Star Properties (Chapter 8)

Student Learning Objectives Classify stars Explain how star properties are related.

What is the parallax method for measuring distance?  Stellar parallax is a change in an object’s apparent position caused by a change in the observer’s position.

Example: Example: Sirius is a very bright star in our sky. It has a parallax angle of arcseconds.  Distance in parsecs is inversely related to the parallax angle in arcseconds. d = 1 p

Practice Calculate the distance in Light Years (1 pc = 3.26 LY) 1)Parallax angle = 0.2 arcseconds 2)Vega: Parallax angle = 0.12 arcseconds 3)Polaris: Parallax angle = 0.01 arcseconds

How are stellar properties related?  Magnitudes are a measure of brightness, and brightness is the direct result of energy output.  Brightness refers to the number of photons.  Intrinsic to Star  Apparent on Earth

Mathematical relationship between brightness & energy B = L 4  d 2

Magnitudes & Distance  Absolute Magnitude is the magnitude a star would have at a distance of 10 parsecs.  Magnitudes can be used to determine distance. m – M = 5log 10 (d)

 Luminosity and Flux measure energy from the star. L = (Area)  T 4

Mass → Temperature → Energy → Brightness  The more mass a star has, the greater the energy output. L = M 3.5

Practice Star A: Large Surface Area at 6,000 K Star B: Small Surface Area at 6,000K 1) Which would have a greater luminosity? 2) Which would have a greater flux? 3) Which would be brighter if both were the same distance from us?

What does a star’s spectral class indicate?  The spectral class is determined from a star’s atomic spectrum.  All stars have similar compositions  Stars have different temperatures OBAFGKMLT

TypeColorTemperature OBlue-Violet30,000+ K BBlue18,000 K ABlue-White10,000 K FWhite7,000 K GYellow5,500 K KOrange4,000 K MOrange-Red3,000 K LRed2,000 K TDark Red700-1,300 K

What is shown by an H-R diagram?  An H-R diagram is a graph of luminosity compared to temperature.

 The locations of the stars on the graph indicates where they are in the stellar evolution process. HR Graph Shows Distance Radius Mass Luminosity Class Age of Cluster

Globular Cluster M55

Cluster Ages

 The HR Diagram is further divided into Luminosity Classes based on line width.

Why are binary stars important?  The mass of a binary system can be determined from the orbital period. (m 1 + m 2 )P 2 = a 3

Visual Binaries See orbital periods

Spectroscopic Binaries Doppler shift leads to orbital period

Eclipsing binaries Light curves lead to orbital periods