ISM, Page 61

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An Introduction to Nebula

Gas in the Interstellar Medium (ISM)
Of the gas in the Milky Way, 90% by mass is hydrogen, with the remainder mostly helium. Approximately 99% of the mass of the interstellar medium is in the form of gas with the remainder primarily in dust. The gas appears primarily in two forms

Cold clouds of atomic or molecular hydrogen
Hot ionized hydrogen near hot young stars

Different Kinds of Nebulae:

1) Dark Nebula: The dark nebulae are clumps or clouds that become opaque because of their internal dust grains. The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way.

2) Emission Nebula: The HII regions (emission nebulae) are composed mostly of a plasma of ionized hydrogen (HII) and free electrons, with trace levels of other chemicals. The hydrogen atoms of the interstellar medium are ionized by the ultraviolet radiation from a nearby very hot stars (typically young stars). As the hydrogen ions de-excite to lower energy levels, they emit their characteristic spectral lines. The most prominent of these in the visible spectrum is the red line of hydrogen, giving most emission nebulae a characteristic red glow. Green is from ionized oxygen, and green is from nitrogen.

3) Reflection Nebula: These nebula are visible because of light from nearby bright stars scattering off of dust particles. The nearby star or stars are not hot enough to cause ionization in the gas of the nebula but are bright enough to give sufficient scattering to make the dust visible. Since wavelengths in the blue are scattered more readily than red, reflection nebula tend to have a blue shading.

4) Planetary Nebula: Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. Our Sun will probably produce a planetary nebula in about 5 billion years. They have nothing at all to do with planets; the terminology was invented because they often look a little like planets in small telescopes. A typical planetary nebula is less than one light-year across.

1) Dark Nebula: The dark nebulae are clumps or clouds that become opaque because of their internal dust grains. The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way.
2) Emission Nebula: The HII regions (emission nebulae) are composed mostly of a plasma of ionized hydrogen (HII) and free electrons, with trace levels of other chemicals. The hydrogen atoms of the interstellar medium are ionized by the ultraviolet radiation from a nearby very hot stars (typically young stars). As the hydrogen ions de-excite to lower energy levels, they emit their characteristic spectral lines. The most prominent of these in the visible spectrum is the red line of hydrogen, giving most emission nebulae a characteristic red glow. Green is from ionized oxygen, and green is from nitrogen.
3) Reflection Nebula: These nebula are visible because of light from nearby bright stars scattering off of dust particles. The nearby star or stars are not hot enough to cause ionization in the gas of the nebula but are bright enough to give sufficient scattering to make the dust visible. Since wavelengths in the blue are scattered more readily than red, reflection nebula tend to have a blue shading.
4) Planetary Nebula: Planetary nebulae are shells of gas thrown out by some stars near the end of their lives. Our Sun will probably produce a planetary nebula in about 5 billion years. They have nothing at all to do with planets; the terminology was invented because they often look a little like planets in small telescopes. A typical planetary nebula is less than one light-year across.