Part 12: Interstellar Atoms, Dust and Nebulae
John P. Pratt
All space contains some matter
- Although space is a better "vacuum" than we can create on earth, it still contains some matter.
- Most space is nearly perfectly transparent, but there are still minor effects on light passing through it.
- Denser areas of such matter can appear cloud-like, and are called nebulae (plural of nebula, latin for "cloud").
Instellar Matter Affects Light
- Interstellar atoms can absorb light and then re-radiate it as emission lines at the set wavelengths for that atom.
- An observer looking through a cloud of those atoms at a star will see absorption lines at that wavelength.
- An observer looking at the cloud, but not directly at the star, will see emission lines at that wavelength.
- Interstellar molecules (two or more atoms combined) give rise to longer wavelength absorption or emission
bands rather than lines.
- Interstellar dust grains tend to redden the light passing through them, like a reddened sunset.
- Dense clouds of atoms and molecules can also redden a beam of light much more than dust, and
cause the gas to appear blue.
- This is why the sky is blue, and the sunset red, even on totally clear, dustless days.
- This is called Rayleigh scattering.
- Dust pollution in the air causes the sky to be much less blue because it is caused by dust scattering.
- Thus, interstellar reddening means that starlight looks redder, usually because of dust particles.
Observations of Interstellar Matter
- Some atomic absorption lines from interstellar atoms have different Doppler shifts from the stars.
- Some forbidden emission lines are seen
- Electrons cannot spontaneously drop between certain electron levels in atoms.
- Electrons can make those forbidden transitions by collisions with other atoms.
- Such forbidden emission lines of oxygen were once thought to be from a new element "nebulium."
- Hydrogen emits a 21 cm radio emission line when its electron's spin becomes opposite from its proton's spin.
- This line was predicted before it was ever observed.
- This 21 cm line has been used to map the great clouds of hydrogen gas throughout the galaxy.
- Many interstellar molecules have been discovered.
- By 1940 the first molecules were discovered.
- During the 1960s - 1970s many more were found, including complicated amino acids
- Interstellar Grains tend to polarize light (make if vibrate in preferred directions).
- The obscure enough star light to make the sky dark at night.
- A lot of the grains contain carbon, thought to have been created in the interiors of stars.
There are four types of interstellar regions
- Molecular clouds are the large cold clouds of gas and dust which can condense into stars.
- H I regions are clouds of neutral hydrogen gas.
- H II regions are clouds of ionized hydrogen gas.
- They are usually found around hot O and B stars.
- Such stars give off enough ultraviolet light to strip the electron away from the Hydrogen atoms.
- They are characterized visually by the red H-alpha emission lines, form by recombining electrons.
- Superbubbles of extremely hot interstellar matter caused by the explosion of supernovae.
- Messier was a comet hunter who made a list of cloud-like nuisance objects that were not comets.
- His life's work on comets has been forgotten, but his list of over 100 "Messier objects" is famous.
- Some of those objects are gaseous nebulae, most are star clusters or galaxies.
- They are often named for their shape, such as the Crab Nebula (M 1, meaning Messier's first object ).
The Orion Nebula
- It is the middle "star" in the sword of Orion.
- It is thought to be a classic example of a region of star formation.
- It contains many massive O and B stars (like Orion's belt), thought to be very young.
- There are also dark nebulae in the area (dark clouds seen silhouetted against a bright background),
such as the Horsehead Nebula.
Other Nebulae Worth Knowing which are all in the summer sky.
- Three beautiful nebulae are in Sagittarius, looking toward the center of our Milky Way galaxy.
- The Lagoon Nebula (M8) at the end of the handle of the "Milk Dipper" in Sagittarius in the
summer sky is nearly as bright as the Orion Nebula.
- The Trifid Nebula (M20) is just north of the Lagoon Nebula, nearly in same low power telescope
field. (Trifid means "three-lobed).
- The Horseshoe Nebula (M17) is at the northmost point of Sagittarius, nearly to Scutum.
- The Dumbbell Nebula (M27) above the point of the arrow Sagitta (between Aquila and Cygnus) is
named for its shape.
- The Ring Nebula (M57) in Lyra is a planetary nebula appears in small telescopes like a little smoke ring.