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Stars Characteristics of Stars Star Classification • Stars are classified using three main characteristics: – Size – Temperature – Brightness Size • Although stars look to be the same size from here on Earth, they can be many different sizes. • Neutron Stars are the smallest. They are made of the material left behind after a larger star explodes; about 20 kilometers in diameter. Size (cont’d) • A white dwarf is about the size of our Earth, but has as much mass as our sun. It is made of the blue-white hot core left behind from a bigger star whose outer layers expanded and drifted out into space when the star began to burn out. – Example: Sirius B Size (cont’d) • Our Sun is a mediumsized star. – Made mostly of hydrogen at very high temperatures and pressures (5,500oC). – Neither solid, liquid or gas, it is plasma - all the atoms have been stripped of their electrons and the nuclei are packed together. • Example: Our Sun, Alpha Centauri A Size (cont’d) • Very large stars, such as Aldebaran, Arcturus and Capella are categorized as giant stars. • The largest are called supergiant stars. If our Sun were a supergiant, it would fill our solar system as far out as Jupiter. – Examples: Betelgeuse, Rigel Temperature • The temperature of a star is indicated by the color it glows. Type of Star O Color B Blue-White 10,000 – 25,000 A White 7,500 – 10,000 F Yellow-White 6,000 – 7,500 G Yellow 5,000 – 6,000 K Orange 3,500 – 5,000 M Red Below 3,500 Blue Surface Temp (oC) Above 25,000 Brightness • Brightness of a star depends on its size, temperature, and its distance from Earth. – Apparent magnitude – Absolute magnitude Apparent Magnitude • A star’s brightness as seen from Earth. • Measured with electronic devices. • Though our Sun seems very bright to us, it is not the brightest star in the sky, it is just the one closest to Earth. Absolute Magnitude • The brightness the star would have if it were at a standard distance from Earth. • Astronomers must find out the star’s apparent magnitude and its distance from Earth then calculate its absolute magnitude. Homework • Write a “Step-Up-To-Writing” paragraph using this as your topic sentence: “The three main characteristics used for classifying stars are size, temperature and brightness.” • Your paragraph should include, in addition to the topic sentence, three detail sentences each followed by an example sentence and finished off with a conclusion sentence.