Fact Impact - The Milky Way

Fact Impact: All you need to know about the Milky Way...
25 February 2011

Interview with 

Carolin Crawford, Cambridge University


  • All the stars you can see at night with the unaided eye lie in our home, the Milky Way
  • In total, the Galaxy contains around 400 billion stars
  • Many of them suns like our own, surrounded by their own planetary systems.

  • The Milky WayThe Milky Way is shaped like two fried eggs back-to-back
  • A large central bulge is surrounded by a flat disc of stars measuring some 120,000 light-years across
  • But only a few thousand light-years thick.

  • The central bulge of stars is about 12,000 light years in diameter
  • It consists of a much older population of stars than found in the disk
  • ...and is substantially elongated, making The Milky Way a barred spiral galaxy

  • The disc contains the spiral arms, which appear prominent as they contain hot, luminous stars
  • They are sites of active star formation, and are lined with glowing gas nebulae and young blue star clusters
  • Where stars have formed in groups, all originally having collapsed from the same dense molecular cloud.

  • The Galaxy has four spiral arms; known as the Perseus, Scutum-Crux, Carina & Sagittarius, and Norma & Cygnus arms
  • The Sun lies halfway out from the centre to the edge of the stellar disc
  • And is located in the Orion arm, a small outer spur of the Cygnus arm

  • All the stars in the disc - including the Sun - are rotating in near-circular orbits
  • The Sun is orbiting at a speed of around 220 km/s, moving in the direction of the star known as Vega
  • Even at that speed, it takes us around 250 million years to travel once round the centre of the galaxy.

  • Detailed study of the speeds of stars in the rotating disc reveals that the mass of the Galaxy is around eight hundred thousand million times the mass of the Sun
  • But over 90% of this mass is in the form of an invisible giant halo of dark matter, which only reveals itself by its gravitational pull on the visible matter
  • Its mass is so great that it is gravity of the dark matter (rather than of all the stars) that holds the galaxy together.Artist's conception of the spiral structure of the Milky Way with two major stellar arms and a bar.
  • The centre of the Milky Way lies in the direction of the constellation of Sagittarius, and is some 30,000 light-years  away from us
  • It is marked by intense radio feature, called Sagittarius A*, which is surrounded by a cluster of hot young stars
  • Many of these stars are moving very fast in tight orbits around a mass some 4 million times that of the Sun
  • Which must be contained within a volume less than the size of our Solar System
  • This is the evidence that the core of our Galaxy hosts a super-massive black hole
  • Which remains dormant, not actively accreting any matter.

  • The Milky Way is not alone in space
  • Along with its sister galaxy Andromeda and the Triangulum Galaxy, it is part of the 'Local Group', a collection of around 25-30 galaxies spread over a region some 10 million light-years across
  • Which in turn is at the outskirts of the Virgo supercluster of galaxies, some 150 million light-years in diameter.

  • Two of the small galaxies in orbit around the Milky Way can be seen in the Southern hemisphere night sky
  • Known as the Magellanic clouds, they lie 160,000 and 200,000 light years away
  • ...and are slowly being distorted and pulled apart by the tidal forces from the Milky Way's gravity.

  • A new satellite galaxy was only discovered in 1994, and it is known as the Sagittarius Dwarf Elliptical Galaxy (SagDEG)
  • It moves in a polar orbit very close to the Milky Way, but is currently on the opposite side of the Galactic Center from the Solar System
  • ...and is also being shredded by the Galaxy's gravitational pull, shedding a long stream of stars in its wake.

  • The close proximity of such satellite galaxies as the Magellanic clouds and SagDEG have in return affected the Milky Way
  • Their tidal gravitational forces have slightly warped the disc of the MW so it is not completely flat.

  • The Milky Way is on a collision course with its near neighbour, the Andromeda galaxy
  • They are heading towards each other at a rate of about 500,000 kmph
  • ...and the two galaxies are expected to begin the process of merging within a few billion years.


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