Credit: NASA and The Hubble Heritage Team ( STScI / AURA )
Bow shock around the very young star LL Ori.
Stars form in dense molecular clouds within galaxies. These clouds of dust and gas obscure the early stages of stellar formation from optical telescopes. Fortunately recent advances in radio and infrared astronomy now allow astronomers to peer inside these clouds and gain a greater understanding of the processes involved in starbirth. Computationally intensive computer simulations also allow them to model the processes and test the results against observations.
Gravity is the force responsible for stellar formation and the mass of material that forms at star largely determines its life and fate.
Nebulae are simply clouds of interstellar gas and dust and appear either as dark regions blotting out background stars - the so-called dark or absorption nebulae or as brighter clouds of gas that emit or reflect light. They are the most visible components of the interstellar medium.
Despite what you might think, space is not a perfect vacuum. The space between the stars is filled with a tenuous range of material that provides the building blocks of stars. This material is gas and dust and collectively is known as the interstellar medium ( ISM ). The ISM gas is predominantly hydrogen whilst the dust is about 1% by mass and includes carbon compounds and silicates. Dust is responsible for the interstellar reddening and extinction of starlight. The more of the ISM a star's light travels through on its way to an observer on Earth the more it gets scattered and absorbed, decreasing the star's apparent brightness and reddening its appearance.
Properties of the ISM vary widely depending upon its location within a galaxy. At its most tenuous, in hot regions between denser clouds, it may have a density of only 100 particles per cubic metre, mostly ionised hydrogen atoms. In the inner regions of shells of gas surrounding stars the density can be as high as 10 17 particles per m 3 although this is still a million times less dense than a normal vacuum on Earth.
Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Dark dust lanes and bright HII regions in the spiral arms of M51, the Whirlpool Galaxy.
Stars form in regions of the ISM where there is sufficient material available. These are the giant molecular clouds or GMCs . The GMCs contain large amounts of matter; 10 5 to 2 × 10
6 solar masses from 12 to 120 pc across and are very cold, typically only 10 -30 K. Their main constituent is molecular hydrogen gas but other molecules such as water, carbon monoxide, CO, ammonia, NH
3
and methanol, CH
3
OH are also present. Although denser than most of the ISM they still only have a few hundred hydrogen molecules per cubic centimeter making them
10 17
× less dense than our atmosphere. The dust within these clouds is sufficient to block out starlight from stars and thus also keeps the cloud temperature very cold. Dust grains are roughly 30 nm in size although they may also have a mantle of ice or frozen ammonia on them, increasing their size tenfold. One of our best methods of studying these clouds is by the characteristic emissions from CO molecules at mm wavelengths in the radio waveband.
In spiral galaxies such as our own Milky Way the GMCs tend to be concentrated along the spiral arms and can be seen as the dark dust bands in the arms. We see some of the closer dense molecular clouds as absorption or dark nebulae .
Examples include the Coalsack Nebula in Crux and the famous Horsehead Nebula in Orion. Dark nebulae are often alongside hotter regions where the radiation from nearby stars is already heating the gas so that it glows. These are emission regions.

Credit: NASA , NOAO, ESA and The Hubble Heritage Team ( STScI / AURA )
The Horsehead Nebula, a dense molecular cloud (dark nebula) in the Orion region. It is in front of an emission nebula, an HII region
An emission nebula is a cloud of dust and gas that shines due to the emission of its own light. Gas is these nebulae is hot enough to be ionised and the emitted light is due to interactions between the free electrons and the ions and to electron transitions within ions. Actually there are three main types of emission nebulae; HII regions associated with starbirth regions such as M42, the Great Nebula in Orion, planetary nebulae associated with the death of lower-mass stars and supernova remnants (SNRs). These later two are discussed elsewhere. Here we focus on the HII starbirth regions.
M42, the Great Nebula in Orion. This image is about 32 arc minutes across. The nebula is 460 parsecs distant and is the closest HII region to us. It is a giant stellar nursery.
In astronomy, HI refers to neutral hydrogen whilst HII is ionized hydrogen. HII regions generally display distinctive red and green colours. .The red is caused by hydrogen-alpha emission and forbidden transitions of singly-ionized nitrogen. OIII, doubly-ionized oxygen ions in which both the outer electrons have been lost, produces a green emission due to forbidden transitions. This process only takes place if photons have high enough energy such as the UV photons emitted from hot O and B stars within or close to the nebula.
Credit: Jeff Hester and Paul Scowen (Arizona State University), and NASA
"M16, the Eagle Nebula shows newborn stars emerging from "eggs" - not the barnyard variety - but rather, dense, compact pockets of interstellar gas called evaporating gaseous globules (EGGs). The EGGs are at the tip of finger-like features protruding from monstrous columns of cold gas and dust in the Eagle Nebula. The columns - dubbed "elephant trunks" - protrude from the wall of a vast cloud of molecular hydrogen, like stalagmites rising above the floor of a cavern. Inside the gaseous towers, which are light-years long, the interstellar gas is dense enough to collapse under its own weight, forming young stars that continue to grow as they accumulate more and more mass from their surroundings." NASA press release
The intense radiation released by these hot, young stars eventually sweeps away the dust and gas, leaving the stars exposed.
Credit: NASA and The Hubble Heritage Team ( STScI )
Reflection nebula, NGC 1999 in Orion.
Reflection nebulae are much less dense than emission nebulae. They appear bluish due to the dust grains preferentially scattering blue light from nearby hot stars. Our sky appears blue for much the same reason, blue light from the Sun is scattered in all directions. The scattering of light also polarises it, a fact astronomers utilise to study the properties of the dust.
Different types of nebulae are often found close together. Just as absorption and emission nebulae may be associated, the regions further out from hot stars may appear as reflection nebulae. Clusters of stars that are still relatively young may be old enough to have dispersed most of the surrounding dust and gas but still have traces left as a reflection nebula. A classic example of this is the blue nebulosity seen around the Pleiades .