Earth Science Ms. Cordaro San Marcos High School Stars Earth Science Ms. Cordaro San Marcos High School

Lesson objectives: I will learn what stars are made of, how to measure their distance. And how to classify them. Success skills:                     I understand what the electromagnetic spectrum is.                     I can explain what the Doppler effect is and how it works.                     I know what the elements hydrogen and helium are.

Characteristics of Stars A star is a body of gasses that gives off radiant energy in the form of light and heat. Stars differ in size, mass, color, composition, temperature, distance from earth and brightness.

Characteristics of Stars The closest star to us is our sun!!! Some stars are less than 20 km in diameter (around), far smaller than the Earth. Some stars are 1,000 times larger than the sun. The sun is 1,392,000 km in diameter and is considered a medium star. Most stars we see in the night sky are medium sized stars.

Composition and Temperature Astronomers learn about stars by analyzing the light they give off. Astronomers use a spectrometer that separates the light into different colors or wavelengths. Star lights passing through the spectrometer produces a display of colors and lines called a spectrum.

Composition and Temperature There are 3 types of spectra: Emission or bright light Absorption or dark-line Continuous Astronomers use the dark-line spectra to reveal certain characteristics about stars such as composition and temperature.

Composition and Temperature Every chemical element has a characteristic spectrum. The colors and lines in the spectrum of a star show which elements make up that star. The most common element in stars is Hydrogen. The second most common element in stars is Helium. Elements such as carbon, oxygen, nitrogen and calcium make up the remaining mass of the stars.

Composition and Temperature The surface temperature of a star is shown by it’s color. Just like the electromagnetic spectrum, the more blue, the more energy and in this case, the hotter the star is. Star temperatures are measured in Kelvins (k).

Composition and Temperature Blue stars are very hot and usually have a surface temperature of 35,000 k but can go as high as 50,000 k. Red stars are the coolest with surface temperatures of 3,000 k or less. Yellow stars like our sun have a surface temperature of around 5,500 k

Motion There are 2 kinds of motion related to stars. Actual motion ~ measured with high power telescopes and are the actual motion of the stars. Apparent motion ~ is what stars look like from earth and is visible to the unaided eye due to the movement of the earth.

Motion The Earths rotation around the sun causes the stars to appear to move in another way and this is how we have the astrological signs. As the earth orbits the sun, different stars become visible during different seasons. The visible stars seem to shift slightly to the West every night. Some stars are always visible in the night sky such as Polaris (the North star) in our sky. This star is said to be circumpolar because it is directly above earths axis and it appears to never move. The big and little dipper are also circumpolar because they are so close to Polaris.

Motion The Doppler effect shows whether stars are moving toward us or away from us. Blue Shift = shorter wavelengths, more energy and appear to moving toward us. Red Shift = longer wavelengths, less energy and appear to moving away from us.

Distance to the Stars Distances in space are measured in Light Years (ly) A light year is the distance light can travel in one year. The speed of light is 300,000 km/s. In one year light can travel approximately 9.5 trillion km. Light from the sun takes about 8 minutes to reach the earth.

Distance to the Stars The closest star to Earth is Proxima Centauri and is 4.2 light years away (300,000 times the distance of the earth to the sun.) Polaris is 700 light years away. One way astronomers measure distance is a method called parallax. This method is used for stars that are less than 1,000 light years away.

Distance to the Stars During a six month period, a nearby star will appear to shift slightly, relative to the stars that are farther away The closer the nearby star, the greater the mount of shift. From the amount of shift, astronomers can calculate the distance to any star within 1,000 light years. (Simulation with thumbs.)

Stellar Magnitudes Apparent Magnitude: Absolute Magnitude The brightness of a star as it appears from earth. Absolute Magnitude How bright a star would be if seen from a distance of 32.6 light years away

Stellar Magnitudes Each star has both an apparent magnitude and an actual magnitude. The relationship between these 2 measures of brightness depends on the distance between the earth and the star. Stars that are less than 32.6 LY away appear brighter than they would if they were 32.6 LY away. These stars have an apparent magnitude lower than their absolute magnitude. An example is our sun. It’s apparent magnitude is -26.8 but it’s actual magnitude is +5. The lower the number, the brighter the star.

Classification of Stars Plotting the surface temperatures of star against their absolute magnitudes reveals an interesting pattern. The graph is called a Hertzsprung-Russell diagram or H-R diagram and is named after the astronomers who discovered the pattern.

Classification of Stars The majority of stars fall within a band running through the middle of the H-R diagram and are called the main sequence stars. This sequence or pattern runs from the cool dim red stars at the lower right and extends to the hot, bright blue stars in the upper left.

Classification of Stars A group of cool bright stars appear in the upper right corner and are called giants. Some are so large they are called super giants. Stars in the lower left of the chart are called white dwarfs.