# PCS 181 Lecture Notes - Radial Velocity, Parsec, Vir Chakra

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Published on 13 Apr 2013
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PCS 181
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PCS 181
Friday, January-18-13
The spectral line is λ = 656.2850nm in the lab (red line)
The same line (in the spectrum of regulus is λ = 656.2927nm)
Questions to ask: is regulus approaching or receding? What is its radial velocity?
The regulus is longer than in the lab and is red shifted (moving away)
P this Ha/α has a longer wave length in the spectrum (red shifted) and is moving away from us
Vr = C*( Δλ/ λlab) = (3*10^8m/s)*(656.2927nm 656.2850nm/656.2858nm)
= 3519.81m/s or 3500m/s or 3.5km/s
Doppler Effect
Using the Doppler Effect how can you tell that the sun rotates? (it rotates every 30 days)
Cover up half of the star + check spectrum-moving up to you
Cover other half of the star + check spectrum moving away
**moving toward and away at the same time = rotating
Unit 2: The Universe
Distance Units in Astronomy
Light year = 1ly: the distance travelled by light in a year
1ly=9.5*10^15m
Parsec = 1pc: 1pc = 3.26ly (1 Mega parsec = 1000000ly)
Astronomical Unit = 1AU: average distance between the sun and the earth
The Hubble Law
A galaxy is a large assemblage of stars which orbit a common centre and is all held together by
gravity
Some contain a few thousand and some contain a few trillion stars
Hubble and Humason determined the distances and radial velocities in faraway galaxies
The further away the galaxy is, the faster it’s travelling
Hubble Law: radial velocity of a galaxy is directly proportional to its distance
Hubble and Humason’s findings
They concluded that all galaxies are moving away from one another (that the universe is
expanding)
PCS 181
Page 2
All faraway galaxies are red shifted and are receding from us
The Andromeda galaxy is blue shifted and will collide with the Milky Way galaxy
Farther galaxies are receding faster

## Document Summary

The spectral line is = 656. 2850nm in the lab (red line) The same line (in the spectrum of regulus is = 656. 2927nm) The regulus is longer than in the lab and is red shifted (moving away) P this ha/ has a longer wave length in the spectrum (red shifted) and is moving away from us. Vr = c*( / lab) = (3*10^8m/s)*(656. 2927nm 656. 2850nm/656. 2858nm) Using the doppler effect how can you tell that the sun rotates? (it rotates every 30 days) Cover up half of the star + check spectrum-moving up to you. Cover other half of the star + check spectrum moving away. **moving toward and away at the same time = rotating. Light year = 1ly: the distance travelled by light in a year. Parsec = 1pc: 1pc = 3. 26ly (1 mega parsec = 1000000ly) Astronomical unit = 1au: average distance between the sun and the earth.