1.Chapter 15: Measuring Stellar Properties
List of useful properties for understanding how stars work
2.More or less in order of importance…
3.Want distance? Measure the parallax angle…
The basic idea: Closer objects shift in direction through a bigger angle than do farther ones
Simple geometry (see next slide) gives the formula…
Distance (in pc) = 1/(parallax angle (in arcsec))
Must use the right units here: define a new distance unit, for convenience… a parsec
1 parsec = distance of a star with a parallax of 1 arc second
Typical parallax shift for the nearest stars… less than one arc second! That’s REALLY tiny and hard to measure
Distances are the HARDEST thing we do in astronomy!
5.Knowing distance, apparent brightness can be converted to true energy output = Luminosity
Apparent brightness of a star is quantified by a number called “magnitude”, similar to the old Greek idea: brightest stars were “First magnitude”, those a bit dimmer “second magnitude”, etc.
19th century astronomer N.R. Pogson proposed a formula which captures the essence of the Greek idea.
A mag=2 star is 2.5 times brighter than a mag=3 star, and a mag 3 star is 2.5 times brighter than a mag=4 star, etc.
If you imagine moving a star to a standard distance of 10 parsecs, the apparent magnitude is called the absolute magnitude. Since in this case the variable of distance has been removed, differences in absolute magnitude are differences in true luminosity.
Absolute magnitude is a convenient, dimensionless way to quantify luminosities.
Read about apparent magnitudes here, and absolute magnitude here, as well as in the text. We don’t have time in the course to get more mathematical than this
6.Spectral Types and Surface Temperature
In the late 1800’s, Annie Jump Cannon at Harvard found patterns in the growing collection of stellar spectra
This is pre-quantum mechanics, pre-discovery of the atom, electron orbitals etc., so… we had NO idea what spectral lines meant.
7.The Random Walk towards the Spectral Types…
White Stars, with prominent hydrogen lines. Called them “A stars.”
Blue-white stars, with less prominent H lines, and weak helium lines. Called them “B stars”
Then, no more in this color direction, so skip some letters and… Cream colored stars, with weaker H lines and lots of very weak other lines. Called them “F stars”
Yellow stars, with prominent double line in the yellow part of spectrum. Called them “G stars”.
Orange stars, with very weak H lines and tons of other lines. Skip some more letters and call them “K stars”.
Red stars, with no H lines, tons of lines, even big thick bands of light taken out by what we now know as TiO. Skip L and call them M stars
Then, by now, found a few stars very blue and with very weak H lines and strong He lines, and nothing else. Call them O stars.
8.Voila’! The Spectral Sequence!
OBAFGKM from hot to cool. Memorize it!
Now we know it’s a surface temperature sequence, nothing to do with chemical composition (chem comp is about the same for all stars in the solar neighborhood)
“Oh, Be A Fine Girl, Kiss Me!”
Don’t blame me, it’s been around for a hundred years!
9.On Average, cooler means more absorption lines
11.Logic of Absorption Lines vs Temperature
Hot stellar atmospheres mean bluer color, and also most atoms are ionized and show few lines.
The cooler the star, the more atoms have their electrons and are available for absorption line transitions. And recall that an electron being ionized out of the atom to some random (not quantized!) high energy, will not create an absorption LINE, its absorption loss of light will be spread out.
Therefore OBAFGKM hotter->cooler is also a sequence of few lines -> many lines in the spectrum
12.OBAFGKM spectra – real stars
13.Now We’ve got Two Fundamental Properties of Stars…
First thing to do - make a Scatter Plot. Plot one against the other and see if any interesting pattern shows up
This is, the Hertzsprung Russell Diagram – the H-R Diagram - a fundamental tool of stellar astronomy
Making the first one was HARD!
Had to carefully measure photographs distance for a bazillion stars, trying to find that tiny few which had detectable parallax shifts, which gives distance, which then gives luminosity (when combined with brightness)
14.HR fair sample
15.Now Plot stars you can SEE above a given brightness. This brightness-limit gives very different sample. Now the rare brilliant blue and red giants appear since they can be seen from so far away, you’re sampling a much bigger volume)
Note, a given volume of space has a “fair sample” of what kind of stars are out there. Mostly dim, reddish stars
Looking at a brightness-limited sample is what you end up doing when you look at the night sky – you see everything above some limiting brightness accessible to your eye or telescope. This is a very Unfair sample!
It’s heavily skewed toward the most luminous stars, which you can see from much farther away and hence sampling a much bigger volume of space
HR diagram dominated by the Main Sequence
17.About 10% of the stars are not on the Main Sequence. Let’s come up with some good names for them, through Pure Reason!
19.The H-R Diagram
90% of stars fall on the Main Sequence – why?
Because Main Sequence Stars = core hydrogen fusion stars, and hydrogen is both the most high octane of nuclear fuels, and also the star starts out with the large majority of its mass as hydrogen.
Therefore makes perfect sense that a star will spend almost all it’s life burning H into He and so a random photo of the sky will mostly be H-burning (=Main Sequence) stars.
HR diagram can be used to show stellar evolution. We’ll do this in the next chapter
20.How Do We Find the Mass of Stars?
Or better to ask… what directly observable things could we measure which depend sensitively and directly on mass?
When we have unambiguous observed masses, we can then see what our stellar computer models may need in order to reproduce stars correctly
21.As I told you once before…
Kepler’s 3rd Law and variations of it, are the most useful formulae in Astronomy – because mass is so fundamental to understanding stars and galaxies and the universe, and…
Kepler III is how we find mass! We apply it to binary stars – a nice clean laboratory for finding stellar mass w/o complications.
Almost half the stars in the Galaxy are in binary systems, so plenty of opportunities
22.Binary Star Types
Visual Binaries: You can actually see two separate stars on your images. These are relatively rare – need wide separations, like out to Pluto and beyond – to separate the stars on images
Spectroscopic binaries: by far the most common, binary pairs are usually so close you can only see a blended image of both stars. It is the Doppler Shifts in the spectra of one or both stars which tell you it’s actually 2, not 1 star.
Eclipsing Binaries: One star passes in front of another, causing light loss in a characteristic pattern. These are the MOST USEFUL!
Just about anything you want to find out about a star, you can find out if it’s in an eclipsing binary system (well, maybe a slight exaggeration, but not much)
23.A famous visual binary: Sirius (brightest star in the sky)
24.Big disadvantage for Visual Binaries: If can literally SEE two stars, they’re so far apart they make take a century or more to orbit once
25. You can also get the sizes of the stars in eclipsing binaries. Can you see how?
26.Partial Eclipses; light never levels and flattens at the bottom, but instead comes right back up
27.Flat-bottom = total eclipses: one star is completely in front of the other. These give the size of the stars much more easily
28.Measure the Doppler Shift…
….that’ll tell you the velocity of one star relative to the other.
Multiply the velocity by the period, that gives you the circumference of the orbit, and therefore the semi-major axis
BUT, a fatal flaw in this reasoning…. Can you see what it is?
29.Here’s the Fatal Flaw
The Doppler Effect only tells you the velocity component along the line-of-sight, but what you need to measure the semi-major axis is the true velocity in space; the space velocity
And to get the space velocity from the Doppler velocity, you need to know the tilt of the orbit
Most of the time, you canNOT measure the tilt, and so all you can get is a lower limit to the masses
30.HR old mass slide
31.HR showing mass, lifetime
32.25 nearest stars drawing
33.Chap 15: Key Points “Stellar Properties”
Parallax, the only direct method of finding distances to stars, except for rare eclipsing binaries
Spectral types are a Temperature sequence: OBAFGKM hot to cool.
The Main Sequence of the HR Diagram: core H burning produces it.
Main Sequence is a mass sequence, lower right to upper left
All MS stars are doing core hydrogen burning into helium. That defines the Main Sequence
Total eclipses: flat bottomed.These are the easiest to use in getting stellar sizes
Half of all stars are in binaries