Chapter 5 Light Astronomers can explore most of the Universe by studying the ________ that comes to Earth from distant objects. light

Properties of Light Light behaves as both a ________ and a _____ Light particles are called __________, which can be counted individually. Light is also an ___________________ particle wave photons electromagnetic wave

Light is an electromagnetic wave = Wavelength

Electromagnetic Radiation ______________________ can be described in terms of a stream of _______, each traveling in a wave-like pattern, moving at the _______________ and carrying some amount of energy. The only difference between radio waves, visible light, and gamma-rays is the energy of the photons. ______ waves have photons with low energies, ___________ have a little more energy than radio waves, ________ has still more, then ______, ___________, ________, and ___________. Electromagnetic radiation photons infrared Radio ultravioletvisible microwaves speed of light gamma-raysX-rays

Examples from Space!

Light is Fast! 186,282 miles per second 670,616,629 miles per hour 300,000 kilometer per second Earth Sun

At this speed it can go around the world ________ in one second. 8 times

Radio Emitted by –Astronomical Objects –Radio Station transmitters Detected by –Ground based radio telescopes –Radios

Microwave Emitted by: –Gas clouds collapsing into stars –Microwave Ovens –Radar Stations –Cell Phones Detected by –Microwave Telescopes –Food (heated) –Cell phones –Radar systems

Infrared Emitted by –Sun and stars (Near) –TV Remote Controls –Food Warming Lights (Thermal) –Everything at room temp or above Detected by –Infrared Cameras –TVs, VCRs, –Your skin

Visible Emitted by –The sun and other astronomical objects –Laser pointers –Light bulbs Detected by –Cameras (film or digital) –Human eyes –Plants (red light) –Telescopes

Ultraviolet Emitted by –Tanning booths (A) –The Sun –Black light bulbs (B) –UV lamps Detected by –Space based UV detectors –UV Cameras –Flying insects (flies)

X-ray Emitted by –Astronomical objects –X-ray machines –CAT scan machines –Older televisions –Radioactive minerals –Airport luggage scanners Detected by –Space based X-ray detectors –X-ray film –CCD detectors

Chandra X-ray Observatory Chandra is designed to observe X-rays from high energy regions of the universe, such as the remnants of exploded stars. One of the most sophisticated observatory built. Deployed by the Space Shuttle Columbia on July 23, Chandra X-ray Observatory

Gamma Ray Emitted by –Radioactive materials –Exploding nuclear weapons –Gamma-ray bursts –Solar flares Detected by –Gamma detectors and astronomical satellites –Medical imaging detectors

X-rays and Gamma Rays Black holes Active Galaxies Pulsars Diffuse emission Supernovae Gamma-ray bursts Unidentified

Apparent Brightness Inverse Square Law

Stellar Luminosity __________ is the total amount of power the star radiates into space. It is measured in power units (Watts). __________of a star in the sky depends on the distance to a star and its luminosity. The __________________ is the amount of light reaching us per unit area. Luminosity apparent brightness Brightness

Magnitude system for brightness ________numbers imply ________ stars. “Apparent magnitude” is a measure of apparent brightness. Antares has mag. 1; Polaris has mag. 2; naked eye limit is about ___. Sirius has mag. –1.5. The faintest stars observed with HST are of ~ _____ magnitudes. A star of magnitude 1.00 is ____ times brighter than a star of magnitude Smallerbrighter 30th 2.5 6

The Nearest Stars Name Distance (light years) Apparent magnitude Luminosity (compared to sun) Sun–26.71 Alpha Centauri A Alpha Centauri B Alpha Centauri C Barnard’s star Wolf HD Luyten 726-8A Luyten 726-8B Sirius A8.6– Sirius B Ross Ross

The Brightest Stars (as viewed from earth) Name Distance (light years) Apparent magnitude Luminosity (compared to sun) Sun–26.71 Sirius A Canopus ,000 Arcturus Alpha Centauri A Vega Capella Rigel ,000 Procyon Betelgeuse Achernar Hadar (Beta Cen) ,000

Brightness of Stars True brightness (“_________”) is a star’s actual rate of energy output, measured (for example) in watts. The Sun’s luminosity is about 4 x watts. Apparent brightness is determined by the “________” of starlight striking a detector. It is measured (for example) in watts per square meter. The Sun’s apparent brightness from Earth’s location is about 1400 watts per square meter. Formula: Apparent brightness = True brightness 4π(distance) 2 luminosity intensity

Brightness of Stars Formula: Apparent brightness = True brightness 4π(distance) 2

The _______________ is due to the dilution of the light. At each radius you have the _____ total amount of light going through the surface of an imaginary sphere. Surface area of a sphere increases by R 2. The light/area therefore decreases by ____. inverse square law same 1/R 2

Light has 1/4 the intensity compared to a distance of R Light more concentrated R 2R Inverse Square Law

Apparent Brightness Apparent brightness obeys an inverse square law with distance. At the distance of Jupiter (__ A.U.), the Sun is _______ times dimmer than on Earth. 5 25

Alpha Centauri How does the apparent brightness of Alpha Centauri compare to the Sun? –Alpha Centauri radiates almost the same amount of light as the Sun. –Distance to Sun = 1.5 x 10 8 km (1 A.U.) –Distance to Star = 3.8 x km (4 ly) 1ly = 63,000 A.U. 3.8 x km 1.5 x 10 8 km = 252,000 times further

Alpha Centauri Alpha Centauri is located 252,000 times Earth’s distance from the Sun. Inverse Square Law 1/distance 2 Thus, its apparent brightness is __________ times less than that of the Sun. 1/(252,000) billion

Suppose we move the Sun to three times its current distance. How much fainter will the Sun appear?

QUESTION QUESTION: Two stars have the same luminosity, but one is 2X further away from the earth. It will appear –1/4 as bright –1/2 as bright –2X brighter –4X brighter Repeat for the case that the second star is 2X closer. Use choices from above.

QUESTION QUESTION: Two stars have the same luminosity, but one is 2X further away from the earth. It will appear –1/4 as bright –1/2 as bright –2X brighter –4X brighter Repeat for the case that the second star is 2X closer. Use choices from above.

QUESTION QUESTION: Two stars have the same luminosity, but one is 2X further away from the earth. It will appear –1/4 as bright –1/2 as bright –2X brighter –4X brighter Repeat for the case that the second star is 2X closer. Use choices from above.

QUESTION QUESTION: Two stars have the same luminosity, but one is 2X further away from the earth. It will appear –1/4 as bright –1/2 as bright –2X brighter –4X brighter Repeat for the case that the second star is 2X closer. Use choices from above.

Predicted Observed Distance (cm) Fraction of Light/Square Number of Illum. Squares Equivalent Bulbs