ASTR 101 Chapter Notes - Chapter 1: Cosmic Distance Ladder, Main Sequence, Helium Atom
How do we go from that first list to the second?
Distance
By measuring the star's parallax
Parallax only works close to us
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By knowing the luminosity ad measuring apparent
brightness
Standard candle method
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Luminosity
By knowing the distance and measuring apparent
brightness
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Apparent Brightness = L/4pid^2
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Temperature
Can be estimated in relative way by the visual colors of stars
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Spectral Type
Look at absorption lines
Presence or absence of lines tell you the temperature of the
star and the star's atmosphere
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Spectral Type A
Sirius 10000 degrees
Sweet for hydrogen
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Dark lines
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Alpha Centauri
Spectral Type G
So is out sun
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O stars is so hot, hydrogen can't make lines
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Proxima Centauri
Spectral Type M
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Mass
Most accurately measured by observing a star moving under
gravity, orbiting another star
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Fortunately such arrangements tend to be quite common
Almost half of all stars like the Sun are in multiple star
systems
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Radius
Even more difficult to directly measure
You need binaries that actually eclipse each other, as
illustrated in the animation on the left
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But if you know the star's luminosity and temperature, you can
use well-known laws of thermal radiation to estimate the star's
size fairly accurately
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H-R diagram
Show temperature vs. luminosity
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Hot on the left, cold on the right
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Bright on top
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Faint on bottom
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Bright and not hot = larger
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Hot and not bight = smaller
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Main sequence = 90% of stars have some relationship between
temperature and luminosity
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Most stars are on the main sequence and most of the stars on
the sequence are the M TYPE stars
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WHY ARE THERE SO MANY M DWARFS?
How stars change over time, and therefore ages of different
stars
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We study large populations of stars and perform demographic
studies of populations
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Helps to have isolated populations with unique characteristics
that you can use to compare
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Nature provides these specialized groups to astronomers in the
form of STAR CLUSTERS
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There are two types of these open clusters
Irregular open shapes
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Hundreds to thousands of stars
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Gas and dust is sometimes seen between the stars
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GLOBULAR CLUSTER
Round spherical shape
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Denser towards the center
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Less stars as you move out
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Far less space between the stars themselves
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MUCH STARS, more than open clusters
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Both types of clusters are very useful for studying stellar properties
All of the stars within a cluster are effectively the same distance
away from the earth as the other stars are
We can compare their apparent brightness to each other as
through they were true luminosities
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Allows us to plot reliable HR diagram of the cluster,
knowing that the brighter stars in the cluster really are
brighter
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Since all of the stars in a cluster form within 100 million years
of each other, they are very approximately the same age, as
least in stellar terms
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Our HR diagram of a cluster is a snapshot of what a group of
stars
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HR diagrams of globular and open clusters are very different
from each other suggesting very different ages
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Open cluster
B and A type stars
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Globular Cluster
Concentrated in F, G
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Reason between mass and luminosity in stars first began to become
clear in the early 20th century, when physicists like Einstein
revealed that atoms possess a type of POTENTIAL ENERGY based
only on how massive they are
MASS ENERGY
Could naturally get the hydrogen in the sun and cramming 4
together can make helium, released energy
Helium atom is less massive than the four protons that went
into it
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Core fusion generates enormous THERMAL PRESSURE, kept in
check only by the inward GRAVITY
More massive stars must generate more energy per second than
smaller stars do
This forces higher mass stars to burn their available fuel more
quickly than lower mass stars have to
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Lecture 10 - 4/25
Wednesday, April 25, 2018
10:59 AM
Document Summary
By knowing the luminosity ad measuring apparent brightness. By knowing the distance and measuring apparent brightness. Can be estimated in relative way by the visual colors of stars. Presence or absence of lines tell you the temperature of the star and the star"s atmosphere. O stars is so hot, hydrogen can"t make lines. Most accurately measured by observing a star moving under gravity, orbiting another star. Fortunately such arrangements tend to be quite common. Almost half of all stars like the sun are in multiple star systems. You need binaries that actually eclipse each other, as illustrated in the animation on the left. But if you know the star"s luminosity and temperature, you can use well-known laws of thermal radiation to estimate the star"s size fairly accurately. Hot on the left, cold on the right. Main sequence = 90% of stars have some relationship between temperature and luminosity.
