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Section 1 StarsChapter 15

Bellringer

List ways that stars differ from one another.

How is the sun like other stars? How is it different?

Write your answers in your Science Journal.



• Stars differ in size, temperature, composition, brightness, and color.

• Distances between stars are very large and are measured in light-years.

What You Will Learn



Color of Stars

• The color of a star indicates the star’s temperature.

• Red stars are the coolest, and blue stars are the hottest.

• If two stars differ in color, you can conclude that they differ in temperature too.



Composition of Stars

• Astronomers use an instrument called a spectroscope to separate a star’s light into a spectrum (plural, spectra).

• A spectrum is the band of colors produced when white light passes through a prism.



Composition of Stars, continued

• A continuous spectrum shows all of the colors, while an absorption spectrum shows which wavelengths of light are absorbed.

• The spectrum of a star is an absorption spectrum because the atmosphere of the star absorbs certain portions of the light produced by the star.




• When a chemical element emits light, only some colors in the spectrum show up. The colors that appear are called emission lines.

• Every element has a unique set of emission lines that act like a fingerprint for that element.




• The pattern of lines in a star’s absorption spectrum is unique to that star.

• A star’s absorption spectrum can be used to determine the elements in that star’s atmosphere and the stage the star occupies in its life cycle.

• Stars are made of mostly hydrogen and helium gases.


Stars, Galaxies, and the UniverseChapter 15




Classifying Stars

• Stars are now classified by how hot they are.

• Temperature differences between stars result in color differences that can be seen. For example, class O stars are blue—the hottest stars.



Classifying Stars, continued

• Magnitude is used to compare the brightness of one object with the brightness of another object.

• To express the brightness of stars, astronomers use a system of magnitudes.



Classifying Stars, continued

• Positive magnitude numbers represent dim stars. Negative magnitude numbers represent bright stars.

• The brightest star in the night sky, Sirius, has a magnitude of -1.4.



Classifying Stars



How Bright Is That Star?

• The apparent magnitude is the brightness of a star as seen from Earth. The absolute magnitude is the brightness that a star would have at a distance of 32.6 light-years from Earth.

• If all stars were the same distance away, their absolute magnitudes would be the same as their apparent magnitudes.



Absolute and Apparent Magnitude



Distance to the Stars

• Because stars are so far away, astronomers use a unit called a light-year to measure the distance from Earth to the stars.

• A light-year is the distance that light travels in one year, about 9.46 trillion kilometers.



Distance to the Stars, continued

• Parallax is an apparent shift in the position of an object when viewed from different locations.

• Astronomers use parallax and trigonometry to find the actual distance to stars that are close to Earth.



Motions of Stars

• All of the stars in the sky appear to make one complete circle around Polaris every 24 h.

• This apparent motion of the stars is due to Earth’s rotation and its revolution around the sun.

• Each star is actually moving in space. But because stars are so distant, their actual motion is hard to see.

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