Astronomy 115 Section 4 Week 6 Adam Fries SF State afries@sfsu.edu
Planetarium shows at Noon (every two weeks) begins Friday 13th in the Hagar Planetarium (4th floor Thorton Hall) Only 45 seats! First come first serve!
Hw Questions?
A Slight Detour... Finish Chapter 3; Telescopes and Optics Tides; read Comins Chapter 15-8: 15-9 Start Chapter 4; Atomic Physics and Spectra HW #2 is due Friday or next Tuesday?
Light can act as a wave (Ex. standing wave), or as a particle (photon).
Crab Nebula: 4 different λ s. c = λf f = c/λ E = hf E = hc/λ h = 6.67 10 34 Js
Last time we determined that red light has a longer wavelength than blue light. Which light has more energy, red or blue? By how much?
Telescopes Have 3 Powers Light Gathering Power Resolving Power Magnifying Power
Which pizza is a better deal? A. 9 inch pizza for $10.00 B. 18 inch pizza for $20.00 C. They are the same.
Light Gathering Power Light gathering power determines how faint an object a telescope can see. It depends on the the Area of the primary lens or mirror. Area = πr 2 Area = π d2 4
Ex. How does the light gathering power of these two lenses (or mirrors) compare? Take a ratio of the square of their diameters!
Resolving Power - abilty to see fine detail It s proportional to the telescopes primary mirror diameter. The larger the mirror, the finer the detail.
Magnification Power Magnification depends on the focal length of the eyepiece and the primary mirror. M = f primary f eyepiece Images get larger, but the resolution does not increase!
Types of Telescopes and Examples
Hubble 2.4 m mirror
Arecibo Radio Telescope 305 m dish First to observe neutron stars, search for ET
Keck 10 m mirrors Used to spot exoplanets
6.5 m mirror $8 billion cost 2018?
Tidal Forces Another Look at Gravity
Ancient coastal dwellers noticed that tides are more pronounced when the Sun and Moon were together or 180 apart in the sky...... this corresponds to when the Moon is New or Full. The Bay of Fundy is a great place to observe the tides! So what s going on?
simulation
Two main effects from this interaction: First, Earth s rotation is slowing down; day is shortening about 1 ms a century. Second, the tiny force felt by the Moon from the Earth s bulge causes the Moon to spiral outward, about 4 cm a year.
Will this go on forever (will we lose our Moon?) The Earth s rotation will continue to slow down...... the tidal friction then decreases and the bulge will slowing start to align itself with the Moon...... as a result, the tiny acceleration felt by the Moon from the Earth s bulges decreases over time and causes the Moon to decrease its outward spiraling rate until it eventually stops moving away.
After about 50 billion years, the Earth will be tidally locked with the Moon (same face always points at the Moon!) the Moon will be about 1.5 times further away from Earth...... and a day on Earth will be the same as a day on the Moon, and both will be equal to a lunar month at that time, 47 of our present days!
Tidal Forces Can be Destructive
Phobos is decelerating (unlike our Moon) from tidal forces from Mars. This causes its orbit to decrease. In about 100 million years, Phobos will get so close to Mars that the difference between the gravity felt by the near side and the far side of Phobos will be so strong that it will be ripped apart! Note: the only thing holding Phobos together is its own self-gravity.
What if we take a ride on Phobos while its being ripped apart. Will we be ripped apart as well from Mars tidal forces?
Chapter 4: Atomic Physics and Spectra
By analyzing light from a star, we can learn about its: surface temperature composition motion
Which star has a hotter surface temperature, Betelguese or Rigel?
Betelgeuse: 3500 K (orange) Rigel: 12000 K (blue) Note: 80 F 300 K 30 C 300 K
Objects that are hotter than their surroundings radiate: energy flows from hot object to the colder surroundings via EM radiation or light. Light from objects (sources) can have a variety of colors. We d like to ask, at which wavelength (color) does the object emit the most light? To find out, we need to use an instrument called a spectrograph.
Simple Spectrograph: Light passes through a prism which separates the light out into its constituent colors: a spectrum. A spectrum is a representation of how much energy is radiated at each wavelength. It s what the spectrograph measures.
Hot objects radiating into their cooler surroundings have a blackbody spectrum: absorbs all incoming energy, then reradiates it.
In reality, nothing is an ideal blackbody, but stars are pretty close!
SOURCES Astronomical images courtesy of http://apod.nasa.gov/ http://www.nature.com/ http://www.stellarium.org/ http://www.skyandtelescope.com http://www.earthsky.org