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Red Stars, Blue Stars, Old Stars, New Stars Session 1 Julie Lutz University of Washington

Red Stars, Blue Stars, Old Stars, New Stars Session 1 Julie Lutz University of Washington

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Red Stars, Blue Stars, Old Stars, New Stars Session 1

Julie Lutz

University of Washington

Orion: A Bunch of Stars

• Greek: Hunter• Japanese: A water

drum• Brightest Stars:

Betelgeuse, Rigel• Orion Nebula: The

Smoking Star

Orion: A Deeper View

Session 1: Basic Properties of Stars

• Imagine that you are an astronomer who undertakes studies of stars.

• What sorts of things would you like to know about them?

Some Properties of Stars

• Temperature• What makes them

shine? Energy source?• Chemical composition• Are they moving? If

so, how?• Do they change

(timescale)?

• Distance from solar system

• Apparent brightness in the night sky

• “True” brightness or luminosity

• Diameter (assuming round)

Astronomer’s Toolkit

• Principles of physics, chemistry, math

• Telescopes and auxiliary instruments

• Computers for data collection and analysis

Some Solar Properties

• About 93 million miles from Earth

• 109x diameter Earth• 330,000x as massive

as Earth• Surface temperature

10,000F

Distances to Nearby Stars

• Use “parallax”--small shifts in star position due to Earth moving around sun

• Unit: parsec• 1 parsec=206265

astronomical units

Units for Star Distance

• Parsec (pc)• Light Year

(ly)=distance that a light beam would travel in one year, about 6 trillion miles

• Nearest star to sun is 4.26 ly (Proxima Cen)

Magnitude Scale

• First used by Greeks for star catalogs.

• Smaller numerically the mag, the brighter the star.

• 1st mag stars are brighter than 5th mag stars

• Some objects are brighter than mag 0. Sun is -26.5; full moon is -11; Venus -3.5

Apparent Mag, Absolute Mag

• Apparent mag (m) is the brightness as seen directly in the sky. Goes from -26.5 to +28

• Absolute mag (M) is a measure of the TRUE brightnesses (luminosities) of objects relative to each other. Particularly useful for comparing stars.

Definition of Absolute Mag

• Brightness that a star would have if it were at a distance of 10 pc.

• 10 pc was an arbitrary choice

• Idea is to line ‘em up and compare brightness at some standard distance.

• M, m and distance are related

Mizar in Ursa Major

• m=2.2• M=0.3• Distance=24 parsecs

or 78 light years

Rigel in Orion

• M=0.2• M=-6.7• Distance=240 parsecs

or 800 light years

The Sun

m=-26.5

M=4.5

Distance=0.000005 pc or 0.000018 ly

IF the Sun were 10 pc away, it would have a brightness like the Pleiades stars

Luminosity

• Luminosity is a “physics term” for the true brightness of a star, so L is related to absolute magnitude

• Some stars are many times more luminous than the sun, others much less

Star Color Indicates Surface Temperature

Star Card Activity

Information Gathering

• Observe many stars to find their temperatures and luminosities.

• Make a graph of this information

• Called a Hertzsprung-Russell (HR) Diagram

What We Know So Far

• Stars seem to have particular patterns in a diagram of Temp vs Luminosity

• Most stars are on the “Main Sequence”

• A few are giants (10s x solar diameter), supergiants (100s x solar diameter), white dwarfs (planet-sized)

• What does this mean? Need more info!

How About Finding the Masses of Stars?

• Use principles of gravitational attraction.

• Binary stars--2 stars in orbit

• Different types of binary systems

Binary Star

Eclipsing Binary

Stellar Masses

• Range from about 0.07 to 120 times mass of sun

• On the Main Sequence, most massive stars are in upper left (blue), least in lower right

How did astronomers figure out the patterns of stellar evolution?

• HR Diagram• Thermonuclear

reactions• Star masses and sizes• Star clusters• Nebulae--gas and dust

Summary of Session 1

• Most stars have temps between 3000 K and 30,000 K.

• Stars have wide range in luminosity. Some are 10s of 1000s of times more luminous than sun; others much less luminous.

• Masses range from 0.07 to 120 times mass of sun

• Diameters planet-sized to 100s x sun

Some Properties of Stars

• Temperature• What makes them

shine? Energy source?• Chemical composition• Are they moving? If

so, how?• Do they change

(timescale)?

• Distance from solar system

• Apparent brightness in the night sky

• “True” brightness or luminosity

• Diameter (assuming round)