Astronomy 1 – Fall 2014

Lecture 5; October 16, 2014

Previously on Astro-1• The Nature of Light

– Electromagnetic radiation– Relation of speed to wavelength and frequency– Wavelength dependence of scattering– Doppler effect and Doppler formula

• Blackbody Radiation– Describes the spectrum of light emitted by opaque sources– The temperature of the blackbody determines

• The spectrum (Wien’s Law)• The energy flux (Stefan-Boltzman Law)

• Kirchoff’s Laws– A hot body produces a continuous spectrum– A hot transparent gas produces emission lines– Cool transparent gas in front of a hot body produces absorption lines

Today on Astro-1•Light can have particle-light properties.– The particles of light are called photons.– Planck’s Law: E = h = hc/– Atoms absorb & emit photons at discrete energies

• Spectroscopy and the composition of objects

•Geometrical Optics– Reflection/Mirrors– Refraction/Lenses

•Telescopes– Optical– Other wavelengths

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1. A hot, dense object such as a blackbody emits a continuous spectrum covering all wavelengths.

2. A hot, transparent gas produces a spectrum that contains bright (emission) lines.

3. A cool, transparent gas in front of a light source that itself has a continuous spectrum produces dark (absorption) lines in the continuous spectrum.

Kirchoff’s Laws

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What causes spectral lines?The structure of atoms

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Rutherford’s Experiment

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Rutherford’s model of the atom.

Today we know this is not exactly correct – electrons do not orbit the nucleus, but the basic idea is right -- protons and neutrons exist in the nucleus, and electrons are outside of it.

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Planck’s Law “Light is also a Particle”

or

E = Energy of a photonh = Planck’s constant = 6.625×10-34 J sc = speed of lightλ = wavelength of lightν = frequency of light

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What is the Energy of a Photon?

Example: DNA molecules are easily broken when hit with ultraviolet light at 260 nm. How much energy does a single photon at this wavelength have?

A. 7.6 x 10-19 JB. 7.6 x 10-17 JC. 7.6 JD. 5.7 x 10-49 JE. 7.6 x 1019 J

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Niels Bohr1885-1962

Was a postdoc with Rutherford. In 1912, to explain discrete nature of spectral lines, hypothesized that electron orbits are quantized (quantum mechanics!). Bohr and Einstein, 1925

The Bohr model of the atom

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The quantum nature of light is related to the quantum nature of atoms!

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R = Rydberg constant = 1.097×107 m-1

n = any integer greater than 2

In 1885 Swiss schoolteacher Johann Jakob Balmer, by trial and error, created a formula that can predict where lines of hydrogen fall in the spectrum of a star.

We still call these Balmer lines.

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The Balmer series and fomula.R = Rydberg constant = 1.097×107 m-1

N = lower orbitaln = higher orbital

Bohr figured out the physical explanation for Balmer’s formula – the spectra from stars depends on the structure of atoms!

Electron Transitions in the Hydrogen Atom The same wavelength occurs whether a photon is emitted or absorbed.

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Atomic number is the number of protons in an atom.

Every Element Has a Unique Set of Spectral Lines

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The Ring Nebula is a shell of glowing gases surrounding a dying star.

Hydrogen & Oxygen

Nitrogen & Sulpher

Spectroscopy Reveals the Chemical Composition of Celestial Objects

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Light is a wave… it is affected by motion too.

Bottom line: you can tell how fast something is moving from its spectrum… sort of

Extra-solar planets: most have been discovered using Doppler shift measurements of their parent stars (since 1995)

Spectral Lines (iclicker Question)

Professor Martin used a spectrograph on the Keck telescope to observe a distant galaxy. She detected 2 absorption lines from sodium atoms. The wavelengths she measured were 0.22 nm bluer than the wavelengths of 589.0 and 589.6 nm where she expected to find the lines. What should she conclude?

A. There are cool clouds between the observer and the galaxy.

B. There gas between the galaxy and the observer is hotter than the galaxy.

C. The gas clouds are moving away from the galaxy towards the observer.

D. The gas clouds are falling into the galaxy.E. Both A and C

Structure of Atoms(icliker Question)

• Most of the mass of ordinary matter resides in the

• A) electrons and nuclei, shared equally• B) nuclei of atoms• C) electrons around the nuclei of atoms• D) energy stored within the atom in electromagnetic

forces• E) Atoms have no mass.

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Geometrical Optics

The Light Rays from Distant Objects are Parallel

…and creates extended images.

Light from Every Point on an Extended Object Passes through Every Point in the Lens

Refraction Demo:Change in Direction of a Light Ray

RefractionWhich Way Does the Path Bend?

Demo:Law of Reflection

Importance of the telescope

Galileo’s Telescope

‘Three great events stand at the threshold of the modern age and determine its character: 1) the discovery of America; 2) the Reformation; 3) the invention of the telescope and the development of a new science that considers the nature of the Earth from the viewpoint of the universe’ (Hannah Arendt, ‘The Human Condition’)

The Keplerian Telescope

Telescope Demo(iclicker Question)

The objective lens of our telescope demo has a focal length of 500 mm. Suppose we replace the 250 mm eyepiece with a 50 mm eyepiece. How will the magnification of the meter stick change?

A. Increase 5xB. Increase to 10xC. Decrease to 50xD. Increase to 50xE. The magnification does not change.

Telescope Demo(iclicker Question)

The objective lens of our telescope demo has a focal length of 500 mm. Suppose we replace the 250 mm eyepiece with a 50 mm eyepiece. How should we move the objective lens?

A. Move the objective lens closer to the eyepiece.B. Move the objective lens further from the eyepiece.C. Move the objective lens away from the optical axis.D. There is no need to refocus the telescope.E. Back and forth by trial and error; it cannot be

predicted.

Telescope Demo(iclicker Question)

Suppose the professor covers the bottom half of the objective lens. What will the lass see through the telescope on the sceen?

A. The top of the ruler, and this image will be inverted left to right.

B. The bottom of the ruler because the image is inverted, and this image will be inverted left to right.

C. The top of the ruler, and this image will NOT be inverted left to right.

D. The bottom of the ruler because the image is inverted, and this image will NOT be inverted left to right.

E. The same image we saw previously.

Name some shortcomings of lenses

Newtonian Reflector

Telescopes:Light gathering power

Light gathering power depends on size of objective lens or primary mirror

How Do You Make a Lightweight 10m Diameter Mirror?

How much more light gathering power does a 10m telescope have

than an 0.5 m telescope?

Answer: The light gathering power is proportional to the square of the mirror’s diameter.(10m)2/(0.5m)2 = 100m / 0.25m = 400

So you can see objects about 400 times fainter with the 10m telescope in the same amount of time.

Reflection Telescopes

The Secondary Mirror Does Not Cause a Hole in the Image

This illustration shows how even a small portion of the primary (objective) mirror of a reflecting telescope can make a complete image of the Moon. Thus, the secondary mirror does not cause a black spot or hole in the image. (It does, however, make the image a bit dimmer by reducing the total amount of light that reaches the primary mirror.)

Reflecting Telescopes

This view of the Gemini North telescope shows its 8.1-meter objective mirror (1). Light incident on this mirror is reflected toward the 1.0-meter secondary mirror (2), then through the hole in the objective mirror (3) to the Cassegrain focus

Angular Resolution

Angular resolution of the telescope

Limited by:

• Blurring effects of the atmosphere (“seeing”), i.e. the twinking of stars

• The quality of the optics and detector on the telescope.

• The size of the telescope – the “diffraction limit.”

The diffraction limitθ= diffraction-limited angular resolution of the telescope, in arcsecondsλ= wavelength of light, in metersD = diameter of telescope objective, in meters

Example: What is the diffraction limit for red light (640nm=6.4×10-7m) for a telescope with with a 0.5m objective/primary.

So even if you had a perfect atmosphere and perfect optics, you couldn’t resolve details finer than 0.32” with a 0.5m telescope.

Today astronomers build telescopes at the best sites in the world, then travel to the telescope to observe, or have someone else on-site observe for them, or observe remotely over the internet.

Mauna Kea, an extinct volcano in Hawaii that reaches 13,400 feet, is the best site in the world for optical and infrared telescopes. It has mostly clear, dark skies, little atmospheric turbulence, and is above most of the water vapor in the Earth’s atmosphere. Notice the snow and lack of vegetation.

Adaptive Optics Help Telescopes on Earth Remove the Blurring Caused by the Atmosphere

Laser Beacon Makes an Artificial Star

Adaptive Optics System

The percentage of radiation that can penetrate the Earth’s atmosphere at different wavelengths. Regions in which the curve is high are called“windows,” because the atmosphere is relatively transparent at thosewavelengths. There are also three wavelength ranges in which theatmosphere is opaque and the curve is near zero: at wavelengths less than about 290 nm, which are absorbed by atmospheric oxygen and nitrogen; between the optical and radio windows, due to absorption by water vapor and carbon dioxide; and at wavelengths longer than about 20 m, which are reflected back into space by ionized gases in the upper atmosphere.

Astronomy Uses the Entire EM Spectrum

Hubble Space Telescope

James Webb Space Telescope(see movie on class website)

Saturn Reflects the Sun’s Light;And It Also Emits Light as Do All Blackbodies

Orion

Discovery Enabled by YearThe heavens are not perfect and unchanging; (ultimately) the Earth is not the center of the universe.

The telescope and Galileo’s observations. ~1609

The sun and stars are giant balls of hydrogen undergoing fusion.

Fraunhofer’s invention of the spectrograph. 1814

Our galaxy is not the center of the universe, and the universe is expanding.

Edwin Hubble and the giant Palomar 200-inch and large-format photographic plates.

1929

The universe started as a hot “Big Bang”

Penzias and Wilson using a radio “telescope,” confirmed by satellites.

1965

Planets are common in the universe. Modern charge-couple-device detectors (CCD); Iodine cell for spectrograph.

1995

Dark Energy dominates the universe.

Large-format CCD detectors; 10m Keck telescope.

1998

And there are many more involving infrared, x-ray, ultraviolet and gamma-ray discoveries.

Summary• Light can have particle-light properties.

• Particle energy: E = h = hc/• Every element (even every ion) has a unique spectral

fingerprint.

• Spectroscopy reveals the composition of distant objects

• Geometrical optics– Reflection & Refraction– Focus, Spherical aberration, Chromatic aberration

• Telescopes – Light gathering power– Magnification– Resolving power

Homework (Due Monday 10/20)

• On your own: answer all the review questions in chapter 5 & 6.s

• To TAs: answer questions, – 5.34 (Note that Io’s surface temperature is -150o C

and not 2150o C), – 5.37, 5.43, 5.44 – 6.34, 6.36, 6.40, 6.41