Stars can form orbital systems with other astronomical objects, as in the case of
planetary systems and
star systems with
more stars. When two such stars have a relatively close orbit, their gravitational interaction can significantly impact their evolution. Stars can form part of a much larger gravitationally bound structure, such as a
star cluster or a galaxy. (Full article...)
Alpha Centauri AB is 1.34
parsec or 4.37
light years away from our
Sun. The two stars are the closest stars to the Sun after their companion
Proxima Centauri, at 0.21 light-year away from the two, and at 4.243 light-years away from the Sun.
visual magnitude, Alpha Centauri appears to the naked-eye as a single star and is fainter than
Canopus. The next brightest star in the night sky is
Arcturus. When considered among the individual
brightest stars in the sky (excluding the
Sun), Alpha Centauri A is the fourth brightest at −0.01 magnitude being only fractionally fainter than Arcturus at −0.04v magnitude. Alpha Centauri B at 1.33v magnitude is twenty-first in brightness.
The field is so small that only a few foreground
stars in the
Milky Way lie within it; thus, almost all of the 3,000 objects in the image are
galaxies, some of which are among the youngest and most distant known. By revealing such large numbers of very young galaxies, the HDF has become a landmark image in the
study of the early universe, with the associated scientific paper having received over 800 citations by the end of 2008.
Three years after the HDF observations were taken, a region in the south celestial hemisphere was imaged in a similar way and named the
Hubble Deep Field South. The similarities between the two regions strengthened the belief that the
universe is uniform over large scales and that the
Earth occupies a typical region in the universe (the
cosmological principle). A wider but shallower survey was also made as part of the
Great Observatories Origins Deep Survey. In 2004 a deeper image, known as the
Hubble Ultra Deep Field (HUDF), was constructed from a total of eleven days of observations. The HUDF image is the deepest (most sensitive)
astronomical image ever made at visible wavelengths.
The motion of uniformly accelerated objects, taught in nearly all high school and introductory college physics courses, was studied by Galileo as the subject of
kinematics. His contributions to observational astronomy include the telescopic confirmation of the
phases of Venus, the discovery of the four largest satellites of
Jupiter (named the
Galilean moons in his honour), and the observation and analysis of
sunspots. Galileo also worked in applied science and technology, inventing an improved
military compass and other instruments.
Galileo's championing of Copernicanism was controversial within his lifetime, when a large majority of philosophers and astronomers still subscribed (at least outwardly) to the
geocentric view that the Earth is at the centre of the universe. After 1610, when he began publicly supporting the
heliocentric view, which placed the Sun at the centre of the universe, he met with bitter opposition from some philosophers and clerics, and two of the latter eventually denounced him to the
Roman Inquisition early in 1615. In February 1616, although he had been cleared of any offence, the
Catholic Church nevertheless condemned heliocentrism as "false and contrary to Scripture", and Galileo was warned to abandon his support for it—which he promised to do. When he later defended his views in his most famous work, Dialogue Concerning the Two Chief World Systems, published in 1632, he was tried by the Inquisition, found "vehemently suspect of heresy", forced to recant, and spent the rest of his life under house arrest.