Astronomy 101The Solar System

Tuesday, Thursday

Tom Burbinetomburbine@astro.umass.edu

Course

• Course Website:– http://blogs.umass.edu/astron101-tburbine/

• Textbook:– Pathways to Astronomy (2nd Edition) by Stephen Schneider

and Thomas Arny.• You also will need a calculator.

• There is an Astronomy Help Desk that is open Monday-Thursday evenings from 7-9 pm in Hasbrouck 205.

• There is an open house at the Observatory every Thursday when it’s clear. Students should check the observatory website before going since the times may change as the semester progresses and the telescope may be down for repairs at times. The website is http://www.astro.umass.edu/~orchardhill/index.html.

HWs #6, #7, #8, and #9

• Due by Feb. 23rd at 1 pm

Exam #2

• February 25th

• Covers from last exam up to today

Sun• Brightest star in the sky

• Closest star to Earth

• Next Closest is Alpha Centauri, which is 4.3 light years away

Sun video

• http://www.space.com/common/media/video/player.php?videoRef=sun_storm

Solar Constant

• Energy received at Earth’s distance from the Sun

• ~1400 W/m2

• 50-70 % reaches Earth’s surface

• 30% absorbed by atmosphere

• 0-20% reflected away by clouds

http://en.wikipedia.org/wiki/File:Sun_Life.png

Absorption lines

Energy Source for Sun

• Fusing hydrogen into helium– Hydrogen nucleus – 1 proton– Helium nucleus – 2 protons, 2 neutrons

• Need high temperatures for this to occur

• ~10 to 14 million degrees Kelvin

http://www.astronomynotes.com/starsun/s3.htm

http://www.astronomynotes.com/starsun/s3.htm

How does Fusion Convert Mass to Energy

• What is the most famous formula in the world?

E = mc2

• m is mass in kilograms

• c is speed of light in meters/s

• E (energy) is in joules

• very small amounts of mass may be converted into a very large amount of energy

Law

• Law of Conservation of mass and energy– Sum of all mass and energy (converted into the same

units) must always remain constant during any physical process

http://observe.arc.nasa.gov/nasa/exhibits/stars/star_6.html

1 kg

0.993 kg

1 kg0.993 kg0.007 kg

Reaction

• 4 protons → helium-4 + 2 neutrinos + energy

Neutrino-virtually massless, chargeless particles

Positron-positively charged electron – annihilated immediately by colliding with an electron to produce energy

Antiparticles

• Antiparticle – particle with the same mass and opposite electric charge

• Antiparticles make up antimatter• Annihilation – when a particle and an antiparticle

collide• Antimatter is said to be the most costly substance

in existence, with an estimated cost of $62.5 trillion per milligram.

Fusion reaction

• Much more complicated than

4 protons → helium-4 + 2 neutrinos + energy

Deuteron – Deuterium (hydrogen with a neutron)nucleus

Proton-Proton Chain Reaction

• This reaction occurs ~1038 times each second

• It if occurred faster, Sun would run out of fuel

Neutrinos• Neutrinos – almost massless particles• No charge• It takes a neutrino about 2 seconds to exit the Sun• The neutrino was first postulated in 1930 by Wolfgang

Pauli to preserve conservation of energy, conservation of momentum, and conservation of angular momentum during the decay of a neutron into a proton where an electron is emitted (and an antineutrino).

• Pauli theorized that an undetected particle was carrying away the observed difference between the energy, momentum, and angular momentum of the initial and final particles.

How was the Homestake Gold Mine used to detect neutrinos?

• A 400,000 liter vat of chlorine-containing cleaning fluid was placed in the Homestake gold mine

• Every so often Chlorine would capture a neutrino and turn into radioactive argon

• Modelers predict 1 reaction per day• Experiments found 1 reaction every 3 days• Newer detectors used water

to look for reactions

What was the solar neutrino problem?

• Less neutrinos appeared to have been produced from the Sun than expected from models

Solution of Problem• Neutrinos come in three types (slightly different

masses)– Electron neutrino– Muon neutrino– Tau Neutrino

• Experiment could only detect electron neutrinos• Fusion reactions in Sun only produced electron

neutrinos• Electron neutrinos could change into other types

of neutrinos that could not be detected• Neutrino oscillations – one type of neutrino could

change into another type

Fusion

• The rate of nuclear fusion is a function of temperature

• Hotter temperature – higher fusion rate

• Lower temperature – lower fusion rate

• If the Sun gets hotter or colder, it may not be good for life on Earth

What is happening to the amount of Helium in the Sun?

• A) Its increasing

• B) its decreasing

• C) Its staying the same

What is happening to the amount of Helium in the Sun?

• A) Its increasing

• B) its decreasing

• C) Its staying the same

So how does the Sun stay relatively constant in Luminosity

(power output)

http://www-ssg.sr.unh.edu/406/Review/rev8.html

Figure 15.8

Figure 15.4

Tem

pera

ture

Den

sity

Parts of SunCore

• Core – 15 million Kelvin – where fusion occurs

Figure 15.4

Radiation zone

• Radiation zone – region where energy is transported primarily by radiative diffusion

• Radiative diffusion is the slow, outward migration of photons

Figure 15.13

Photons emitted from Fusion reactions

• Photons are originally gamma rays

• Tend to lose energy as they bounce around

• Photons emitted by surface tend to be visible photons

• Takes about a million years for the energy produced by fusion to reach the surface

Figure 15.4

Convection Zone

• Temperature is about 2 million Kelvin

• Photons tend to be absorbed by the solar plasma

• Plasma is a gas of ions and electrons

• Hotter plasma tends to rise

• Cooler plasma tends to sink

Figure 15.14

Granulation – bubbling pattern due to convectionbright – hot gas, dark – cool gas

Figure 15.14

Figure 15.10

Figure 15.4

Classification of Stars

• Stars are classified according to luminosity and surface temperature

• Luminosity is the amount of power it radiates into space

• Surface temperature is the temperature of the surface

Stars have different colors

Surface Temperature

• Determine surface temperature by determining the wavelength where a star emits the maximum amount of radiation

• Surface temperature does not vary according to distance so easier to measure

1913

Who were these people?

• These were the women (called computers) who recorded, classified, and catalogued stellar spectra

• Were paid 25 cents a day

• Willamina Fleming (1857-1911) classified stellar spectra according to the strength of their hydrogen lines

• Classified over 10,000 stars

Fleming’s classification

• A - strongest hydrogen emission lines

• B - slighter weaker emission lines

• C, D, E, … L, M, N

• O - weakest hydrogen lines emission lines

Annie Jump Cannon (1863-1941)

• Cannon reordered the classification sequence by temperature and tossed out most of the classes

• She devised OBAFGKM

More information

• Each spectral type had 10 subclasses

• e.g., A0, A1, A2, … A9 in the order from the hottest to the coolest

• Cannon classified over 400,000 stars

OBAFGKM

• Oh Be A Fine Girl/Gal Kiss Me• http://www.mtholyoke.edu/courses/tburbine/ASTR223/O

BAFGKM.mp3

http://physics.uoregon.edu/~jimbrau/BrauImNew/Chap04/FG04_05.jpg

http://spiff.rit.edu/classes/phys301/lectures/spec_lines/spec_lines.html

http://scope.pari.edu/images/stellarspectrum.jpg

But

• absorption line - A dark feature in the spectrum of a star, formed by cooler gas in the star's outer layers (the photosphere) that absorbs radiation emitted by hotter gas below.

Cecilia Payne-Gaposchkin (1900-1979)

• Payne argued that the great variation in stellar absorption lines was due to differing amounts of ionization (due to differing temperatures), not different abundances of elements

Cecilia Payne-Gaposchkin (1900-1979)

• She proposed that most stars were made up of Hydrogen and Helium

• Her 1925 PhD Harvard thesis on these topics was voted best Astronomy thesis of the 20th century

It takes progressively more energy to remove successive electrons from an atom. That is, it is much harder to ionize electrons of He II than He I.

Hertzsprung-Russell Diagram

• Both plotted spectral type (temperature) versus stellar luminosity

• Saw trends in the plots

• Did not plot randomly

Remember

• Temperature on x-axis (vertical) does from higher to lower temperature

• O – hottest

• M - coldest

• Most stars fall along the main sequence

• Stars at the top above the main sequence are called Supergiants

• Stars between the Supergiants and main sequence are called Giants

• Stars below the Main Sequence are called White Dwarfs

Hertzsprung-Russell Diagram

wd white dwarfs

• giant – a star with a radius between 10 and 100 times that of the Sun

• dwarf – any star with a radius comparable to, or smaller than, that of the Sun

Classifications

• Sun is a G2 V

• Betelgeuse is a M2 I

Radius

• Smallest stars on the main sequence fall on the bottom right

• Largest stars on main sequence fall on the top left

• At the same size, hotter stars are more luminous than cooler ones

• At the same temperature, larger stars are more luminous than smaller ones

Main Sequence Stars

• Fuse Hydrogen into Helium for energy

• On main sequence, mass tends to decrease with decreasing temperature

What does this tell us

• The star’s mass is directionally proportional to how luminous it is

• More massive, the star must have a higher nuclear burning rate to maintain gravitational equilibrium

• So more energy is produced

Main Sequence Lifetimes

• The more massive a star on the main sequence, the shorter its lifetime

• More massive stars do contain more hydrogen than smaller stars

• However, the more massive stars have higher luminosities so they are using up their fuel at a much quicker rate than smaller stars

Ages

• Universe is thought to be about 14 billion years old

• So less massive stars have lifetimes longer than the age of the universe

• More massive stars have ages much younger

• So stars must be continually forming

Things to remember

• 90% of classified stars are on main sequence

• Main sequence stars are “young” stars

• If a star is leaving the main sequence, it is at the end of its lifespan of burning hydrogen into helium

Remember

• Largest stars on main sequence are O stars

• Largest stars that can exist are supergiants

You need to know stellar classifications

• O, B, A, F, G, K, M

• A0, A1, A2, … A9 in the order from the hottest to the coolest

wd white dwarfs

Classifications

• Sun is a G2 V

• Betelgeuse is a M2 I

• Vega is a A0 V

• Sirius is a A1 V

• Arcturus is a K3 III

Any Questions?