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Lecture 2

PCS 181 Lecture Notes - Lecture 2: Radial Velocity, London Academy Of Music And Dramatic Art, Raisin Bread


Department
Physics
Course Code
PCS 181
Professor
Margaret Buckby
Lecture
2

Page:
of 5
PCS 181 Class 3
THE DOPPLER EFFECT IN ASTRONOMY
- Short wavelength: blue light/blue star that’s moving= blue shifting(moving closer to you
peaks are squished)….whole pattern of hydrogen lines moved towards the blue in the
spectrum
- Long wavelength = red light/red shifted….moving away peak are further
- Obtain spectrum of stationary (=not moving) source (e.g. hydrogen in the laboratory
- Obtain spectrum of a moving object (e.g. a star)
- Compare the 2 spectra
- Diagram lab spectra emission spectrum compared to stellar spectrum
(absorption)….lines in the star spectrum are shifted to the red side compared to the lab
spectrum therefore its red shifted (moving away)
Doppler effect in Astronomy to determine velocity
- C=speed of light = 3*108 m/s
- Symbol delta lamda = wavelength difference between lab and stellar spectra
- Symbol lamda = wavelength on lab spectrum
- Vr = radial velocity of star
- Vr=c* delata lamda/lamda lab
Velocity of star
Vr= (3x108 m/s)(434.1-434)nm/434nm
Vr= 6.91 x 104 m/s
To get in kilometers, divide by 1000
Vr= 69.1 km/s
***** on test for sure -…..radial velocity……and if the star is moving away from you or
towards you. If its shorter than lab wavelength, it is blue and towards you.
Radial velocity problem
H spectral line is *lamda = 656.2850 nm in the lab….this is red wavelength
-violet is in 400 range
Yellowish green is in 550 range
Same line is lamda alpha= 656.2927 nm in the spectrum of regulus. Is regulus approaching
or receding? Radial velocity
This H alpha line has a longer wavelength in the spectrum of Regulus
- Spectrum is red shifted
- Moving away from us
Vr = c* delta lambda /lamda lab = (3X108 m/s) (656.2927-656.2850) nm/656.2850 nm=
Vr = 3500 m/s = 3.5 km/s
Using the Doppler effect, how can you tell that our sun rotates?
UNIT 2: THE UNIVERSE
Cosmology: origin and evolution of universe from beginning to the end
Distance units in astronomy
- Lightyear = ly: distance that light travels in one year 1 ly = 9.5 x 1015m
- Parsec = pc: 1 pc = 3.26 ly
- Astronomical unit = AU: Average distance between the Earth and Sun
- Einstein rosen bridge- topic for research
- Nebulae = latin for clouds (plural)
The Hubble Law
A galaxy is a large assemblage of stars which orbit a common center all held together by
gravity contains from a few thousand to trillions of stars
Hubble and Humason determined the DISTANCES and RADIAL VELOCITY of FARAWAY
galaxies
- Every galaxy the farther away from us the faster its moving
Hubble and Humason’s Findings
All faraway galaxies are redshifted and receding from us
Hubble law: radial velocity of a galaxy is directly proportional to distance.
- Farther galaxies are receding faster
1. The universe is expanding…space itself is expanding
2. There is a…..
Raisin bread analogy
Cosmic singularity a state of infinite density. Everything was in one place at one time
Steady state cosmology
Cosmic microwave background radiation = CMB
1965: Amo Penzias and Robert Wilson radio astronomers at bell labs in N.J.
Worked with the first communications satellite, TELESTAR.
Detected a persistent radio hiss from
All directions in the sky
At a wavelength of lamda = 1.063X10-3m
CMB TEMPERATURE
T = 0.0029m*K/lamdadom
T= 2.728+/- 0.004 K
= 3K
=CMB
= temperature of empty interstellar space
- Some event must have HEATED the universe.
COSMOLOGY
Penzias and Wilson discovered the remnant (leftover) heat from the birth of the universe.
1. The universe is expanding
2. There was a heat event at the beginning of space and time as proved by the CMB
Thus the universe originated in an explosive event called the BIG BANG.
CMB CONFIRMATION
1989: Cosmic Background Explorer Satellite (COBE) launched.
Provided an above-atmosphere measurement of the CMB.
Confirmed that the CMB is 2.28 +/- 0.004 K