Stars

1. Stars • Stars are very far away. • The nearest star is over 270,000 AU away! (Pluto is 39 AU from the Sun) • That is equal to 25 trillion miles! • At this distance it takes light 4.3 years to travel from this star. In other words the star is 4.3 light years away. • The space shuttle travels 17,500 miles/hour, at this speed it would take over 160,000 years to get to the nearest star! The Pleiades Star Cluster about 400 light years away

2. How Do We Know the Distance to the Stars? • The ancient Greeks realized that if the Earth moved we should see shifts in the positions of the stars over one year. • We do see these shifts in star positions but they are very,very small. • This shift in an object’s position due to the motion of the observer is called parallax. • The farther an object the smaller the shift and the harder it is to find its distance. Using the size of the shift due to parallax and knowing the distance from the Earth to the Sun we can find the distance to nearby stars.

3. How Do We Know Anything About Stars? • Since stars are so far away they almost always appear just as points of light. • But as we already know we can learn a lot from light! • Light can tell us about a star’s: • surface temperature • distance • motion • rotation • composition • and if the star is part of binary star system we can determine its mass. The Trapezium Star Cluster in the Orion Nebula about 1500 light years away

4. Luminosity • Luminosity is the amount of energy a body radiates each second. • Stars appear brighter or dimmer to us for two reasons: • they are at various distances from us and • some stars are naturally more luminous than other stars • If we know a stars distance and we measure its apparent brightness we can determine its luminosity. • A stars luminosity is related to both its temperature and its radius. • So if we also know a star’s temperature we can determine its radius. Large stars have a higher luminosity than small stars. Hot stars have a higher luminosity than cool stars.

5. Stellar Spectra • The temperature of a star can be determined two ways. • from Wien’s law • from the presence or absence of certain spectral lines. • Strong Hydrogen lines are only seen in stars that have surface temperatures between 8,000 and 15,000 Kelvin. • The Sun does not show strong Hydrogen lines even though it is more than 70% Hydrogen! A) A spectrum from a star hotter than the Sun with strong Hydrogen lines. B) A spectrum from a star like our Sun. C) A spectrum from a star cooler than our Sun.

6. Spectral Classification • To understand the properties of stars astronomers gathered hundreds of thousands of stellar spectra. • To understand the patterns they saw they developed spectral classification systems in order to help understand the nature of stars. • The first system was developed in 1866 by Pietro Angelo Secchi an Italian priest and scientist. He grouped stars by their color. • The system used today was developed by Annie Jump Cannon who ordered stars by temperature. Annie Jump Cannon 1863 - 1941

7. Spectral Classes • In 1901 Cannon developed a system where letters were assigned to stars of different temperature. • In the 1920’s another astronomer Cecilia Payne explained why spectral lines change with temperature and confirmed the system that Cannon developed. In order from hottest to coldest stars the letter classification is O, B, A, F, G, K, M In this system our Sun is a G star.

8. Binary Stars • Most stars in the sky actually exist as part of a group of 2 or more stars. • Two stars gravitationally bound are called binary stars. • Binary stars provide a means of determining the masses of stars. • Other properties can also sometimes be determined from binary stars. The two stars of a binary star system orbit around their common center of mass. Using Newton’s modified form of Kepler’s 3rd Law the mass of the stars can be found.

9. Types of Binary Stars Example of a visual binary • If both stars in a binary star system are visible this is a visual binary. The motion of the two stars can be observed directly. • Sometimes only one point of light is seen but the spectra of this star shows two sets of lines. This is a spectroscopic binary. The motion of the two stars can found from the Doppler shift of the lines. Example of a spectroscopic binary

10. Eclipsing Binaries • When the orbit of a binary is viewed edge-on from Earth the stars may actually eclipse one another. This is an eclipsing binary. • Eclipsing binaries can give information on the radius, mass and shape of the stars.

11. The Hertzprung-Russell Diagramor The H-R Diagram • The H-R diagram puts into one figure many of the properties of stars already discussed. • Where a star is located on this diagram depends on • its luminosity and • its spectral class or temperature • Stars tend to show up in distinct groups when plotted in this way.

12. Main Sequence, Giants and Dwarfs • Nearly 90% of the stars lie along a line on the H-R diagram called the Main Sequence. • Those stars in the upper right of the figure are very luminous but also very cool. Therefore they must be very large. These are the Red Giant stars. • Those stars in the lower left are very hot but have low luminosity. They must be very small. These are White Dwarf stars. Betelgeuse is almost as big as the orbit of Jupiter. Sirius B is as small as Earth.

13. The Mass of Stars on the Main Sequence • Where stars are located along the Main Sequence depends almost exclusively on the star’s mass. • More massive stars are more luminous, they release energy at a higher rate and are located towards the upper left of the Main Sequence. (They also consume Hydrogen much faster). • Less massive stars are located at the lower right.