lec7 su06 HRD

Questions of the Day: 

Questions of the Day How does mass affect properties of a star? What is the main sequence? Why do stellar lifetimes depend on mass? How does the HR Diagram of main sequence stars change with time? Why are star clusters useful?

Let’s review some important things we want to know about stars…: 

Let’s review some important things we want to know about stars… Given enough time and information, we can figure out their… Brightness - easily observed Distance - need a parallax Spectral Type - can get from the spectrum Luminosity - need to know brightness and distance Temperature - can get from spectrum Mass - hard to figure out Age - exact age is hard, but can estimate

Slide3: 

The mass of a normal star almost completely determines its LUMINOSITY and TEMPERATURE! Mass sets the rate of hydrogen fusion The structure of the star adjusts to make a “stable” rate of fusion. The resulting structure will be the same for any star of the same mass

Categorizing the stars…: 

Categorizing the stars… We’ve already talked about measuring intrinsic properties like luminosity and temperature We’ve also ordered stars by their spectra Today we will learn about a very useful graph: the Hertzsprung-Russell Diagram (“HR Diagram” to its friends)

The HR Diagram: 

The HR Diagram The HR Diagram is a graph of luminosity versus temperature… The first thing to notice is that the stars aren’t randomly scattered on this graph-- they form a line! TEMPERATURE Luminosity

Slide6: 

Stars of different masses fall along a narrow path when luminosity is plotted against temperature The Main Sequence

Slide7: 

If you measure the luminosity and the color of a star, you know its mass!!! The Main Sequence PS. This had to be calibrated, but that’s a long story.

Slide8: 

If a random star falls on the Main Sequence, you also know that it’s Hydrogen burning! The Main Sequence

Slide9: 

Sometimes, instead of plotting temperature & luminosity, one plots color and magnitude: “color magnitude diagram” We can measure color and magnitude directly (they’re observables, remember?), so this is often easier than trying to figure out theoretical properties.

The more massive a star is, the more luminous it is…: 

The more massive a star is, the more luminous it is… Hotter Brighter Bigger Shorter-lived More massive stars are… But a higher rate of fusion means it’s burning its fuel faster!

Slide11: 

Low mass stars have lifetimes comparable to the Age of the Universe The Main Sequence!

Slide12: 

High mass stars have very short lifetimes, and disappear quickly! The Main Sequence!

Slide13: 

After time passes…

Slide14: 

H-R diagrams can be used like a clock The lifetime of these “turnoff” stars tells you how old the group of stars is. Figure out the mass of the most luminous stars on the Main Sequence (Only works if the group of stars were all born at the same time)

Slide15: 

An “Open Cluster” in the disk of the Milky Way Most of these stars have the same distance & age!

Slide16: 

An “Open Cluster” in the disk of the Milky Way The brightest stars are: A: Red B: Blue

Slide17: 

An “Open Cluster” in the disk of the Milky Way The spectral type of the brightest stars are more likely to be: A: M-stars B: O-stars

Slide18: 

An “Open Cluster” in the disk of the Milky Way The brightest stars are probably: A: more massive than the Sun B: less massive than the Sun

Slide19: 

An “Open Cluster” in the disk of the Milky Way The brightest stars probably: A: formed before the Sun (i.e. are older) B: formed after the Sun (i.e. are younger)

Slide20: 

Young clusters have HR diagrams something like this one. (Long main sequence extending to high masses)

Slide21: 

An old “Globular Cluster” orbiting the Milky Way A bunch of stars at the same distance, and the same age! Q: What color are the brightest stars???

Slide22: 

What are the stars on the upper right???? Only a bit of the main sequence is left (redder overall)! There are also “extra” very bright red stars! Stars down here were too faint to see

Slide23: 

Upper Right: “Red Giants” “Supergiants” Lower Left: “White Dwarfs”

Slide24: 

None of these “extra” stars are Hydrogen burning!

Slide25: 

The Mystery of Red Giants and White Dwarfs… Many of these stars have the same temperature as normal Main Sequence stars, but they’re much brighter or fainter! How is this possible???

Slide27: 

Cool, but bright. Same temp as some main sequence stars  same surface brightness! Must be bigger AREA  BIGGER star! (and thus the name, red giant) Red Giants:

Stellar Sizes on the HR Diagram…: 

Stellar Sizes on the HR Diagram…

Slide29: 

The range of sizes is huge. Not so much along the main sequence, but non-Hydrogen burning stars get very, very big, or very, very small.

Slide30: 

The H-R diagram for a group of stars is not constant! Massive stars use up the Hydrogen in their cores. They evolve off the main sequence. Evolving stars turn into red giants, red & blue supergiants, and white dwarfs.

Slide31: 

Stars cannot stay on the main sequence forever…

Slide32: 

Stars leave the Main Sequence… Luminosity Temperature

Slide33: 

Stars leave the Main Sequence… Luminosity Temperature

Slide34: 

Stars leave the Main Sequence… Luminosity Temperature

Slide35: 

Stars leave the Main Sequence… Luminosity Temperature

Slide36: 

Clusters of stars of different ages. Young: a few giant stars, some stars starting to move off the main sequence

Slide37: 

Clusters of stars of different ages. Old: many more red giants, even more stars starting to move off the main sequence

Slide38: 

When most stars are finished with Hydrogen burning, they leave the main sequence and become red giants.

Slide39: 

Clusters of stars of different ages. With increasing age, the tip of the main sequence shifts to fainter, redder stars, and more bright red stars show up.

Slide40: 

We can represent these clusters as lines on the HR diagram showing the location on which stars would lie. “isochrones”: latin for “same time”

Slide41: 

Clusters of stars of different ages. The only reason we don’t see the white dwarfs is that they’re too faint to detect in these particular observations.

Slide42: 

An HR diagram of all the stars for which we have parallax distances. Red giants are not uncommon! Q: Why does the Main Sequence extend bluer than the turnoff? Color (RED) Absolute Magnitude (BRIGHTER) Results from the Hipparcos satellite

Slide43: 

Low mass main sequence stars and white dwarfs are the most common! Note: the stars for which we have parallaxes are not representative of the whole population! The universe prefers to make small things

Slide44: 

Which star(s): Color (RED) Absolute Magnitude (BRIGHTER) Is the most massive? Is the lowest mass main sequence star? Is definitely young? Are not experiencing Hydrogen fusion in their cores?

Slide45: 

Which pairs of stars: Color (RED) Absolute Magnitude (BRIGHTER) Have the same luminosity? Have the same temperature? Are likely to have the same size?

Slide46: 

Where do all the non-main sequence stars come from? Color (RED) Absolute Magnitude (BRIGHTER) Must be due to lack of Hydrogen in the core. What happens when fusion stops?