Astrophysics --- History of Exploring the Universe

3P303 Chen Xie3I304 Chen Li

CON

TENT

•Celestial Sphere •Coordinate Systems•Constellation•Newton’s law of gravitation•Kepler’s law of planet motion •Cosmic Rays•Classification of Stars•The Lifecycle of Stars•Solar System •Moon’s motion•Eclipses•Telescopes

In astronomy, celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis.

• The celestial sphere is a very practical tool for positional astronomy

Celestial Sphere Definition

http://en.wikipedia.org/wiki/File:Celestial_sphere.svg

•The point on the celestial sphere that is directly over our heads at a given time is termed the zenith. •The imaginary circle passing through the North and South points on our horizon and through the zenith is termed the celestial meridian.

Celestial Sphere

Celestial Coordinate Systems

We can define a useful coordinate system for locating objects on the celestial sphere by projecting onto the sky the latitude-longitude coordinate system that we use on the surface of the earth

Celestial Coordinate Systems

East and West on the Celestial Sphere

Generally, one object is west of another object if it "rises" before the other object over the eastern horizon as the sky appears to turn, and east of the object if it "rises" after the other object.

ConstellationIn modern astronomy, a constellation is an internationally defined area of the celestial sphere.

In colloquial usage, a constellation is a group of celestial bodies, usually stars, which appear to form a pattern in the sky. Astronomers today still utilize the term, though the current system focuses primarily on constellations as grid-like segments of the celestial sphere rather than as patterns.

The constellation Orion is one of the most recognizable in the celestial sphere. The name is associated with the region of the star map marked in yellow, as well as with the pattern of stars within this area, marked in green.

ConstellationIn 1922, Eugène Delporte aided the IAU in dividing the celestial sphere into 88 official constellations. Typically, these modern constellations share the names of their Graeco-Roman predecessors, such as Orion, Leo and Scorpius.

With the technical advancement of astronomy, it became important to move from a pattern-based system of constellations to one based on area-mapping, which led to several historic formations becoming obsolete.

88 modern Constellation

Constellation — Ursa MajorUrsa Major is a constellation visible throughout the year in most of the northern hemisphere.It is dominated by the widely recognized asterism known as the Big Dipper or Plough, which is a useful pointer toward north, and which has mythological significance in numerous world cultures.

Constellation — LyraLyra is a constellation. Its name derived from the lyre, a stringed musical instrument well known for its use in classical antiquity and later. Lyra is a small constellation, but its principal star, Vega, is one of the brightest in the sky. Beginning at the north, Lyra is bordered by the Dragon Draco, the Greek hero Hercules, the little fox Vulpecula and Cygnus the swan.

Newton’s law of gravitationStatement: Every massive particle in the universe attracts every other massive particle with a force.

This force is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

•F is the magnitude of the gravitational force between the two point masses, •G is the gravitational constant, •m1 is the mass of the first point mass, •m2 is the mass of the second point mass, and •r is the distance between the two point masses.

Newton’s law of gravitationLet a1 be the gravity acceleration of one massive particle, according to Newton’s Second Law, we can get that:

→Substitute into the former equation, we can get:

We can see that the magnitude of a1 does not depend on the value of m1. So we can deduce that for any object, it will fall down at the same acceleration, which is the gravity constant. (Ignore the air resistance)

Kepler’s law of planet motion

In astronomy, Kepler's laws give an approximate description of the motion of planets around the Sun.

Kepler’s law of planet motion First Law:

The orbit of every planet is an ellipse with the Sun at a focus.

where (r, θ) are heliocentric polar coordinates for the planet, p is the semi-latus rectum, and ε is the eccentricity.

This law describes the locus of planet orbit around the Sun.

Kepler’s law of planet motion Second Law:

The line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

This is also known as the law of equal areas.

Kepler’s law of planet motion Third law:

The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit

where P is the orbital period of planet and a is the semi major axis of the orbit.

This law is a very important basis of Newton’s Law of Gravitation

Cosmic RaysParticles that bombard the Earth from anywhere beyond its atmosphere are known as cosmic rays. Cosmic rays don't take pretty pictures, but studying the quantity and type of these particles helps us to understand the acceleration processes involved and to measure the composition of the Sun, as well as sources at the far distant reaches of the galaxy.

Cosmic RaysGalactic Cosmic Rays -- coming from outside the solar system

Anomalous Cosmic Rays -- coming from the interstellar space at the edge of the heliopause

Solar Energetic Particles -- associated with solar flares and other energetic solar events 

Cosmic Rays

The relative numbers of different isotopes found in the galaxy are established by the life cycle of massive stars. Star formation, evolution and explosion results in the creation of many of the heavier isotopes found in space. The process is shown in the figure below.

Classification of StarsThe current stellar classification system originated in the early 20th century, when stars were classified fromA to Q based on the strength of the hydrogen line. It was not known at the time that the major influence on the line strength was temperature; the hydrogen line strength reaches a peak at over 9000 K, and is weaker at both hotter and cooler temperatures.

Class Temperature Sample star

O 33,000 K or more Zeta Ophiuchi

B 10,500–30,000 K Rigel

A 7,500–10,000 K AltairF 6,000–7,200 K Procyon AG 5,500–6,000 K SunK 4,000–5,250 K Epsilon Indi

M 2,600–3,850 K Proxima Centauri

The Lifecycle of Stars

Stars expand as they grow old. As the core runs out of hydrogen and then helium, the core contacts and the outer layers expand, cool, and become less bright.

Stars are born in nebulae. Huge clouds of dust and gas collapse under gravitational forces, forming protostars.

Solar SystemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, formed 4.6 billion years ago.

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.

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, Jupiter, Saturn, Uranus and Neptune, also called the gas giants, are composed largely of hydrogen and helium and are far more massive than the terrestrials.

Solar SystemThe Solar System is also home to two regions populated by smaller objects. The asteroid belt, is similar to the terrestrial planets as it is composed mainly of rock and metal.

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.

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.

Solar System — Structure

Solar System — Structure

The orbits of the bodies in the Solar System to scale (clockwise from top left)

Formation and evolution of the Solar System

The Solar System formed from the gravitational collapse of a giant molecular cloud 4.6 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars.

Moon’s motion

The Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days

Moon’s motion

Moon’s motionAppearance from Earth

Eclipses

Eclipses can only occur when the Sun, Earth, and Moon are all in a straight line.

Solar EclipsesSolar eclipses occur near a new Moon, when the Moon is between the Sun and Earth.

Solar Eclipses

Lunar Eclipseslunar eclipses occur near a full Moon, when the Earth is between the Sun and Moon.

Lunar Eclipses

Optical telescopesAn optical telescope gathers and focuses light mainly from the visible part of the elemmctromagnetic spectrum Optical telescopes increase the apparent angular size of distant objects as well as their apparent brightness. In order for the image to be observed, photographed, studied, and sent to a computer, telescopes work by employing one or more curved optical elements—usually made from glass—lenses, or mirrors to gather light and other electromagnetic radiation to bring that light or radiation to a focal point

50 cm refracting telescope at Nice Observatory.

Three main types of Optical telescopes The refracting telescope which uses lenses to form an image.

The reflecting telescope which uses an arrangement of mirrors to form an image.

The catadioptric telescope which uses mirrors combined with lenses to form an image.

Radio telescopesRadio telescopes are directional radio antennas used for radio astronomy. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the wavelength being observed. Multi-element Radio telescopes are constructed from pairs or larger groups of these dishes to synthesize large 'virtual' apertures that are similar in size to the separation between the telescopes. Radio telescopes are also used to collect microwave radiation, which is used to collect radiation when any visible light is obstructed or faint.

The Very Large Array at Socorro, New Mexico, United States.

High energy particle telescopesHigh-energy astronomy requires specialized telescopes to make observations since most of these particles go through most metals and glasses.X-ray and Gamma-ray telescopes are usually on Earth-orbiting satellites or high-flying balloons since the Earth's atmosphere is opaque to this part of the electromagnetic spectrum.

The Einstein Observatory, an X-ray telescope originally named the HEAO B

REFERENCES

•http://en.wikipedia.org/wiki/Telescope

•http://en.wikipedia.org/wiki/Star#Formation_and_evolution

•http://en.wikipedia.org/wiki/Constellation

•http://en.wikipedia.org/wiki/Solar_System

•http://en.wikipedia.org/wiki/Celestial_sphere

•http://en.wikipedia.org/wiki/Moon#Orbit_and_relationship_to_Earth

•http://en.wikipedia.org/wiki/Kepler%27s_law

•helios.gsfc.nasa.gov/cosmic.html

•http://www.enchantedlearning.com/subjects/astronomy/

•http://csep10.phys.utk.edu/astr161/lect/celestial/celestial.html