The Milky Way Galaxy

Question 1

Explain stellar populations make up in the MW

Answer 1

• Nearly all stars appear to be composed mostly of hydrogen and helium, but their abundances of the heavier elements differ
• Population II stars in the outer galactic halo and in globular clusters have much lower abundances of the heavy elements—often less than one-hundredth the concentrations found in the Sun and in rare cases even lower

Question 2

Explain the formation of population II stars

Answer 2

• Formed when the abundance of elements heavier than hydrogen and helium was
  low
• Population I stars formed later, after mass lost by dying members of the first generations of stars had seeded the interstellar medium with elements heavier than hydrogen and helium.

Question 3

Explain Population I stars

Answer 3

• Are found only in the disk and follow nearly circular orbits around the galactic centre.
• Examples are bright supergiant stars, main-sequence stars of high luminosity (spectral classes O and B), which are concentrated in the spiral arms, and members of young open star
  clusters
• Interstellar matter and molecular clouds are found in the same places as population I
  stars
• Some are as old as 10 billion years others are still forming today

Question 4

Explain Population II stars

Answer 4

• Show no correlation with the location of the spiral arms. These objects are found
  throughout the Galaxy
• Some are in the disk, but many others follow eccentric elliptical orbits that carry them high above the galactic disk into the halo. Examples include stars surrounded by planetary nebulae and RR Lyrae variable stars
• The stars in globular clusters, found almost entirely in the Galaxy’s halo, are also classified as population II
• Typical ages are 11 to 13 billion years

Question 5

Where are young stars found

Answer 5

Lie in the thin disk, are rich in metals, and orbit the Galaxy’s centre at high speed

Question 6

Where are old stars found

Answer 6

• In the halo are old, have low abundances of elements heavier than hydrogen and
  helium, and have highly elliptical orbits randomly oriented in direction.
• Halo stars can plunge through the disk and central bulge, but they spend most of their time far above or below the plane of the Galaxy
• The stars in the thick disk are intermediate between these two extremes

Question 7

Explain what the center of the MW is

Answer 7

• Contains a black hole with a mass equivalent to 4.6 million Suns
• Light from the centre is dimmed by dust by a factor of a trillion making it hard to study the center
• Infrared and radio telescopes are used to study the centre of our Galaxy

Question 8

Explain Sagittarius A*

Answer 8

• Lies right at the centre our Galaxy
• Very bright radio source in the nucleus of the Galaxy
• Both radio and infrared observations are required to give us the necessary evidence that Sagittarius A* is a supermassive BH

Question 9

Explain where the BH comes from

Answer 9

• One possibility is that a large cloud of gas near the centre of the Milky Way collapsed directly to form a black hole
• Another possibility is that a massive star might have exploded to leave behind a seed black hole, or a dense cluster of stars might have collapsed into a black hole

Question 10

Explain dark matter

Answer 10

• Current best guess: DM must be some exotic particle that isn’t yet part of our standard model of particle physics
• Up to 95% of the mass in the MW and other galaxies is made up of DM … yet we have no idea of what it is

Question 11

Explain globular clusters

Answer 11

• Are nearly symmetrical round systems of hundreds of thousands of stars
• They live in the halo of our galaxy, and of other galaxies
• Most massive globular cluster in our own
  Galaxy is Omega Centauri
• Are very old