What's interesting about the "Heart Nebula" is what lies in its own heart: an "open cluster" of newly formed super-hot stars.
Stars in "open clusters" are close enough to each other to be attracted by each others gravity. They are traveling through space as a group together.
Open clusters are formed when a dense cloud of gas, many thousand times the mass of our sun, collapses. The idea of a gas cloud collapsing is strange because we think of gas as lightweight and invisible.
The gas clouds used in star formation are thousands of times the mass of our sun. The gravitational pull within these clouds is staggering.
As the gas cloud collapses in on itself, fragments of the cloud splinter off into small clumps. These clumps are what we know as stars (stars are just balls of gas held together by gravity).
If the gas cloud is really big, it cannot collapse into a single star. Instead, it collapses into a "cluster" of stars. These clusters emit massive amounts of ultraviolet radiation.
This radiation interacts with the hydrogen gas of the surrounding cloud, forming the (now "ionized") hydrogen gas that we see in the photo (appearing in the photo as the color blue).
The "portal in the sky" look to the Heart Nebula is ionized hydrogen gas coming from a cluster of stars at its center.
This cluster is so important to the Heart Nebula that it has its own name: "Melotte 15."
The Heart Nebula is relatively close to us--just 7,500 light years away (for reference, the Andromeda Galaxy is 2.5 million light years away). The little appendage pictured at the bottom right of this image is the "Fishhead" nebula.
Full size image: https://www.astrobin.com/full/dtd21n/
