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February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and...

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February 18, 200 3 Lynn Cominsky - Cosmology A350 1 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655 Best way to reach me: [email protected] Astronomy 350 Cosmology
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Page 1: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

1

Professor Lynn Cominsky

Department of Physics and Astronomy

Offices: Darwin 329A and NASA EPO

(707) 664-2655

Best way to reach me: [email protected]

Astronomy 350Cosmology

Page 2: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Disks around stars

There is much evidence of disks with gaps (presumably caused by planets) around bright, nearby stars, such as Beta Pic

Page 3: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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What makes a world habitable?

Select your top three candidates for lifeClass votes:

Earth (duh)Europa (25 votes)Titan (17 votes)Mars (16 votes) Io (13 votes)Callisto (12 votes)

Page 4: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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The Nearest Stars

Distance to Alpha

or Proxima

Centauri is

~4 x 1011 km or

~4.2 light years

Distance between

Alpha and

Proxima Centauri

is ~23 AU

Page 5: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

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The Solar Neighborhood

Some stars within about 2 x 1014 km(~ 20 light years)

Page 6: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

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Distances to Stars

Parallax : determined by the change of position of a nearby star with respect to the distant stars, as seen from the Earth at two different times separated by 6 months.

Page 7: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Calculating Parallax

Measure angle in radians: it is very small The tangent and the sine of the angle are therefore about

the same as the angle in radians The Earth-Sun distance of 1 AU = 1.5 x 108 km Distance to star = (Earth-Sun distance) / parallax

parallax angleParallax for Proxima Centauri is 0.76 arc-seconds

Page 8: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

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Parallax movie

Page 9: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Parallax, parsecs and light years

1 parsec is defined as the distance at which a star would have a parallax angle of 1 arc-second

1 arc-second = (1 degree/3600) = (1 degree/3600) (radians/ 180 degrees ) = 4.85 x 10-6 radians

1 parsec = (1.5 x 108 km)/(4.85 x 10-6 ) = 3.086 x 1013 km = 3.26 light years

1 light-year is the distance light will travel in one year 1 light-year = (2.998 x 108 m/s)(86400 s/d)(365 d/y) = 9.46

x 1012 km = 9.46 x 1015 m A LIGHTYEAR IS A DISTANCE, NOT A TIME!

Page 10: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Absolute vs. Apparent magnitude

Apparent magnitude - How bright does the star appear (from the Earth)? Uses symbol “m”

Absolute magnitude - the apparent magnitude of a star if it were located at 10 pc. Uses symbol “M”

Absolute and apparent magnitude are related to the true distance “D” to the star by:

m – M = 5 log (D/10 pc) = 5 log (D/pc) – 5 OR

D = 10 pc * 10((m-M)/5)

Magnitudes seem backwards – the bigger the number, the fainter the star.

Page 11: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Classifying Stars

Hertzsprung-Russell diagram

Page 12: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Classes of Stars

Bigger stars are brighter than smaller stars because they have more surface area

Hotter stars make more light per square meter. So, for a given size, hotter stars are brighter than cooler stars.

• White dwarfs - small and can be very hot (Class VII)

• Main sequence stars - range from hotter and larger to smaller and cooler (Class V)

• Giants - rather large and cool (Class III)

• Supergiants - cool and very large (Class I)

Page 13: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Properties of Stars

Temperature (degrees K) - color of star light. All stars with the same blackbody temperature are the same color. Specific spectral lines appear for each temperature range classification. Astronomers name temperature ranges in decreasing order as:

Surface gravity - measured from the shapes of the stellar absorption lines. Distinguishes classes of stars: supergiants, giants, main sequence stars and white dwarfs.

O B A F G K M

Page 14: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Populations of Stars

Population I – young, recently formed stars. Contain more metals than older stars, as they were created from debris from previous stellar explosions.

Population II – older stars that have evolved and are almost as old as the Universe itself.

Population III – the original stars that were formed about 200 million years after the Big Bang. They should be nearly all H and He

Page 15: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Life Cycles of Stars

Page 16: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Life Cycles of Stars

Page 17: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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The very first stars

Simulations by Tom Abel, Mike Norman and Greg Bryan 13 million years after the Big Bang, a piece of the Universe has collapsed

due to a slightly higher density of dark matter. It forms a 100 million solar mass protogalaxy, and at the center of this protogalaxy, a star is born!

Density movie

Temperature movie

Page 18: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

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Life and death of the very first star

From The Unfolding Universe, directed by Tom Lucas, simulation by Tom Abel

Page 19: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Molecular clouds and protostars

Giant molecular clouds are very cold, thin and wispy– they stretch out over tens of light years at temperatures from 10-100K, with a warmer core

They are 1000s of time more dense than the local interstellar medium, and collapse further under their own gravity to form protostars at their cores

Orion in mm radio (BIMA)

Simulation with narration by Jack Welch (UCB)

Page 20: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Protostars Orion nebula/Trapezium stars (in the sword) About 1500 light years away

HST/ 2.5 light years Chandra/10 light years

Page 21: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Stellar nurseries Pillars of

dense gas

Newly born stars may emerge at the ends of the pillars

About 7000 light years away

HST/EagleNebula in M16

Page 22: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Main Sequence Stars

Stars spend most of their lives on the “main sequence” where they burn hydrogen in nuclear reactions in their cores

Burning rate is higher for more massive stars - hence their lifetimes on the main sequence are much shorter and they are rather rare

Red dwarf stars are the most common as they burn hydrogen slowly and live the longest

Often called dwarfs (but not the same as White Dwarfs) because they are smaller than giants or supergiants

Our sun is considered a G2V star. It has been on the main sequence for about 4.5 billion years, with another ~5 billion to go

Page 23: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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How stars die

Stars that are below about 8 Mo form red giants at the end of their lives on the main sequence

Red giants evolve into white dwarfs, often accompanied by planetary nebulae

More massive stars form red supergiants Red supergiants undergo supernova

explosions, often leaving behind a stellar core which is a neutron star, or perhaps a black hole (more in later lectures about these topics)

Page 24: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Red Giants and Supergiants

Hydrogen burns in outer shell around the core

Heavier elements burn in inner shells

Page 25: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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White dwarf stars

Red giants (but not supergiants) turn into white dwarf stars as they run out of fuel

White dwarf mass must be less than 1.4 Mo

White dwarfs do not collapse because of quantum mechanical pressure from degenerate electrons

White dwarf radius is about the same as the Earth A teaspoon of a white dwarf would weigh 10 tons Some white dwarfs have magnetic fields as high as 109

Gauss White dwarfs eventually radiate away all their heat and end

up as black dwarfs in billions of years

Page 26: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Planetary nebulae

Planetary nebulae are not the origin of planets

Outer ejected shells of red giant illuminated by a white dwarf formed from the giant’s burnt-out core

Not always formed

HST/WFPC2Eskimo nebula5000 light years

Page 27: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Variable stars

Most stars vary in brightness Periodic variability can be due to:

Eclipses by the companion star Repeated flaring Pulsations as the star changes size or temperature

Novae are stars which repeatedly blow off their outer layers in huge flares

Flare stars have regions which explode Pulsating stars have an unstable equilibrium between the

competing forces of gas pressure and gravity

Page 28: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Cepheid variables

Henrietta Leavitt studied variable stars that were all at the same distance (in the LMC or SMC) and found that their pulsation periods were related to their brightnesses

L =K P1.3

Polaris (the North Star)

is not constant, it

is a Cepheid variable!

Page 29: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Distances to Cepheids

Distance to closest Cepheid (Delta Cephei) in our Galaxy can be found using parallax measurements. This determines K in the period-luminosity relation (L = KP1. 3)

Cepheids are very bright stars – they can be seen in other galaxies out to ~10 million light years (with HST)

Since the period of a Cepheid is related to its absolute brightness, if you observe its period and the apparent brightness, you can then derive its distance (to within about 10%)

Page 30: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Pleiades Star Cluster

A star cluster has a group of stars which are all located at approximately the same distance

The stars in the Pleiades were all formed at about the same time, from a single cloud of dust and gas

D = 116 pc

Page 31: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Open Star Clusters

Open ClusterNGC 3293

d = 8000 c-yr

20 -1000 stars

diameter ~ 10 pc

young stars (Pop I)

mostly located in spiral arms of our Galaxy and other galaxies

solar metal abundance

Page 32: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Globular Star Clusters

Globular Cluster 47 Tuc

d=20,000 c-yr 104 - 106 stars

diameter ~ 30 pc

centrally condensed

old stars (Pop II)

galaxy halo

low in metals

Page 33: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Finding the age of star clusters

This graphing activity from the University of Washington allows you to figure out the age of 2 clusters of stars by plotting stellar data on color-magnitude forms of the H-R diagram

47 Tuc

M45

Page 34: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

February 18, 2003 Lynn Cominsky - Cosmology A350

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Web Resources

Astronomy picture of the Day http://antwrp.gsfc.nasa.gov/apod/astropix.html

Imagine the Universe http://imagine.gsfc.nasa.gov Ned Wright’s ABCs of Distance

http://www.astro.ucla.edu/~wright/distance.htm National Geographic Star Journey

http://www.nationalgeographic.com/features/97/stars/index.html

Zoom Star Types Site http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml

Page 35: February 18, 2003Lynn Cominsky - Cosmology A3501 Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655.

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Web Resources

John Blondin’s supercomputer models http://www.physics.ncsu.edu/people/faculty.html

Cepheid variables http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html

U Washington Star Age Lab http://www.astro.washington.edu/labs/clearinghouse/labs/Clusterhr/color_mag.html

First star simulations http://cosmos.ucsd.edu/~tabel/GB/gb.html

Molecular cloud - protostar simulations http://archive.ncsa.uiuc.edu/Cyberia/Bima/StarForm.html


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