I85PREP
⌘ K
DSST Astronomy study materials
DSST Astronomy study resources

DSST Astronomy: The Galaxy

DSST Astronomy · Work through your exam, one topic at a time.

All guide sections

The Galaxy

This domain examines the Milky Way as an object: its structure, stellar populations, interstellar contents, rotation, and central black hole. Questions test your mental map of the Galaxy and your ability to use rotation and population evidence to infer mass and history.

Structure and populations

  • Components: a thin disk of gas, dust, and young stars with spiral arms; a central bulge of older stars; and a roughly spherical halo of old, metal-poor stars and globular clusters.
  • Diameter about 100,000 light-years; the Sun sits roughly 26,000 light-years (8 kpc) from the center, in the Orion Arm.
  • Population I stars are young, metal-rich disk stars; Population II stars are old, metal-poor halo and globular-cluster stars. Metallicity rises over galactic history as supernovae enrich the gas.
  • The interstellar medium includes molecular clouds, atomic hydrogen traced by the 21-cm line, ionized gas, and dust that reddens and extinguishes starlight.

Rotation and mass

  • The Sun orbits at about 220 km/s, completing a revolution in roughly 225–250 million years.
  • Rotation curves stay nearly flat at large radii rather than falling as Kepler's law predicts, implying mass that extends far beyond the visible disk - dark matter.
  • The center hosts Sagittarius A*, a supermassive black hole of about 4 million solar masses, identified from stellar orbits.

Worked example

Find the Sun's orbital period from its radius and speed. r = 8 kpc = 8 × 3.086 × 10¹⁶ km = 2.47 × 10¹⁷ km. The circumference is 2πr = 1.55 × 10¹⁸ km. Dividing by 220 km/s gives 7.05 × 10¹⁵ s. Since a year is 3.156 × 10⁷ s, the period is about 2.2 × 10⁸ years, roughly 220 million years - the "galactic year." Note that this requires assuming a circular orbit at constant speed, which is itself evidence for a flat rotation curve.

Traps that cost marks

  • Dark matter is not dust; dust is a small fraction of the interstellar medium and is detected by reddening, not by gravity.
  • Globular clusters belong to the halo, not the disk; open clusters are disk objects.
  • The Galaxy's rotation is not Keplerian outside the bulge, which is exactly the dark-matter argument.
  • The Milky Way is a barred spiral, not a pure grand-design spiral.
  • Andromeda is approaching, not receding; not every galaxy shows redshift.