| Sky & Telescope An
international team of astronomers have studied the atmosphere of a star 25,000
light-years away. This feat was accomplished thanks in large part to two smaller
stars that happened to be in the way.
For several years, teams of astronomers
have monitored fields of stars looking for gradual brightenings. The sought magnitude
changes aren't due to any variable nature in the stars themselves, but because
of gravitational lensing. When a massive, but dim, object crosses our line of
sight to a background star, the gravity of the intervening star distorts the light
from the more-distant object. The effect is a focusing of the starlight. The cycle
of brightening and dimming of this so-called microlensing can last several weeks.
Astronomers hope that such microlensing searches will help estimate the amount
of dark matter in the galaxy by finding evidence for dwarf stars and other bodies
we can't detect through other means.
On May 5, 2000, astronomers of the
EROS program found a microlensing candidate and soon other observing programs
were monitoring the event, designated EROS-BLG-2000-5. After about a month, the
star brightened significantly, indicating that the event was in fact a pair of
dwarf stars passing in front of a red giant in the central bulge of the Milky
Way. Furthermore, researchers predicted that the star would have another brightening
a few weeks later. Astronomers at the European Southern Observatory's Very Large
Telescope geared up for the event, and when the star did indeed brighten again,
they took spectra throughout several nights in early July 2000. The lensing effect
enhanced emission from different parts of the giant star as the foreground dwarfs
moved across the disk, in effect peering into the structure of the star. The ESO
astronomers traced changes in hydrogen emission from different atmospheric depths,
which were consistent with stellar models.
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