10. Milky Way and Other Galaxies

Question 1

Describe how we can determine the shape of the Milky Way…from the inside! e.g. which wavelengths are useful for what purposes

Answer 1

Hydrogen emits radiation with a wavelength of about 21 centimeters, and this radiation is easy to detect. And where there’s hydrogen, there are stars. Doppler shift. If the emitting atoms are moving away from us, the wavelength will be slightly larger than 21 cm. If they’re moving towards us, slightly less. So instead of just getting radiation at 21 cm, you get a distribution of wavelengths around 21 cm. We know how fast stars orbit the center given their orbit radius, from galaxy rotation curves. And if we know how fast a star is moving relative to us, we can figure out how much their 21 cm line is shifted.

Question 2

Describe the main components of the Milky Way galaxy

Answer 2

(1) a nucleus
(2) a central bulge
(3) a disk (both a thin and a thick disk)
(4) spiral arms
(5) a spherical component
(6) a massive halo. Some of these components blend into each other.

Question 3

Know the size of the Milky Way and the distance from the Sun to the centre of the Galaxy

Answer 3

The Milky Way is about 100,000 light years or about 30 kpc across. The Sun does not lie near the centre of our Galaxy. It lies about 8 kpc from the centre on what is known as the Orion Arm of the Milky Way.

Question 4

Explain the difference between stars of Population I, II and III

Answer 4

Population I, or metal-rich, stars are young stars with the highest metallicity out of all three populations and are more commonly found in the spiral arms of the Milky Way galaxy.

Population II, or metal-poor, stars are those with relatively little of the elements heavier than helium. These objects were formed during an earlier time of the universe. Intermediate population II stars are common in the bulge near the centre of the Milky Way, whereas population II stars found in the galactic halo are older and thus more metal-deficient. Globular clusters also contain high numbers of population II stars.[19]

Population III stars[23] are a hypothetical population of extremely massive, luminous and hot stars with virtually no metals, except possibly for intermixing ejecta from other nearby population III supernovae. Such stars are likely to have existed in the very early universe

Question 5

Interpret the rotation curve of the Milky Way

Answer 5

A galactic rotation curve is the radial velocity of the stars, dust, and gas that make up a galaxy plotted as a function of their distance from the galaxy’s center. Based on visible matter alone, one would expect that stars closest to the center of the galaxy would move faster than the stars near the galaxy’s outer edge (dashed line). However, in most galaxies inner and outer stars move at roughly the same velocity (solid line). There is some additional gravitational pull on the outer stars that isn’t fully described by the amount of visible matter in a galaxy. Most scientists have interpreted these rotation curves to mean that galaxies are surrounded by a halo of invisible dark matter