Computers Astronomer Edward Charles Pickering's Harvard “computers" Annie Jump Cannon Classified 200,000 stars in 3 years: OBAFGKMLTY (Average: 2 minutes per star, six days per week) Could eventually classify three stars per minute.
Classified 350,000 stars in her lifetime.
Obviously, Brilliant Astronomers Feel Glad Knowing Many Luminous Types, Yo Q: When stars are sorted according to spectral type based on the width of the Hydrogen absorption lines what physical property underlies the order?
Meet Astronomer Barbie, Available Fall, 2019 Q: When stars are sorted according to spectral type based on the width of the Hydrogen absorption lines what physical property underlies the order?
A: Most broadening is due to Doppler shifts caused by bulk motion or nonzero temperature of the emitting regions.
Recently, OBAFGKM à OBAFGKMLTY because we can see fainter stars now
Obviously, Brilliant Astronomers Feel Glad Knowing Many Luminous Types, Yo Main- Main- Main- Fraction of sequence Effective Vega-relative sequence sequence Hydrogen all main Class Chromaticity mass Temperature chromaticity radius luminosity lines sequence (solar masses) (solar radii) (bolometric) stars
O ≥ 30,000 K blue blue ≥ 16 M☉ ≥ 6.6 R☉ ≥ 30,000 L☉ Weak ~0.00003%
deep blue B 10,000–30,000 K blue white 2.1–16 M 1.8–6.6 R 25–30,000 L Medium 0.13% white ☉ ☉ ☉
A 7,500–10,000 K white blue white 1.4–2.1 M☉ 1.4–1.8 R☉ 5–25 L☉ Strong 0.6%
F 6,000–7,500 K yellow white white 1.04–1.4 M☉ 1.15–1.4 R☉ 1.5–5 L☉ Medium 3%
G 5,200–6,000 K yellow yellowish white 0.8–1.04 M☉ 0.96–1.15 R☉ 0.6–1.5 L☉ Weak 7.6%
pale yellow K 3,700–5,200 K light orange 0.45–0.8 M 0.7–0.96 R 0.08–0.6 L Very weak 12.1% orange ☉ ☉ ☉
light orange M 2,400–3,700 K orange red 0.08–0.45 M ≤ 0.7 R ≤ 0.08 L Very weak 76.45% red ☉ ☉ ☉ The bolometric magnitude of a star is a measure of the total radiation of a star emitted across all wavelengths of the electromagnetic spectrum. See: Chromaticity: color https://en.wikipedia.org/wiki/Stellar_classification Luminosity is the total amount of electromagnetic energy emitted per unit of time by a star, galaxy, or other astronomical object. (Watts) Boltzman equation: What proportion of electrons are in each energy state? Saha equation: What proportion of atoms are ionized?
Hertzsprung and Russell: Giants and dwarfs Absolute Magnitude Smaller total Larger total energy output energy output Hotter Spectral Type Cooler Radius Q: What goes this way? Mass
Radius
Q: What is the density?
(Thanks, Ken) We’ve got better HR diagrams now! Stars on the main sequence are burning Hydrogen.
As stars exhaust their Hydrogen, they move off the main sequence.
How they evolve from there depends on their mass. The color of a star gives a measure of its temperature .
The human eye only sees part of the blackbody radiation spectrum. The color we see for a star depends on the average only over the visible part. As the temperature increases, the visual effect traces a line across the possible colors.
The line misses green entirely, and even at infinite temperature we would only see blue- white. Luminosity
Spectra at the same spectral type show variation with size.
Denser stars have higher surface gravity. The increased pressure leads to pressure broadening of spectral lines. Thus, the broadening of spectral lines from a giant star is much less than from a dwarf star, even if the stars have the same color (and hence temperature).
Therefore, broadening of spectral lines varies with the luminosity.
We can estimate both the luminosity and the temperature of a star from its spectrum, and place it on the HR diagram.
https://www.youtube.com/watch?v=K2fZxOPWbDU
http://hubblesite.org/video/704/news Elizabeth Langdon Williams (February 8, 1879 in Putnam, Connecticut – 1981[1]) was an American human computerand astronomer. She graduated from MIT in physics in 1903 as one of their earliest female graduates,[2] and was hired by Percival Lowell in 1905 to work from an office in Boston.[3] Her role was to perform the mathematical calculations required to predict the location of a proposed Planet X that Lowell hypothesised affected the orbits of the known planets Neptuneand Uranus.[4] She was the head human computer for Lowell in 1915. Her calculations led to predictions for the location of the unknown planet, but Lowell died in 1916 meaning that the project was discontinued. In the late 1920s the project was resumed and Clyde Tombaugh was hired to lead it. Tombaugh used Lowell's predictions to locate an image in a region of the sky photographed in 1915 that he identified as a new planet, named Pluto in 1930.[5] Williams continued to work at Lowell Observatory after Lowell's death, moving from Boston to the observatory itself at Flagstaff in 1919. In 1922, Williams married another astronomer, George Hall Hamilton.[1][6] She was then dismissed from her position at the observatory by Constance Lowell since it was considered inappropriate to employ a married woman.[7]Williams and her husband were subsequently employed at an observatory in Mandeville, Jamaica run by Harvard College Obervatory where they worked together. In 1935, William's husband died. She moved to New Hampshire and subsequently died in poverty.
Evolution of a Planetary Nebula formation solar mass star
Thermal Pulse Asymptotic Giant Branch
Early Asymptotic Giant Branch
Red Giant Branch
Sub-Giant Branch
Hydrogen fusion in core Zero Age Main Sequence 90 of stellar lifetime Evolution of a Planetary Nebula formation solar mass star
Thermal Pulse Asymptotic Giant Branch
Early Asymptotic Giant Branch
Red Giant Branch
Sub-Giant Branch
Hydrogen fusion in core Zero Age Main Sequence 90 of stellar lifetime Evolution from the main sequence: Low (∼ 1 M⊙) and intermediate (∼ 5 M⊙ – 8 M⊙ ) mass stars
• As Hydrogen fuses to Helium the proportion of Helium to Hydrogen in the core increases, but not enough to fuse Helium. The temperature decreases à outer layers of the star compress the core à the temperature of the core increases enough to support the star The average density of the core increases.
• Cessation of Hydrogen burning in the core Hydrogen fusion ceases in the core, continuing in a Hydrogen burning shell. The Helium core is nearly constant temperature, with increasing density toward the center.
• Increased temperature drives Hydrogen fusion faster
• Luminosity of the star increases
• The outer layers of the star force expand Evolution from the main sequence: Low (∼ 1 M⊙) and intermediate (∼ 5 M⊙ – 8 M⊙ ) mass stars
• The Subgiant Branch
At the Schönberg-Chandrasekhar Limit the isothermal Helium core cannot support the outer layers of the star.
The core is compressed, hea ng the Hydrogen burning layer. Luminosity increases rapidly, driving the outer layers of the star outward.
David Taylor, Evanston, IL
The star moves to the right in the Hertzsprung-Russell diagram. This is the subgiant branch. Evolution from the main sequence: Low (∼ 1 M⊙) and intermediate (∼ 5 M⊙ – 8 M⊙ ) mass stars
Planetary Nebula formation
Thermal Pulse Asymptotic Giant Branch
• The Red Giant Branch
Early Asymptotic The star develops convection Giant Branch currents that mix deeper and deeper layers of the star with the Red Giant surface layers. This increases energy Branch transport; the star greatly increases in size and magnitude.
This “first dredge up” brings Sub-Giant Branch processed elements from the core out to the surface, providing an important spectroscopic test of the predictions of stellar models. Zero Age Main Sequence Evolution from the main sequence: Low (∼ 1 M⊙) and intermediate (∼ 5 M⊙ – 8 M⊙ ) mass stars • Triple alpha fusion The triple alpha process begins almost explosively, releasing 1011 mes the luminosity of the sun. The burst lasts only a few seconds and most of the energy is absorbed by the outer layers of the star.
• Helium burns into Carbon and Oxygen At the top of the red giant branch, the core temperature and density become high enough for quantum tunneling between alpha particles.
Helium nuclei can fuse with another alpha particle to produce Beryllium, 8Be.
This unstable and decay backs to Helium unless, within the lifetime of the Beryllium, a third alpha particle fuses to form Carbon, 12C. This is the triple alpha process.
An additional fusion of an alpha particle with the Carbon can produce Oxygen. Evolution from the main sequence: Low (∼ 1 M⊙) and intermediate (∼ 5 M⊙ – 8 M⊙ ) mass stars
Bluer
• The Horizontal Branch
Helium fusion con nues at the core of the star. The burning proceeds much faster than the main sequence Hydrogen fusion.
The star contracts and moves to the le Horizontal branch (blueward) along the horizontal branch, and may pulse periodically.
As the Helium is converted into Carbon and Oxygen, the Schönberg-Chandrasekhar limit is again approached at the core. The CO core contracts, a zone of Helium outside the core forms and con nues burning, driving the outer layers to expand and cool. Evolution from the main sequence: Low (∼ 1 M⊙) and intermediate (∼ 5 M⊙ – 8 M⊙ ) mass stars
Planetary Nebula formation
Thermal Pulse Asymptotic Giant Branch
• Early Asymptotic Early Asymptotic Giant Giant Branch Branch A burning Helium shell now Red Giant drives expansion. Branch
• The core temperature reaches 2×108K. The outer layers absorb much of the energy, expanding and Sub-Giant Branch cooling. • Convection begins again and deepens, giving the second dredge up. Zero Age • The temperature of the star Main Sequence continues to rise, driving it higher in the Hertzsprung-Russell diagram Evolution from the main sequence: Low (∼ 1 M⊙) and intermediate (∼ 5 M⊙ – 8 M⊙ ) mass stars
Planetary Nebula • Thermal-Pulse Asympto c Giant Branch formation As the star grows ho er the hydrogen shell Thermal Pulse reignites and dominates the star’s luminosity. Asymptotic Giant Branch The narrow layer of burning helium thins as it produces carbon and oxygen, but thickens as hydrogen burning adds more helium.
Early Asymptotic The non-uniformity of this process turns helium Giant Branch burning on and off in a series of pulses.
Red Giant In more detail, as hydrogen burning increases the mass of the helium shell, Branch the base of the helium shell develops some electron degeneracy. As the temperature increases in response, a helium flash occurs. The flash drives the outer layers of the star outward, cooling them and extinguishing hydrogen burning until the energy of the helium flash dissipates. Then hydrogen reignites and the process starts again. The pulses last for thousands of years in intermediate size stars and for hundreds of thousands of years for low mass stars. Sub-Giant Branch Since hydrogen fusion is responsible for most of the star’s luminosity, the luminosity decreases after the helium flash. When the pressure drops enough that the degeneracy is lifted, helium burning diminishes and the star settles back into its pre-flash configuration. Zero Age Main Sequence The overall trend throughout the recurring flashes is toward higher luminosity and lower temperature. • Carbon stars The thermal pulses from helium flashes leads to the development of convection currents that eventually mix carbon and oxygen from the inner layers of the star with the surface. Since carbon is as much as ten times as abundant as oxygen in the star, this can shift the overall spectrum of the star from oxygen abundant to carbon abundant. Carbon rich giants, that is, giants with more carbon than oxygen in the atmosphere, are called carbon stars.
During the asymptotic giant branch, stars lose mass at a high rate. Their surface temperatures remain quite cool, and the mass ejections enrich the interstellar medium with carbon and oxygen compounds. There are now two types of evolution:
• Stars with mass M < 4 M⊙ Only Helium fusion occurs. The Carbon-Oxygen core grows, and grows denser, leading to electron degeneracy. These stars are not large enough to overcome electron degeneracy.
• Stars with 4M⊙ < M < 8M⊙ In the absence of other processes they could reach the Chandrasekhar limit and therefore collapse further, but two things prevent this:
Mass loss of the asymptotic giant stage continues. As the mass of the star drops, it is easier for matter to escape, and mass loss accelerates
Fusion continues beyond Carbon and Oxygen, producing a core consisting of Oxygen, Neon and Magnesium.
Core collapse does not occur for stars up to about 8 solar masses. Evolution from the main sequence: Low (∼ 1 M⊙) and intermediate (∼ 5 M⊙ – 8 M⊙ ) mass stars
Planetary Nebula formation
hotter
• Superwind
Early Asymptotic ForGiant the Branch next ~25,000 years, low and intermediate mass stars develop a −4 superwind, which carries away ∼ 10 M⊙ per year. Red Giant An opaque cloud con nuesBranch to build around the remaining star.
The remnant star grows ho er with energy from gravita onal contrac on.
Eventually Subthe- surroundingGiant Branch cloud thins enough to see the remnant, consis ng principally of the CO or ONeMg core and a very thin layer of remaining hydrogen and helium
Zero Age Main Sequence The mass of the remnant of is reduced to .00027 of the original mass. Luminosity remains roughly constant until the core itself starts to cool.
The remnant, a slowly cooling Earth-sized core of carbon and sometimes oxygen, neon and magnesium, drops to the lower left corner of the Hertzsprung-Russell diagram as a white dwarf.
The expanding shell of gas around the white dwarf is called a planetary nebula. Planetary nebulae glow as the gas is struck by the ultraviolet light emanating from the core.
Wikipedia
The planetary nebula NGC 5189. Credit: NASA, ESA European Space Agency and the Hubble Heritage Team (STScI/AURA)