C19. Astrophysics

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Another update for C19


Astrophysicists (and their cousins the astronomers, solar physicists, planetary
scientists, cosmologists, and so forth) study objects from comets to the cosmos,
with stops along the way at stars, galaxies, pulsars, black holes, interstellar
chemistry, and much else. The most recent summary of (almost) the entire field
appeared as "Astrophysics in 2004" in Publications of the Astronomical Society
of the Pacific, volume 117, pp 311-394. A similar review o 2005 will appear
in about June 2006. Meanwhile, here are some extracts.

The Deep Impact Mission hit Comet Tempel 1 on July 4, 2005. It scattered
up a great deal of material, revealing that there are small patches of ice
on the surface, most of the rest of which is very dusty. The material just
barely clings together, like ashes rising from a fire.
Pluto turned up with two more very small moons, for a total of three. All
were probably produced in a single collision with another object in the outer
parts of the solar system. Indeed in the last two years, we have found
several Pluto-like bodies at least as large and orbiting somewhat further from
the sun. The community has not quite decided what to call these, apart from
designations like 2003 EL61. Additional very small moons of Jupiter, Saturn,
Uranus, and Neptune have also been imaged by the Galileo and Cassini missions
and by Hubble Space Telescope. The total is about 50 for Saturn.

The basic numbers describing the universe on large scales have remained
stable at least since 1997, though very few of us would have estimated them
correctly much before that. The age is about 14 billion years; the size
may well be infinite and is, in any case, very large compared to the part
we can survey. Only about 4% of it is made up of atomic material like
us and the earth and sun. The rest is dark matter (23%) and dark energy
(73%) both of which can be studied from the way they affected the formation
of the galaxies and clusters we observe. In March 2006, the WMAP satellite
team released data from their first three years of operation, confirming
these numbers and also telling us that stars and other sources of light
turned on when the universe was only about 365 million years old.


In 1990, we had never found any of these. 51 Peg B was discovered in
1995, and now there are more than 160, most of them about as massive as
our Jupiter and Saturn and most of them located closer to their stars
than Jupiter and Saturn are to the sun. New methods of searching for
exoplanets, which involved either the planet passing in front of its star
("transits") or the star-planet system passing in front of another star
("microlensing") have begun to reveal planets only about 5 times more
massive than Earth, some closer to their stars, some further away. Earth-
like planets may become detectable over the next decade.


How do you look for something that emits no light, no X-rays, no radio
waves, or anything else? Either by its effect on its surroundings or from
the fireworks of its birth. Some black holes orbit other stars and are
stong sources of X-rays, because gas approaching the black hole gets very
hot. In 1997, astronomers recognized that previously puzzling rapid bursts
of gamma rays were the result of very massive, rapidly rotating black holes
being formed from the collapse of very massive, rapidly rotating stars.
These are so rare that each galaxy has one every few million years, but so
bright that we can see them from all over the observable universe. The most
distant one so far, in September 2005, had a redshift of 6.29, as distant as
the furthrest galaxies and quasars.
Also in 2005, a second class of gamma ray bursters (thanks to a new
satellite called SWIFT) were shown to be the product of pairs of neutron
stars in orbit around each other spiraling together and again forming a
black hole. Such neutron star pairs exist in our Milky Way (with the
next gamma burst of that type due in a few million years).
Several other new manifestations of neutron stars and the remnants left
when they form were also discovered in the past year. These include
(a) sources of even higher energy gamma rays, discovered by a detector
called HESS in Namibia, and (b) two kinds of transient radio sources,
a single example of one near the galactic center and a bakers dozen of
others along the plane. Boht are probably old pulsars, so that they are
active only very occasionally, perhaps with companions. They too are part
of the Milky Way Galaxy.

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