GALACTIC NEBULAE
Presented by Mohammad Nehal
MSC PHYSICS 4th SEM
REG ID- 21UMPY09
This Photo by Unknown Author is licensed under CC BY
CHAPTER 12
INTRODUCTION
Welcome to the fascinating world of Galactic Nebulae! This ppt will take you on a journey through the vast regions of space between the stars, where interstellar matter plays a crucial role in the birth of new stars. Discover the importance of ionized and neutral gas, interstellar dust, and cosmic rays in the cosmic recycling process.
Interstellar dust consists of minute solid particles of undetermined structure and composition that mingle with gas in the vast interstellar space.�
New stars are born in regions of large local concentrations of interstellar matter.
NEBULAE
This Photo by Unknown Author is licensed under CC BY-NC-ND
This Photo by Unknown Author is licensed under CC BY-NC-ND
This Photo by Unknown Author is licensed under CC BY-SA
This Photo by Unknown Author is licensed under CC BY-SA
A nebula is a giant cloud of dust and gas in space. Some nebulae (more than one nebula) come from the gas and dust thrown out by the explosion of a dying star, such as a supernova. Other nebulae are regions where new stars are beginning to form.
How do stars form in a nebula?
Nebulae are made of dust and gases—mostly hydrogen and helium. The dust and gases in a nebula are very spread out, but gravity can slowly begin to pull together clumps of dust and gas. As these clumps get bigger and bigger, their gravity gets stronger and stronger.
Eventually, the clump of dust and gas gets so big that it collapses from its own gravity. The collapse causes the material at the center of the cloud to heat up-and this hot core is the beginning of a star
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Where are these nebulae?
Nebulae exist in the space between the stars—also known as interstellar space. The closest known nebula to Earth is called the Helix Nebula. It is the remnant of a dying star—possibly one like the Sun. It is approximately 700 light-years away from Earth. That means even if you could travel at the speed of light, it would still take you 700 years to get there!
Classification and Galactic Distribution of Nebulae
Early observers could not distinguish between a luminous gaseous cloud belonging to our Milky Way and a far-off external stellar system like the Andromeda galaxy because both appear alike when viewed through a telescope.
The true nature of the latter objects was known mainly through the work of Edwin P. Hubble in the 1920s when he discovered large red-shifts of the spectral lines in their spectrum.
The large red-shifts at once revealed that these apparently nebulous objects must be hundreds of kiloparsecs or even many megaparsecs away.
This led to the classification of nebulae into two main types: extragalactic nebulae (also known as galaxies) and galactic nebulae (also known as interstellar nebulae). Galactic nebulae are further classified into diffuse and planetary nebulae based on their appearance and physical properties.
Observational Techniques
Early observers could not distinguish between a luminous gaseous cloud belonging to our Milky Way and a far-off external stellar system like the Andromeda galaxy because both appear alike when viewed through a telescope.
The true nature of the latter objects was known mainly through the work of Edwin P. Hubble in the 1920s when he discovered large red-shifts of the spectral lines in their spectrum.
The large red-shifts at once revealed that these apparently nebulous objects must be hundreds of kiloparsecs or even many megaparsecs away.
This led to the classification of nebulae into two main types: extragalactic nebulae (also known as galaxies) and galactic nebulae (also known as interstellar nebulae). Galactic nebulae are further classified into diffuse and planetary nebulae based on their appearance and physical properties.
Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology.
Hubble proved that many objects previously thought to be clouds of dust and gas and classified as "nebulae" were actually galaxies beyond the Milky Way.
Hubble's law, also known as the Hubble–Lemaitre law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving away from Earth. The velocity of the galaxies has been determined by their redshift, a shift of the light they emit toward the red end of the visible spectrum.
Dark Nebulae
Dark nebulae are clouds of dust and gas that are so dense that they obscure the light from stars behind them, making them appear dark.
Additionally, normal nebulae are often associated with star-forming regions and contain young hot stars that ionize the surrounding gas, whereas dark nebulae can also be associated with star-forming regions but do not contain any visible young hot stars due to their obscuring nature.
However, determining the area of dark nebulae is a challenging task because it is sensitive to the limiting magnitude accessible as well as to the wavelength sensitivity of photographic plates.
The outer parts of dark nebulae are transparent to red light, which means that areas determined from red plates are systematically smaller than those determined from blue plates.
This is because blue light has a shorter wavelength than red light and can penetrate deeper into the cloud before being scattered or absorbed by dust particles. Therefore, astronomers must take into account these factors when studying dark nebulae in order to accurately determine their properties and understand their role in interstellar space.�
Observation of Dark Nebulae
They do not emit any visible light themselves but can be detected by their absorption of background starlight.
Dark nebulae do not emit any visible light themselves, but they can be detected by their absorption of background starlight.
When the light from a background star passes through a dark nebula, some of it is absorbed by the dust particles in the cloud, making the star appear dimmer and redder than it would otherwise. This effect is known as interstellar extinction or reddening and can be used to study the properties of dark nebulae.
By measuring the amount of extinction at different wavelengths, astronomers can estimate the size, density, and composition of the dust particles in the cloud and infer its physical properties.
More on Dark Nebulae
Reflection Nebulae
Reflection nebulae are a type of nebula that can only reflect the light received from associated stars, and the reflection is produced by the dust particles associated with the nebula. These nebulae appear blue because they preferentially scatter blue light more than red light
In other words, reflection nebulae are illuminated by nearby stars, which cause them to reflect some of their light and appear as glowing clouds in space. Unlike emission nebulae, which emit their own light due to ionization by hot stars, reflection nebulae do not emit any visible light themselves but instead reflect the light from nearby stars.
Diffuse Emission Nebulae: Theory of Emission Lines
The theory of emission lines in nebulae is well understood and has been explained by the early works of several astronomers. The results based on theoretical predictions clearly explain the observed physical characteristics in these nebulae.
Two different mechanisms are known by which the observed emission lines are formed.
One mechanism is called collisional excitation, which occurs when electrons collide with atoms or ions in a gas cloud and transfer energy to them, causing them to emit light at specific wavelengths.
The other mechanism is called photoionization, which occurs when high-energy photons from hot stars or other sources ionize atoms or molecules in a gas cloud, causing them to emit light at specific wavelengths.
Diffuse emission nebulae are a type of nebula that results from random distributions of galactic gas and dust and do not have any regular structure. They are illuminated by nearby stars or groups of stars lying close to or embedded in them, which cause them to emit light due to ionization by hot stars.
Planetary Nebulae
Planetary nebulae is described as emission nebulae of fairly regular shapes, each having an extremely hot blue star at the center called the nucleus of the nebula.
In telescopic view, a planetary nebula appears as a circular or oval disc having finite size and well-defined boundary, thus imparting it the appearance of a planet.
The name planetary nebula was derived from this similarity. Planetary nebulae are different from diffuse emission nebulae in that they have a more regular shape and are often ring-shaped or disc-shaped.
They are formed when a low-mass star like our Sun reaches the end of its life and expels its outer layers into space, leaving behind a hot, dense core called a white dwarf. The ejected material forms a shell around the white dwarf that is ionized by its intense ultraviolet radiation, causing it to emit light at specific wavelengths and creating the characteristic colors and shapes of planetary nebulae. Planetary nebulae can have complex structures including intersecting rings and wispy projections, but they all share the common feature of having a central star or nucleus that powers their emissio
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