Properties of Stars N. Sharp (REU/NOAO/AURA/NSF)
What properties of the stars can we determine just from this image? Measuring Stars
Measuring Stars Information you can get from 1 image: Position on the sky Relative brightness Color, which gives you Temperature
Measuring Stars Information you can t get from an image: Chemical composition Motion through space, or toward/away from us Distance from us
Apparent vs. Absolute Brightness A bright star in the night sky could be bright or faint for two reasons: It is close. It is truly bright or luminous.
Measuring Brightness The brightest stars in Gemini are Castor & Pollux. In our sky, they have roughly the same apparent brightness. Castor Pollux Just from this image we can t tell how far away they are...
Brightness vs. Distance Fundamental relationship: More distant objects look fainter! So a star may appear faint because its not emitting much light OR because it is far away To determine a star s absolute brightness (known as its luminosity), you need: apparent brightness AND distance Finding stellar distances is a colossal problem!
Measuring Apparent Brightness A system for describing the brightness of stars was first developed by the Greek astronomer Hipparchus in the 2 nd century B.C. He ranked the brightest stars as first magnitude. Those stars just barely visible were 6th magnitude. There were 6 magnitude rankings, 1 st through 6 th. Then, along came modern instruments
The Magnitude Scale Notice that the scale runs backwards since the old magnitude 1 was brighter than the magnitude 2. You do not need to memorize these values, but have some sense of common objects. -26.7 Sun -12.5 Full Moon -4.4 Venus at brightest -1.5 Sirius, brightest star at night 0.000 Vega 0.8 Betelgeuse 2.5 Polaris 6 Human eye limit 9.5 Barnard s star, faint red star 10 Binocular limit 14 10-inch telescope limit 20 1-meter telescope limit 26 4-meter telescope with CCD 30 Hubble, Keck limit with CCD
Apparent vs. Absolute Brightness Luminosity, or absolute brightness, is a measure of the total energy radiated by a star per unit time. As the starlight travels into space, it spreads out in all directions over a spherical surface (area=4πr 2 ). The apparent brightness is the amount of energy received per unit area. As distance increases, the apparent brightness decreases. apparent brightness = This is the inverse square law. luminosity 2 distance
Apparent vs. Absolute Brightness For example, a certain amount of light will strike 1 unit of area on a sphere of radius 1. This same amount of light will strike 4 units of area on a sphere twice as large (radius = 2). The same energy strikes 4 times the area, so the apparent brightness has dropped by a factor of 4 (twice the distance, 4 times weaker, or ¼ the brightness).
Apparent vs. Absolute Magnitude We can measure a star s apparent magnitude and its distance from us. With this information we can correct the apparent magnitude for the effect of distance. But what distance to use as a standard? 10 is a nice round number. A star s absolute magnitude is the apparent magnitude that the star would have if it were viewed from a distance of 10 parsecs.
Apparent vs. Absolute Magnitude An equation relates: apparent magnitude (written as m ) and absolute magnitude (written as M ) and distance (written as d ): d m M = 5log( 10pc You do not need to memorize this equation, but you will use it in AST 114 lab. )
Which star appears the brightest? Apparent magnitude of Star A is 4 Apparent magnitude of Star B is 0 Apparent magnitude of Star C is +4 A. Star A B. Star B C. Star C D. Information is not sufficient.
Which star is the most luminous? Apparent magnitude of Star A is 4 Apparent magnitude of Star B is 0 Apparent magnitude of Star C is +4 A. Star A B. Star B C. Star C D. Information is not sufficient.
Consider these two stars: Star Trillian: app. mag. = -1, abs. mag. = +1 Star Zaphod: app. mag. = 0, abs. mag = +6 Which star appears brighter? A. Star Trillian B. Star Zaphod C. They appear equally bright. D. Information is not sufficient.
Consider these two stars: Star Trillian: app. mag. = -1, abs. mag. = +1 Star Zaphod: app. mag. = 0, abs. mag = +6 Which star has a greater luminosity? A. Star Trillian B. Star Zaphod C. They have equal luminosities. D. Information is not sufficient.
Consider these two stars: Star Trillian: app. mag. = -1, abs. mag. = +1 Star Zaphod: app. mag. = 0, abs. mag = +6 Which star is closer to Earth? A. Star Trillian B. Star Zaphod C. They are equal distances from Earth. D. Information is not sufficient.
Consider these two stars: Star Trillian: app. mag. = -1, abs. mag. = +1 Star Zaphod: app. mag. = 0, abs. mag = +6 What can be said about these stars distances? A. They are both closer than 10 parsecs. B. They are both farther than 10 parsecs. C. They are both 10 parsecs away from Earth. D. Information is not sufficient.