MGMT 1050 Lecture Notes - Lecture 9: Ms Star, Proper Motion, Absorption Spectroscopy
7 views22 pages
Part One: The First Distances to Stars
Beginning from the 1700s, there were many attempts to determine the distances to the stars in order to establish our
location in the larger Universe.
1.
The detection of stellar parallax was sought after as it could be used to calculate the distance to the stars
•
To figure out what our universe really looks like, needed to find out how far our stars were
•
The distance to a star can be derived from its parallax shift due to Earth's motion around the Sun.
2.
Detection of stellar parallax
Since parallax decreases with distance and since our baseline is limited, parallax distances can only be measured fro nearby
stars.
3.
Longest baseline we can achieve is 2 AU (diameter of the earth's orbit) -- limits how big a parallax we can detect
○
Problem with detecting stellar parallax: there is a limit to the size of our baseline
•
Only the nearest stars have parallax shifts that are detectable from earth
○
How can we determine which stars are nearest if we don’t know their distances
○
Largest parallax we can induce is about 1 arcsecond, however most stars have parallax shifts smaller than 1 arc second
•
Between the 17th and 18th century, there were many failed attempts to detect stellar parallax, owing to the incorrect
assumption that the brightest stars are the nearest stars
4.
Incorrect assumption, as stars vary in brightness and size
•
1718: Halley compared ancient star catalogues with current star positions and found 3 moving stars.
5.
The common assumption was that stars were fixed and any have no motion of their own
•
Each of which moved in different a direction (therefore not due to precession or parallax)
○
Edmund Halley discovered 3 stars whose coordinates changed over a long period of time
•
Early 1800s: repeated star cataloguing revealed increasing numbers of moving stars
6.
Clear that stars are moving throughout space (in a nonsystemic way with varying speed and direction)
•
Distances to stars began to be described by their parallax
○
The distance of a star with a parallax shift of 1 arcsecond would be expressed as parsec (pc)
○
Since stars were so far away, using the unit AU would be way to large to express easily
•
CH 9 - Discoveries About the Stars
Ch 9 Page 1
By the 1830s, stellar parallax was finally seen in the nearest stars, now chosen for their brightness and large motion (and
preferably, widely-space binaries)
7.
A star with noticeable motion is likely be closer, and following that a brighter star could possibly be closer
•
Binary star: two stars coupled together in the star so that they appear brighter
•
Part Two
The nearest star (Alpha Centauri) has a distance of 270,000 AU (4.4 ly, or 1.3 pc)
8.
1990s: the Hipparcos satellite measured parallaxes of ~100,000 of the nearest stars (less than a millionth of the stars in out
galaxy)
9.
Telescopes in space avoid our atmosphere
○
Can detect parallax shifts as small as 1/1000th
○
Allows us to detect even smaller parallax shifts than can be detected from on earth
•
Our galaxy contains billions of stars (is 100,000 ly in length)
•
Ch 9 Page 2
Introduction to Light
Late 1600s: Hooke proposed that light travels as a wave, and Huygens later presented light as a wave of oscillating electric
and magnetic energy fields.
1.
Huygens: all light sources contain charged particles that move around (creating electric fields)
•
And moving electric fields create corresponding magnetic fields which oscillates at the same rate as the electric fields
but in a perpendicular direction
○
Net result: wave of electromagnetic energy moving away from the moving source that produced
○
Motion of the charged particles produces an oscillating electric field
•
Waves travel away from the light source
•
Amplitude: determines the intensity of a light source
2.
Higher amplitude tells us t comes from a higher energy wave source
•
Higher amplitude, more intense (brighter) colour
•
Wavelength (λ): determines the colour of a light source
3.
Different colours are associated with different wavelengths
•
Frequency (f) = # of wavelengths leaving the source per second (i.e. high fmeans short λ, low fmeans long λ)
4.
Wavelength decreases from red to blue (therefore, frequency increases from red to blue)
5.
Ch 9 Page 3