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# Conceprezi: Observable Properties of Stars

what we measure and how that is used to obtain physical properties of the stars
by

## Deborah Skapik

on 3 June 2018

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#### Transcript of Conceprezi: Observable Properties of Stars

Observable Properties
of Stars

How do we measure the distance to the stars?
Two types of distance measurement techniques:
geometric (
parallax
)
brightness based (
standard candles
)
Parallax
is the apparent shift of a nearby object relative to a distant background based on the movement of the observer.
So, if you are able to measure the amount the star "shifts" back and forth, then you can figure out how far away it is (only works for stars less than roughly 100pc away).
The smaller the parallax, the more distant the star.
Standard candles
are objects whose intrinsic ("built in" brightness is known. So, if you observe them from a distance and measure how faint they appear to be, you can calculate how far they are from you.
note: this clip also covers a 3rd technique that does not apply to stars in our galaxy
How do we measure the amount of energy that stars make?
If you measure how bright a star appears to be (
flux
), and if you know how far from you it is, then you can calculate its energy output per second (
luminosity
).
How do we measure the temperature of the stars?
spectra
see these absorption lines?
HOT stars
COOL stars
Notice the overall trend that hotter stars are indeed brighter in blue than red, and cool stars are brighter in red than blue. The Sun is a G0 star.
If we make a graph of the luminosity vs the temperature of the stars, amazing things are revealed! This graph is called an "HR Diagram".
What's so amazing? Stars tend to populate only certain places on this graph! In other words, stars of certain luminosities seem to be restricted to maintaining certain temperatures. Soon, we will find out why this is so!
But wait! There's more!
diagonal lines are lines of constant radius!
How do we measure sizes of stars?
If we have luminosity and temperature information, we can use the HR diagram to help.
HOT stars
COOL stars
BIG stars
SMALL stars
DIM stars
BRIGHT stars
HR diagram key
How can we determine masses of stars?
if we have information about things orbiting them
if we know they are on the Main Sequence
Some stars orbit around other stars. Stars with companions are called
binary stars
.
By studying the paths of the orbits of the stars in binary systems, astronomers can determine the masses of the stars in that system.
In this video, notice how the two stars orbit around a common center of mass (the x). Astronomers use orbital paths to determine binary star masses.
It turns out that
if
a star is on the Main Sequence of the HR diagram (that is, if its temperature and luminosity are in a certain range), then there is a relationship between the luminosity of the star and its mass.
Who's in our neighborhood?
Full transcript