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S&T's News Bulletin - Jun 16



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         SKY & TELESCOPE'S NEWS BULLETIN -- JUNE 16, 1995
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BROWN DWARF IDENTIFIED

Pointing the world's largest optical telescope at one of our galaxy's
dimmest sources has revealed the best evidence yet for a brown dwarf.
These are starlike bodies whose masses don't quite generate the central
pressure and temperature required to sustain nuclear fusion. At this
week's meeting of the American Astronomical Society, Gibor Basri, Geoffrey
Marcy and James Graham announced that the 10-meter Keck telescope had
obtained spectra of the faintest, reddest stars in the Pleiades star
cluster, M45, in Taurus. The signature of lithium turned up in only one
object, the dimmest one sampled, which is dubbed PPL 15. Since it has
lithium while its slightly brighter neighbors do not, this object
presumably has not developed the fusion engine to convert the lithium into
other elements and allow it to graduate from brown-dwarfhood to true
stellar rank.

ANCIENT HELIUM

The signature of intergalactic helium in the early universe has been seen
in data obtained last March during the Astro-2 mission.  Arthur Davidsen,
Gerard Kriss and Wei Zheng announced last week that the Hopkins
Ultraviolet Telescope found firm spectroscopic evidence for helium gas
lying along the line of sight to a distant quasar.  In theory, reactions
during the first minutes after the Big Bang would have produced roughly
one helium atom for every twelve of hydrogen. This ratio is seen in stars
throughout our galaxy. However, Astro-2 was looking for primordial gas
that never collected into stars, and the helium spied by Astro-2 left its
mark on the itinerant ultraviolet light when the universe was less than
one fifth its current age.  Although helium absorption in another quasar's
spectrum was seen last year by European observers using the Hubble Space
Telescope, the new, higher-quality data have enabled the Hopkins team to
estimate its density. That value implies that the intergalactic medium in
the early universe contained nearly all the baryonic (normal) matter from
which galaxies, stars and planets later formed.

WATCHING MIR & ATLANTIS

Here's an update on watching Mir and the Space Shuttle *Atlantis* as they
coast over the U.S. in late June. The shuttle launch is planned for June
23rd and docking with Mir about two days later.  Their paths over the U.S.
and southern Canada are depicted on page 69 of SKY & TEL's July issue.
Recent orbital changes require would-be trackers to make corrections to
the lower map on that page.  In general each pass should be shifted 5
millimeters in a direction perpendicular to the track as plotted, toward
the northeast or north. A time correction must also be applied.  For
observers near the U.S.-Canada border, the spacecraft will come by about
30 minutes earlier than noted on the map.  Those in the midsection of the
U.S. will need to look about 20 minutes earlier, those in far-southern
states about 12 minutes.

"SUMMER" SOLSTICE

The Sun reaches the June solstice, its farthest migration northward in the
heavens, on the 21st at 20:34 Universal time (4:34 p.m. Eastern Time).

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   *------------------------------------------------------------*
   | Stuart Goldman         Internet: sgoldman@cfa.harvard.edu  |
   * Associate Editor                 mrastro@aol.com           *
   | Sky & Telescope                                            |
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   | Belmont, MA  02178           Magazine of Astronomy         |
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