Nebula vs Galaxy
Nebulae and galaxies are deep sky celestial objects which can only be seen clearly with the aid of a telescope. With the naked eye or low powered telescopes both types of objects can be seen as fuzzy patches in the night sky. Therefore in the early developmental stages of astronomy confusions existed and in some instances they are carried even today.
Nebula
Nebulae are large collections of interstellar gas and dust particles. Most nebulae can be interpreted as a denser region of the interstellar medium accreting under gravity; others are remnants of stars after the end of their lifetime. They mainly consist of hydrogen and helium. But other elements may also be included in smaller but varying amounts. If the nebula is located near a highly active astronomical objects such as young stars and other form of radiation sources, the gasses in the nebulae may become ionized.
Nebulae are often observed as bright patches in the night sky. They appear in many colors and shapes, often leading to their commonly used names (not astronomical designations) such as Cat’s Eye, Ant, California, Horse Head and Eagle nebulae.
The three main categories of nebulae are emission nebulae, dark nebulae, and reflection nebulae. Emission nebulae are interstellar gas clouds with characteristic emission line spectrum. An energy source, such as hot young stars and accretion disks of black holes, ionize the dense interstellar medium around them, and the excited gasses emit radiation in different wavelengths. We observe this region as a nebula. The Orion nebula is a classic example of an emission nebula; it is the third apparent star in the sword of Orion, The Hunter. The Orion nebula spans .5° in the night sky and lies about 1500 light years away. It contains about 300 solar masses of material, and it is a region with young O and B type stars born within the nebula. These young stars cause the gasses to glow. Four visible bright stars embedded inside the nebula is known as the Trapezium.
Dark nebulae are dense gas clouds that do not emit radiation in the visible frequencies, but they are silhouetted in bright regions of space, making them observable. Horse head nebula and Bernard 86 are examples of dark nebulae. Reflection nebula scatters and reflects light from nearby stars and do not emit light. NGC 6726 and NGC 2023 are reflection nebulae.
Nebulae are closely related to the lifecycle of stars. Stars are created (born) within nebulae. A nebula or a gaseous region contracts to form a protostar. After the start of nuclear fusion, it again emits some mass into surroundings creating a protoplanetary nebula. After a star finishes its life with a supernova, the outer gaseous layers are shot into the surrounding space. Again the remnants are visible as a nebula, often called a planetary nebula.
Galaxy
Galaxies are massive collections of stars and large interstellar gas clouds. These large superstructures of stars were not identified and studied properly until the late 18th and 19th centuries. Then these were considered as nebulae. These collections of stars lie beyond the vicinity of Milky Way, which is our collection of stars. Therefore, it is difficult to distinguish between a galaxy and nebula with the naked eye or a small telescope. Majority of the objects in the night sky belongs to our galaxy, but if you observe closely, you can identify the twin galaxy of the Milky Way, The Andromeda Galaxy.
Edwin Hubble made an extensive study of the galaxies and classified those based on their shape and structure and categorized them. The two main categories of the galaxies were spiral and the elliptical galaxies. Based on the shape of the spiral arms, Spiral galaxies were further classified into two sub-categories as Spiral galaxies (S) and Barred Spiral galaxies (Sb).
Spiral galaxies have spiral arms with a central bulge. The center of the galaxy has a very high star density and appears bright with bulge extending above and below the galactic plane. Spiral arms are also regions with the higher star density, which is why these regions are visible as bright winding lines. The interstellar medium in these regions is illuminated by the energy of the stars. The darker areas also contain interstellar medium, but the star density is low to illuminate these regions, making them appear darker than the other areas. In general, spiral galaxies contain roughly 109 to 1011 solar masses and have luminosity between 108 and 2×1010 solar luminosity. The diameter of the spiral galaxies can vary from 5 kiloparsecs to 250 kiloparsecs.
Elliptical galaxies have the characteristic oval shape in their outer perimeter and any formation such as spiral arms are not visible. Even though elliptical galaxies display no internal structure, they also have a denser nucleus. Roughly 20% of the galaxies in the universe are elliptical galaxies. An elliptical galaxy may contain 105 to 1013 solar masses and may create luminosity between 3×105 to 1011 solar luminosities. The diameter may range from 1 kiloparsec to 200 kiloparsecs. An elliptical galaxy contains a mixture of Population I and Population II stars within the body.
What is the difference between Nebula and Galaxy?
• Dense regions in the interstellar medium that are distinguishable from the surrounding region is known as a nebula.
• Galaxies are large structures of stars and star clusters bound by gravity. They also contain interstellar medium, which gives rise to nebula.