One of the joys of astronomy is the opportunity to view the spectacular, alien objects strewn throughout the universe. Nebulae are one of the more awe inspiring of these. The term nebulae means 'cloud' but in astronomy it can refer to several different objects. Most distant astronomical objects appear cloudy though small telescopes and in the past astronomers called any seemingly cloudy object a nebula, after ruling out that it wasn't a comet. For example, galaxies were called nebulae because they looked like fuzzy spiral blobs from here on Earth. The term nebulae is a bit dated to refer to galaxies, given the differences between distant star systems and the much smaller clouds of gas that populate the interstellar medium. Today, we generally use the term to refer to true interstellar clouds. Nebulae come in several different forms. There are ionization nebulae(also called emission nebulae), reflection nebulae, and solar nebulae.
A solar nebulae is an area of interstellar gas created by star formation. Our own solar system likely originated in a solar nebulae, according to the Nebular Hypothesis. Solar nebulae begin as large clouds of cold, low density gas. once gravity brings the gas cloud closer together, collapse begins. As the solar nebula shrinks in size, it begins to heat up, spin, and flatten. This creates an orderly solar system like the one we live in today.
Ionization Nebulae (Emission Nebulae)
Regions of active star formation can be markedly picturesque. Colorful, wispy blobs of glowing gas can be found near hot stars. These are known as ionization, or emission, nebulae. They glow because ultra-violet photons from the nearby hot stars raise their electrons to high energy levels (ionization) and the atoms emit light as the electrons return to lower levels. The Orion Nebula is among the most famous ionization nebula. It is located in the sword of the constellation Orion. Ionization nebulae appear predominantly red because of the red photons released bu the ionization of abundant hydrogen atoms.
Reflection nebulae also occur in star forming regions. They have blue and black tints, making them dark nebulae, because starlight reflects from dust grains in the gas cloud, and dust scatters more blue light than red light. The light from the nearby star is insufficient, in the case of reflection nebulae, to ionize the gas, as in ionization nebulae, but produces enough scattering to make the dust visible. One of the brightest reflection nebulae in the sky is M78, located in an outer spiral arm of our galaxy.
Nebulae are just another example of how beautiful and complex our vast universe is. We can thank technology and the dedication of scientists, engineers, and even amateur stargazers for allowing us the opportunity to view these and other fantastic celestial phenomena.