Astronomy Notes

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 Objects

• 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.