Properties of Stars: Distance, Temperature, Luminosity
text: Chapter 12 (sections 12.1 and 12.2)
What properties can we learn about?
Use these to determine:
Typical Ranges of values:
For nearby stars, we can measure distance by parallax:
parallax = apparent motion of a star caused by motion of the Earth
[DEMO: hold thumb up a arms length in front of your face. Close one eye and then switch eyes, your thumb appears to move relative to a distance background.]
Same happens as Earth orbit the Sun. Examine position of star at two different times about 6 months apart and examine the apparent change of position of the nearby star relative to the distant stars.
In January, the star appears in the sky near the right hand distant star. In July, it appears in the sky near the left hand distant star.
We can measure parallax to get distance information:
Geometry says that if I know all three angles of a triangle and the length of one side, I know the other side.
We can measure all of the angles, and we know the distance from Earth to Sun, so we know the distance to the star.
(These pictures show the star much too close to us. The parallax angles are really VERY small since even nearby stars are quite far away.)
Click on the button to bring up an animation of parallax from Ohio State University. It will appear on a separate window in the upper right corner of your screen. You can close the new window by clicking on the small box in the upper corner - or just click on this main window to return here.
Here is a similar JAVA applet on parallax: http://instruct1.cit.cornell.edu/courses/astro101/java/parallax/parallax.html
This link has a JAVA applet which shows the concept of parallax: http://www.astro.washington.edu/labs/parallax/solar.html
Define a distance unit related to the parallax angle:
1 parsec = distance of a star with a parallax angle of 1 arcsecond= 3.26 light years = 3 x 1013 km
1 arcsecond = 1/3600 of a degree
Parallax angles are ALWAYS VERY SMALL (< 1 arc second)
this makes them hard to measure
this technique only works for the closest stars
measuring from Earth, we can measure about 1000 stars at distances < 100 pc away
measuring from satellites in orbit we can measure about 100,000 stars out to about 1000 pc
This animation shows a picture of stars with the motion from parallax (sped up considerably!): http://www.astro.washington.edu/labs/parallax/parallax_distance.html
We will need other methods for measuring stars which are farther away (MOST STARS).
One method which we will discuss in more detail later is to use objects whose actual brightness we think we know.
"method of standard candles"
if brightness is known then brighter stars are closer
hot stars emit more blue and ultra-violet light
cool stars emit more red and infrared light
the line spectra of stars also contain information about temperature (more on this later)
hard to measure directly since most stars appear as points of light in telescopescan not measure angular size of image for most stars
can make direct measurements (using interferometry or Space Telescope) for a few nearby stars and a few very large stars (< 50 total)
Link to a photograph of red supergiant star Betelgeuse: http://antwrp.gsfc.nasa.gov/apod/ap980419.html
Link to an exercise allowing you to calculate the size of Betelgeuse: http://imagine.gsfc.nasa.gov/YBA/HTCas-size/betelgeuse.html
will learn an indirect method in the next section
luminosity = amount of energy per second emitted by an object
the wattage of a light bulb is an example of measuring luminosity
luminosity depends on
brightness = how bright something appears to us
depends on temperature, size and distance of a star
PROBLEM : The Brightness we can measure from Earth depends on the distance to the stargreater distance = less brightness (inverse square law)
for nearby stars, we can measure the distance from parallax and determine the luminosity from the brightness
use a system developed by Hipparchus in about 150 BC
Brightness from Apparent Magnitude
let the "brightest star" have magnitude = 1
let the faintest star visible by eye have magnitude = 6
Each magnitude is 2.5 times fainter than the previous one.
There are brighter stars (not visible in Mediteranean region) - so use negative numbers
magnitude of -1 is BRIGHTER than magnitude of 1
OBJECT APPARENT MAGNITUDE
brightest star (Sirius)
faintest star observable with telescopes
Apparent magnitude measures brightness which depends on both Luminosity and Distance.
Luminosity from Absolute Magnitude
Define something that does NOT depend on distance
absolute magnitude = apparent magnitude of a star if it was a distance of 10 pc away
If we know brightness (apparent magnitude) (look at the sky) and luminosity (absolute magnitude) - we can find DISTANCE - discuss this further soon (Method of Standard Candles)
If we know brightness (apparent magnitude) (look at the sky) and distance - we can find luminosity (absolute magnitude) - possible for stars close enough to use parallax (about 100,000 of them)
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