Introduction to Astronomy

Q1:Annie's jobQ2: strength of linesQ3: temp and colorQ4: temp and colorQ5: temp and color
Lecture slides
Observable Properties of Stars: Temperature, Motion, Spectral Class, Luminosity Class
Spectral Class: Annie Jump Cannon story...

Stars with the strongest hydrogen lines (designated with H in the above picture) were originally classed as type A, and stars with the weakest hydrogen lines were classed as type O. Originally, there were 15 types (A-O), but later some of the classes were omitted, because they had been invented to fit some of the poor quality spectra. In 1920, an Indian astronomer named Saha figured out that the spectra could be sorted in order of temperature, and the classification scheme was re-ordered:
OBAFGKM
O stars are the hottest stars and M stars are the coolest stars. A stars have the strongest hydrogen lines (which is why Annie classified them this way!).

The spectral classes are subdivided into 10 subclasses, 0-9. An O0 star is the hottest, followed by an O1, O2, etc. O9 is just a little bit hotter than a B0, and so on.

Most stars can be classified in this way. There are a few that just don't fit, sort of like the duck-billed platypus, which doesn't fit into zoology classifications. With those, we just do what we can.

So the strength of the lines indicates the temperature, as well as the abundance of the element. In a way, this is bad, because it makes it difficult to figure out if a star has strong lines because there is a lot of that atom there, or because it's just the right temperature. So we need an INDEPENDENT way to determine the temperature.

Temperature: Objects have colors for one of two reasons. Think about your clothes. Your clothes are all different colors, because they are made of all different things. They have intentionally been dunked in dye so that they are red or yellow or blue or some combination of these. But objects can have color for another reason. Think about the burner on an electric stove. As you turn up the temperature, the burner changes from black to red. This tells you that the temperature can also change the color.

This is also true for stars. In this case, we find the temperature of a star by comparing the continuum spectrum of the star (the part that's NOT lines) with a blackbody spectrum. Here are a set of blackbody curves:

From these curves, you can see that hot objects are bluer, and cool objects are redder. So red stars are cooler than blue stars. "What!?" You say, "Stars have colors?!" Absolutely they do. If you go out at night and find Orion in the sky, you will see Betelgeuse and Rigel, a red star and a blue star, in one constellation. It's quite spectacular, and once you've noticed it, you'll see color in lots of stars.

The other important thing that you should notice about this figure is that the lines do not have exactly the same shape. If you shift them up or down, they will not peak in the same place, nor will they line up if you shift them left or right! This means that you can find the temperature of a star just by measuring the continuum spectrum in a couple of places, and comparing to the slopes of the blackbody curves for different temperatures.

Motion: All this time, I've been talking about the stars as though they are fixed in the sky. But that's not quite true. They DO move, just very slowly! Stars move in all kinds of directions in the sky relative to the Earth. When astronomers try to figure out how a star is moving, they generally do it in two steps, because each part has to be determined in a different way. The motion across the sky is called the proper motion, and the motion towards or away from us is called the radial motion.

  1. Proper Motion: This is the motion across the sky, and is measured as a change in the Right Ascension and Declination of a star. Proper Motion was discovered by Halley (of Halley's comet). He compared the positions of Arcturus, Sirius and Aldebaran with positions noted by the ancient Greeks, and with that several thousand year baseline, was able to determine that they had moved 0.5 degrees, which is not very far, if you think about it.

    He REASONED:
    1. These three stars are among the brightest.
    2. This means that they are perhaps also among the closest.
    3. These closest stars are the only stars that I can see moving.
    4. Therefore, perhaps ALL stars move, and if my measurements were more accurate, I could see that.
    He was right! All stars in the sky move, but some move very little, and the motion is barely observable.

    Barnard's star is the fastest mover in the sky, and moves about 10".25 per year. (Remember how small an arcsecond is---a tennis ball eight miles away!)

  2. Radial Motion:

    To understand this, we need a digression.

    The Doppler Effect: Imagine that you are lying on the sofa, watching soaps. A fire engine comes down the street. While it's approaching you, the pitch of the siren is higher. While it's receding from you, the pitch of the siren is lower. If you could hear stars, they would do the same thing. The stars that are approaching you would have a higher pitch, and the ones that were going away from you would have a lower pitch.

    Unfortunately, noise does not travel through space. Fortunately, the same effect also applies to light, if we just replace the word pitch with the word frequency.

    The Doppler effect shifts the light's frequency, depending on whether the object is moving towards you or away from you. If the object moves towards you, it is catching up a little bit to the light as it is emitted, and the frequency gets higher. This is called "blue-shifting", and the light is bluer. If the object is moving away from you, the light gets stretched out, and the frequency gets lower. This is called "red-shifting", and the light is redder. The source in the following image is the yellow dot. It is moving to the left.

    How can you tell the difference between an object which has been blue-shifted, and one that is hotter?

    The answer is to use the 'lines' discussed above, which show what the star is made of. These have a particular pattern, and also a particular set of wavelengths when they are at rest. We look at the spectra of stars, and measure how much the lines have shifted.

    The shift in frequency, the rest frequency and the velocity are all related: 

    (Change in Frequency)     velocity
---------------------  =  --------
  (Rest Frequency)            c

    where c is the speed of light.

    So. Finally. What did we just figure out? Oh right. The radial velocity, or the motion of the star towards or away from you. To figure this out, you need to know that an atom's signature lines get shifted when the object moves, and the amount of the shift is determined by the speed of the star.

Luminosity Class: The final property of stars that we want to talk about is the Luminosity class (you forgot what we were doing, didn't you?). The luminosity class is a way of talking about the radius and therefore the mass of a star. It is divided into 5 categories, labeled by Roman numerals. (Classes II and IV are not useful to us.) The vast majority of stars are dwarf stars. Our Sun is a dwarf star, and if we wanted to describe it to other astronomers, we would call it a G2V star. It's spectral class is G2, and it's a dwarf. By the way, I didn't just spell "dwarfs" wrong. It really is spelled that way. I don't know why.

So those are all the things that we can observe about stars. Next, we'll start talking about what we can determine ABOUT stars from these observable features.

Concept Question 1:
How much would I have to pay you to get you do Annie Jump Cannon's job?

  1. $10/hour
  2. $5 million
  3. Make me supreme ruler of the Universe.
  4. You could not make me do this job.
Concept Question 2:
The strength of spectral lines tells you which two things?

  1. Temperature and composition
  2. Temperature and distance
  3. Distance and composition
  4. Temperature and abundance
Concept Question 3:
Hot objects are

  1. Blue and faint
  2. Blue and bright
  3. Red and faint
  4. Red and bright
Concept Question 4:
Suppose you observe a 900 nm line that has been shifted by 90 nm. How fast is the object moving?

  1. 0.01c
  2. 0.1c
  3. c
  4. 10c
Concept Question 5:
Suppose you observe a star with a proper motion of 3 km/s. It's radial motion is 4 km/s. What is its total speed?

  1. 3.5 km/s
  2. 1 km/s
  3. 7 km/s
  4. 5 km/s