8. Birth of planetary systems and exoplanets

Question 1

Outline the material between the stars, and relative amounts

Answer 1

Entire collection of interstellar matter is called the interstellar medium (ISM).

99% of the material between the stars is in the form of a gas.

The most abundant elements in this gas are hydrogen and helium

The remaining 1% of the interstellar material is solid—frozen particles consisting of many atoms and molecules that are called interstellar grains or interstellar dust

Question 2

Explain how dust reddens and scatters light

Answer 2

Dust grains in the interstellar medium have a typical size that is comparable to the wavelength of blue light. The result is that the blue light coming from distant objects is strongly absorbed and scattered by the dust.

The reddening of an object is inversely proportional to the wavelength of optical light, so shorter wavelengths (blue) are more heavily reddened than longer (red) wavelengths

Question 3

Explain why nebulae are mostly red and blue

Answer 3

Emission nebulae tend to be red in colour because of the abundance of hydrogen. Additional colours, such as blue and green, can be produced by the atoms of other elements, but hydrogen is almost always the most abundant

Question 4

Identify the four main types of gas clouds in the ISM and their approximate temperature and density

Answer 4

1. Ionized Hydrogen (H II) Regions—Gas Near Hot Stars (10,000K)
2. Neutral Hydrogen Clouds. Most of the volume of the interstellar medium is filled with neutral (nonionized) hydrogen. 100 K (–173 °C) to as high as about 8000 K.
3. Ultra-Hot Interstellar Gas. One million degrees, but no visible source of heat - caused by supernovae.
4. Molecular Clouds. High density low volume. 20-30% mass of Milkyway. typical temperatures near 10 K (−263 °C).

Question 5

Identify ways of observing each of the main types of gas clouds

Answer 5

21 centimetre neutral hydrogen line, spectral lines from numerous interstellar molecules, and emission lines from hot ionized emission nebulae.

Question 6

Explain why infrared observations are so useful

Answer 6

Near-infrared: dust is transparent to it.

Far-infrared: emission from the dust itself becomes observable.

Relatively cold objects invisible to optical telescopes become visible in the infrared. Interstellar gas, planets and dust discs around other stars, asteroids, brown dwarfs (failed stars) and stars being born are all examples of objects that are too cold to shine in visible wavelengths but become conspicuous when viewed in the infrared

Question 7

Describe cosmic rays and where they are thought to originate

Answer 7

Cosmic rays are high-energy protons and atomic nuclei that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own galaxy, and from distant galaxies