Hoag's Object // Georg N. Nyman

Hoag's Object is a peculiar ring galaxy consisting of a nearly perfect ring of young blue stars surrounding an older yellow nucleus. Below is the Hubble Space Telescope image of it.

The beautiful and mysterious galaxy known as Hoag's Object has a nucleus of older yellow stars surrounded by a ring of young blue stars separated by a gap spanning thousands of light-years.

📸: NASA​ /​ ESA​ /​ STScI

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Hoag's Object is an unusual ring galaxy in the constellation of Serpens Caput.

It is named after Arthur Hoag (1921-1999), who discovered it in 1950. He identified it as either a planetary nebula or a peculiar galaxy.

Hoag's Galaxy

600 million light years in the constellation of Serpens lies a very special galaxy, a spherical face on galaxy, with a clear separation from the central bulge, in what is often referred to as a ring galaxy.

What we do know is, it is part of the same structure, and that the outer part shows clear signs of blue stars, new stars forming, while the central section has a yellowed colour, indicating much older stars like our Sun that can live for billions of years before dying.

How this galaxy came to form is still the stuff of debate, assumptions go from an early merger or interaction that messed up a barred spiral, leaving the centre disconnected, to some unknown influence at the creation point.

2005 October 22

Ring Galaxy AM 0644-741 from Hubble

Credit : Hubble Heritage Team (AURA / STScI), J. Higdon (Cornell) ESA, NASA

Explanation: How could a galaxy become shaped like a ring? The rim of the blue galaxy pictured on the right is an immense ring-like structure 150,000 light years in diameter composed of newly formed, extremely bright, massive stars. That galaxy, AM 0644-741, is known as a ring galaxy and was caused by an immense galaxy collision. When galaxies collide, they pass through each other -- their individual stars rarely come into contact. The ring-like shape is the result of the gravitational disruption caused by an entire small intruder galaxy passing through a large one. When this happens, interstellar gas and dust become condensed, causing a wave of star formation to move out from the impact point like a ripple across the surface of a pond. The intruder galaxy has since moved out of the frame taken by the Hubble Space Telescope and released to commemorate the anniversary of Hubble's launch in 1990. Ring galaxy AM 0644-741 lies about 300 million light years away.

Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)

NASA Official: Jay Norris.

A service of: EUD at NASA / GSFC

Lindsay-Shapley Ring Galaxy

Astronomy's Most Perfect Ring Galaxy, Hoag's Object, Is Still A Mystery After 70 Years

“In a way, the mystery of Hoag's object only adds to its beauty. When we look at it in detail, we find that there's an older population of stars in the center, corresponding to an elliptical-like core that appears to have very little gas and no evidence of a central bar feature. Beyond that, there's a gap, with imperceptibly small amounts of gas and stars, lasting tens of thousands of light-years. And in the outskirts, a large halo of young stars persists, created by a mechanism that is not yet decided.

Perhaps there was a very low-speed collision that happened many billions of years ago, and this bizarre configuration is the only remaining clue. Perhaps there was gas or a series of small galaxies that got consumed by the larger elliptical galaxy, creating this outer ring. Or, perhaps, it's something else entirely, that will require more data and additional objects to fill in these gaps. Whatever the case, 70 years after its discovery, Hoag's object continues to mystify and astonish all those who investigate it.”

We know how many, maybe even most, of the Universe’s ring galaxies came into existence. For a majority of them, we see the same features: a central population of older stars with a shock wave of gas propagating outwards from a smaller galaxy colliding with it. That gas produces young, blue stars in a ring, or halo, around the central object. Over time, the gas goes back to being neutral, and only the stars remain as a visual sight.