Origin of Modern Astronomy

Origin of Modern Astronomy

Ancient Greeks

Early Astronomy

Astronomy is the science that studies the universe. It includes the observation and interpretation of celestial bodies and phenomena.

The Greeks used philosophical arguments to explain natural phenomena.

The Greeks also used some observational data.

Ancient Greeks

Early Astronomy

Geocentric Model = Ptolemy Greek Astronomer• In the ancient Greeks’ geocentric model, the

moon, sun, and the known planets—Mercury, Venus, Mars, and Jupiter—orbit Earth.

Heliocentric Model = Nicolaus Copernicus

• In the heliocentric model, Earth and the other planets orbit the sun.

Ancient Greeks

Early Astronomy

Ptolemaic System• Ptolemy created a model of the universe that

accounted for the movement of the planets.• Retrograde motion is the apparent westward

motion of the planets with respect to the stars.

Retrograde motion of Mars

East West

Sept.

Aug. July

June

Dec.Jan.Feb.March

AprilMay

Retrograde Motion

99 Years of Astronomy

The Birth of Modern Astronomy

Early Astronomy

Nicolaus Copernicus• Copernicus concluded that Earth is a planet. He

proposed a model of the solar system with the sun at the center. Heliocentric Model

This model explained the retrograde motion of planets better than the geocentric model.


Early Astronomy

Tycho Brahe• Tycho Brahe designed and built instruments to

measure the locations of the heavenly bodies. Brahe’s observations, especially of Mars, were far more precise than any made previously.

Johannes Kepler• Kepler discovered three laws of planetary motion:1. Orbits of the planets are elliptical.2. Planets revolve around the sun at varying speed.3. There is a proportional relationship between a planet’s

orbital period and its distance to the sun.


Early Astronomy

German astronomer Johannes Kepler (1571-1630) helped establish the era of modern astronomy by deriving three laws of planetary motion.

Johannes Kepler

• 1599 – Kepler hired by Tycho Brahe – Work on the orbit of Mars

• 1609 – Kepler’s 1st and 2nd Laws– Planets move on ellipsesellipses with the Sun at one focusfocus– The radius vectorradius vector sweeps out equal areas in equal

times

• 1618 – Kepler’s 3rd Law– The squaresquare of a planet’s orbital period Porbital period P is

proportional to the cubecube of its semi-major axis Rsemi-major axis R.

Early Astronomy

Johannes Kepler used Tycho Brahe’s data to develop three laws that explained the motions of the planets.

June 15th

July 15th

January 15th

December 15th

(30 days) (30 days)

Sun

Equal areas

Earth’s orbit

KEPLER’S EQUAL AREA LAW states that a line connecting Earth to the sun will pass over equal areas of space in equal times. Because Earth’s orbit is elliptical, Earth moves faster when it is nearer to the sun.

Early Astronomy

Faster Slower

Equal areas law

KEPLER’S EQUAL AREA LAW states that a line connecting Earth to the sun will pass over equal areas of space in equal times. Because Earth’s orbit is elliptical, Earth moves faster when it is nearer to the sun.

Early Astronomy

Galileo Galilei

Italian scientist Galileo Galilei (1564—1642) used a new invention, the telescope, to observe the Sun, Moon, and planets in more detail than ever before.


Early Astronomy

Galileo Galilei

• Galileo’s most important contributions were his descriptions of the behavior of moving objects.

• He developed his own telescope and made important discoveries:

1. Four satellites, or moons, orbit Jupiter. 2. Planets are circular disks, not just points of light.3. Venus has phases just like the moon.

4. The moon’s surface is not smooth.

5. The sun has sunspots, or dark regions.

Early Astronomy

Sir Isaac Newton

English scientist Sir Isaac Newton (1642—1727) explained gravity as the force that holds planets in orbit around the Sun.


Early Astronomy

Sir Isaac Newton• Although others had theorized the existence of

gravitational force, Newton was the first to formulate and test the law of universal gravitation. The universal law of gravitation, helped explain the motions of planets in the solar system.

Universal Gravitation

• Gravitational force decreases with distance.

• The greater the mass of an object, the greater is its gravitational force.

Gravity’s Influence on Orbits

Newton’s Laws of Motion

• 1st Law– A body at rest, or in uniform motion, will

remain so unless acted upon by an unbalanced forceunbalanced force.

• 2nd Law– The change in motion (accelerationacceleration) is

proportional to the unbalanced force

• 3rd Law– For every actionaction there is an equal and

opposite reactionreaction

Gravity

• Gravity is the force that– holds us to the Earth– causes a rock to fall towards the ground– causes the Earth to go around the Sun– causes the Sun to be pulled towards the center

of the Milky Way galaxy

• Gravity acts between any two objects even if they are far apart. – “action at a distanceaction at a distance”

Summary• Kepler’s and Galileo’s Laws provided Newton

with important clues that helped him formulate his laws of motion

• Newton arrived at 3 laws that govern the motion of objects– The law of inertia

– The law of force

– The law of action and reaction

• Newton also arrived at a law of gravity– But it seemed to require action at a distance!

Light and Astronomical Observations

Earth Science

• An ellipse is an oval-shaped path.

An astronomical unit (AU) is the average distance between

Earth and the sun; it is about 150 million kilometers.Light-year The distance that light travels in one year, about 9.5 trillion kilometers.

Parsec: A unit of measurement used to describe distances between celestial objects, equal to 3.258 light-years.

Important Astronomical Measurements

Electromagnetic radiation

•Visible light is only one small part of an array of energy

•Electromagnetic radiation includes•Gamma rays

•X-rays

•Ultraviolet light

•Visible light

•Infrared light

•Radio waves

The study of light

*Energy radiated in the form of a wave, resulting from the motion of electric charges and the magnetic fields they produce.

The study of light Electromagnetic radiation •All forms of radiation travel at 300,000 kilometers (186,000 miles) per secondLight (electromagnetic radiation) can be

described in two ways Wave modelWavelengths of radiation vary

Radio waves measure up to several kilometers long

Gamma ray waves are less than a billionth of a centimeter long

White light consists of several wavelengths corresponding to the colors of the rainbow

A continuum depicting the range of electromagnetic radiation, with the longest wavelength at one end and the shortest at the other.

Light (electromagnetic radiation) can be described in two ways

•Particle model

•Particles called photons

•Exert a pressure, called radiation pressure, on matter

•Shorter wavelengths correspond to more energetic photons

Spectroscopy

•The study of the properties of light that depend on wavelength

•The light pattern produced by passing light through a prism, which spreads out the various wavelengths, is called a spectrum (plural: spectra)

The study of light

A spectrum is produced when white light passes through a prism

The study of light

Spectroscopy

The study of light

•Types of spectra

•Continuous spectrum: A spectrum that contains all colors or wavelengths.

•Produced by an incandescent solid, liquid, or high pressure gas

•Uninterrupted band of color

•Dark-line (absorption) spectrum

•Produced when white light is passed through a comparatively cool, low pressure gas

•Appears as a continuous spectrum but with dark lines running through it

Formation of the three types of spectra

A spectrum consisting of individual lines at characteristic wavelengths produced when light passes through an incandescent gas; a bright-line spectrum.

Emission Spectrum

A continuous spectrum crossed by dark lines produced when light passes through a nonincandescent gas.

Absorption Spectrum

Emission spectrum of hydrogen

Absorption Spectrum of Hydrogen

Doppler effect

•The apparent change in wavelength of radiation caused by the relative motions of the source and observer

•Used to determine•Direction of motion

•Increasing distance – wavelength is longer ("stretches")

•Decreasing distance – makes wavelength shorter ("compresses")

•Velocity – larger Doppler shifts indicate higher velocities

The study of light

The Doppler effect

Originally discovered by the Austrian mathematician and physicist, Christian Doppler (1803-53), this change in pitch results from a shift in the frequency of the sound waves.

•Redshift, a phenomenon of electromagnetic waves such as light in which spectral lines are shifted to the red end of the spectrum.

The electromagnetic radiation emitted by a moving object also exhibits the Doppler effect.

The Doppler effect

http://en.wikipedia.org/wiki/Redshift

The radiation emitted by an object moving toward an observer is squeezed; its frequency appears to increase and is therefore said to be blueshifted. In contrast, the radiation emitted by an object moving away is stretched or redshifted. Blueshifts and redshifts exhibited by stars, galaxies and gas clouds also indicate their motions with respect to the observer.

The Doppler effect

Redshift: This spectrum shows hydrogen shifted to the red end of the spectrum. This star is moving away from Earth.

Blueshift: This spectrum shows hydrogen shifted to the blue end of the spectrum. This star is moving toward Earth.

Optical (visible light) telescopes

•Two basic types (1) Refracting telescope •Uses a lens (called the objective) to bend (refract) the light to produce an image

•Light converges at an area called the focus

•Distance between the lens and the focus is called the focal length

•The eyepiece is a second lens used to examine the image directly

•Have an optical defect called chromatic aberration (color distortion)

Astronomical tools

A simple refracting telescope


•Two basic types (2) Reflecting telescope

•Uses a concave mirror to gather the light

•No color distortion

•Nearly all large telescopes are of this type

Astronomical tools

A prime focus reflecting telescope

Cassegrain focus reflecting telescope

Newtonian focus reflecting telescope

The 200" (5m) Hale Reflector of Palomar Observatory is shown above. Until recently it was the world's largest optical/infrared telescope.


•Properties of optical telescopes

•Light-gathering power

•Larger lens (or mirror) intercepts more light

•Determines the brightness

•Resolving power

•The ability to separate close objects

•Allows for a sharper image and finer detail

Astronomical tools


•Properties of optical telescopes

•Magnifying power

•The ability to make an image larger

•Calculated by dividing the focal length of the objective by the focal length of the eyepiece

•Can be changed by changing the eyepiece

•Limited by atmospheric conditions and the resolving power of the telescope

•Even with the largest telescopes, stars (other than the Sun) appear only as points of light

Astronomical tools

Detecting invisible radiation

•Radio radiation

•Gathered by "big dishes" called radio telescopes

•Large because radio waves are about 100,000 times longer than visible radiation

•Often made of a wire mesh

•Have rather poor resolution

•Can be wired together into a network called a radio interferometer

Astronomical tools

A steerable radio telescope at Green Bank, West Virginia

Radio Telescope

Detecting invisible radiation

•Radio radiation

•Gathered by "big dishes" called radio telescopes

•Advantages over optical telescopes

•Less affected by weather

•Less expensive

•Can be used 24 hours a day

•Detects material that does not emit visible radiation

•Can "see" through interstellar dust clouds

Astronomical tools

The 300-meter radio telescope at Arecibo, Puerto Rico

Radio Telescope

The theory holding that the universe originated from the instant expansion of an extremely small agglomeration of matter of extremely high density and temperature.

The Big Bang Theory

Photons converted into particle-antiparticle pairs and vice-versa

E = mc2

Early universe was full of particles and radiation because of its high temperature

The Big Band Theory

•Evidence for Big Bang

•This is the theory of the universe’s earliest moments.•It presumes that the universe began from a tiny, hot, and dense collection of matter and radiation.•It describes how expansion and cooling of particles could have led to the present universe of stars and galaxies. •It explains several aspects of today’s universe with a very good accuracy.

The Big Band Theory

The Big Bang theory is a model, which explains some facts (observations).

It should be able to make predictions that can be verified through observations or experiments.

Two important predictions:

• Cosmic microwave background radiation.2. Fusion of original hydrogen into helium.

Evidence for the Big Bang

The Cosmic Background Radiation (Microwaves)

Penzias & Wilson (1962) discovered an isotropic background microwave signal during testing a microwave antenna at Bell Labs in 1965. The noise was found to be coming from every direction. At the same time, physicists from Princeton calculated the expected radiation from the initially hot universe.They suggested that this radiation could be detected with a microwave antenna.

The result was a Nobel Prize in physics for 1978.

The Cosmic Microwave Background

The Cosmic Background Radiation (Microwaves)

Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/IR

The Cosmic Microwave Background

The background consists of photons (radiation) arriving at Earth directly from the end of the era of nuclei (when the Universe was about 380,000 years old).Neutral atoms captured most of the electrons.Photons were released and have flown freely through the universe ever since.

This background radiation can be detected with a small TV antenna as part (1%) of static “snow”. The redshifted spectrum of the background radiation has now a temperature of 2.73 K.

Cosmic Background Explorer

The first satellite built dedicated to cosmology. Its goals were to investigate the cosmic microwave background radiation (CMB) of the universe and provide measurements that would help shape our understanding of the cosmos.

This work helped cement the big-bang theory of the universe. According to the Nobel Prize committee, "the COBE-project can also be regarded as the starting point for cosmology as a precision science". Two of COBE's principal investigators, George Smoot and John Mather, received the Nobel Prize in Physics in 2006.



The "famous" map of the CMB anisotropy formed from data taken by the COBE spacecraft.


In 1927, the Belgian priest Georges Lemaître was the first to propose that the universe began with the explosion of a primeval atom.



Edwin Hubble found experimental evidence to help justify Lemaître's theory. He found that distant galaxies in every direction are going away from us with speeds proportional to their distance (the redshift). The big bang was initially suggested because it explains why distant galaxies are traveling away from us at great speeds. The theory also predicts the existence of cosmic background radiation (the glow left over from the explosion itself). The Big Bang Theory received its strongest confirmation when this radiation was discovered in 1964 by Arno Penzias and Robert Wilson, who later won the Nobel Prize for this discovery.

Hubble’s Evidence

• Doppler shifting - wavelength emitted by something moving away from us is shifted to a lower frequency

• Sound of a fire truck siren - pitch of the siren is higher as the fire truck moves towards you, and lower as it moves away from you

• Visible wavelengths emitted by objects moving away from us are shifted towards the red part of the visible spectrum

• The faster they move away from us, the more they are redshifted. Thus, redshift is a reasonable way to measure the speed of an object (this, by the way, is the principal by which radar guns measure the speed of a car or baseball)

• When we observe the redshift of galaxies outside our local group, every galaxy appears to be moving away from us - universe is expanding.

Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves

Big Bang Theory - Evidence for the TheoryWhat are the major evidences which support the Big Bang theory?


•First of all, we are reasonably certain that the universe had a beginning. •Second, galaxies appear to be moving away from us at speeds proportional to their distance. This is called "Hubble's Law," named after Edwin Hubble (1889-1953) who discovered this phenomenon in 1929. This observation supports the expansion of the universe and suggests that the universe was once compacted.


•Third, if the universe was initially very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat. In 1965, Radioastronomers Arno Penzias and Robert Wilson discovered a 2.725 degree Kelvin (-454.765 degree Fahrenheit, -270.425 degree Celsius) Cosmic Microwave Background radiation (CMB) which pervades the observable universe. This is thought to be the remnant which scientists were looking for. Penzias and Wilson shared in the 1978 Nobel Prize for Physics for their discovery.


•Finally, the abundance of the "light elements" Hydrogen and Helium found in the observable universe are thought to support the Big Bang model of origins.

Synthesis of Helium

•The current CMB temperature tells us precisely how hot the universe was when it appeared.

•It tells us how much helium was initially produced.

•A helium nucleus contains 2 protons and 2 neutrons.

•At T > 1011 K, nuclear reactions converted protons into neutrons and back, keeping their numbers nearly equal.•Between 1010 and 1011 K, neutron – proton reactions favor protons, because neutrons are heavier than protons.

•Energy is required to convert protons to neutrons.

•At T < 1010 K, only neutrons can be changed into protons.

•However, fusion continued to operate

•and protons and neutrons combined into deuterium.

•Then deuterium fused into helium.

•During the early era of nucleosynthesis, helium nuclei were being destroyed by gamma-rays.

•At ~1 minute, gamma-rays were gone and the proton – neutron ratio was set to 7:1.

Synthesis of Helium

Big Bang theory prediction: 75% H, 25% He (by mass)

Matches observations of nearly primordial gases

Synthesis of Helium

Abundances of other light elements agree with Big Bang model having 4.4% normal matter – more evidence for WIMPS!

Synthesis of Helium

Nebular Hypothesis of Solar System Formation.

Origin of Modern Astronomy

Documents

retrograde motion of

heliocentric model

early astronomyjohannes

era of modern astronomy

johannes kepler kepler

planets orbital period

orbit of mars

equal times