In astrophysics, MACHO is an acronym for MAssive Compact
Halo Object, what is believed to be a component of dark matter. MACHO
is also the name given to an observing project dedicated to the search for
dark matter.
The nature of MACHOs themselves is unknown, though one view is that they are
brown dwarf stars, or gas giant planets floating freely through the
galaxy. These objects would be so faint that we couldn't detect
them directly, but their collective mass may influence the kinematic
behavior and rotation of
the Milky Way and other galaxies.
Though we cannot see them shine by their own light, we can detect their
presence by gravitational lensing. Imagine we point a telescope at a
distant star, and measure that star's brightness over time.
Now suppose that one of the MACHOs passes between us and that distant star.
According to general relativity, the curvature of space around any mass
can act like a lens for background light. Curved space "bends" the rays
of light, sending them in a slightly different direction, much like glass
lens does. If the MACHO is located close to the line of sight between us
and the background star, we will see an increase in the light from that
star as the MACHO bends the light around it. We see this effect change over
time, because the background star, the MACHO, and the Earth all move through
space over time. If the background star is not variable,
we will first find it has constant brightness, followed by an increase in
light, and then a decay down to the normal light level. These lensing events
last from a few days to a few weeks, and do not repeat.
While there are lots of stars in the sky, and presumably a lot of dark matter
as well, this happens very infrequently because space is so vast, and these
objects are very far apart; the number of such microlensing(*) events is measured
only in the hundreds since their first detection in the early 1990's. The
way such events are detected is rather ingenious. You use a telescope
(or two), and take an image of a dense star field a few times a night. For
example, you can take pictures of the Milky Way's central bulge, or the Large and Small Magellanic Clouds. You then use a
computer to measure
the brightness of each star on the image. Over days and months and years you
keep imaging the same patch of sky, and obtain a very long light curve of
each star. You then use a computer to search each light
curve for the characteristic shape of a microlensing event. It is a bit like
searching for a needle in a haystack, but it works.
Several independent projects were undertaken to perform searches for
dark matter in this way. Two of the most famous ones are the
MACHO Project, a collaboration between many universities in the U.S.,
Canada, and Australia with a telescope in Australia, and the
OGLE Project (Optical Gravitational Lens Experiment), a collaboration
between the Carnegie Institution in Washington, Princeton University, and
the Warsaw University Observatory in Poland with a telescope at Las Campanas
Observatory in Chile.
While these projects were established to search for dark matter, one of the
most important fringe benefits has been their detection of many thousands of
variable stars in the Galactic Bulge, in
globular clusters,
and in the Magellanic Clouds. Since the light curves of millions of stars
are measured every night, it is straightforward to search them for variability
not due to gravitational lensing. Both the MACHO and OGLE projects
have detected thousands of variable stars including RR Lyrae, Cepheid and delta Scuti variables, and binary stars.
These stars have been used for many purposes, such studying the physics of
stellar variability, and measuring the distance
to the Milky Way's center and the Magellanic Clouds. Data on the Magellanic
Clouds have also been used to make
color-magnitude diagrams of millions of stars
in the Magellanic clouds, to study its star formation history.
(*) We call these events microlensing, because we usually reserve
gravitational lensing for what is done by massive clusters of
galaxies on very large scales, or by single foreground galaxies on distant
quasars.
Gritchka points out that MACHOs are much better dark
matter candidates than WIMPs, for obvious reasons. :)
Actually, a friend of mine thinks it's all bunk, and that
topological defects are the way to go. I think cosmology gives me a headache....
Sources:
http://www.macho.mcmaster.ca
http://bulge.princeton.edu
My PhD dissertation: An Observational And Theoretical Study of
High-Amplitude Delta Scuti Stars (June 2000)
I was an affiliate of the MACHO Project for part of my dissertation work.
| 
