Anne’s Image of the Day: Herbig–Haro objects HH1 and HH2


March 29, 2014

HH1 and HH2, gas jets from a proto-star in Orion

HH1 and HH2

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

Herbig–Haro objects HH1 (left) and HH2 (right) are gas jets from a proto-star, located about 1,500 light-years away from Earth in the equatorial constellation of Orion (the Hunter), near the Orion Nebula. The jets have a speed of 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 (HHs) – named after astronomers George Herbig and Guillermo Haro who studied the outflows in the 1950s – are such narrow, high-speed jets of gas and matter ejected by young stars, 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, symmetrically opposite a young star which is ejecting material along its polar axis, but is hidden from view behind a dark cloud of dust. The nearly symmetrical 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.

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 the jets offer clues to events which also occurred in our own Solar System, 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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