A nebula, in simple terms, is a cloud of gas and dust in space. In the past, with less advanced optical telescopes, astronomers identified and named some nebulae that were actually entire galaxies. So how are nebulae understood today? How many types of nebulae have been identified?
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(Photo: akhtarnama) |
For example, the Andromeda Nebula (M31), commonly referred to as the Andromeda Nebula, is not actually a nebula by the above definition but rather a galaxy. The Andromeda Galaxy is the largest galaxy in our local group of galaxies, while our Milky Way Galaxy is the second largest, and the third is another galaxy often mistaken for a nebula – M33.
According to modern definitions, nebulae can be clouds of dust that have gathered due to gravitational attraction (with insufficient mass to form a star or a larger celestial body), as mentioned, or they can be material expelled from the end stages of a star’s life (such as novae and supernovae).
Types of Nebulae
Diffuse Nebula
This is the most common type of nebula. A diffuse nebula consists of gas and dust clouds with no clear boundaries, and they are usually classified into two types: emission nebulae and reflection nebulae.
- Emission Nebula: This type of nebula is composed of gas and dust that becomes ionized and emits light when near large stars due to strong excitation. The temperature at the core of these nebulae can reach 8000 – 10000K, with diameters ranging from tens to hundreds of light-years. Some well-known examples include the Orion Nebula (M42) and the Eagle Nebula (M16).
Eagle Nebula/M16.
- Reflection Nebula: These nebulae gain their light from reflecting light from nearby stars. This type of nebula consists of gas and dust that can reflect light well. The light from reflection nebulae is not as intense as that from emission nebulae; however, reflection nebulae display a continuous spectrum (since the light is reflected), while emission nebulae have spectral lines due to stimulated emission.
Pleiades Nebula/M45.
Some nebulae consist of gas and dust that are so dense and opaque that they can obscure the light from background stars or other nebulae (like the Horsehead Nebula in the constellation Orion) or block background stars (like the Coal Sack Nebula in the Southern Cross constellation). In the sky, they appear as dark silhouettes since the light from stars is largely absorbed by the gas, and these are referred to as dark nebulae.
The Horsehead Nebula in Orion is a dark nebula.
Planetary Nebula and Supernova
Planetary Nebula or planetary cloud refers to a type of luminous nebula consisting of an ionized gas shell produced by red giant stars in their late stages. This phenomenon is relatively short-lived, occurring over a few tens of thousands of years compared to the typical lifespan of a star, which is billions of years.
Supernova, or supernova explosion, refers to a type of stellar explosion that results in extremely bright objects primarily composed of plasma, which flares up for a short time, with brightness increasing suddenly by billions of times, then gradually decreasing over weeks or months. The total energy released can reach up to 1044 Joules. Absolute magnitudes can reach -20m.
These stars sustain themselves by consuming internal nuclear fusion energy to create a force that balances gravitational forces. When there is nothing left to consume, gravity pulls all material into the core, causing the star to collapse and end in a supernova explosion. This explosion has immense heat, burning all material and creating super-hot, bright plasma streams. Depending on their brightness, they are recorded as novae or supernovae.
For massive stars, the material expelled into space disperses, forming a large cloud of gas and dust surrounding the star, emitting radiation in the X-ray and strong radio bands, known as the remnant of a supernova explosion.
The Crab Nebula/M1.
Solar Nebula
The Solar Nebula is the cloud of gas from which our Solar System is believed to have formed. This solar nebula hypothesis was first proposed in 1734 by Emanuel Swedenborg. A similar model was presented by Pierre-Simon Laplace in 1796.
Formation of the Solar System.
With the help of amateur telescopes or binoculars, you can observe some of the identified nebulae.
Images of Nebulae
The Monkey Head Nebula is nearly round and located at the edge of a giant molecular cloud in the constellation Monoceros. This nebula is approximately 5200 light-years away from the Sun.
The Wizard Nebula is in the constellation Cepheus, approximately 7000 light-years from Earth. This nebula was discovered by Caroline Herschel in 1787. Her brother, William Herschel, discovered infrared light in 1800.
The Hourglass Nebula is a young planetary nebula located in the constellation Musca, approximately 8000 light-years from Earth.
The Tarantula Nebula is located in the Large Magellanic Cloud, approximately 180,000 light-years away. If the Tarantula Nebula were only 1500 light-years away (like the Orion Nebula), it would cover about half of the Earth’s sky.
The Boomerang Nebula is the coldest place in the universe. In 1995, using a large Swedish telescope located in Chile, astronomers discovered it has a temperature of -272 degrees Celsius, only 1 degree warmer than absolute zero (the lowest limit of temperature).
The California Nebula is named so because this cosmic cloud coincidentally resembles the shape of California’s map on the western coast of the United States and is about 1000 light-years away.
The Heart Nebula is located in the constellation Cassiopeia, approximately 7500 light-years from Earth.
The Boomerang Nebula is the coldest nebula in the universe, while the Jellyfish Nebula is 10,000 times hotter than the Sun and is about 5000 light-years away. According to scientists, this is what remains from fireballs ejected by a supernova thousands of years ago.
The Horseshoe Nebula is formed by a layer of gas and dust surrounding a very bright star at its center. One theory suggests this nebula was formed about 250,000 years ago.
