Press release images are available at http://eagle.la.asu.edu/hester/trifid.html

Highlights:

• HST observations of the Trifid Nebula offer a spectacular look at a region where radiation from a massive star is destroying a nursery for stars like our sun.
• A stellar jet emerging from the wall of the Trifid extends for three quarters of a light year into the hot, tenuous interior of the nebula. The jet is illuminated by light from the nebula's central star, allowing a first look at the structure of a stellar jet along its entire continuous length.
• The source of the jet will be uncovered and cut off from the material it is accreting sometime in the next 20,000 years or so. A dense finger protruding from the wall of the dark cloud contains a possible young stellar object that has already suffered this fate.

Spectacular new Hubble Space Telescope images of the Trifid Nebula show a region of ongoing star formation as it is being torn apart by radiation from the star that powers the nebula. A stellar jet streaming outward from the walls of the nebula is a sure sign that the star at its source is still growing. But the jet's days - and the star's time in its womb - are numbered. Within the next 20,000 years or so the star's natal environment will be destroyed by the radiation from its powerful neighbor. A possible second young star sits atop a stalk protruding into the nebula's interior - a former victim of the fate that awaits the star at the source of the jet. The images show a beautiful example of how the life cycle of stars like our sun are intimately connected with their more powerful siblings.

In astronomical circles, stars like our own sun are considered runts in comparison to the massive, powerful stars that light up spectacular nebulae like Orion and the Eagle. When a massive star turns on, it shines with tens of thousands or even hundreds of thousands of times more power than the sun. Much of this energy comes pouring out in the form of ultraviolet light - light capable of tearing electrons away from their parent atoms and heating the interstellar gas surrounding the star to twice the temperature of the surface of the sun. When this happens, the result is like a bomb going off. The bubble of hot, high-pressure gas pushes its way outward through the "dense" interstellar cloud from which the star formed. When the expanding bubble of hot gas finally bursts free from the interstellar cloud, it leaves behind a wound - a cavity or bowl in the side of the natal cloud. Imagine a bubble bursting on the surface of a "mud pot" in Yellowstone National Park and you will not be far wrong in your mental picture of how the blister breaks free from the cloud.

Yet even after the bubble has burst, radiation from the massive star continues to blast the walls of the cavity it carved, eating away at its surroundings like a blow torch carving away at a block of dry ice. The material streaming away from the cloud wall is ionized by the radiation from the star, and glows in much the same way that the gas inside of a fluorescent light bulb glows. The advancing front of destruction moves outward from the star at a snail's pace - only about 2,000 miles per hour. Even so, over the millions of years that the massive star lives, the bubble that it carves grows to be many light years in size.

Several years ago we were using the Hubble Space Telescope to study the nature of the advancing "ionization front" in the Eagle Nebula, and were amazed to discover something unexpected. As the radiation from the massive stars ate away at the stars' surroundings, small dense globules of gas were being uncovered and then evaporated by the intense ultraviolet light. Inside some of these "evaporating gaseous globules," or EGGs for short, there appeared to be young stellar objects. We realized that we were seeing a stellar nursery as it was being destroyed.

As a continuation of our earlier work on the Eagle, we have now used the Hubble Space Telescope to look at the Trifid Nebula. Located about 9,000 light years from the earth, the Trifid is another region where radiation from a massive star is reshaping that star's environment. The HST images show a small part of the cavity wall, located about 8 light years away from the nebula's central star. Images were taken through filters that isolate emission from hydrogen atoms, ionized sulfur atoms, and doubly ionized oxygen atoms. The images were combined in a single color composite image. While the resulting picture is not true color, it is suggestive of what a sensitive human eye might see.

The new images show part of the interstellar cloud as it is being destroyed by radiation from the star. But at the same time, the new images show unmistakable evidence of the stars that are forming within the ill-fated cloud.

A jet extending three quarters of a light year out into the interior of the nebula points back to a very young stellar object that lies buried within the cloud. Jets such as this are ephemeral features - interstellar "contrails" - the exhaust gases of star formation. The HST observations of this jet (which was designated HH 399 following its independent discovery last year by a group of European astronomers) are newsworthy in their own right. Most jets from young stellar objects are illuminated from within, glowing only where violent collisions heat things up enough for the gas to glow. Pictures of such jets tell us a great deal, but leave us wondering what we are missing. When such a jet "ends" does that mean there is no jet there, or only that the jet is no longer glowing? It is often difficult to tell.

In contrast, the jet seen in the Trifid is lit up from outside. Radiation from the massive star at the center of the nebula causes the gas in the jet to glow, just as it causes the rest of the nebula to glow. It is as if someone turned on the lights, showing us the jet in its full glory.

A ship passing in the night might slip by unnoticed if it shines no light, but during the daytime it is seen by all. In most stellar jets the gas in the jet cools within about 100 years of the time it left the source. Such jets seem to gradually fade from view along their length, remaining invisible until they hit some dense gas much farther downstream. Taking into account how rapidly jet material normally cools, if it were not for the light from the Trifid's central star probably less than 20% of the length of the jet would be seen.

The story that the jet in the Trifid tells is fascinating. For the last 600 years the source of the jet has spit out a knot of material roughly once every 15 years. During that time the source has also wobbled like a top once every 250 years, leading to the jet's corkscrew appearance. Perhaps most significantly, when the jet "turned on" 600 years ago it did so in a sudden violent outburst. A second knot of emission much further out in the nebula (out of the field of view of the HST images) indicates that the young stellar object underwent a similar outburst about 3,000 years ago.

The jet in the Trifid is a ticker tape telling the history of one particular young stellar object that is continuing to grow as its gravity draws in gas from its surroundings. But this particular ticker tape will not run for much longer. Within the next 10,000 years or so the advancing ionization front at the edge of the nebula will overrun the forming star, bringing its growth to an abrupt and possibly premature end.

Another nearby star may have already faced this fate. The HST images show a "stalk" pointing from the cloud directly back toward the star that powers the Trifid. This stalk is a prominent example of the evaporating gaseous globules, or "EGGs," that were seen previously in the Eagle Nebula. The stalk has survived because at its tip there is knot of gas that is dense enough to resist begin dispersed by the powerful radiation. This dense knot cast a shadow, protecting the gas behind it as the destruction swept past.

The tip of this EGG is especially fascinating. Reflected starlight at the tip of the EGG may be due to light from the Trifid's central star, but it may also be due to a young stellar object buried within the EGG. Similarly, a tiny spike of emission pointing outward from the EGG looks a great deal like a stellar "microjet." While confirmation of a young stellar object in this EGG awaits infrared observations later this summer, the HST data are very suggestive.

If our interpretation is correct, the microjet may be the last gasp from a star that was cut off from its supply lines 100,000 years ago.

The vast majority of stars like our sun form not in isolation, but in the neighborhood of massive, powerful stars. HST observations of the Trifid Nebula provide a window on the nature of star formation in the vicinity of massive stars, as well as a spectacular snapshot of the "ecology" from which stars like our sun emerge.

Collaborators:
Paul Scowen, Arizona State University
Karl Stapelfeldt, Jet Propulsion Laboratory
John Krist, Space Telescope Science Institute
HST Wide Field Planetary Camera 2 Investigation Definition Team

Images available on the web site:

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Hubble Space Telescope image of a portion of the Trifid Nebula. Located about 9,000 light years from the earth, the Trifid is one of the most prominent nebulae in the night sky. Radiation from the powerful central star is eating away at the surrounding dense interstellar material. The field of view of this HST image includes a region of star formation that will be destroyed by the advancing ionization front in the next 20,000 years or so. This image is a composite of three separate exposures. Red shows emission from singly ionized sulfur atoms. Green shows emission from hydrogen. Blue shows emission from doubly ionized oxygen atoms.

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Blowup of the HST image of the head of a dense cloud in the Trifid Nebula. A prominent jet from a young stellar object and a long finger with a possible young stellar object at its tip are apparent in the image.

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A stellar jet emerging from the wall of a cloud in the Trifid Nebula. The jet is remarkable because, unlike most stellar jets, it can be seen along its entire length. This is because the jet is being lit up by radiation from the massive, luminous star that powers the Trifid.
 

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The tip of a finger-like evaporating gaseous globule, or "EGG", pointing back at the Trifid's central star. A tiny jet emerging from the EGG and a patch of reflected light suggest that a young stellar object is buried in the tip of the jet. If we are correct in our interpretation, this young stellar object was uncovered a few tens of thousands of years ago as radiation from the Trifid's central star disrupted the dense cloud from with the star formed.

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A ground based image of the Trifid Nebula taken with the Palomar 1.5-m telescope. The Hubble Space Telescope WFPC2 field of view is shown.

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The ground based image of the Trifid Nebula without the annotation in the preceding figures.