NATS 1745 02/27/2014
The discovery of Uranus
1. William Herschel (17381822) was an amateur astronomer who built the largest telescope of his time
2. While cataloguing stars to aid the search for stellar parallax (and therefore the 1 stellar distances), he
became the 1 person to discover a planet.
3. 1781: he observed a starlike object which increased in size when he increased his telescope
4. Shortly after, he observed its motion relative to the stars
5. Further investigation revealed its nearcircular orbit at ~20 AU from the sun (orbital period = 84 years).
6. It was accepted as a new planet and named Uranus (Greek god of the sky, father of Saturn, grandfather
7. He also discovered Uranus’ 2 brightest moons, Titania and Oberon
8. The moon orbits revealed Uranus’ mass: 15x Earth’s mass
9. Its mass/size = low density, therefore a gas giant (but mostly frozen; also known as an ‘ice giant’).
10. By 1979, it was known: Jupiter, Saturn and Uranus have rings.
11. Unlike the other planets, Uranus’ orbits the sun on its side, possible due to a past collision.
The discovery of the Asteroid belt
1. When Kepler observed the large gap between Mars and Jupiter, he proposed: it contains an
2. 1772: Bode showed that planetary distances follow a numeric sequence (‘Bode’s law’)
3. When Uranus was found to follow the law, a search began for an undiscovered planet at the empty
number between Mars and Jupiter. 4. 1801: Piazzi detected a starlike object in a near circular orbit around the sun at 2.7 AU (near Bode’s
number after Mars).
5. It was named ‘Ceres’ and downgraded from planet to asteroid when it was found to be smaller than the
6. ‘Asteroid’ is Greek for ‘starlike’ (coined by Herschel due to its pointlike appearance through a
7. Over the next century, a circular belt containing hundreds of asteroids was discovered between Mars and
8. Today, there are thousands of known asteroids in the belt, ranging in size from pebbles to small moons
(Ceres is the largest).
The discovery of Neptune
1. By the 1830s, deviations were detected in the orbit of Uranus.
2. This led to a search for a perturbing planet beyond Uranus.
3. 1846: the planet was found independently by French and English astronomers, in a nearcircular orbit at
30 AU (i.e., with an orbital period of 164 years).
4. The planet was named ‘Neptune’ (Roman sea god), owing to its blueish color
5. The orbit of Neptune’s largest moon, Triton, revealed: Neptune’s mass is 17x Earth (another gas/ice
6. Neptune has 13 known moons (Triton is retrograde, therefore not native to the Neptune system).
7. Neptune has a ring system.
The Origin of the Solar System
1. Current theory: after the sun was bon from a spinning collapsing gas cloud, the remaining gaseous and
rocky particles in the Sun’s spinning circumstellar disk coalesced into planets.
2. This explains why all planets orbit in the same direction and in roughly the same plane.
3. The proof: disks of matter have been found around newborn stars in starforming nebulae. 4. The asteroid belt is likely leftover debris from the Sun’s circumstellar disk that couldn’t coalesce into a
planet due to the opposing gravitational pulls of the Sun and nearby giant Jupiter.
5. Planetary rings are likely the leftover debris from protoplanetary disks which couldn’t coalesce into a
The Gas Giants vs. The Terrestrial Planets
1. The 4 planets beyond Mars are all gas giants, how numerous moons, and all have rings.
2. The 4 innermost planets are small with a rocky composition (‘terrestrial planets’), have 02 moons, and
3. The reason: the outer circumstellar disk contained more material, allowing the outer planets to collect
more mass and to attract more debris into orbit as moons and rings.
4. The planets’ solid cores were likely the 1 to form. Since the outer planets grew faster, they had more
gravity to draw in the gas particles, leaving the inner planets with much less gas
5. Mars’ 2 moons are similar to asteroids in shape and composition and may therefore be nonnative to
6. The large size and composition of Earth’s 1 moon suggests that I came from a large Earth impact.
The discovery of Pluto
1. Percival Lowell (18551916): An American businessman and astronomer, particularly interested in finding
life on other planets.
2.Lowell popularized the idea of intelligent life on Mars, but by the 1960’s his theories were rejected after
closeup observations of mars
3. An initial underestimate of Neptune’s mass led to the theory that an additional planet is perturbing
4. Lowell conducted a 30year search from his private observatory for a planet beyond Netpune
5. 1930: a planet beyond Neptune was found at Lowell Observatory by Tombough (orbital radius = 40 AU,
orbital period = 248 years).
6. The planet was named ‘Pluto’ (Greek god of the underworld) by a young girl from England
7. Late 1970s: the discovery of Charon (largest of Pluto’s 3 moons) revealed Pluto’s mass (.002x Earth, too
small to perturb Uranus). Pluto’s discovery was therefore a coincidence. 8. 1950s: Kuiper proposed the existence of a dusk of icy debris around the planets, left over from the solar
9. Since the 1990s, hundreds of Kuiper Belt Objects (KBOs) have been discovered, some larger than Pluto.
10. In 2006, this motivated the IAU to define a planet
i. It must be in a solar orbit (excludes moons)
ii. It must be a sphere by its own gravity (excludes comets and some asteroids)
It must have cleared the debris around its orbit (excludes all asteroids and KBOs)
11. Pluto failed criterion #3, and is now considered a dwarf planet (i.e., an object which only meets the 1 st
two criteria for a planet).
12. There are currently 5 identified dwarf planets in our solar system (including Pluto and Ceres), but nearly
2000 are expected to be out there. 02/27/2014
The First Distances to Stars
1. 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
2. The distance to a star can be derived from its parallax shift due to Earth’s motion around the Sun
3. Since parallax deceases with distance and since our baseline is limited, parallax distances can only be
measured for nearby stars.
4. Between the 17 – 18 C, there were many failed attempts to detect stellar parallax, owing to the
incorrect assumption that the brightest stars are the nearest stars.
5. 1718: Halley compared ancient star catalogues with current star positions and found 3 moving stars.
6. Early 1800s: repeated star cataloguing revealed increasing numbers of moving stars.
7. By the 1830s, stellar parallax was finally seen in the nearest stars, now hosen for their brightness and
large motion (and preferably widelyspace binaries).
8. The nearest star (Alpha Centauri) has a distance of 270,000 AU (4.4 ly, or 1.3 pc).
9. 1990s: the Hipparcos satellite measured parallaxes of ~100,000 of the nearest stars (less than a millionth
of the stars in our galaxy). 02/27/2014
Introduction to Light
1. Late 1600s: Hooke proposed that light travels as a wave, and Huygens later presented light as a wave of
oscillating electrics and magnetic energy fields.
2. Amplitude: determines the intensity of a light source
3. Wavelength (λ): determines the color of a light source
4. Frequency (f) = # of wavelengths leaving the source per second (i.e. high f means short λ, low f means
5. Wavelength decreases from red to blue (therefore, frequency increases from red to blue). 02/27/2014
6. The light we can see is only a tiny fraction of the full spectrum of light.
7. Radio waves are the longest and lowestenergy waves, while gamma waves are the shortest and highest
8. The high energy from shortwavelength waves is not necessarily dangerous – eg., the Sun’s UV waves
are damaging to humans because they are high in amplitude, whereas lowamplitude UV waves can be
safe for humans.
1. Early 1800s: Fraunhofer passed sunlight through glass and observed the spectrum through a telescope
(ie., a "spectroscope").
2.He saw ~600 thin dark lines and labeled them with letters to designate their wavelength ("Fraunhofer's
3. 1859: Bunsen and Kirchoff produced the spectra of various heated gases
4. They found: the spectrum of each gas contains a unique sequence of bright coloured lines.
5. Spectroscopy is the analysis of the spectrum of a light source for the purpose of determining its chemical
6. The dark lines in the Sun's spectrum match the wavelength of the bright lines in known gases
7. To figure out why, B&K produced the spectrum of sunlight after it passed through a gas flame. They
found: the dark lines got thicker.
8. They concluded: the sun's core, which produces the full spectrum of light, is surrounded by a gas layer
9. Molecules in the Sun's atmosphere absorb light at their signature wavelengths, producing the dark lines
10. The proof: during solar eclipses in the 1860s70s, the spectrum of the Sun's atmosphere was produced,
revealing the "emission lines" predicted by B&K
11. 1860s: the spectrum of hydrogen gas was produced and matched with a series of unidentified dark lines
in the Sun's spectrum, revealing hydrogen in the Sun's atm
12. 1868: Lockyer discovered a new emission line in the Sun's atm and named it "helium" (Helios = Greek
god of the Sun)
13. A visual spectrum: a photograph of the spectrum of a light source; dark lines are absorption lines and
bright lines are emission lines.
14. A graphical spectrum: a plot of the amount of light produced at each wavelength; dips are absorption
lines and peaks are emission lines.
15. 1864: Huggins produced the spectrum of a planetary nebula. It contained unidentified emission lines.
The lines were named 'Nebulium', until matched with Oxygen in the 1920s.
16. From the spectra of a sample of nebulous celestial bodies, Huggins found:
Emissionline nebulae (gas clouds)
Continuum and absorptionline nebulae (star systems)
Continuum, emission line and absorptionline nebulae (galaxies: i.e., gas/star systems)
The Motion of Stars
1. Space velocity: a star's true motion through space.
2. A star's space velocity can be split into 2 components:
Radial velocity: the approaching or receding motion; only detectable from star's light spectrum via
the "Doppler Effect".
transverse velocity: the motion across the sky; can be calculated from star's observed proper
motion and distance.
3. The Doppler Effect: the stretching (or compressing) of a wave due to the approaching or receding motion
of the source of the wave
A stretched wave is redshifted, and a compressed wave is blueshifted. 02/27/2014
The larger the wavelength shift, the faster the radial velocity
Spectral Classification of Stars
1. Henry Draper:
a. Pioneer of astrophotography (the production of longexposure sky photos using a camera
attached to a telescope). Sky photos are deeper and can be stored on film for later analysis
b. Photographed the 1st stellar spectrum, and began a photographic spectroscopic survey of all
2. E.C. Pickering: As Director of the Harvard College Observatory, he hired a team of women 'computers' to
complete the Henry Draper Catalogue of spectra of over 225,000 stars.
3. Initially, the Harvard team classified stellar spectra into alphabetcal spectral types by the strength of their
4. Since the late 1700s, when Herschel observed stars through a glass prism, it was understood: stars have
different colours (i.e. s star's colour is the wavelength with the highest intensity in its spectrum).
5. The bluer the star, the hotter its temperature
6. Annie Cannon: reduced and reordered the spectral types from blue to red (hottest to coldest): OBAF
7. This revealed: the reddest stars (K, M) have more metals than other spectral types
8. Each spectral type was subdivided into 10 subtypes from 09 (i.e., O0, O1, ... O9, B0, B1, .... B9, A0, ...)
Atoms and Light
1. A photon is a particle of light (another way of representing light, like the wave).
2. As with waves, red photons are "big" (and low energy). blue photons are "small" (and high energy), and a
highamplitude wave is a "big packet" of photons
3. Atoms consist of a nucleus of protons (+charge) and neutrons (0charge) surrounded by a shell of
orbiting electrons ( charge).
4. The # of protons (p) determines an atom's # of electrons (e) and its chemical element; 1p1e =
hydrogen, 2p2e = Helium, etc.
8.Cecilia Payne: used atomic physics to calculate chemical abundances and temperatures of stars from
their spectral lines. 02/27/2014
9. Hlines appear weak in K and M stars, but by accounting for the different absorbencies of atoms, Payne’s
calculations correctly showed: all stars are predominately composed of H (and He).
The Engines of Stars
1. Einstein (18791955):
In his General Relativity Theory (GR), mass (m) & energy (E) convert into each other according to E=mc2
(where c is the speed of light).
GR predicts: “gravity” is created by the curvature of space.
2. Eddington (18821944):
During a total solar eclipse, he proved GR by observing that when light rays pass near the Sun, their path is
bent by the curved space around the Sun.
He proposed: the energy from stars is produced from the fusion of Hydrogen nuclei into Helium at high
temperatures (i.e. nuclearfusion)
3. Bethe (19062005): derived the nuclearfusion process from Hydrogen to Helium:
4H = 1He + Energy (since mass 4H > mass 1He)
where the energy (E=mc2) is in the form of gammawaves.
4. Nuclearfusion explains why stars:
Produce enormous energy (Hatoms are plentiful in stars) 02/27/2014
Emit a full spectrum of light (the gammarays downgrade to other λs as they interact with atoms in the star)
Last for billions of years (the initial fusion reaction of Hydrogen takes a billion years)
Light and Distance
1. Light dims with distance according to the inversesquare law (i.e. the amount of light we receive from a
source is 1/d2 of its actual light production).
2. Apparent brightness:
The brightness of a light source as it appears from Earth 02/27/2014
Quantified by apparent magnitude (m); the lower w, the brighter the source
3. Intrinsic brightness:
The total amount of light produced by the light source
Quantified by luminosity (L) = total energy output per sec; or absolute magnitude (M); the lower the M, the
brighter the source 02/27/2014
Properties of Stars
1910s: found that the luminosity of cluster stars depends on their spectral type (O stars are most luminous,
M stars least luminous).
By restricting his study to stars in the same cluster, the apparent magnitudes reflect the stars’ relative
1910s: found the same relationship between luminosity and spectral type among noncluster stars with
Found a population of cold but luminous AM stars which strayed from his trend (the Red Giants; eg.
Helped discover a population of whitehot, lowluminosity stars with masses of ~1Msun packed into a radius
of ~1 Rearth (White Dwarfs; eg. Sirius B). 02/27/2014
3. The HR diagram allows a star’s luminosity (L), temperature, ra