Massive compact halo objects
as dark matter candidates

• Made of ordinary matter, hydrogen and helium.
• Could be very dim stars.
• Could be brown dwarfs
  • Condensed from gas clouds like ordinary stars
  • But mass was too small, M < 0.1 M_sun.
  • So nuclear burning never turned on.
  • A lightweight example is Jupiter, with M ~ 0.001 M_sun.
• Of course, the lighter they are, the more you need to make a significant contribution to the mass of the Galaxy.

A strategy for looking for MACHOS.

A MACHO could act as a gravitational lens. When it passes in front (or almost in front) of a background star, it could focus the light from that star, making it seem brighter for awhile.

There is an experiment to look for this.