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: Sirius is a very bright star in our sky. It has a parallax angle of 0.379 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 StarApparent on Earth

Mathematical relationship between brightness & energy

B = L 4pd2

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 = 5log10(d)

Luminosity and Flux measure energy from the star.

L = (Area)sT4

Mass → Temperature → Energy → Brightness

The more mass a star has, the greater the energy output.

L = M3.5

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 compositionsStars have different temperatures

OBAFGKMLT

Type Color TemperatureO Blue-Violet 30,000+ K

B Blue 18,000 K

A Blue-White 10,000 K

F White 7,000 K

G Yellow 5,500 K

K Orange 4,000 K

M Orange-Red 3,000 K

L Red 2,000 K

T Dark Red 700-1,300 K

http://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectral_class.html

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

DistanceRadiusMass

Luminosity ClassAge of Cluster

Globular Cluster M55

Cluster Ages

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

I Super Giants

II Bright Giants

III Giants

IV Sub Giants

V Main-Sequence

Why are binary stars important?

The mass of a binary system can be determined from the orbital period.

(m1 + m2)P2 = a3

Visual Binaries

See orbital periods

Spectroscopic Binaries

Doppler shift leads to orbital period

Eclipsing binaries

Light curves lead to orbital periods