How many nebulae are there in the universe

A star is born The life cycle of a star spans billions of years. All that glitters Some stars shine more brightly than others.

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Travel A road trip in Burgundy reveals far more than fine wine. Travel My Hometown In L. Travel The last artists crafting a Thai royal treasure. 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. Another popular favorite is the Lagoon Nebula, M8.

It is much larger than the Orion Nebula, reaching over light-years across the heavens. The Trifid Nebula, M20, is one of the most colorful. This reflection nebula contains a combination of elements that render it in rich hues of red, blue, and pink.

Dark lanes of dust divide it into three distinct parts, giving rise to its name. One of the most famous planetary nebulae is the Ring Nebula, M This is a beautiful object that resembles a circular rainbow around a small central star. Another popular planetary nebula is the Dumbbell Nebula, M Its unmistakable bow tie shape gives it its name.

The Crab Nebula, M1, is probably the best known supernova remnant. It is a shell of gas expelled by a supernova explosion. The lost mass is converted into energy, and this energy is released, providing the star's luminosity. Over billions of years, however, the residual helium in the star's core accumulates. When enough helium has accumulated, the helium can also undergo nuclear reaction. In this reaction, three helium atoms are converted into one carbon atom.

The helium-burning nuclear reaction can occur only when the star's interior reaches a higher temperature, and this higher temperature causes the star's outer surface to expand to a much larger size than it was while it remained on the main sequence. Even though the core of the star is much hotter, the surface is now cooler, making the star redder.

Thus, over time, a star becomes a red giant, moving from the main sequence area in the center of the H-R diagram to the red giant area in the upper right. The evolution from main sequence to red giant occurs at different times for different stars.

Stars that are much heavier and hotter, like O-stars, become red giants in only 10 million years. Cooler, lighter stars like our sun take 10 billion years to become red giants.

This fact actually provides a way of testing how old a group of stars is - jut make an H-R diagram for the stars, and see which classes of stars have evolved off the main sequence! Eventually, all the helium in the core of the star is used up. At this point, what happens next depends on the mass of the star. The heaviest stars, over six to eight times as massive as our sun, have enough pressure in their cores to start fusing carbon. Once carbon is gone, they explode as supernovae, leaving behind neutron stars or a black holes.

Less massive stars simply burn out, shedding their outer layers into beautiful planetary nebulae, and leaving the core as a hot white dwarf. White dwarfs lie in the lower left corner of the H-R diagram, a cosmic burial ground for dead stars.

An H-R diagram showing the evolutionary track of a sun-like star. Nebulae Originally, the word "nebula" referred to almost any extended astronomical object other than planets and comets. The word "nebula" comes from the Greek word for "cloud. Today, we reserve the word nebula for extended objects consisting mostly of gas and dust. Nebulae come in many shapes and sizes, and form in many ways.

In some nebulae, stars form out of large clouds of gas and dust; once some stars have formed inside the cloud, their light illuminates the cloud, making it visible to us. These star formation regions are sites of emission and reflection nebulae, like the famous Orion Nebula shown in the picture on the right. Emission nebulae are clouds of high temperature gas.

The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states neon lights glow in much the same way. Emission nebulae are usually red, because hydrogen, the most common gas in the universe, most commonly emits red light.

Reflection nebulae are clouds of dust that simply reflect the light of a nearby star or stars. Reflection nebulae are usually blue, because blue light scatters more easily. Emission and reflection nebulae are often seen together and are sometimes both referred to as diffuse nebulae.

In some nebulae, the star formation regions are so dense and thick that light cannot get through. Not surprisingly, these are called dark nebulae.