Properties of Stars & H-R Diagram - loch on sciencelochonscience.weebly.com/uploads/2/2/6/8/22686328/... · Hertzsprung-Russell Diagram • Parts of an H-R diagram – Main-sequence

Properties of Stars & H-R Diagram

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)

Properties of Stars

4 major properties of stars that we study:

– Distance – Stellar brightness– Color and

Temperature– Stellar mass

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

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

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

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 1026 Watts

• To make things easier we measure the brightness of stars relative to the Sun.

Units of temperature

• Temperature is measured in Kelvin• The Kelvin temperature scale is the same as

the Celsius scale, but starts from -273o.– This temperature is known as “absolute zero”

-273 oC -173 oC 0 oC 100 oC

0 K 100 K 273 K 373 K

1000 oC

1273 K

Kelvin = Celsius + 273

Measuring the temperature

• The temperature of a star is indicated by its color

• Blue stars are hot, and red stars are cold

Red star

3,000 K

Yellow star

5,000 K

Blue star

10,000 K

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

Stellar brightness• Controlled by three

factors – Size – Temperature– Distance

• Also called Magnitude, there are 2 types:– Apparent magnitude – Absolute magnitude

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

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.

Absolute Value Chart

StarAbsolute

Magnitude(TRUE)

ApparentMagnitude

(FROM EARTH)

Distance from Earth

(light-years)

The Sun 4.8 -26.72 0.000016Sirius 1.4 -1.46 8.6

Arcturus 0.2 -0.04 34Vega 0.6 0.03 25Rigel -8.1 0.12 900

Betelgeuse -7.2 0.7 1,500

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

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?

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 K = 4,000 K

A = 10,000 K M = 3,000 K

F = 7,000 K

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

Hertzsprung-Russell Diagram

• Shows the relationship between stellar brightness & temperature/ spectra

• Used to study stellar evolution (i.e life cycles of stars)

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

Draw the Hertzsprung-Russell Diagram

• 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.

Quick Write

• On the H-R Diagram, where are most stars located?