Don Savage
Headquarters, Washington, DC November 2, 1995
(Phone: 202/358-1547)
Fred Brown
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-5566)
Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone: 410/338-4514)
RELEASE: 95-190
EMBRYONIC STARS EMERGE FROM INTERSTELLAR "EGGS"
Dramatic new pictures from NASA's Hubble Space
Telescope show newborn stars emerging from dense, compact
pockets of interstellar gas called evaporating gaseous
globules (EGGs). Hubble found the "EGGs," appropriately
enough, in the Eagle nebula, a nearby star-forming region
7,000 light-years away in the constellation Serpens.
"For a long time astronomers have speculated about what
processes control the sizes of stars -- about why stars are
the sizes that they are," says Jeff Hester of Arizona State
University, Tempe. "Now we seem to be watching at least one
such process at work right in front of our eyes."
Pictures taken by Hester and co-investigators with
Hubble's Wide Field Planetary Camera-2 (WFPC2) resolve the
EGGs at the tip of finger-like features protruding from
monstrous columns of cold gas in the Eagle nebula (also
called M16 -- 16th object in the Messier column). The
columns -- dubbed "elephant trunks" -- protrude from the
wall of a vast cloud of molecular hydrogen, like stalagmites
rising above the floor of a cavern. Inside the gaseous
towers, which are light-years long, the interstellar gas is
dense enough to collapse under its own weight, forming young
stars that continue to grow as they accumulate more and more
mass from their surroundings.
Hubble gives a clear look at what happens as a torrent
of ultraviolet light from nearby young, hot stars heats the
gas along the surface of the pillars, "boiling it away" into
interstellar space -- a process called "photoevaporation."
The Hubble pictures show photoevaporating gas as ghostly
streamers flowing away from the columns. But not all of the
gas boils off at the same rate. The EGGs, which are denser
than their surroundings, are left behind after the gas
around them is gone.
"It's a bit like a wind storm in the desert," said
Hester. "As the wind blows away the lighter sand, heavier
rocks buried in the sand are uncovered. But in M16, instead
of rocks, the ultraviolet light is uncovering the denser
egg-like globules of gas that surround stars that were
forming inside the gigantic gas columns."
Some EGGs appear as nothing but tiny bumps on the
surface of the columns. Others have been uncovered more
completely, and now resemble "fingers" of gas protruding
from the larger cloud. (The fingers are gas that has been
protected from photoevaporation by the shadows of the EGGs).
Some EGGs have pinched off completely from the larger column
from which they emerged, and now look like teardrops in space.
By stringing together these pictures of EGGs caught at
different stages of being uncovered, Hester and his
colleagues from the Wide Field and Planetary Camera
Investigation Definition Team are getting an unprecedented
look at how stars and their surroundings appear before they
are truly stars.
"This is the first time that we have actually seen the
process of forming stars being uncovered by
photoevaporation," Hester emphasized. "In some ways it
seems more like archaeology than astronomy. The ultraviolet
light from nearby stars does the digging for us, and we
study what is unearthed."
"In a few cases we can see the stars in the EGGs
directly in the WFPC2 images," says Hester. "As soon as the
star in an EGG is exposed, the object looks something like
an ice cream cone, with a newly uncovered star playing the
role of the cherry on top."
Ultimately, photoevaporation inhibits the further
growth of the embryonic stars by dispersing the cloud of gas
they were "feeding" from. "We believe that the stars in M16
were continuing to grow as more and more gas fell onto them,
right up until the moment that they were cut off from that
surrounding material by photoevaporation," said Hester.
This process is markedly different from the process
that governs the sizes of stars forming in isolation. Some
astronomers believe that, left to its own devices, a star
will continue to grow until it nears the point where nuclear
fusion begins in its interior. When this happens, the star
begins to blow a strong "wind" that clears away the residual
material. Hubble has imaged this process in detail in so-
called Herbig-Haro objects.
Hester also speculated that photoevaporation might
actually inhibit the formation of planets around such stars.
"It is not at all clear from the new data that the stars in
M16 have reached the point where they have formed the disks
that go on to become solar systems," said Hester, "and if
these disks haven't formed yet, they never will."
Hester plans to use Hubble's high resolution to probe
other nearby star-forming regions to look for similar
structures. "Discoveries about the nature of the M16 EGGs
might lead astronomers to rethink some of their ideas about
the environments of stars forming in other regions, such as
the Orion Nebula," he predicted.
The Space Telescope Science Institute is operated by
the Association of Universities for Research in Astronomy,
Inc., for NASA, under contract with the Goddard Space Flight
Center, Greenbelt, MD. The Hubble Space Telescope is a
project of international cooperation between NASA and the
European Space Agency.
EDITOR'S NOTE: Three images depicting the dramatic pillars
in the Eagle Nebula and "EGGs" are available to news media
representatives by calling the Headquarters Imaging Branch
at 202/358-1900. NASA Photo Numbers are:
Color B&W
M16 3 Pillars 95-HC-631 95-H-643
M16 1 Pillar 95-HC-632 95-H-644
M16 B&W Detail 95-H-645
Image files in GIF and JPEG format and captions may be
accessed on the Internet via anonymous ftp from
ftp.stsci.edu in /pubinfo:
M16 3 Pillars gif/M16Full.gif jpeg/M16Full.jpg
M16 1 Pillar gif/M16WF2.gif jpeg/M16WF2.jpg
M16 B&W Detail gif/M16HaBW.gif jpeg/M16HaBW.jpg
Higher resolution versions (300 dpi JPEG) of the release
photographs will be available temporarily in
/pubinfo/hrtemp: 95-44a.jpg, 95-44b.jpg and 95-44c.jpg.
GIF and JPEG images, captions and press release text are
available via World Wide Web at URL
http://www.stsci.edu/pubinfo/PR95/44.html, or via links in:
http://www.stsci.edu/Latest.html and
http://www.stsci.edu/pubinfo/Pictures.html.
