Objects supported by electron degeneracy pressure
span a broad range of masses.  The low-mass end of this
range, which is near the mass of Saturn, is set by the
transition from pressure exerted by atoms to pressure
exerted by degenerate electrons.  The high end of this range,
which is 1.4 solar masses, is set by the gravitational
instability that arises when the degenerate electrons have
kinetic energies equal to the electron rest-mass energy.
These limits are given by several fundamental constants
of physics.  Despite the neutron stars being supported
by neutron and proton degeneracy pressure rather than
electron degeneracy pressure, they have an upper mass
similar to that of the degenerate dwarf.

The Chinese government is investing in
a rocket engine called the Emdrive that generates thrust
with microwaves.  There appears to be a slight problem,
however, with this engine: it violates conservation of
momentum.

The stars and planets have radii that are set by
the balance of internal pressure against self-gravity.
Because internal pressure has several sources, the stars
and planets fall into several classes, each characterized
by a specific source of pressure.  The consequence is that
the objects of each class obey a unique relationship
between radius and mass.

The Large Hadron Collider at CERN, a machine
that accelerates protons to very high energies and then
bangs them together, began operating on September 10, 2008.
Some believe this machine threatens Earth. They need not
worry, because the particle collisions created in this
machine occur daily when cosmic rays strike Earth's
atmosphere. Man can't yet rival nature's extremes.

The structure of a brown dwarf is set by
degeneracy pressure.  Unlike a star, where the mass
sets both the radius and the photospheric temperature,
a brown dwarf has a radius and temperature that is nearly
independent of its mass.  All brown dwarfs are about
the same size as Jupiter.  The photospheric temperature
of a brown dwarf is set by its age, although the lifetime
of a brown dwarf is set by the mass.  Because the
low-mass brown dwarfs cool much faster than the
high-mass brown dwarfs, infrared surveys preferentially
find the more-massive brown dwarfs.

Recently some people have claimed that the Sun
is entering a new Maunder Minimum—a decades-long period
of few sunspots—and that this will cause the Earth's
atmosphere to cool.  The Sun is certainly quiet in 2008,
but this is the normal quiet of a minimum in the 11 year
sunspot cycle.  Clearly the tendency to interpret normal
variations as fundamental changes is not confined to the
global warming alarmists.

A class of object, long predicted by astrophysicists,
sits in the mass range between the giant gaseous planets
and the M dwarf stars.  These objects are called brown
dwarfs.  They are massive enough to burn deuterium, but they
are too light to burn hydrogen.  The first brown dwarf was
observed orbiting an M dwarf star in 1988, and since that
time, hundreds of additional brown dwarfs have been found.
They are cool, so they are primarily emitters of infrared
radiation.  In the early stages of their lives, they are
powered by deuterium fusion and gravitational potential
energy, but when they consume their deuterium, and when
the electron degeneracy pressure stops their shrinkage,
they grow cold and dark.

In the nineteenth century, astronomers recognized
that stars could be classified by their spectra into a handful
of types.  Over time, this system was refined to characterize
a star in terms of prototypical stars with similar spectra.
This is the meaning of the jargon that the Sun is a G2 V
star: the G2 refers to the pattern of lines in the Sun's
spectrum, which is directly dependent on temperature, and
the V refers to the widths of these lines, which are
dependent on luminosity.  The advantage of this system
is that astronomers can determine what stars are like the
Sun in temperature and luminosity simply by looking at
the patterns of lines in the stars' spectra.

The magnitude system used by astronomers ranks
stars by brightness, with the brightest stars having the
lowest values of magnitude.  A star's magnitude is generally
measured after the starlight has passed through a colored
filter, which gives a measure of a star's color. Literally
dozens of filter systems are used by astronomers.  The
most common system for measuring color over the infrared,
visible, and ultraviolet wavelengths is the Johnson-Cousins
UBVRI system.

The HR diagram of the stars within 10 parsecs is
presented on this page.  The diagram reveals that we are
surrounded largely by two types of star: dark main-sequence
stars and degenerate dwarfs.  Stars like the Sun are the
exception rather than the rule, and the more luminous A stars
and red giants are rather rare.  The brilliant and massive
supergiant O and B stars, of which Rigel in the constellation
Orion is an example, are completely absent from the local
stellar neighborhood, despite their prominence in the night
sky.  Most stars in the Galactic disk are much less luminous
than the Sun, and most of the stellar mass of the Galactic
disk is in these stars.

Little more than 350 stars are known to be
within 10 parsecs of the Sun.  Most of these are too dim
to see with the unaided eye. Several, however, are among
the brightest stars in the night sky.  The 10 brightest are
listed in a table on this page, along with their distances,
apparent visual magnitudes, absolute visual magnitudes,
color indices, and stellar types.

The nearby stars are of all ages, which gives them
a broad variety of luminosities and colors.  To see stars of the
same age, to see the effects of mass and composition alone on
a star's color and luminosity, one must examine star clusters.
All of the stars in a star cluster are born at about same time.
The open clusters scattered in the Galactic disk provide us with
collections of young stars.  The ancient globular clusters that
swarm around the Galactic center provide us with collections of
old stars.  By creating Herzsprung-Russell diagrams for both types
of star cluster—plots of the colors and luminosities of
stars—astrophysicists gain insight into how stars, especially
stars more massive than the Sun, change over billions of years.

Authors: B. Pérez-Rendón, G. García-Segura and N. Langer
A&A 506, 1249 (2009) Received 5 April 2008 / Accepted 20 March 2009
Keywords: stars: evolution, ISM: kinematics and dynamics, ISM: supernova remnants, ISM: bubbles, ISM: individual objects: Cassiopeia A

Authors: R. C. Gamen, E. Fernández-Lajús, V. S. Niemela and R. H. Barbá
A&A 506, 1269 (2009) Received 2 May 2008 / Accepted 1 August 2009
Keywords: stars: binaries: close, stars: binaries: eclipsing, stars: binaries: spectroscopic, stars: Wolf-Rayet

Authors: D. Ruždjak, H. Boži?, P. Harmanec, R. Fi?t, P. Chadima, K. Bjorkman, D. R. Gies, A. B. Kaye, P. Koubský, D. McDavid, N. Richardson, D. Sudar, M. Šlechta, M. Wolf and S. Yang
A&A 506, 1319 (2009) Received 4 July 2008 / Accepted 24 August 2009
Keywords: stars: early-type, binaries: spectroscopic, stars: emission-line, Be, stars: individual: \zeta Tauri

Authors: G. Consolini, R. Tozzi and P. De Michelis
A&A 506, 1381 (2009) Received 2 October 2008 / Accepted 5 June 2009
Keywords: Sun: activity, Sun: sunspots, methods: statistical, chaos

Authors: V. Bommier, M. Martínez González, M. Bianda, H. Frisch, A. Asensio Ramos, B. Gelly and E. Landi Degl'Innocenti
A&A 506, 1415 (2009) Received 18 November 2008 / Accepted 16 July 2009
Keywords: magnetic fields, polarization, turbulence, techniques: polarimetric, methods: data analysis, Sun: magnetic fields

Authors: S. Witte, Ch. Helling and P. H. Hauschildt
A&A 506, 1367 (2009) Received 11 December 2008 / Accepted 21 August 2009

Keywords: astrochemistry, methods: numerical, stars: atmospheres, stars: low-mass, brown dwarfs

Authors: B. Ascaso, J. A. L. Aguerri, M. Moles, R. Sánchez-Janssen and D. Bettoni
A&A 506, 1071 (2009) Received 29 December 2008 / Accepted 2 July 2009
Keywords: galaxies: clusters: general, galaxies: structure, cosmology: observations

Authors: N. Crimier, C. Ceccarelli, B. Lefloch and A. Faure
A&A 506, 1229 (2009) Received 12 January 2009 / Accepted 5 August 2009
Keywords: ISM: abundances, ISM: molecules, stars: formation