NATS 1745 Study Guide - Midterm Guide: Henry Draper Catalogue, Chemical Element, Astrophotography
NATS745 Y2020-2021 History of Astronomy
Discoveries About our Solar System
1. The Search for Stellar Parallax:
a. Why did all attempts to detect stellar parallax fail until the 1830s?
●All scientists were trying to detect stellar parallax on the
brightest stars thinking they were the most nearby stars. This is
false because some stars are brighter than others and they’re
too far to detect parallax. They just appear brighter due to their
bigger size. Since parallax decreases with distance and since
our baseline is limited, parallax distances can only be measured
for nearby stars.
●Telescopes could only detect parallax in closer stars,
astronomers assumed the brightest stars were the closest and
they couldn’t detect parallax
● Hooke turned his entire house into a telescope in an attempt to
detect a stellar parallax. He failed.
●In the 1830s, stellar parallax was finally seen in the nearest
stars, now chosen for their brightness and large motion. (nearby
stars would display larger motion travelled than a more distant
star).
b. What did Halley discover about stars in the early 1700s? How did this
discovery aid the detection of stellar parallax (i.e., what 2 attributes of a
star indicate that it is relatively nearby)?
● In 1718 Halley compared ancient star catalogues with current
star catalogues and found 3 moving stars, contradicting
previous knowledge that stars were fixed. They changed
randomly meaning their motion couldn’t be from the Earth’s
motion or stellar parallax (they would shift in the same
direction).found 1. They have unique motions in space and 2.
hard to detect because they can only be seen in long period of
time PTOLEMY
● Suggesting that stars have unique motion across the sky. He
suggested using old star charts to determine which stars moved
the most as this would help display parallax.
2. The First Distances to Stars:
a. What is the definition of a light year? What is the definition of a parsec?
(NOTE: the specific values in km aren't necessary). Which unit is
bigger, and by approximately how many times?
● A light year is the distance light travels in 1 year (⅓ of a parsec).
So, a star 10 light years away is seen as it appeared 10 years
ago.
● A parsec is the distance of an object with a parallax of 1
arcsecond.
● A parsec is 3 times a light year.
b. What is our nearest star beyond the Sun, and how far away is it
(approximately) in light years?
● The nearest star beyond the sun is Alpha Centauri, and it’s
270,000 at away = 4.2 Ly (roughly)
3. Light and the Electromagnetic Spectrum:
a. According to the wave theory of light, what are all light waves
composed of?
● Hooke proposed that light travels as a wave, and Huygens later
presented light as a wave of oscillating electric and magnetic
energy fields that are perpendicular to each other.
b. Describe, in terms of amplitude and wavelength, how a wave from a
bright blue light source is different from a wave from a faint red-light
source.
● Amplitude: Determines the intensity of a light source (The height
of a wave).
● Wavelength: Determines the colour of a light source. (The
distance between the wave peaks).
● Frequency: Number of wavelengths leaving the source per
second
● Within the visible spectrum, our experience of red is associated
with longer wavelengths, greens are intermediate, and blues
and violets are shorter in wavelength. (An easy way to
remember this is the mnemonic ROYGBIV: red, orange,
yellow, green, blue, indigo, violet). The amplitude of light
waves is associated with our experience of brightness or
intensity of color, with larger amplitudes appearing brighter
● A bright blue light source wave will have a large amplitude, low
wavelength (and so high frequency). A dim red light will have a
low amplitude, long wavelength (and so a low frequency).
c. In the full spectrum of light, what kind of light is shortest in wavelength?
What kind of light is longest in wavelength? What kind of light has the
most energy?
● Wavelengths decrease from red to blue (therefore, frequency
increases from red to blue)
● Gamma waves are the shortest and highest-energy waves,
Radio waves are the longest and lowest-energy waves
● The light we see is only a tiny fraction of the full spectrum of
light. (We only see light with wavelengths between red and
violet)
4. Spectroscopy:
a. What is a spectroscope?
● A spectroscope is an apparatus that produces a light spectrum
that can be observed through a telescope. Fraunhofer was first
to observe this when he passed sunlight through a glass and
observed the spectrum through a telescope.
b. What is Fraunhofer’s Spectrum a spectrum of?
● He found black lines on his Sun spectrum every time and the
lines always appeared at the same colors on the spectrum. He
mapped the lines and labeled them with a letter. He never
explored what those lines meant but they were absorption lines
c. Describe how the spectrum of a star can tell us its chemical
composition.
● Through spectroscopy the chemical composition can be
measured through a spectra of light where different elements
emit different amounts of light of different colors on the spectra.
Each element emits light at certain wavelengths and thus would
have its own unique identifiable “fingerprint”.
5. Spectroscopic Discoveries about the Sun:
a. Explain how Bunsen and Kirchhoff correctly concluded that the Sun
has an atmosphere.
