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An overview of the nebula in astronomy

A schematic plot of the H-R diagram. Stellar Evolution Stars are not static objects. As a star consumes fuel in its nuclear reactions, its structure and composition changes, affecting its color and luminosity. Thus, the H-R diagram not only shows us the colors and luminosities of many stars, it shows these stars at different stages in their evolutionary histories. All stars on the main sequence have interiors hot enough fuse four hydrogen atoms into one helium atom, and this one helium atom is 0.

Star Colors and Luminosities: The H-R Diagram

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.

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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. 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.

What is a nebula?

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.

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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 an overview of the nebula in astronomy 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. Another type of nebula, called a planetary nebula, results from the death of a star. When a star has burned through so much material that it can no longer sustain its own fusion reactions, the star's gravity causes it to collapse.

As the star collapses, its interior heats up. The heating of the interior produces a stellar wind that lasts for a few thousand years and blows away the outer layers of the star. When the outer layers have blown away, the remaining core remnant heats the gases, which are now far from the star, and causes them to glow. The resulting "planetary nebulae" so named because they look like gas giant planets through a telescope are shells of glowing gas that surround a small core.

Astronomers estimate that our galaxy contains about 10,000 planetary nebulae. Planetary nebulae are a common part of the normal stellar life cycle, but they are short-lived, lasting only about 25,000 years.

The life of a star whose mass is greater than 1. When such a star runs out of fuel and collapses, an enormous shock wave sweeps through the star at high speed, blasting away various layers and leaving behind a core called a neutron star and an expanding shell of matter known as a supernova remnant. A supernova's shock wave is much more violent than the stellar wind that marks the end of a low mass star.

Near the core of the remnant, electrons emit radiation called "synchrotron radiation" as they spiral toward the neutron star at speeds close to the speed of light. The ultraviolet portion of this radiation can strip electrons off, or "ionize" the outer filaments of the nebula, causing them to glow. The most famous supernova remnant is the Crab Nebula in Taurus M1shown in the image above.

The light of the inner core is from synchrotron radiation, while the outer regions glow in many colors from emission of many gases, including red for hydrogen.

Absorption due to methane CH4 is evident. Some stars fizzle out before their evolutionary life cycle begins; these failed stars are called brown dwarfs. Brown dwarfs are balls of gas not heavy enough for fusion reactions to ignite in their cores, so their energy output comes only from gravity. Although their existence has been predicted by theory for a long time, they are so cool, dark, and hard to see that the first one was discovered only five years ago.

The brown dwarfs with the lowest masses are actually an overview of the nebula in astronomy similar to Jupiter, showing absorption due to methane in their spectra. The last two letters in the temperature classification system for stars, L and T, have been added recently to include brown dwarfs.

The SDSS, in combination with near-infrared follow-up studies, has been found many brown dwarfs because it covers a large area of sky, it can see quite dim objects, and it has a filter in the red part of the spectrum z'. Brown dwarfs are interesting for two reasons: Second, brown dwarfs may compose some of the missing mass, or "dark matter" in our galaxy.