chapter 16 the nature of stars astro1010-lee.com [email protected] uvu survey of astronomy
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Chapter 16Chapter 16The Nature of Stars A
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Chapter 16Chapter 16The Nature of Stars
Stellar Distance ScalesLight Year = the distance that light travels in one yearParsec = the distance to a point where 1 AU subtends one second of arc
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Chapter 16Chapter 16The Nature of Stars
Remember that nearby stellar distances can be measured using parallax
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Chapter 16Chapter 16The Nature of Stars
Nearest star to the Sun is Proxima Centauri which is a member of a 3-star system: Alpha Centauri complex
Model of distances:
Sun is a marble, Earth is a grain of sand orbiting 1 m away
Nearest star is another marble 270 km away
Solar system extends about 50 m from Sun; rest of distance to nearest star is basically empty
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Chapter 16Chapter 16The Nature of Stars
The 30 closest stars to the Sun
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Chapter 16Chapter 16The Nature of Stars
Brightest stars were known to, and named by, the ancients (Procyon) In 1604, stars within a constellation were ranked in order of brightness, and labeled with Greek letters (Alpha Centauri)
In the early 18th century, stars were numbered from west to east in a constellation (61 Cygni) As more and more stars were discovered, different naming schemes were developed (G51-15, Lacaille 8760, S 2398)
Now, new objects are simply labeled by their celestial coordinates
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Chapter 16Chapter 16The Nature of Stars
Apparent magnitudeHippachus 1st to 6th Spica (1st Mag), Vega (0 Mag)
Brightness (Luminocity) measured by light meter An Apparent Magnitude difference of 5 represents a Brightness ratio of 100/1
Brightness Scales
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Chapter 16Chapter 16The Nature of Stars
Apparent Magnitudes of several objects
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Chapter 16Chapter 16The Nature of Stars
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The Absolute Magnitude (M) of stars is defined as the apparent magnitude that the star would have if were at 10 parsecs distance. Then the following ratio holds:
Note: This is not exact – there are constants left out.
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Chapter 16Chapter 16The Nature of Stars
Luminosity, or absolute brightness, is a measure of the total power radiated by a star.
Apparent brightness is how bright a star appears when viewed from Earth; it depends on the absolute brightness but also on the distance of the star
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Chapter 16Chapter 16The Nature of Stars
Two stars that appear equally bright might be a dimmer, nearer star and a brighter, farther star
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Chapter 16Chapter 16The Nature of Stars
The color of a star is indicative of its temperature. Red stars are relatively cool, while blue ones are hotter.
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Chapter 16Chapter 16The Nature of Stars
Spectral classes make up a Temperature Sequence
O, B, A, F, G, K, MO hottestM coolest
Oh Be A Fine Girl Kiss Me
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Chapter 16Chapter 16The Nature of Stars
These are representative spectra of each class.
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Chapter 16Chapter 16For the vast majority of stars that cannot be imaged directly, size must be calculated knowing the luminosity and temperature:
Supergiant stars are more than 100 solar radii
Giant stars are between 10 and 100 solar radii
Upper main sequence stars are 8 to 100 solar radii
Average stars are 0.5 to 8 solar radii
Dwarf stars are 0.1 to 0.5 solar radii
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Chapter 16Chapter 16The Nature of Stars
Stellar radii vary widely.
In the chart, note the great changes in scale required to show the different sizes.
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Chapter 16Chapter 16The Nature of Stars
Ejnar Hertzsprung (8 Oct, 1873 - 21 Oct, 1967) was a Danish chemist and astronomer.
Henry Norris Russell (Oct 25, 1877 – Feb 18, 1957) was an American Astronomer.
Together they invented one of most useful graphs in Astronomy
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Chapter 16Chapter 16The Nature of Stars
The H–R diagram plots stellar luminosity against surface temperature. This is an H–R diagram of a few prominent stars
The Hertzsprung– Russell Diagram
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Chapter 16Chapter 16The Nature of Stars
Once many stars are plotted on an H–R diagram, a pattern begins to form. These are the 80 closest stars to us; note the dashed lines of constant radius.
The darkened band is called the main sequence, as this is where most stars are.
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Chapter 16Chapter 16The Nature of Stars
An H–R diagram of the 100 brightest stars looks quite different, These stars are all more luminous than the Sun. Two new categories appear here – the red giants and the blue giants.
Clearly, the brightest stars in the sky appear bright because of their enormous luminosities, not their proximity.
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Chapter 16Chapter 16The Nature of Stars
Major Sections of the H-R Diagram. They start with the Main Sequence , then the two Giant Stages then finally the White Dwarf Stage
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Chapter 16Chapter 16The Nature of Stars
This is an H–R plot of about 20,000 stars. The main sequence is clear, as is the red giant region.
About 90% of stars lie on the main sequence; 9% are giants and 1% are white dwarfs.
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Chapter 16Chapter 16Spectroscopic parallax: has nothing to do
with parallax, but does use spectroscopy to extend our ability to determine the distance to a star
1. Measure the star’s apparent magnitude (brightness) and spectral class (temperature)
2. Use temperature to estimate luminosity
3. Apply inverse-square law to find distance
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Chapter 16Chapter 16A
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Spectrographic Parallax can be seen on this H-R Diagram and the definition of Absolute Magnitude
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Chapter 16Chapter 16The Nature of Stars
Spectroscopic parallax can extend the cosmic distance scale to several thousand parsecs.
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Chapter 16Chapter 16The Nature of Stars
The spectroscopic parallax calculation can be misleading if the star is not on the main sequence. The width of spectral lines can be used to define luminosity classes
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Chapter 16Chapter 16
Many stars are in binary pairs; measurement of their orbital motion allows determination of the masses of the stars.
Visual binaries can be measured directly; this is Kruger 60:
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y Determination of Stellar Masses
Chapter 16Chapter 16The Nature of Stars A
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M1 M2d1 d2
2 3(M1 + M2)P = d
M = M1+ M2
d = d1 + d2
M1d1 = M2d2
Equations needed to determine individual masses
Chapter 16Chapter 16The Nature of Stars
Study of spectral lines reveals the motion of spectroscopic binaries and hence their spacing. From that the masses are calculated.
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Chapter 16Chapter 16The Nature of Stars
The mass of a star is also correlated with its radius, and very strongly correlated with its luminosity.
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Chapter 16Chapter 16The Nature of Stars
Mass is also related to stellar lifetime
Using the mass–luminosity relationship
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Chapter 16Chapter 16The Nature of Stars
So the most massive stars have the shortest lifetimes – they have a lot of fuel but burn it at a very rapid pace.
On the other hand, small red dwarfs burn their fuel extremely slowly, and can have lifetimes of a trillion years or more.
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Chapter 16Chapter 16The Nature of Stars A
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In –Class QuizSpectroscopic parallax: has nothing to do with parallax, but does use spectroscopy to extend our ability to determine the ______________ to a star
