Adam Frank for McGraw-Hill

When I was a kid I read a lot of comic books. Much to my parents horror I spurned the classics like Great Expectations for X-men and the Swamp Thing (hey, is it my fault that none of Dickens's characters had super powers?). One of the coolest things you could encounter as comics fan was the so-called "Origin Story". This was the comic book where they revealed how the superhero came to posses his amazing strength, x-ray vision or bulletproof underwear. There is a special fascination for Origin Stories that extends beyond comic books to almost all aspects of human activity. From the special episode of Friends were we learn how all the characters ended up sharing the same apartment to my daughters insistence on being told her the tale of her birth again and again, we all want to know how things began.

Astronomy is no different. There are many stories to tell of the heavens - exploding supernova, colliding asteroids, cosmic rays crashing through the atmosphere. Stories of beginnings however seem to hold a special interest. That is why so much of the world sat up and took notice last month when NASA astronomers showed us all a new chapter in one of the most compelling of all cosmic origin stories: the birth of the stars themselves.

Astronomers think they understand how stars are born. Every so often a chunk of a giant interstellar cloud of gas and dust falls victim to its own gravity. The inward pull of gravity grabs a small section of the cloud (lets call it a cloudlet) and collapses it in on itself. Matter rains down on the cloudlet's center and slowly a proto-stellar core is assembled from the infalling gas. As more matter rains down on the newly formed core, the pressure and temperature at its center increase until nuclear reactions begin. Once the nuclear flame is lit the lights go on in earnest and a new star is born. Energy generated by the nuclear engine at the young star's core give it support against its own ever-increasing gravity. Material continues to fall down onto the star for as long as a million years until the surrounding cloudlet's reservoir of gas is depleted.

This basic storyline for star formation has been around for a long time. For more than 70 years astronomers expected that when they finally saw a new star forming they would see gas falling inward. Imagine their surprise when their telescopes finally became powerful enough to actually observe an infant star and what they saw was gas spewing outward!

Astronomers now have had to admit the startling fact that almost every new born star ejects narrow streams of hypersonic gas known as "stellar jets" or "bipolar outflows" (bipolar means along two poles). These jets and outflows are astonishing for their size and power. The jets often travel at speeds of more than 500 kilometers per second (that's more than a million miles per hour) while the outflows can span more than a light-year and contain as much mass as 100 suns. In spite of their size and power, these strange beasts appeared to astronomers only as fuzzy streaks when seen on images taken with older ground based telescopes. Then the Hubble Space Telescope was rocketed into orbit and everything changed. Instead of fuzz, astronomers were able to see the jets from young stars as they really were - long beautiful chains of brightly glowing gossamer knots. The HST pictures revolutionized the study of stellar jets by giving astronomers their first detailed snapshots of the fireworks attending the birth of a star.

Of course, as every young parents will tell you, its far better to have a movie than a snapshot and that is exactly NASA astronomers now have for their stellar jets. XZ Tau and HH30 are two newly born stars that sport dramatic outflows. By accumulating HST images over the last five years and stringing them together a NASA team from the Jet Propulsion Lab in Pasadena has just created two short movies showing the evolution of a jet (HH30) and a wider bubble-like outflow (XZ Tau). It is nothing short of astonishing that, in spite of the hundreds of light-years that separates Earth from these young stars, we are able to watch in detail as knots of gas are shot into space racing up the jet's beam like water from a firehose.

These movies put a whole new twist on our understanding of how stars form. By animating the amazing HST images we see a cosmic Origin Story unfolding right before our eyes. No one yet knows why stars create their jets and outflows. That is a story that still needs to be written. XZ Tau and HH30 are, however, telling us something important right now, even if it's only the importance of wonder. Log on to the HST webpage and enjoy the show.

HST Jets Webpage
http://oposite.stsci.edu/pubinfo/PR/2000/32/index.html