Types of Nebulae

Emission nebulae emit their own light due its star forming regions. The ultra violet light emitted by clusters of large, young stars ionize the surrounding gas into various colors. The light emitted will depend on the type of gas, many emission nebulae appear red due to the presence of large amounts of hydrogen gas.

Dark nebulae are simply dense areas of gas and dust that block out any light coming from behind. They are often seen in conjunction with emission or reflection nebulae. Very large dark nebulae can often be seen with the naked eye as dark patches against the brighter areas of the Milky Way. The famous Horsehead Nebula (pictured left) is a dark nebula.

As the name suggests reflection nebulas do not emit their own light, they simply reflect light from a nearby star. The energy emitted by the nearby star or stars is not sufficient to ionize the surrounding gas, so we don’t get the range of colors emitted by other nebulae. Instead the light is reflected off the dust present in the nebula so it is similar to the visible light given off by stars.

Planetary nebulae are formed when stars similar to the size of the Sun use up all of their hydrogen and begin to fuse helium causing them to expand to hundreds of times their original size. When the helium runs out the star becomes unstable and its mass is ejected leaving only the core remaining in the form of a white dwarf. The radiation emitted by the white dwarf ionizes the surrounding gas, producing spectacular color displays.

Supernova remnants are formed when large stars come to the end of their lifespan. When these stars become unstable a violent explosion rips them apart scattering matter in all directions at incredible speeds. The energy created by a supernova is enormous, causing it to briefly outshine its own galaxy. The material ejected will eventually cool down and form into a nebula.

Nebulae and Star Formation

The Interstellar Medium

The space between stars is not ‘empty space’ it actually contains gas and dust that makes up around 15% of the visible mass in the Milky Way. This is known as the Interstellar Medium or ISM. Even though matter in the ISM is abundant it is highly dispersed over vast areas, it is in fact far more dispersed than any vacuum that can be created on Earth. The ISM has an average density of only 1 atom for every cubic centimeter, compare that to Earth’s atmosphere which contains a 100 billion billion atoms per cubic centimeter.

The vast majority of the ISM is composed of the most abundant gas in the Universe, hydrogen, which makes up around 90% of its mass, around 9% is composed of other gases, mostly helium. The remaining 1% of the ISM is made up of dust particles, this is not like the dust you will find in your home, these tiny particles are only 1 to 20 millionth of a centimeter in size.

The nature of gravity means that over time these materials will be attracted to one another, forming into clumps of matter which in turn will join other clumps of matter eventually forming into huge dark nebula clouds that can be seen against the bright backdrop of the bright Milky Way (picture above). These nebulae can eventually give birth to stars, planets and moons such as we see in our own solar system.

Stellar Nurseries

These dark molecular clouds of gas can become stellar nurseries, providing an environment which allows stars to be born. When pockets of the dark nebula have sufficient density, hydrogen molecules will begin to condense, this is possibly triggered by the shockwaves of a nearby supernova explosion. Clumps of hydrogen will grow large enough to begin collapsing under their own weight, heating up and creating the early stage of a star called a Protostar. Over thousands of years the young star continues to grow and heat up until its core is hot enough to allow nuclear fusion, this is when hydrogen atoms are fused together producing enormous amounts of energy. The star is now in its main sequence and will remain in this state for most of its life.

Around 5 billion years ago this process began at the edge of the Milky Way, eventually producing a yellow dwarf star. Out of the large disk of gas and dust that formed around the new star a small rocky planet was created which was in just the right location to allow water to flow on its surface. This was Planet Earth, these conditions allowed life to flourish ultimately leading to the evolution of mankind.

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Images & Video

The bizarre looking Elephant's Trunk Nebula

3D view of the Carina Nebula

The Cone Nebula

Posters From Solar System Quick