Properties of Stars & H-R Diagram

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Molly Matthews
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1 Properties of Stars & H-R Diagram

2 What is a star? A cloud of gas, mainly hydrogen and helium The core is so hot/dense that nuclear fusion can occur. The fusion converts light nuclei (elements) into heavier ones (H+H=He)

3 Properties of Stars 4 major properties of stars that we study: Distance Stellar brightness Color and Temperature Stellar mass

4 Stellar Distance Measuring a star's distance can be very difficult Distances to the stars are very large Units of measurement Kilometers or astronomical units are too cumbersome to use Light-year is used most often

5 Quick Check WHAT IS THE CLOSEST STAR? The closest star to us is the sun! The next closest star is Proxima Centauri, AKA Alpha Centauri C (the dimmest star in the Alpha Centauri system) Proxima Centauri is 4.2 light-years from the Sun

6 Every star is different All the stars have different Brightness: Tells us how luminous the star is, i.e. how much energy is being produced in the core Color: Tells us the surface temperature of the star

7 Units of luminosity We measure the luminosity of every day objects in Watts. How bright is a light bulb? By comparison, the Sun outputs: 380,000,000,000,000,000,000,000,000 Watts (380 million million million million Watts!) This is easier to write as 3.8 x Watts To make things easier we measure the brightness of stars relative to the Sun.

8 Units of temperature Temperature is measured in Kelvin The Kelvin temperature scale is the same as the Celsius scale, but starts from -273 o. This temperature is known as absolute zero -273 o C -173 o C 0 o C 100 o C 1000 o C 0 K 100 K 273 K 373 K 1273 K Kelvin = Celsius + 273

9 Measuring the temperature The temperature of a star is indicated by its color Blue stars are hot, and red stars are cold Red star Yellow star Blue star 3,000 K 5,000 K 10,000 K

11 Quick Check 1. How is the temperature of stars measured? a) Fahrenheit b) Celsius c) Kelvin 2. How is luminosity measured? a) Spectra b) Watts c) Lumens

13 Stellar brightness Controlled by three factors Size Temperature Distance Also called Magnitude, there are 2 types: Apparent magnitude Absolute magnitude

14 Apparent magnitude Brightness when a star is viewed from Earth Decreases with distance Dim stars have large numbers Negative numbers are also used Our Sun: -26.7

15 Absolute Magnitude "True" or intrinsic brightness of a star Most stars' absolute magnitudes are between -5 and +15 Our Sun: 5 Remember, the smaller or more negative, the brighter.

16 Absolute Value Chart Star Absolute Magnitude (TRUE) Apparent Magnitude (FROM EARTH) Distance from Earth (light-years) The Sun Sirius Arcturus Vega Rigel Betelgeuse ,500

17 The HERTZSPRUNG-RUSSEL (H-R) DIAGRAM Classification system for stars Depicts the relationship between luminosity (absolute magnitudes) and temperature Named after Ejnar Hertzsprung (Danish) and Henry Norris Russell (United States) Developed in the 1920 s

18 What do you think color has to do with this diagram? Hint: Which color has the most energy and how does that energy relate to temperature?

19 Spectral Class Spectral classification (light spectrum) is more accurate then measuring the temperature of a star by its color. Sometimes the temperature scale on the horizontal axis is replaced by spectral types, OBAFGKM. Oh Be A Fine Girl/Guy, Kiss Me Spectral types refers to the state of the atoms on the star. This contains more information about the star than just its temperature (the state of its atoms). Even though it is a scale based on stellar spectra it does contain correlations between the stellar spectra and the surface temperature of the star. O = 30,000 K G = 6,000 K B = 20,000 K A = 10,000 K F = 7,000 K K = 4,000 K M = 3,000 K

20 Color and Temperature Hot star: Temperature above 30,000 K Appears blue Cool star: Temperature less than 3,000 K Appears red Between 5000 and 6000 K Stars appear yellow H-R Diagram

21 Hertzsprung-Russell Diagram Shows the relationship between stellar brightness & temperature/ spectra Used to study stellar evolution (i.e life cycles of stars)

22 Hertzsprung-Russell Diagram Parts of an H-R diagram Main-sequence stars: approx. 86% of all stars Giants & Super Giants Large Upper-right on the H-R diagram Only a few percent of all stars (approx. 4%) Dwarfs (white & others) Small (approximate the size of Earth) Perhaps 10% of all stars

23 Draw the Hertzsprung-Russell Diagram

24 Luminosity (brightness) goes up the y-axis Temperature x-axis. Hotter (bluer) stars lie to the left Cooler (redder) stars lie to the right *Note that the higher temperatures are to the left and cooler stars are to the right.

26 Quick Write On the H-R Diagram, where are most stars located?

A1101, Lab 5: The Hertzsprung- Russell Diagram Laboratory Worksheet

Student Name: Lab TA Name: A1101, Lab 5: The Hertzsprung- Russell Diagram Laboratory Worksheet One of the most basic physical properties of a star is its luminosity, the rate at which it radiates energy