Visible-light images from NASA’s Hubble Space Telescope have revealed a stellar incubator in the Eagle nebula 7000 light years distant in the constellation Serpens. The spacecraft’s Wide Field and Planetary Camera 2 found dense, compact pockets of interstellar molecular hydrogen (H2) termed evaporating gaseous globules (EGGs). The EGGs are found at the tips of the finger-like features extending from the huge columns of cold gas and dust of the nebula.
Inside these vast trunks, the hydrogen is dense enough to collapse under its own gravitational attraction, forming stars that continue growing by accumulating more and more mass from their surroundings. Ultraviolet light from nearby hot, young stars heats the “surface” of these pillar-like gas fingers, causing it to photoevaporate and “boil” away looking like veiled streamers flowing out from the columns.
Not all the gas boils off at the same rate. Thus the denser EGGs are left behind after the gas around them is gone. Arizona State University (Tempe, AZ) investigator Jeff Hester likens the process to a desert wind storm that blows away the lighter sand exposing buried, heavier rocks. “Instead of rocks, the ultraviolet light is uncovering the denser globules of gas that surround stars that were forming inside the gigantic gas columns,” says Hester.
Some of these EGGs are seen as bumps on the column surface. Others, uncovered more, are at the ends of the gas fingers. The fingers result from regions of gas being shielded from ultraviolet light by the shadow of the EGG. And some of the EGG structures have been pinched off from the column from which they were born to form teardrop shapes. Eventually photoevaporation mitigates growth of the stellar embryos by dispersing the gas cloud feeding their formation.
Observing these different evolutionary stages allows an unprecedented look at what emerging stars and their surroundings look like before they form mature stars. Hester says, “It seems more like archaeology than astronomy. The ultraviolet light does the digging for us, and we study what is unearthed.” The process seen is different from that governing growth of stars that form in isolation. Here, it is theorized, a star will grow, accreting surrounding gas, until nuclear fusion begins in its interior. The resulting wind of solar particles and light streaming from the star sweep away any residual surrounding material.
In the images, the ultraviolet light illuminates the convoluted surface of the columns and gas streamers. The images were recorded in three atomic-emission colors: red from singly ionized sulfur, green from hydrogen, and blue from doubly ionized oxygen.