Transcript The Milky Way

The Milky Way
The Greek philosopher Democritus (450–370 BC) proposed
that the bright band on the night sky known as the Milky Way
might consist of distant stars. Aristotle (384–322 BC), however,
believed the Milky Way to be caused by "the ignition of the
fiery exhalation of some stars which were large, numerous and
close together" and that the "ignition takes place in the upper
part of the atmosphere, in the region of the world which is
continuous with the heavenly motions.“ The Neoplatonist
philosopher Olympiodorus the Younger (c. 495–570 AD)
criticized this view, arguing that if the Milky Way were
sublunary it should appear different at different times and
places on the Earth, and that it should have parallax, which it
does not. In his view, the Milky Way was celestial. This idea
would be influential later in the Islamic world.
According to Mohaini Mohamed, the Arabian astronomer, Alhazen
(965–1037), made the first attempt at observing and measuring
the Milky Way's parallax, and he thus "determined that because
the Milky Way had no parallax, it was very remote from the Earth
and did not belong to the atmosphere.“ An Persian astronomer
proposed the Milky Way galaxy to be "a collection of countless
fragments of the nature of nebulous stars.“ An Andalusian
astronomer proposed that the Milky Way was made up of many
stars that almost touch one another and appear to be a
continuous image due to the effect of refraction from sublunary
material, citing his observation of the conjunction of Jupiter and
Mars as evidence of this occurring when two objects are near. In
the 14th century, a Syrian-born scientist proposed the Milky Way
galaxy to be "a myriad of tiny stars packed together in the sphere
of the fixed stars".
The Proof
Actual proof of the Milky Way consisting of many stars
came in 1610 when Galileo Galilei used a telescope to
study the Milky Way and discovered that it is composed
of a huge number of faint stars. In 1750 Thomas Wright,
in his An original theory or new hypothesis of the
Universe, speculated (correctly) that the galaxy might
be a rotating body of a huge number of stars held
together by gravitational forces, akin to the solar
system but on a much larger scale. The resulting disk of
stars can be seen as a band on the sky from our
perspective inside the disk. In a essay in 1755,
Immanuel Kant elaborated on Wright's idea about the
structure of the Milky Way.
The first attempt to describe the shape of the
Milky Way and the position of the Sun in it was
carried out by William Herschel in 1785 by
carefully counting the number of stars in
different regions of the sky. He produced a
diagram of the shape of the galaxy with the
solar system close to the center. Using a refined
approach, Kapteyn in 1920 arrived at the
picture of a small (diameter about 15
kiloparsecs) ellipsoid galaxy with the Sun close
to the center.
A different method by Harlow Shapley based
on the cataloguing of globular clusters led to a
radically different picture: a flat disk with
diameter approximately 70 kiloparsecs and the
Sun far from the center. Both analyses failed to
take into account the absorption of light by
interstellar dust present in the galactic plane,
but after Robert Julius Trumpler quantified this
effect in 1930 by studying open clusters, the
present picture of our galaxy, the Milky Way,
emerged.
The shape of the Milky Way as deduced from star counts by
William Herschel in 1785; the solar system was assumed to
be near the center.
The Solar System consists of the Sun and the
astronomical objects gravitationally bound in
orbit around it, all of which formed from the
collapse of a giant molecular cloud
approximately 4.6 billion years ago. The vast
majority of the system's mass (well over 99%)
is in the sun. Of the many objects that orbit
the Sun, most of the mass is contained within
eight relatively solitary planets whose orbits
are almost circular and lie within a nearly flat
disc called the ecliptic plane.
PLANETS
The four smaller inner planets, Mercury, Venus,
Earth and Mars, also called the terrestrial planets,
are primarily composed of rock and metal. The four
outer planets, the gas giants, are substantially more
massive than the terrestrials. The two largest,
Jupiter and Saturn -, are composed mainly of
hydrogen and helium; the two outermost planets,
Uranus and Neptune, are composed largely largely
of ices, such as water, ammonia and methane, and
are often referred to separately as "ice giants".
The Solar System is also home to a number of regions
populated by smaller objects. The asteroid belt, which lies
between Mars and Jupiter, is similar to the terrestrial
planets as it is composed mainly of rock and metal. Beyond
Neptune's orbit lie the Kuiper belt and scattered disc; linked
populations of trans-Neptunian objects composed mostly of
ices such as water, ammonia and methane. Within these
populations, five individual objects, Ceres, Pluto, Haumea,
Makemake and Eris, are recognized to be large enough to
have been rounded by their own gravity, and are thus
termed dwarf planets. In addition to thousands of small
bodies in those two regions, various other small body
populations, such as comets, centaurs and interplanetary
dust, freely travel between regions.
Six of the planets and three of the dwarf planets are orbited
by natural satellites, usually termed "moons" after Earth's
Moon. Each of the outer planets is encircled by planetary
rings of dust and other particles.
The solar wind, a flow of plasma from the Sun, creates a
bubble in the interstellar medium known as the
heliosphere, which extends out to the edge of the scattered
disc. The hypothetical Oort cloud, which acts as the source
for long-period comets, may also exist at a distance roughly
a thousand times further than the heliosphere.
The Solar System is located in the Milky Way galaxy, which
contains about 200 billion stars.
Our Solar System
Discovery and Exploration
For many thousands of years, humanity, with a few
notable exceptions, did not recognize the existence
of the Solar System. People believed the Earth to be
stationary at the center of the universe and
categorically different from the divine or ethereal
objects that moved through the sky. Although the
Greek philosopher Aristarchus of Samos had
speculated on a heliocentric reordering of the
cosmos, Nicolaus Copernicus was the first to develop
a mathematically predictive heliocentric system.
His 17th-century successors, Galileo Galilei, Johannes
Kepler and Isaac Newton, developed an understanding
of physics that led to the gradual acceptance of the
idea that the Earth moves around the Sun and that the
planets are governed by the same physical laws that
governed the Earth. Additionally, the invention of the
telescope led to the discovery of further planets and
moons. In more recent times, improvements in the
telescope and the use of unmanned spacecraft have
enabled the investigation of geological phenomena
such as mountains and craters, and seasonal
meteorological phenomena such as clouds, dust storms
and ice caps on the other planets.
Structure
The principal component of the Solar System is
the Sun, a main-sequence G2 star that contains
99.86 percent of the system's known mass and
dominates it gravitationally. The Sun's four
largest orbiting bodies, the gas giants, account
for 99 percent of the remaining mass, with
Jupiter and Saturn together comprising more
than 90 percent.
Most large objects in orbit around the Sun lie
near the plane of Earth's orbit, known as the
ecliptic. The planets are very close to the
ecliptic while comets and Kuiper belt objects
are frequently at significantly greater angles
to it. All the planets and most other objects
orbit the Sun in the same direction that the
Sun is rotating (counter-clockwise, as viewed
from above the Sun's north pole). There are
exceptions, such as Halley's Comet.
How long does it take the sun to reach
objects in our solar system?
Mercury ~ 3.16 minutes (190 seconds)
Venus ~ 6 minutes (360 seconds)
Earth ~ 8.3333 minutes (500 seconds)
Moon: Approximately the same as Earth
Mars ~ 12.6 minutes (760 seconds)
Jupiter ~ 43 minutes
Saturn ~ 1 hour 20 minutes
Uranus ~ 2 hours 40 minutes
Neptune ~ 4 hours 40 minutes
----------Dwarf Planets
Pluto ~ 5 hours 30 minutes
Ceres ~ 23 minutes
Eris ~ 9 hours 23 minutes
Comet Hale-Bopp
March 3, 1997
Comet PANSTARRS March 2, 2013
The overall structure of the charted regions of the
Solar System consists of the Sun, four relatively small
inner planets surrounded by a belt of rocky asteroids,
and four gas giants surrounded by the outer Kuiper
belt of icy objects. Astronomers sometimes informally
divide this structure into separate regions. The inner
Solar System includes the four terrestrial planets and
the asteroid belt. The outer Solar System is beyond
the asteroids, including the four gas giant planets.
Since the discovery of the Kuiper belt, the outermost
parts of the Solar System are considered a distinct
region consisting of the objects beyond Neptune.
Kepler's laws of planetary motion describe the orbits
of objects about the Sun. Following Kepler's laws,
each object travels along an ellipse with the Sun at
one focus. Objects closer to the Sun (with smaller
semi-major axes) travel more quickly, as they are
more affected by the Sun's gravity. On an elliptical
orbit, a body's distance from the Sun varies over the
course of its year. A body's closest approach to the
Sun is called its perihelion, while its most distant point
from the Sun is called its aphelion. The orbits of the
planets are nearly circular, but many comets,
asteroids and Kuiper belt objects follow highly
elliptical orbits.
Due to the vast distances involved, many
representations of the Solar System show orbits
the same distance apart. In reality, with a few
exceptions, the farther a planet or belt is from
the Sun, the larger the distance between it and
the previous orbit. Attempts have been made to
determine a relationship between these orbital
distances but no such theory has been
accepted.
Most of the planets in the Solar System possess
secondary systems of their own, being orbited
by planetary objects called natural satellites, or
moons (two of which are larger than the planet
Mercury), or, in the case of the four gas giants,
by planetary rings; thin bands of tiny particles
that orbit them in unison. Most of the largest
natural satellites are in synchronous rotation,
with one face permanently turned toward their
parent.