What makes nebulae

In essence, a nebula is formed when portions of the interstellar medium undergo gravitational collapse. Mutual gravitational attraction causes matter to clump together, forming regions of greater and greater density. Most nebulae are vast in size, measuring up to hundreds of light years in diameter. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created in an Earthen environment. In fact, a nebular cloud that was similar in size to Earth would only so much material that its mass would be only a few kilograms.

Stellar objects that can be called Nebula come in four major classes. Most fall into the category of Diffuse Nebulae , which means they have no well-defined boundaries. Emission Nebulae are those that emit spectral line radiation from ionized gas, and are often called HII regions because they are largely composed of ionized hydrogen.

In contrast, Reflection Nebulae do not emit significant amounts of visible light, but are still luminous because they reflect the light from nearby stars.

There are also what is known as Dark Nebulae , opaque clouds that do not emit visible radiation and are not illuminated by stars, but block light from luminous objects behind them. Much like Emission and Reflection Nebulae, Dark Nebulae are sources of infrared emissions, chiefly due to the presence of dust within them.

Some nebulae are formed as the result of supernova explosions, and are hence classified as a Supernova Remnant Nebulae. In this case, short-lived stars experience implosion in their cores and blow off their external layers. Other nebulae may form as Planetary Nebulae , which involves a low-mass star entering the final stage of its life.

In this scenario, stars enter their Red Giant phase, slowly losing their outer layers due to helium flashes in their interior. When the star has lost enough material, its temperature increases and the UV radiation it emits ionizes the surrounding material it has thrown off. During the Asymptotic Giant Branch AGB phase, the star undergoes mass loss, emitting a circumstellar shell of hydrogen gas.

When this phase comes to an end, the star enters the PPN phase, where it is energized by a central star, causing it to emit strong infrared radiation and become a reflection nebula. The PPN phase continues until the central star reaches a temperature of 30, K, after which it is hot enough to ionize the surrounding gas. Many nebulous objects were noticed in the night sky by astronomers during Classical Antiquity and the Middle Ages.

The first recorded observation took place in CE, when Ptolemy noted the presence of five stars in Almagast that appeared nebulous in his book. Supernova Remnant - Supernova remnants are created when a star ends it life in a massive explosion known as a supernova.

The explosion blows a large amount of the star's matter out into space. This cloud of matter glows with the remains of the star that created it. One of the best examples of a supernova remnant is the crab Nebula M1 in Taurus. It is illuminated by a pulsar which was created by the supernova. Nebulae are often the sites of star formation. In fact, all stars, planets, and solar systems are formed from nebulae. A nebula may lie undisturbed for many millions or billions of years as it waits for just the right conditions.

Eventually, the gravity from a passing star or the shock wave from a nearby supernova explosion may cause swirls and ripples within the cloud. Matter begins to coalesce into clumps and grow in size. As these clumps get larger, their gravity increases. Gravity continues to pull in matter from the nebula until one or more of the clumps reach critical mass.

The clumps are forming protostars. As gravity squeezes even tighter, the core temperature eventually reaches 18 million degrees. At this point, nuclear fusion begins and a star is born. The solar wind from the star will eventually blow away all of the excess dust and gas. Sometimes other smaller clumps of matter around the star may form planets. This is the beginning of a new solar system.

Several nebulae have been found to be stellar nurseries. The Eagle Nebula, and the Orion Nebula are both sites of active star formation. There are a few nebulae that can be seen with the naked eye and many more that can be detected with a good pair of binoculars.

A telescope is required to bring our fine details. Unfortunately, the human eye is not sensitive enough to bring out the rich colors of most nebulae. It is the photograph that does the most justice to these incredible objects. Until recently, time exposures on film were the best way to bring a nebula's true colors. Today, digital photography has simplified the process. New tools like the Hubble space telescope are giving us views of nebulae that have never been seen before.

Areas of active star formation have been identified in many galaxies that were once thought to be inert. Perhaps the most well known nebulae is the Orion Nebula, also known as M It is one of the very few that can be seen with the naked eye.

It is a bright emission nebula over 30 light-years in diameter. The nebula is illuminated by a group of stars at its center known as the trapezium. They are sometimes considered nurseries, because oftentimes they are the location where stars are created and born. Nebulae, which is the plural form more than one of nebula, form when gas, like hydrogen and helium, as well as molecules, particles, ions, and electrons are pulled together by gravity.

Some of the gas and dust particles group together to create common regions that have similar densities. In areas that become more dense, stars can form out of the collapsing material. There are a few different kinds of nebula formations. Many nebulae have no clear beginning or end, and no clear boundaries.

These are diffuse nebulae. Other kinds of nebulae are formed out of explosions of dying stars, and these are called supernova remnant nebulae. Another kind of nebula, a planetary nebula, is created with material lost from a red giant phase of a dying star.