Anne’s Picture of the Day: Stellar Gas Jets HH1 and HH2

March 16, 2013

HH1 and HH2, stellar gas jets in Orion

HH1, HH2

Image Credit: J. Hester (Arizona State University), the WFPC 2 Team, and NASA

Herbig–Haro objects HH1 (left) and HH2 are gas jets from a proto-star that lies about 1,500 light-years away in the constellation of Orion. Each of these jets span slightly more than a light-year and are roughly ten times the width of the Solar System. They zip along at more than 700,000 kilometers per hour.

A star forms from a collapsing cloud of cold hydrogen gas. As the star grows, it gravitationally attracts more matter, creating a large spinning disk of gas and dust around it. The disk material gradually spirals onto the star and escapes as high velocity jets along the star’s axis of spin. The high-speed jets may initially be confined to narrow beams by the star’s powerful magnetic field. The jet phase stops when the disk runs out of material, usually a few million years after the star’s birth.

Herbig–Haro objects (HH) – named after astronomers George Herbig and Guillermo Haro who studied the outflows in the 1950s – are such narrow jets of gas and matter ejected by young stars at speeds of 100 to 1000 kilometers per second that collide with interstellar gas, producing bright shock fronts that glow as the gas is heated by friction while the surrounding gas is excited by the high-energy radiation of nearby hot stars.

These objects are transient phenomena, lasting only about 100,000 years. They can evolve visibly over quite short timescales as they move rapidly away from their parent star into the gas clouds in interstellar space. They are ubiquitous in star-forming regions, and several are often seen around a single star, aligned along its rotational axis.

HH1 and HH2 lie about a light-year apart, nearly symmetrically opposite the young star (hidden from view behind a cloud of obscuring dust) which is ejecting material along its polar axis. The blobs of gas at either end are where the jet has slammed into interstellar gas.

The arrowhead structure at the end of HH2, just like the more irregular structure at the top of the jet in HH1, are bowshock patterns produced when high-speed material encounters a slower-speed medium.

In HH1 the bowshock appears to be grazing at the edge of a dense gas cloud. Glowing knots of material may represent gas from the cloud being swept up by the jet, just as a swift-flowing river pulls along mud from the shoreline.

A region near the star in HH2 reveals a string of glowing clumps of gas, ejected by the star in machine-gun like bursts, what provides clues to the dynamics of the star formation process. The jets are ejected from a whirlpool of gas and dust orbiting the young star.

The structures and details visible in these star jets offer clues to events which also occurred when the Sun was formed from a collapsing interstellar cloud 4.5 billion years ago.

This image is taken with the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope.

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