Hoag's object facts for kids
Quick facts for kids Hoag's object |
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| Observation data (J2000 epoch) | |
| Constellation | Serpens Caput |
| Right ascension | 15h 17m 14.4s |
| Declination | +21° 35′ 08″ |
| Redshift | 12740±50 km/s |
| Distance | 600±30 Mly (183±8 Mpc) |
| Type | SA0(r) |
| Apparent dimensions (V) | 0′.28 × 0′.28 |
| Apparent magnitude (V) | 16.0 |
| Notable features | Ring galaxy |
| Other designations | |
| PGC 54559, PRC D-51 | |
| See also: Galaxy, List of galaxies | |
Hoag's Object is a very special type of galaxy called a ring galaxy. It was discovered in 1950 by an astronomer named Arthur Allen Hoag. At first, he wasn't sure if it was a planetary nebula (a cloud of gas and dust) or a truly unusual galaxy.
This amazing object has about eight billion stars! It is located about 600 million light-years away from us. You can find it in the night sky within the constellation Serpens.
Contents
What Makes Hoag's Object Special?
Hoag's Object looks like a perfect cosmic bullseye. It has a bright, round center and a distinct ring around it.
The Galaxy's Structure
The inner part of the galaxy is made of older, yellow stars. This central core is about 17±0.7 kly (17,000 light-years) wide.
Surrounding this core is a nearly perfect ring of young, hot, blue stars. This outer ring is huge! Its inner edge is about 75±3 kly (75,000 light-years) across. The outer edge stretches to about 121±4 kly (121,000 light-years) wide. That's even a bit bigger than our own Milky Way Galaxy!
There's a clear gap between the yellow center and the blue outer ring. This empty space might contain some faint star clusters that are very hard to see.
Another Galaxy in the Picture
Even more amazing, if you look closely at pictures of Hoag's Object, you can see another galaxy! This second galaxy is a reddish, unnamed ring galaxy. It appears right through Hoag's Object, between its center and outer ring. It's like looking through a window in space!
How Did Hoag's Object Form?
Scientists are still trying to figure out how Hoag's Object got its unique shape. It's a bit of a mystery!
Early Ideas About Its Formation
When Arthur Hoag first saw it, he thought the ring might be caused by something called gravitational lensing. This is when a massive object bends light from something behind it. However, scientists later found that the center and the ring of Hoag's Object are moving at the same speed away from us (they have the same redshift). Also, the ring has bumpy parts, which wouldn't happen with gravitational lensing. So, this idea was put aside.
The Collision Theory
Usually, "classic" ring galaxies form when a small galaxy crashes right through the center of a larger, flat, disk-shaped galaxy. This collision creates a wave that spreads out, forming the ring shape. This kind of event might have happened billions of years ago.
However, for Hoag's Object, there's no sign of the "bullet" galaxy that would have caused the crash. Also, the center of Hoag's Object moves very slowly compared to the ring. This makes the usual collision idea less likely for this particular galaxy.
Other Ideas and "Hoag-Type" Galaxies
One idea suggests that Hoag's Object might have formed from an extreme "bar instability" in a barred spiral galaxy. This means the central bar of a spiral galaxy might have stretched out and then collapsed to form the ring. But the center of Hoag's Object is round, not disk-shaped like a barred spiral galaxy's center. So, this idea is also debated.
A few other galaxies look somewhat similar to Hoag's Object, with bright, separate rings of stars. However, their centers are often stretched out or have bars, and they might even show some spiral arms. While none are as perfectly symmetrical as Hoag's Object, these few galaxies are sometimes called "Hoag-type galaxies." Scientists continue to study them to understand how these rare and beautiful cosmic rings are made.
See also
In Spanish: Objeto de Hoag para niños
