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Today:. Star sizes HR diagrams More info from spectra Mass: Mass and luminosity How do we measure mass? Binary stars. Stars come in various sizes. Let’s do a few calculations to get a feel for the range of stellar sizes:. Finding key properties of a nearby star.

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Presentation Transcript
Today:
• Star sizes
• HR diagrams
• Mass:
• Mass and luminosity
• How do we measure mass?
• Binary stars
Stars come in various sizes

Let’s do a few calculations to get a feel for the range of stellar sizes:

Understanding stellar data
• We have a huge pile of data about the stars. What do we need to do to understand it?
• Make a graph!
• What should we graph?
• Two quantities differ substantially from star to star: temperature and luminosity.
• If we make a graph relating these two quantities we can understand the relationship between L and T. As a bonus we also create a plot that helps us understand how stars form, evolve and die.
• This is a really important graph, perhaps the most important in all astronomy!
Hertzsprung-Russell diagrams
• 1911: Ejnar Hertzsprung made a plot of absolute magnitude (luminosity) as a function of color (temperature).
• 1913: Henry Norris Russell made a similar plot but used spectral class instead of color.
• This plot is called a Hertzsprung-Russell or HR diagram.
• Surface temperature largely determines which lines are prominent in a star’s spectrum.
• Classifying stars by spectral type is equivalent to categorizing them by surface temperature.
• Problem: 5800K surface temperature can mean a star is a white dwarf, a main-sequence star, a giant or even a supergiant…
• It turns out that details in the spectrum of a star help us solve this problem.
• Can determine type of star.
• Can even be used to determine distance to star.
Size from spectra
• Balmer lines are narrow in large, luminous supergiants and broad in small, less luminous main-sequence star.
• Higher density (small stars)=broad lines.
• Lower density (large stars)=narrow lines
Spectroscopic parallax
• A star’s spectral type and luminosity combined with info on the HR diagram allow us to estimate a star’s distance from Earth.
• Also need inverse-square law.
• Let’s look at an example:
Stellar mass
• Answer lies in energy source for stars:
• Greater mass means higher pressure and temperature at center of stars leading to more rapid fusion and a greater energy output.
• To maintain hydrostatic and thermal equilibrium in a more massive star, the star must have a larger radius and higher surface temperature.
• This is what we see in the HR diagram.
How do we measure mass?
• About half of all visible stars are parts of multiple star systems.
• Typically pairs called binary stars.
• These stars are gravitationally bound so we can use physics to describe their motions.
Determining stellar masses

In principle, if we can determine a and P we can calculate M1+M2. We want M1 and M2 separately however. How do we find that?

Determining stellar masses
• Binary stars orbit the center of mass of the system.
• Balance point of kids on seesaw. Its position depends on the relative masses of the kids.
• Understanding size of orbits around center of mass gives us M1/M2.
• This, combined with the sum of the masses (and a little algebra) allows us to calculate the individual masses.
• We won’t do this in class today, but you can on the homework for some extra credit.