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| the amount of light emitted by a source divided by the area from which the light is emitted. for example, a galaxy with a high density of stars will have a high surface brightness, but one with the same number of stars more widely spread out will have a lower surface brightness. |
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| a band running around the sky in which few galaxies are visible. it coincides with the plane of the Milky Way and is caused by dust that is within our Galaxy. This dust blocks the light from distant galaxies. |
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| a shift in the wavelength of electromagnetic radiation to a longer wavelength. for visible light, this implies a shift toward the red end of the spectrum. the shift can be caused by a source of radiation moving away from the observer or by the observer moving away from the source. light traveling through expanding space undergoes an expansion redshift, and light traveling away from a strong gravitational source undergoes a gravitational redshift. |
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| the apparent speed of a galaxy (or other distant object) moving away from us, caused by the expansion of the universe. |
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| a relation between a galaxy's distance, D, and its recession velocity, V, which states that more distant galaxies recede faster than nearby ones. mathematically v= h x d, which h is Hubble's constant |
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| a distance unit equal to 1 million parsecs and abbreviated mpc |
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| a lengthening of the wavelength of electromagnetic radiation caused when the waves travel through expanding space. the redshift of distant galaxies is an example. |
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| a small, generally rocky, solid body orbiting the Sun and ranging in diameter from a few meters to hundreds of kilometers |
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| a numerical formula for predicting the approximate distances of most of the planets from the sun |
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| the first-discovered and largest of asteroids, over 900 km in diameter. often called a planet during the early 1800s |
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| regions in the asteroid belt with a lower-than-average number of asteroids. some of the gaps result from the gravitational force of Jupiter removing asteroids from orbits within the gaps |
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| a condition in which the repetitive motion of one body interacts with the repetitive motion of another to reinforce the motion. for example, planets or satellites orbiting with orbital periods that have a simple fractional ratio (2:1, 3:2, etc) are in resonance and can have a strong influence on each other's orbit |
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| a group of asteroids that appear to have originated from a single object that underwent a collision. the asteroids all have orbital paramets and spectroscopic properties that indiciate they have similar compositions. |
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| an asteroid with an orbit that crosses or comes close to the Earth's orbit |
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| the bright trail of light created by small solid particles entering the Earth's atmosphere and burning up; a "shooting star" |
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| small, solid bodies moving within the solar system. when a meteoroid enters the atmosphere and heats up the trail of a luminous gas it leaves is called a meteor. when the body lands on the ground its a meteorite. |
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| the solid remains of a meteor that falls to the earth |
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| a meteorite containing small spherical granules called chondrules |
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| a small spherical silicate body embedded in many meteorites |
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| a type of meteorite containing many tiny spheres (chondrules) of rocky or metallic material embedded in carbon-rich material |
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| a compound containing carbon, especially a complex carbon compound. organic compounds can be produced by chemical processes, so they do not necessarily indicate the presence of living organisms |
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| a stony meteorite of rocky material from the upper layers of a differentiated body. no chondrules |
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| an impact crater more than 1 km in diameter located in Arizona, probably produced 50, 000 years ago |
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| a large aerial explosion that occurred over Siberia in 1908 probably when a large meteor entered the atmosphere and heated up and exploded before reaching the ground |
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| a period of time in the geologic timescale ranging from about 146 million to 65 million years |
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| electromagnetic radiation spread into its component wavelengths or colors. a rainbow is a spectrum produced naturally by water droplets in our atmosphere |
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| the layer in the sun or any other star where photons can escape into space. this appears to be a surface when we look at the sun, but there is higher-density gas below and lower-density gas above the photosphere |
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| a spectrum in which certain wavelengths are darker than adjacent wavelength. the missing light is absorbed by atoms or molecules between the source and the observer |
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| the study of the properties of stars based on information that can be learned from their spectra |
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| an indicator of a star's temperature. a star's spectral type is based on the appearance of its spectral lines, with different strengths and weaknesses indicating stellar composition and temperature. the fundamental types are from hot to cool, O, B, A, F, G, K, M, L, T |
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| a series of absorption or emission lines of hydrogen seen at visible wavelengths |
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| a method for measuring distances. this method is based on constructing a triangle, one side of which is the distance to be determined. that side is then calculated by measuring another side (the baseline) and the two angles at either end of the baseline |
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| the distance between two observing locations used for the purposes of triangulation measurements. the larger the baseline, the better the resolution attainable |
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| an angular measure equal 1/60th of 1 degree |
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| the passage of a planet directly between the observer and the sun. at a transit, we see the planet as a dark spot against the sun's bright disk. from earth, only mercury and venus can transit the sun |
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| the shift in an object's perceived position caused by the observer's motion. also a method for finding distance based on the shift. |
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| the change in the apparent position of an object. for example, an object will show an angular shift relative to background objects when viewed from different places |
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| an angular measure equal to 1/60th of 1 arc minute or 1/3600th of 1 degree |
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| a unit of distance equal to about 3.26 light years. it is the distance at which an object would have a parallax of one arc second |
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| the nearest star to the solar system at a distance of 1.30 parsecs. despite its proximity, this m type main sequence start is too dim to be seen without a small telescope |
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| the position shift of a star on the celestial sphere caused by the star's own motion relative to the sun |
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| the actual speed of a star across, or transverse to, our line of sight, determined from the proper motion and the distance of the star |
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| the velocity of a body along one's line of sight. that is, the part of a body's motion directly toward or away the observer |
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| the overall velocity of a star, determined by vector addition of the star's radial velocity and transverse velocity |
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| the angular shift in the apparent direction of a star caused by the orbital speed of the earth |
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| greek astronomer who made one of the first detailed charts of the stars |
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| the use of an interferometer to measure small angular-size features |
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| a technique to overcome the limitations imposed by the effects of scintillation in our atmosphere by analyzing a set of images collected over very short time intervals |
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| two stars in orbit around each other, held together by their mutual gravity |
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| a plot of the brightness of a body verses time |
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| the amount of energy by one square meter in one second a hot, sense material depends on the temperature T raised to the fourth power |
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| a graph on which stars located according to their temperature and luminosity. most stars on such a plot lie along a diagonal line, called the main sequence, which runs from cool, dim stars in the lower right to hot, luminous stars in the upper left |
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| the region in the HR diagram in which most stars, including the sun, are located. the main sequence runs diagonally across the hr diagram from cool, dim stars to hot, luminous stars |
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| a cool, luminous star with a radius much larger than the sun's. red giants are found in the upper right portion of the hr diagram |
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| a dense star whose radius is approximately the same as the Earth's but whose mass is comparable to the Sun's. white dwarfs burn no nuclear fuel and shine by residual heat. they are the end stage of stellar evolution for low-mass stars like the sun |
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| a relation between the mass and luminosity of star. higher mass starts have higher luminosity |
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| a very large-diameter and luminous star, typically at least 10000 times the sun's luminosity |
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