A Hubble Telescope image of the spectacular Pillars of Creation, part of the Eagle Nebula.
- Nebulae are clouds of gas and dust which form in interstellar space.
- They range in size from millions of miles across to hundreds of light years across.
- Nebulas are mostly composed of hydrogen along with smaller amounts of other gases and dust particles.
- Stars and planetary systems are formed from the gas and dust within nebulae.
- The Orion Nebula is the closest star forming region to Earth, it lies 1,300 light years away and is thought to be around 25 light years across.
- Nebulae which exist outside the Milky Way are known as extra-galactic nebulae.
- Up until the 1920's distant galaxies were classified as nebulae.
- Some nebulae can be seen in the night sky with the naked eye.
- In 5 billion years our Sun will become a planetary nebula.
- The word nebula is derived from the Latin word for cloud, the plural of nebula is nebulae, although nebulas is also used.
Nebulae - Gems of the Universe
Image of the aptly named Butterfly Nebula captured by the Hubble Space Telescope.
Nebulae come in an extraordinary variety of shapes, colors and sizes making them some of the most spectacular and fascinating objects in our Universe.
Thanks to the modern techniques of astrophotography the breath taking beauty of these clouds of gas and dust are being revealed.
The names they are given often reflect the familiar shapes they form into, such as the Butterfly Nebula, Lemon Slice Nebula or North America Nebula.
Nebulae are not just a pretty face, they provide the material and environments from which stars are born, forming the basis from which entire planetary systems can take shape,
such as our own solar system.
The abundance of hydrogen gas within nebulae provides the platform from which stars are created and the tiny dust particles eventually form into orbiting bodies around the new stars such as planets and asteroids.
When those very same stars die the process will continue as the matter they eject into interstellar space will form into new nebulae.
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 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 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 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 the hydrogen in their core and instead begin fusing hydrogen in an outer shell surrounding the core. This results in them expanding to hundreds of times their original size and becoming red giants. Eventually the core heats up enough to begin fusing helium, when the helium runs out the star becomes unstable and its outer layers are 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 form into a nebula.
The Interstellar Medium
The dark nebula known as Barnard 68 blocking out light from the background stars.
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 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.
A massive star forming region in an orbiting galaxy of the Milky Way
These dark molecular clouds of gas described above 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 such a process produced a yellow dwarf star which was located several thousand light years from the center of our galaxy. 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 planet was our very own Earth and through the process of evolution these conditions ultimately allowed life to flourish on our world.