Link: Eagle Nebula

This is arguably the most striking astronomical picture ever made. An image made even more powerful when you realize it is as breathtaking in content as it is in beauty.

To see it is to be a witness to stellar creation.

The tall pillar-like structures are clouds of dust and molecular hydrogen gas. Hydrogen can exist in this state only at relatively low temperatures. When it is bound in this way, hydrogen does not usually give off light. That is why the lower parts of the cloud appear dark. But off the top of the picture are hot stars flooding the nebula with high energy ultraviolet (UV) radiation. This UV light is boiling off the molecular cloud by a mechanism known as photoevaporation. In the process it causes the heated gas to radiate, which is why we see an eerie blue haze around the tops of the pillars.     Just as ultraviolet (UV) rays from our Sun cause skin cancer, they can break apart molecules and atoms in interstellar space. As gas from the clouds is boiled away, knots of denser gas are uncovered in much the same way that rocks are uncovered by the erosion of a stream. These knots were formed when the gas in parts of the cloud started collapsing under its own gravity. They have been named Evaporating Gaseous Globules, or EGGs. It is a particularly fitting name, because inside some EGGs a star is being born.

Due to gravitational attraction, EGGs keep accumulating matter and growing like the goldfish that doesn't know when to stop eating. When these globules of gas get dense enough, they become protostars as they start to shine under their own power. What stops the globules from growing and thus limits the masses of the stars that are formed?

As the traditional theory goes, when fusion reactions start in the cores of these young stars, they are thought to blow out a "stellar wind" which sweeps away any gas that did not make it into the star. It is like the goldfish has burped and the force has blown the rest of his food away. The stellar wind is the gas blown off of the outside layers of a star. This process is believed to be behind the jets of Herbig-Haro objects.

But this picture of the Eagle Nebula is standing traditional theories of star formation on their head. A different process seems to be at work here. As the EGGs are uncovered by UV radiation, the material in the shadow, between the knot of gas and the pillar, is often protected from photoevaporation. This gives the appearence of "fingers" of gas rising from the pillars.

In some cases these fingers have broken off from the main pillars of gas. Now isolated, there is only a limited amount of gas for the EGG to turn into a star. The EGGs can no longer draw from the huge reservoir of gas in the pillars. It is like a goldfish that can't get to the rest of its food in the can beside its tank, and so has to grow smaller than it otherwise would.

This new theory of stellar evolution has also sparked a debate over the nature of the "proplyds" in the Orion Nebula. Are they protoplanetary disks, or EGGs seen face on? Is this how most stars are formed, or only a tiny fraction of them? Astronomers are still sorting out the answers to these exciting questions made possible by the Hubble Space Telescope.