Astronomers have for the first time photographed and weighed a star thought to represent a class of long-sought, mysterious objects that make up part of the "dark matter" in our galaxy.
The newly imaged red dwarf star is rather mundane as dark matter goes. In fact it is not really dark at all.
But no one could see the star until now. The technique used to obtain the picture and to calculate the star's distance and mass represents a step toward cataloguing other objects that are known to exist but which have never been seen.
These elusive quarry may or may not be in our galaxy and are of undetermined mass.
It's there, but where?
Prior to being photographed, the red dwarf star's presence was known for six years, owing to a phenomenon called microlensing. It works like this:
Albert Einstein predicted that gravity would bend light. He was right -- even our own Sun tugs at the light that passes nearby. Astronomers use this knowledge to study incredibly distant and faint stars and galaxies that would not be revealed if light traveled in a straight line.
The meager light from faraway things is sometimes refocused by the gravity of an intervening object, such as the newly imaged red dwarf star. These unseen and mysterious nearer objects -- also thought to include brown dwarf and white dwarf stars and even black holes -- would go undetected except for the fact that they act like cosmic magnifying glasses.
In the new study, researchers trained the Hubble Space Telescope on a spot in the sky where a gravitational lens was known to exist. Hubble photographed the red dwarf, which had moved slightly from its suspected position six years prior.
"If we'd known where to look a long time ago, we'd have been able to see it," said Kem Cook, a Lawrence Livermore National Laboratory researcher who led the effort. But until recently, the red dwarf was lost in the glare of the more distant star, which nonetheless shone with similar brightness.
Cook told SPACE.com that other microlenses can now be photographed. The prospect suggests that at least one type of dark matter will become much less mysterious over the next few years.
The finding, confirmed by the European Southern Observatory's Very Large Telescope, is presented in the Dec. 6 issue of the journal Nature.
What's the matter?
Though likely plentiful, dwarf stars like the one in the new study are considered by most astronomers to make up only
that can be seen.There are two types of dark matter, however.
Some, like the newly imaged red dwarf star, represents regular old matter wrapped up in hard-to-spot packages -- cold, dim stars that aren't readily observable with present technology. Collectively, these objects have come to be called MACHOs (MAssive Compact Halo Objects).
Truly exotic dark matter, on the other hand, is thought to be made of invisible particles that have yet to be detected. These particles are called WIMPs (Weakly Interacting Massive Particles).
Most of the mass in the universe is likely attributable to WIMPs, researchers suspect. Yet only when more MACHOs are found and weighed will their contribution be known for sure.
Where is it?
One large project has detected several MACHOs, each positioned in front of stars in the Large Magellanic Cloud, a nearby galaxy.
"Frustratingly, most microlensing events yield absolutely no information about the distance to the lens, so it is not known whether it lies close to the source star [in the Large Magellenic Cloud], close to the observer in our galaxy, or in the vast space in between," said Andrew Gould, an Ohio State University researcher.
Hubble was directed toward one of these microlensing objects, one whose light signature suggested it might be in the Milky Way.
The investigation determined that it is, said Gould, who was not involved in the study. He added that the success will open up new ways to study both dark and luminous matter in the galaxy and "will eventually transform the field."
The faint red star is about 600 light-years away and has a mass no more than 10 percent that of the Sun, according to the new study. It sits in the main, or "luminous" part of our galaxy, said Cook, the lead researcher. That means the object is not in the halo, a sparsely populated sphere of stars that surround the entire Milky Way Galaxy.
Little is known about the galactic halo, and Cook said scientists will be interested to learn whether or not similar objects exist there.
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