Astronomy · Rung 9 of 12

The Milky Way and the galaxies

The Milky Way is a barred spiral about 100,000 light years across with some hundred billion stars, and we sit 26,000 light years from the centre, on a secondary arm. That milky band you see in summer is the disc seen edge-on from inside. At the centre there is a black hole of four million solar masses, Sagittarius A*. Hubble sorted galaxies into ellipticals, spirals (barred or not) and irregulars; the Local Group has about eighty, dominated by Andromeda and by us, and in some four billion years we will collide with it. Collide is a figure of speech: stars almost never touch.

Where this sits

Comes after
The stars
Opens up
Cosmology

Exercises from this rung

Generated by the app, from easiest to hardest

  1. Globular clusters orbit the galactic halo. What makes them special?

    • They lie outside the galaxy
    • They are the youngest
    • They contain mostly planets
    • They are the oldest things in the Milky Way

    Why They are balls of hundreds of thousands of old, metal-poor stars that formed before the disc. Their age sets a floor on the age of the universe: it cannot be younger than its oldest stars, and that independent check agrees with the 13.8 billion years the microwave background gives.

    easier

  2. How do we know there is a black hole at the centre of the Milky Way?

    • From the orbits of nearby stars
    • From its colour
    • From the dust lane
    • Because it looks dark

    Why The orbits of stars at the galactic centre were tracked for decades, in particular S2, which goes around every 16 years. The orbit gives the mass: four million suns inside a volume smaller than our Solar System. Nothing known can be that except a black hole. Genzel and Ghez took the 2020 Nobel for those measurements.

    in between

  3. When Andromeda collides with us in four billion years, what happens to the stars?

    • Almost none will collide
    • They will all be thrown out of the disc
    • Nearly all of them will collide
    • They will be extinguished by the impact

    Why A galaxy is mostly emptiness. If the Sun were an orange, the nearest star would be another orange 4,000 kilometres away. In a merger the stars pass through without touching; what does collide is the gas, and that sets off a burst of star formation. The two spirals will end fused into a giant elliptical.

    harder