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Solar Composition• The Sun consists of hydrogen, about 73.4 percent
by mass, and helium, 25 percent, as well as a small amount of other elements.
The Sun
• This composition is very similar to that of the gas giant planets.
• The Sun’s composition represents that of the galaxy as a whole.
Groups of Stars• Constellations are the 88 groups of stars
named after animals, mythological characters, or everyday objects.
Measuring the Stars
– Circumpolar constellations can be seen all year long as they appear to move around the north or south pole.
– Summer, fall, winter, and spring constellations can be seen only at certain times of the year because of Earth’s changing position in its orbit around the Sun.
Groups of Stars
Star Clusters
Measuring the Stars
– A group of stars that are gravitationally bound to each other is called a cluster.
• In an open cluster, the stars are not densely packed.
• In a globular cluster, stars are densely packed into a spherical shape.
M55 is a globular cluster in the constellation Sagittarius. Quite large on the sky (about two thirds of the Moon's diameter), but with a very loose, almost non-globular, appearance, M55 is about 18000 light-years away and about 100 light-years across.
Groups of Stars
Binaries
Measuring the Stars
– A binary star is two stars that are gravitationally bound together and that orbit a common center of mass.
– More than half of the stars in the sky are either binary stars or members of multiple-star systems.
Binary Stars
The top image shows a blob without adaptive-optics correction. The bottom one shows the same image after an adaptive-optics correction that resolves the top blob into a double star.
Binary Stars vary in shape and position.
Stellar Position and Distances• Astronomers use several units of measure for long
distances.
a) A light-year (ly) is the distance that light travels in one year, equal to 9.461 × 1012 km.
b) A parsec (pc) is equal to 3.26 ly, or 3.086 × 1013 km.
c) (Write out) – An Astronomical Unit (AU) is equal to 93 million miles, the distance between the sun and Earth. Pluto is 39.5 AU.
Earth’s diameter: 7,926 Miles
Excerpted from a commencement address delivered May 11, 1996. Dr. Sagan's book Pale Blue Dot expands on these ideas.
Image from Voyager 1, 1990 3.7 billion miles away (a little pass Pluto)
Carl Sagan died Dec. 20, 1996. He was an astronomer, astrochemist, and pioneered SETI (Search for ET intelligence).
Reflections on a mote of dust...
Carl Sagen’s “Reflections on a mote of dust”
Earth
Stellar Position and Distances• To estimate the distance of stars from Earth,
astronomers make use of the fact that nearby stars shift in position as observed from Earth.
Measuring the Stars
• Parallax is the apparent shift in position of an object caused by the motion of the observer.
• As Earth moves from one side of its orbit to the opposite side, a nearby star appears to be shifting back and forth.
Stellar Position and Distances• The distance to a star, up to 500 pc using the
latest technology, can be estimated from its parallax shift.
Measuring the Stars
Basic Properties of Stars• The basic properties of stars include diameter,
mass, brightness, energy output (power), surface temperature, and composition.
Measuring the Stars
• The diameters of stars range from as little as 0.1 times the Sun’s diameter to hundreds of times larger.
• The masses of stars vary from a little less than 0.01 to 20 or more times the Sun’s mass.
Basic Properties of Stars
Magnitude
Measuring the Stars
– One of the most basic observable properties of a star is how bright it appears.
– The ancient Greeks established a classification system based on the brightnesses of stars.
– The brightest stars were given a ranking of +1, the next brightest +2, and so on.
Basic Properties of Stars
Apparent Magnitude
Measuring the Stars
a) Apparent magnitude is based on the ancient Greek system of classification which rates how bright a star appears to be.
– In this system, a difference of 5 magnitudes corresponds to a factor of 100 in brightness.
– Negative numbers are assigned for objects brighter than magnitude +1.
Basic Properties of Stars
Absolute Magnitude
Measuring the Stars
– Apparent magnitude does not actually indicate how bright a star is, because it does not take distance into account.
b) Absolute magnitude is the brightness an object would have if it were placed at a distance of 10 pc.
Basic Properties of Stars
Luminosity
Measuring the Stars
– Luminosity is the energy output from the surface of a star per second.
– The brightness we observe for a star depends on both its luminosity and its distance.
– Luminosity is measured in units of energy emitted per second, or watts.
– The Sun’s luminosity is about 3.85 × 1026 W.
Stars: Basic Properties
– Mass determines a star’s temperature, color, and brightness. Mass also determines the age/life span of a star.
– Stars on the Main Sequence (stable stars) are classified into “spectral” classes: O, B, A, F, G, K, M.
– “M” is the coolest, least massive, and dimmest.
– “G” is average – the sun is in this class.
– “O” class is the fewest in number, “M” has the most stars.
– “O” is the hottest, most massive, and brightest (write out).
Spectral
Class
Effective Temperature
(K)Color L/LSun
Main Sequen
ce Lifespa
n
O 28,000 - 50,000 Blue90,000 - 800,000
1 - 10 Myr
B 10,000 - 28,000Blue-white
95 - 52,000
11 - 400 Myr
A 7,500 - 10,000 White 8 -55
400 Myr - 3 Gyr
(Billion Years)
F 6,000 - 7,500White-yellow
2.0 - 6.5 3 - 7 Gyr
G 4,900 - 6,000 Yellow0.66 -
1.57 - 15 Gyr
K 3,500 - 4,900 Orange0.10 - 0.42
17 Gyr
M 2,000 - 3,500 Red0.001 -
0.0856 Gyr
*Mass
60
18
3.1
1.7
1.1
0.8
0.3
*Mass is compared to our sun..
“Oh,
Be
a
Fine Girl, Kiss Me.
Brightness
Highest
Lowest
Brightest
Dimmest
Shorter life
Longer life
Spectra of Stars
H-R Diagrams
Measuring the Stars
– A Hertzsprung-Russell diagram, or H-R diagram, demonstrates the relationship between mass, luminosity, temperature, and the diameter of stars.
– An H-R diagram plots the absolute magnitude on the vertical axis and temperature or spectral type on the horizontal axis.
Red Giants
Spectra of Stars
H-R Diagrams
Measuring the Stars
– The main sequence, which runs diagonally from the upper-left corner to the lower-right corner of an H-R diagram, represents about 90 percent of stars.
– Red giants are large, cool, luminous stars plotted at the upper-right corner.
– White dwarfs are small, dim, hot stars plotted in the lower-left corner.
•In the Hertzsprung-Russell (HR) diagram, the star is no longer on the main sequence. If of modest mass, it is a red giant. If very massive, it is a yellow or red supergiant.•The Sun, whose current age is 4.5 billion years, will become a red giant in about 5.5 billion years.
Main Sequence Star
Red Giant
