Upload
christopher-clark
View
226
Download
2
Embed Size (px)
Citation preview
Orion: A Bunch of Stars
• Greek: Hunter• Japanese: A water
drum• Brightest Stars:
Betelgeuse, Rigel• Orion Nebula: The
Smoking Star
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
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
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
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