2 A Objectives Describe the size and brightness

2 A Objectives Describe the size and brightness of stars. n Explain how the composition of a star is determined. n

Stars n Sizes of stars – – Giant stars Medium stars White dwarfs Neutron stars

Largest Stars Giant Stars n A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature n Compared to the sun – 10 -100 times larger

Largest Stars Supergiant Stars n Compared to the sun – 1000 times larger n Rigel, Betelgeuse, Antares

Smallest Stars White Dwarfs n Small star whose mass is comparable to that of the Sun and its volume is comparable to that of the Earth – It is very dense n n Smaller than the Earth Van Maanen’s star – Smaller than Asia

Smallest Stars n Neutron Stars – Smaller still – 16 km – 9. 9 miles – Sun is 60, 000 times larger than a neutron star

Spectroscope Composition n Optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum – Takes the light and breaks it into the different colors – The presence or absence of different colors are signatures of specific characteristics

Spectroscope When a material is heated to incandescence it emits light that is characteristic of the atomic makeup of the material n Particular light frequencies give rise to sharply defined bands n Can be thought of as fingerprints n

Composition of Stars Depending on what a star is made of… n It will show certain bands of light when a spectroscope is used n So what have we found? n – Hydrogen n 60 -80% of a star – Helium – Both together make up 96 -99% of star’s mass

Temperature n Temperature of a star can be studied by studying the color of a star – Why? n The reason for the different colors is that the light is sensitive to the temperature of the star.

Brightness n Flashlight example n Depends on: – Size – Surface Temperature – Distance

Brightness n Apparent Magnitude – Brightness as seen by an observer on Earth – Normalized to the value it would have in the absence of the atmosphere – The brighter the object appears, the lower the value of its magnitude

Brightness n Absolute Magnitude – Measures a celestial object's intrinsic brightness – Accounts for distance and calculate show bright a star really is

Variable Stars n Intrinsic variables: – Whose luminosity actually changes n Because shrinks. n the star periodically swells and Extrinsic variables: – Whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth n Because the star has an orbiting companion that sometimes eclipses it.