astronomy ch 20 quiz

Astronomers estimate that the ISM makes up about ____ of the total mass of the Milky Way Galaxy.

15%

Elements heavier than hydrogen and helium are found in the ISM. How are they produced?

They are produced inside stars and then enrich the ISM when the stars die.

What proof do we have of ultra-hot ISM temperatures?

Astronomers have recorded X-ray emissions from ISM gas and observed the spectra of oxygen atoms with five levels of ionization.

A star must be hotter than about 25,000 K to produce an H II region. Which of the following stars can produce such regions? (More than one possible answer.)

B1V white dwarf O9V

HI refers to atomic hydrogen. HII refers to ionized hydrogen. H2 refers to molecular hydrogen. Which is the most abundant type of hydrogen observed in the universe?

HI

Radio waves at a wavelength of 21-cm are used to detect....

HI

Why do HII regions appear red?

HII regions are composed of completely ionized hydrogen. When the hydrogen captures an electron, light is emitted. The red Balmer hydrogen line is the strongest wavelength emitted.

What is the ISM?

ISM refers to the material between stars. Some interstellar material is concentrated into giant clouds known as nebulae.

Suppose a single star has spectral features indicating it has a high surface temperature that emits mostly blue light, but when observed, the star appears red. What is an explanation for this?

Light from the hot star passes through clouds containing interstellar dust. The dust scatters blue light but allows red light to pass through. The effect is that the star appears red even though it is emitting mostly blue light.

Astronomers are able to observe hydrogen in HII regions with optical telescopes. How do astronomers observe cold hydrogen clouds (HI regions) that aren't near a star?

They use radio telescopes and observe the hydrogen through 21-cm radiation.

What would be the source of energy producing hot interstellar regions with temperatures of millions of degrees?

Supernova explosions

Which of the following is true concerning HII regions?

They are always found near hot stars which cause the surrounding gas to fluoresce.

True or False? Most of the ISM is composed of ionized hydrogen (HII).

false

Which of the following is the best candidate to explain cosmic rays?

galactic or extra-galactic supernova explosions

The hottest temperatures observed in the ISM are measured in ____ of degrees.

millions