The Interstellar Medium

Chapter 14

Is There Anything Between the Stars?

• The answer is yes! And that “stuff” forms some of the most beautiful objects in all of space!

• The dust and gas that exists between the stars is called the interstellar medium (ISM). Sometimes these form clouds of various densities and temperatures, each with different properties.

Nebulae

• The densest regions of the dust and gas are called nebulae (singular = nebula), which comes from the Latin word for mist or cloud.

• There are 3 types of nebulae that we will study.

1. Emission Nebulae

• Emission nebulae are produced by clouds of gas surrounding hot stars. The star must be hotter than about 25,000 K (B1) so that it is able to emit enough UV radiation to ionize the gas. Most “glow” in pink due to the blending of the red, blue, and violet H Balmer lines. Photons emitted.

Another Name

• Emission nubulae are also called HII regions because of the ionized gas contained in them. Remember: I means neutral, while II means singly ionized. HII regions contain a lot of ionized hydrogen. As “thick” as these clouds might appear, their densities are only 100 – 1000 atoms per cm3 (equivalent to the best vacuums on Earth).

Ex. Lagoon Nebula (M 8)

Note the pink hue!

The Famous Orion Nebula (M 42) The emission

nebula is in “red”; we will talk about the blue nebulae (NGC 1973, 1975, and 1977) next! Note “Trapezium” where new stars are forming!

2. Reflection Nebulae

• A reflection nebula contains a large quantity of dust (and gas, but not excited enough to glow). These clouds get their name because they reflect (technically scatter) light from this dust. We would see an absorption spectrum.

Characteristics

• A reflection nebula tends to appear blue for the same reason that our sky appears blue—shorter (B) wavelengths scatter more easily than longer ones (R). Dust particles have diameters ~10-6 to 10-9 m.

• Blue nebulae in prior slide were reflection nebulae.

Ex. Trifid Nebula (M 20)

The image contains an emission and a reflection nebula. Which one is which?

The Pleiades (M 45)

The hottest star, Merope, is a B3 star, which is not hot enough to ionize the gas. Therefore, M 45 is a reflection nebula instead of an emission.

Merope

3. Dark Nebulae• Dark nebulae are cool, dense clouds

of dust and gas that obstruct our view of more distant stars. They appear as dark regions surrounded by stars or nebulosity.

• Typically, the reason we even see dark nebulae is because our view of brighter, more distant objects, is blocked.

• Knots and twists in these clouds suggests that there are “winds” that push through the ISM.

Ex. Coalsack Dark Nebula

Horsehead Nebula

Note how the horsehead is silhouetted against a more distant emission nebula.

Two Effects of the ISM• The ISM is not confined to these

“dense” clouds, but is spread through-out regions that might seem empty.

• One effect of this is called interstellar extinction, or dimming, of light from more distant stars.

• Extinction dims the apparent magnitude of stars 1000 pc away by 1.9 magnitudes—each 1000 pc! A star 2000 pc away would be dimmed by 3.8 mags, etc.

Another Effect• Hot, blue stars might appear redder

than they should be due to inter-stellar reddening, caused by the scattering of light (particularly blue light) from small dust particles.

• To find the amount of reddening, two stars of the same spectral type are compared.

• The ISM is composed mainly of carbon, silicates, iron, and ice.

Observing the ISM• Much of the ISM is invisible to the

human eye, but can be seen in the infrared part of the spectrum. The IRAS space telescope, launched in 1983, discovered thin “cirrus clouds” of dust throughout space, but mostly near the plane of our galaxy. Temperature ~30K. The Spitzer Space Telescope now does most of the observing in IR.

One Special Wavelength• HI (neutral H) consists of 1 proton

and 1 electron. Both particles spin and have magnetic N/S poles. If the poles have an opposite orientation [attractive, lower E], then both particles will be tighter bound than the case where they have the same orientation [repulsive, higher E].

Continued• When an electron drops from the higher

to lower energy level, a photon with a wavelength of 21 cm (Radio) is emitted. This is called 21-cm Radiation and has been detected. This helps us map regions of neutral H clouds.