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Transcript of Stars
Measurements of Stars
Measuring distance can be tricky, and usually means 2 things:
How bright the star appears from Earth
Some stars can emit the same spectra, meaning that they have the same effective temperature, but have different luminosities and absolute magnitudes
Using a star's spectra to classify it.
Birth, Life and Death of a star
and what they tell you about stars
The actual or absolute distance as things really are out in space.
The apparent distance as you look up at the sky
Apparent magnitude depends on 3 things:
The distance of the star
The luminosity of the star
The absolute, or actual, magnitude
The spectral class for a star depends on the elements that make it up and the temperature that it burns at.
To solve the classification problem this presents, the luminosity classes were developed.
Compared to Humans:
Measured in light years,
the distance light travels in one year (ly)
1 ly = 5.88x10 mi
That's 5.88 trillion miles!
As you look at the night sky, it is measured in degrees, arcminutes, and arcseconds.
º degrees, ' minutes, " seconds
1º = 60' = 3600"
By figuring out the relationship between apparent and absolute distances we can translate between the two.
Parallax Method - the change in an object's apparent position when viewed from 2 locations.
With a lot of patience, this is how Tycho did it:
Example: Hold your finger at an arms length away. Close one eye, and observe your finger's position compared to the background. Then switch eyes, and watch how your finger appears to change position!
The parallax method leads us to a new unit of absolute distance:
the parsec (pc)
Parsec stands for "parallax arcseconds"
1 pc = 3.0857x10 km = 2.0626x10 AU = 3.26156 ly
The parsec was derived using simple trigonometry:
To find distance, you will use the following equation:
p is parallax angle in arcseconds
d is distance in parsecs
Let's try a real example:
The star is 2 parsecs away
Measures the total energy output of a star in Watts
This contributes to the apparent brightness of a star, but brightness also depends on distance.
Our Sun has a luminosity of 3.9x10 Watts
In star graphs, luminosity is usually expressed as a logarithm of the luminosity ratio (the luminosity of the star divided by the luminosity of the Sun)
For example, if a star's luminosity is 3.9x10 Watts
We would express this star's luminosity as -6
The brightness of a star depends on how far away it is.
As light travels, it spreads out in area.
Traditional system invented by Hipparchus of Nicaea, c. 300 BC.
Ranks stars into "magnitudes": 1st, 2nd, 3rd, etc. as follows:
1st magnitude stars are the brightest stars.
2nd magnitude stars are the second brightest.
and so forth...
The faintest stars visible to the naked eye are 6th magnitude.
Gives no reasonable means of independently measuring brightness other than comparing them by-eye to other stars in the sky. (Not very useful for modern astronomy)
Magnitudes defined this way are the relative brightness of stars.
The Modern System:
BIGGER magnitude = FAINTER star
The standard of brightness is the star Sirius A
(0th magnitude - the brightest star in the night sky)
Remember: its BACKWARDS (larger magnitude = fainter star)
The Apparent Magnitude of some familiar celestial objects:
Stars that have known magnitudes and are used as a comparison of stars with unknown apparent magnitudes.
Also known as Standard Candles or Candle Stars
Example: Type 1A Supernova explosions have absolute magnitudes of -19.33
The Essential Measurements:
Distance - How far away it is
Luminosity - How much energy it gives off
Apparent Magnitude - How bright it looks from Earth
Absolute Magnitude - How bright it really is
Spectral Class - How it is classified by its spectra
The actual, or absolute, brightness of a star
If you know the apparent magnitude and the distance of a star, you can calculate the absolute brightness.
Use the equation:
M is the absolute magnitude
m is the apparent magnitude
D is the distance in parsecs
Class O is the hottest and Class M is the coolest
Each class is subdivided into 10 categories
(O0, O1, O2...O9, B0, B1, etc)
Spectral Class Summary
Spectral & Luminosity Classes
Example: Our Sun is in spectral class
and is a main sequence star, putting it in luminosity class
This makes our Sun a
Life Cycle of Stars
Enormous dust clouds, called nebulae, slowly pull together into massive spheres due to the force of gravity
The sphere, called a protostar, will grow until the weight of the matter is so great that fusion ignites.
Once fusion begins, the star is born.
For a star, mass is everything
The amount of dust and gas that accumulated during formation will determine the way the star lives and dies.
For this reason, stars are lumped into 2 categories:
Low mass stars = mass of less than 5 times the mass of the Sun
Massive stars = mass of more than 5 times the mass of the Sun
Regardless of mass, all stars spend their lives fusing their fuel (creating gas pressure) fighting against the pull of gravity.
The star will be happy and stable (hydrostatic equilibrium) until the fuel begins to run out...
Beginning of the End
Once the hydrogen in the core is gone, fusion stops there.
Without the gas pressure, the outer layers collapse due to gravity and the star shrinks.
The heat of the collapse ignites a new round of fusion, this time with helium as the fuel.
The second round of fusion causes increasingly large gas pressure, which counteracts gravity and causes the star to expand.
Now the star is a red giant.
What happens next depends on if the star is a massive star or a low mass star...
Death of a Low Mass Star
The red giant continues to expand its outer layers as the core continues to shrink while fusing helium.
Once all of the helium is gone, the star stops producing new energy.
The outer layers dissipate into space, called a planetary nebula, and the star has 20% of its original mass left.
The star will spend the rest of its days shrinking and cooling until it is a white dwarf.
White dwarfs are stable - repulsion of electrons keeps it from collapsing.
Once completely cool, it is sometimes called a black dwarf.
Death of a Massive Star
The core of the red supergiant begins to shrink due to gravity and it ignites several fusion reactions.
Once only iron is left, fusion stops.
In less than 1 second, the star finally collapses, and the iron is smashed together.
The nuclei repel each other, and the star sends out an explosive shock wave.
The shock wave sends the outer layers of the star out in a tremendous explosion, called a supernova.
The gasses and dust are sent out into space to form new stars, and planets.
The core is left behind and gravity forces electrons to combine with protons to form neutrons.
What happens next depends on the mass of the star...
Death of a Massive Star
Option 1 -
Stars with masses 5-15 times the mass of the Sun
All that is left of the star is a ball of super dense neutrons called a neutron star.
Death of a Massive Star
Stars with masses greater than 15 times the mass of the Sun
The neutrons cannot withstand the force of gravity and even they collapse.
What is left over is a black hole.
About the Diagram
The Hertzsprung-Russell Diagram is a tool that shows relationships and differences between stars.
It is something of a "family portrait"
It shows stars of different ages and in different stages, all at the same time.
In the Hertzsprung-Russell (HR) Diagram, each star is represented by a dot.
The position of each dot on the diagram tells you 2 things about each star:
Its luminosity (or absolute magnitude)
Its temperature (or spectral class)
How to read the diagram
A star in the upper left corner of the diagram is hot and bright.
A star in the upper right corner of the diagram is cool and bright.
A star in the lower left corner is hot and dim.
A star in the lower right corner is cool and dim.
The Sun rests approximately in the middle of the diagram, and it is the star which we use for comparison.
Groups on the HR Diagram
Once most of the stars have been plotted, distinct groups appear:
The red giants are in the upper right corner (cool and bright)
The main sequence makes a line across the middle (various temperatures and magnitudes)
White dwarfs are in the lower left corner (hot and dim)