Astronomy 1020Spring_2015

Day-26Stellar Astronomy

Course Announcements

• Smartworks Chapter 13: Next week sometime

• Apr. 2 – Last day to drop a class.

Astronomy in the Fall, 2015

ASTR-1010/1011 - Planetary Astronomy + Lab (H,R)ASTR-1020/1021 - Stellar Astronomy + Lab (R)ASTR-2010 - Problems in Planet AstronomyASTR-2011 - Intro. to Observational AstronomyASTR-4000/4001 – Astrophotography & LabASTR-3030/3031 – Methods & Techniques in Astronomy

Stars are classified into spectral types according to the appearance of their spectra.

Absorption lines depend mainly on the temperature.

Full sequence: O B A F G K M. Sun = G2.

Hottest stars: weak absorption by hydrogen and helium (type O).

Medium: strong hydrogen absorption (type A). Coolest: absorption by heavy elements or

molecules (type M).

Spectral lines are used to find the composition of stars.

All stars are mostly hydrogen and helium.

Sun: 74.5% H, 23.7% He by mass (92.5% H, 7.4% He by number), and the rest are heavy elements.

Concept Quiz—Spectral Types

Stars such as the Sun (type G) have spectra with many absorption lines from heavy elements. Why?

A.The Sun is made mostly of heavy elements

B.The Sun is a red giant.

C.Heavy elements are efficient absorbers of light at the temperature of the Sun.

D.Hydrogen and helium never absorb light.

With luminosity and temperature, we can calculate the size of the star.

Size: radius (half the diameter), R.

The radius comes from the Stefan-Boltzmann law.

There are many more small stars than large ones.

To measure mass, we must look for the effects of gravity.

Many stars are binary stars orbiting a common center of mass.

A less massive star moves faster on a larger orbit.

Measure velocities of the stars as they orbit.

Calculate total mass of both stars from Kepler’s law and a ratio of one star’s mass to the other.

Lowest-mass stars have M = 0.08 M.

Highest-mass are likely a little bigger than 150 M.

Visual binary: can distinguish both stars visually.

Spectroscopic binary: stars are too far away to distinguish; pairs of Doppler-shifted lines trade places.

Eclipsing binary: The total light coming from the star system decreases when one star passes in front of the other.

Could also potentially measure the radii of the stars in these systems.

Masses of stars can directly be calculated if they are in an eclipsing binary system.

Using observations of the orbital periods and velocities with Newton’s formulation of Kepler’s Third law:

MATH TOOLS 13.3MATH TOOLS 13.3

Binary StarsLecture Tutorial pg. 121

Work with a partner!Read the instructions and questions carefully.Discuss the concepts and your answers with

one another. Take time to understand it now!!!!Come to a consensus answer you both agree on

and write complete thoughts into your LT.If you get stuck or are not sure of your answer,

ask another group.