e-Pulsar - Jun

e-Pulsar Jun - Jul 98

Astronomical Feature of the Month
NEBULAE

M42 Perhaps one of the most spectacular and popular sights in the sky are nebulae. The public has an endless fascination with tremendous clouds of colorful gas. Paintings of nebulae often draw the most discussion and interest. It may help your paintings to know them a little better. There are four primary types of nebulae: emission, reflection, dark, and planetary.

Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states, in much the same way as a neon light. These nebulae are usually red because the predominant emission line of hydrogen happens to be red. Other colors are produced by other atoms, but hydrogen is by far the most abundant. Emission nebulae are usually the sites of recent and ongoing star formation. (see M42 above)

Reflection nebulae are clouds of dust which are reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light, though they can have other colors. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.

Horsehead Dark nebulae are clouds of dust which block the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry between the light source, the cloud, and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across. (Horsehead Nebula at left)

Hourglass Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. They have nothing at all to do with planets; the term was invented because they often look a little like planets in the small telescopes of early astronomers. Recent Hubble Space Telescope pictures have revealed that planetary nebulae often have wild, radical shapes and emit light in many colors. A typical planetary nebula is less than one light-year across.

Data source: http://www.seds.org/billa/twn/types.html