Properties of Stars

http://stardate.org/radio/program/delta-lyrae

Learning goals: Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star. Define arc second, parsec. Define brightness, apparent magnitude, absolute magnitude. Describe the methods used to determine the temperature, luminosity, and radius of a star.

Distances Questions: Which stars are the brightest? Which stars are putting out the most watts? (luminosity = energy per second) NEED TO KNOW: Distances The most fundamental and accurate (within a certain range) means of finding distances is measuring the parallaxes of stars.

You already know about the parallax effect: Demonstrating parallax Parallax of Stars Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star.

Define arc second How many degrees in a circle? How many arc minutes in a degree? How many arc seconds in an arc minute? How many arc seconds in a degree? How many arc seconds in a circle? __?__ radians = 360 degrees 1 radian = 57.3 degrees How many arc seconds in 1 radian? 360, 60, 60, 3600; 1,296,000; 2 pi; 206,265 arc sec/rad

PARSEC: Parallax ARc SECond A star having a parallax of 1 arc second is 1 parsec away 1 parsec (pc) = 3.26 light years 1 kiloparsec (1 kpc) = 1000 pc; 1 megaparsec (1 Mpc) = 1,000,000 pc Baseline is 1 Astronomical Unit Small angle formula for distance in AU’s: Define arc second, parsec

Measured Parallax of Stars 6.7 22 667 2170 ly Works accurately for stars within about 200 pc (Hipparchos satellite) Biggest problem: measuring the miniscule shift of a star against more distant stars 6.7 22 667 2170 ly Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star.

Explain what is meant by the parallax of a star, how we measure it and use it to find the distance to a star.

Using SIMBAD to find the parallaxes of the stars of Exercise 2 41 Cygni data (partial) Parallax = 4.24 ± 0.16 mas or 0.00424 ± 0.00016 arc seconds Distance = 1/parallax = 1/0.00424 = 236 pc or ~770 ly

Inverse square law for light p. 494

  Brightness How the star looks to US HERE ON EARTH. times farther away  100 Watt 1000 Watt 1 Watts 10 times farther away  2 x farther away, 1/4 as bright 3 x farther away, 1/9 as bright Define brightness, apparent magnitude, absolute magnitude

Every 5 magnitudes difference means 100 x difference in brightness Apparent Magnitude Every 5 magnitudes difference means 100 x difference in brightness One magnitude difference is 2.512 times in brightness. (2.5125 = 100) Define brightness, apparent magnitude, absolute magnitude

When you see only “magnitude,” that means APPARENT magnitude. The magnitude (m) of star A is 1, the magnitude (m) of star B is 6. How many times brighter is A than B? a) 5 b) 10 c) 100 d) 1000 m of star C is 12, m of star D is 2: How many times brighter is star D than star C? (Or, equally stated, how many times dimmer is star C than star D?) a) 10 b) 24 c) 100 d) 10,000 The Sun is the brightest star in the sky, with an apparent magnitude of about -26.5 Sirius is next in line, with an apparent magnitude of -1.5; how many times brighter is the Sun than Sirius? a) 25 b) 28 c) 100,000 d) 10,000,000,000

Using SIMBAD to find the apparent magnitudes of the stars of Exercise 2 41 Cygni data (partial) V = apparent magnitude through “visual” filter Think of it as mv . IR UV

Absolute Magnitude Absolute magnitude is the apparent magnitude a star would have if its distance = 10 parsecs. Relates luminosities by “placing” stars on common scale. Smaller the absolute magnitude number, the more luminous the star. 41 Cygni dpc = 236 parsecs mv = 4.016 What does the answer tell you? Define brightness, apparent magnitude, absolute magnitude

Define brightness, apparent and absolute magnitude

Luminosity Classes Supergiant I Bright-Giant II Giant III Sub-Giant IV Main Sequence Star (dwarf) V We estimate the luminosity of a star by measuring how broad the absorption lines are in its spectrum. At a given temperature, the less luminous stars have atoms colliding a lot more than in the giant stars. Describe the methods used to determine temperature, luminosity, radius

High Luminosity Low Low High Temperature

Using SIMBAD to find the parallaxes of the stars of Exercise 2 41 Cygni data (partial) F5 Iab

The H-R Diagram

Relationship between absolute magnitude and luminosity - bring in the Sun!

Luminosity Depends on Size (radius, R) Temperature Explain Describe the methods used to determine temperature, luminosity, radius

The H-R Diagram