1. The Interstellar Medium
Chapter 18
The Interstellar Medium

2. The Interstellar Medium
Occupies the space among the stars
Is made up of:
cold gas , mostly hydrogen and helium
Dust grains (size ~ 10-7 m - 103 x larger than gas particles)

3. Interstellar Matter

4. Starlight passing through a dusty region of space is both dimmed and reddened, but spectral lines are still recognizable in the light that reaches Earth

5. The dimming of the starlight by dust is called “Extinction”
The blocking of the shorter wavelength (blue) light and transmission of the longer wavelength (Red) light gives the star a redder appearance. This is called “reddening”

6. Typical interstellar dust particle.

7. Dust grains are generally elongated rather than spherical.
Astronomers know this by studying their polarizing effects on starlight

8. Emission Nebulae
Nebula – Any “fuzzy” patch (bright or dark) on the sky.
Emission nebulae – extended clouds of hot, glowing interstellar gas.
They are associated with star formation, and result when hot O and B class stars heat and ionize the surrounding gas and dust from which they have formed.

9. Most of these emission nebulae are catalogued in the Messier Catalogue
Most of these emission nebulae are catalogued in the Messier Catalogue. (Ex: M8, M16, M17, M20)
Charles Messier was an 18th century French Astronomer who was mainly interested in comets.
Messier was actually more concerned with making a list of celestial objects that might be confused with comets.

10. The ORION Nebula (M42)

11. Nebular Spectra
Nebular spectra tell us a great deal about the ionized interstellar gas.
Nebula are large enough for their actual sizes to be measured by simple geometry. (unlike stars, which appear to be points of light)

12. M16 – the Eagle Nebula

13. The Butterfly Nebula, a glowing patch of gas a few parsecs across.
Its emission spectrum, showing light intensity over the entire visible portion of the EM spectruum from red to deep violet.

14. Some Nebular Properties
Object
Distance (pc)
Diameter (pc)
Density
(106 part. /m3)
Mass
(Solar Masses)
Temp.
(K) M8
1200 14 80
2600
7500
M16
1800 8 90
600
8000
M17
1500 7
120
500
8700
M20
900 4
100
150
8200

15. The Orion Nebula
The greenish tint of portions of this nebula are produced by an electron transition in doubly ionized oxygen.
“Forbidden” Lines since they rarely occur in a “low density” gas on Earth.

16. Dark Dust Clouds
Rho Ophiuchi
Dark Dust clouds are cold irregularly shaped regions in the interstellar medium that diminish or completely obscure the light from background stars.

17. Horsehead Nebula in Orion.
Dark Dust cloud

18. A simplified diagram of some interstellar clouds between a hot star and Earth

19. 21 cm Radiation

20. Typical 21 –cm radio spectrum observed from several different regions of interstellar space.

21. Interstellar Molecules
The interstellar medium also contains cold, dark molecular clouds which are observed mainly through the radio radiation they emit.

22. Spectra indiates that formaldehyde molecules exist around M20, as indicated by the arrows.

23. Contour Map of the amount of formaldehyde near M20 nebula, demonstrating how formaldehyde is especially abundant in the darkest interstellar regions.