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June 27, 2001
Photo No: H2001-20a
Hubble Uncovers Mystery Objects in the Dense Core of a Nearby Star Cluster
Piercing the heart of a glittering swarm of stars, NASA's sharp-eyed
Hubble Space Telescope unveils the central region of the globular
cluster M22, a 12- to 14-billion-year-old grouping of stars in the
constellation Sagittarius. The telescope's view of the cluster's core
measures 3.3 light-years across.
The stars near the cluster's core are 100,000 times more numerous
than those in the Sun's neighborhood. Buried in the glow of starlight
are about six "mystery objects," which astronomers estimate are no
larger than one quarter the mass of the giant planet Jupiter, the solar
system's heftiest planet.
The mystery objects are too far and dim for Hubble to see directly.
Instead, the orbiting observatory detected these unseen celestial
bodies by looking for their gravitational effects on the light from far
distant stars. In this case, the stars are far beyond the cluster in the
galactic bulge, about 30,000 light-years from Earth at the center of
the Milky Way Galaxy. M22 is 8,500 light-years away. The invisible
objects betrayed their presence by bending the starlight gravitationally
and amplifying it, a phenomenon known as microlensing.
From February 22 to June 15, 1999, Hubble's Wide Field and
Planetary Camera 2 looked through this central region and monitored
83,000 stars. During that time the orbiting observatory recorded six
unexpectedly brief microlensing events. In each case a background
star jumped in brightness for less than 20 hours before dropping back
to normal. These transitory spikes in brightness mean that the object
passing in front of the star must have been much smaller than a normal
star. Hubble also detected one clear microlensing event. In that
observation a star appeared about 10 times brighter over an 18-day
span before returning to normal. Astronomers traced the leap in
brightness to a dwarf star in the cluster floating in front of the
background star.
Credits for Hubble image: NASA, Kailash Sahu, Stefano Casertano,
Mario Livio, Ron Gilliland (Space Telescope Science Institute), Nino
Panagia (European Space Agency/Space Telescope Science Institute),
Michael Albrow and Mike Potter (Space Telescope Science Institute)
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