the Milky Way. The inner ring is 24,000 light-years across, which is around the same size as the inner core of our galaxy. This makes it seem like the object is one galaxy that somehow had a portion of stars from its interior removed.

The underlying question is how such a ring galaxy could have formed, and astronomers still don’t have an answer to that. A few other ring galaxies have been found through the years, but none as strikingly perfect as Hoag’s Object.

One possibility, according to astrophysics blogger Graham Doskoch, is that the object began life as a single barred galaxy, but that the bar was unstable. Over time, the instability destroyed the inner structure of the galaxy. The problem with this theory is that it would mean the core of the object would be disk-shaped, but that’s not what we see — the core is spherical.

Another possibility is what is called a “major accretion event.” If two galaxies passed close to each other but not close enough to collide or merge, then a large amount of matter could have passed from one galaxy to the other. But if this had happened to Hoag’s Object, we would expect to see indicators like tails of matter stretching out in one direction, which we don’t observe here. It could be that the event happened billions of years ago, so such indicators have eroded over time, leaving only the nearly perfect ring we see today.

Editors' Recommendations

Georgina Torbet
Georgina is the Digital Trends space writer, covering human space exploration, planetary science, and cosmology. She…
Hubble spots a rare pair of highly active galaxies
This image from the NASA/ESA Hubble Space Telescope feels incredibly three-dimensional for a piece of deep-space imagery. The image shows Arp 282, an interacting galaxy pair composed of the Seyfert galaxy NGC 169 (bottom) and the galaxy IC 1559 (top).

This week's image from the Hubble Space Telescope shows a pair of interacting galaxies, collectively known as Arp 282. The bottom galaxy, NGC 169, is a type of highly active galaxy known as a Seyfert galaxy. Many similarly active galaxies are hard to observe because they give off so much radiation, but Seyfert galaxies give off energy at wavelengths other than those observed by instruments like those on Hubble. That means the galaxy can be clearly observed, even though it's extremely active.

The other galaxy in the image, IC 1559, located toward the top of the image, is also a very active type. The two galaxies are so close together that they are interacting, with material moving between the two in streams of matter.

Read more
Hubble revisits an irregular dwarf galaxy bursting with young stars
The dwarf galaxy NGC 1705 featured in this image from the NASA/ESA Hubble Space Telescope lies in the southern constellation Pictor, approximately 17 million light-years from Earth. NGC 1705 is a cosmic oddball – it is small, irregularly shaped, and has recently undergone a spate of star formation known as a starburst.

This week's image from the Hubble Space Telescope shows the quirky dwarf galaxy NGC 1705, an unusually shaped small galaxy located 17 million light-years away. Stars have formed in this galaxy across its entire lifetime, but the galaxy went through a very intensive period of star formation, called a starburst, approximately 30 million years ago. Many of the stars born during this period are now located around the central core or within the huge central star cluster.

This particular dwarf galaxy is an irregular shape, but it is still a useful object of study. Dwarf galaxies are thought to be some of the oldest galaxies, so studying them can help us learn about the early universe. This one was observed in order to learn about young stars.

Read more
Hubble spots three galaxies pulled into an unusual shape
The subject of this image is a group of three galaxies, collectively known as NGC 7764A. They were imaged by the NASA/ESA Hubble Space Telescope, using both its Advanced Camera for Surveys and Wide Field Camera 3.

This week's image from the Hubble Space Telescope shows a group of three galaxies that are so close together that they have been given one collective name: NGC 7764A. When galaxies come close together, they can interact as their gravity affects one another, leading to them being pulled into different shapes or even, in extreme cases, merging together or destroying one of the galaxies.

"The two galaxies in the upper right of the image appear to be interacting with one another," the Hubble scientists explained in a statement. "The long trails of stars and gas extending from them give the impression that they have both just been struck at great speed, thrown into disarray by the bowling-ball-shaped galaxy to the lower left of the image. In reality, interactions between galaxies happen over very long time periods, and galaxies rarely collide head-on with one another. It is also unclear whether the galaxy to the lower left is interacting with the other two, although they are so relatively close in space that it seems possible that they are."

Read more