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Citation: (c) Crystal Lake Observatory, All Rights Reserved.


Star Temperature

The temperature of the surface layer of a star is directly related to the color of the star.

The terms white-hot and red-hot are sometimes used to describe the appearance of a metal when it is heated to different temperatures.  A metal that is white-hot is hotter than a metal that is red-hot.

The metal filament inside an incandescent light bulb is a good example of this.  If a small amount of electric current is passed through the filament, the metal heats up a small amount and emits a faint, deep red glow.  As the current increases, the metal filament becomes very hot and emits a bright white light.

Stars behave the same way, they emit radiation because they are hot.  The surface layer temperature of stars can range in temperature from as low as 2,400° Kelvin to as high as 40,000° Kelvin.

A hot star with a surface temperature as high as 40,000° K will emit most of its radiation in the blue and ultraviolet portion of the spectrum.

An average star, like our own sun, with a surface temperature of 6,000° K will emit most of its radiation in the yellow portion of the spectrum.

And a cool star with a surface temperature as low as 2,400° K will emit most of its radiation in the red and infrared  portion of the spectrum.

All of the normal stars in the universe fall in this range somewhere.  If you go outdoors at night and closely examine the stars you will notice the different colors.  It is easy to distinguish the hot blue-white stars from the cool red stars, but with practice you can distinguish the other colors as well.

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Contents

TS-24 Lab Exercises

Exercise #4

Stellar Temperatures

( to return to the main document, click the Done button)

Introduction

At the telescope

Data analysis

Reviewing the results

Additional study

(c) 1995 Crystal Lake Observatory, All Rights Reserved.


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Lab book

Exercise #4, Stellar Temperatures

Date: March 15, 1995

Time: 10:00 UTC

Observatory: Auckland

Object:  The Scorpion

Telescope: 0.5 meter Zeiss

Instrument: Type 2 Photometer

Star #1, [B-V] = 0.912


UBV system

Using a photoelectric photometer mounted on a telescope an astronomer can very precisely measure the brightness (magnitude) of a star.

To determine the color of a star the astronomer needs to measure the magnitude of the star in different parts of the spectrum.

This is done by inserting color filters in front of the photometer and measuring the brightness of the star through each filter.  These measurements are referred to as color magnitudes.

Astronomers generally use three filters: U (ultraviolet magnitude), B (blue magnitude) and V (visible magnitude).

Comparing the color magnitudes determines the color of the star.  The [B-V] color index is calculated by subtracting the V magnitude from the B magnitude:

[B-V] = B magnitude - V magnitude

The [B-V] color index is then used in the following formula to calculate the temperature of a star:

T (°K) = 7,200 / ([B-V] + 0.64)

For example, a cool red star might have a [B-V] color index of 1.97.  The temperature is calculated as:

T (°K)	= 7,200 / (1.97 + 0.64)

= 7,200 / (2.61)

= 2,760°K

A hot blue-white star might have a [B-V] color index of 0.08.  The temperature is calculated as:

T (°K)	= 7,200 / (0.08 + 0.64)

= 7,200 / (0.72)

= 10,000 °K

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Photoelectric Photometer

If an astronomer can determine the color of a star then the temperature of the star can be calculated.  So, how can we measure colors precisely?

Stars emit radiation in the entire spectrum.  Hot stars emit most of their radiation in the blue and ultraviolet portions of the spectrum, cool stars emit most of their radiation in the red and infrared portions of the spectrum.

The first step in determining the color of a star is to measure the intensity of the stars' radiation at different parts of the spectrum.  To do this, the astronomer uses an instrument known as a photoelectric photometer.

A photoelectric photometer is an instrument used to measure light intensity.

An astronomical photometer is used to measure the intensity of star light and is mounted on the telescope at the same location you would normally mount an eyepiece.

The light arriving from a star is focused by the telescope onto a small light sensitive detector, which converts photons into electrons using the photoelectric effect.  The brighter the star the greater the electrical signal.  By precisely measuring the amplitude of the electrical signal the astronomer can calculate the brightness of the star.

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Reviewing the results

What type of stars are these?  Are they cool stars, average stars or hot stars?

How do these stars compare to our own sun, hotter or cooler?

Compare your calculations with other students.  If your results are different, explain why.

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At the telescope

Before we begin, enter in your lab book:

Exercise #4, Stellar Temperatures

If you have not yet closed the License Display dialog in window 1 go ahead and do so now (click the Continue button).

Step 1	Set the date:

set date -> March 15, 1995

Enter in your lab book:

Date: March 15, 1995

Step 2	Set the UTC time:

set time -> 10:00 UTC

Enter in your lab book:

Time: 10:00 UTC

Tonight we will use the telescope at the Auckland Observatory to make our measurements.

See Auckland Observatory.

Step 3	Select the observatory

select observatory -> Auckland

Enter in your lab book:

Observatory: Auckland

Step 4	Select the Telescope

select telescope -> Guide

The Guide telescope is a small telescope mounted on the side of the main telescope.  We will use it first so you can see the difference between a small telescope and a much larger telescope..

Before we continue let's check the video display [Test Display].

Next, we will use the automated computer pointing system on the telescope to point the telescope at an interesting little group of stars known as The Scorpion:

Step 5	Set the Slew Rate

set slew rate -> 10 arc-minutes

Step 6	Select the DT-7 Object Group:

select group -> The Scorpion

Enter in your lab book:

Object:  The Scorpion

Step 7	Select the DT-7 Object:

go to object -> center

The telescope should now be moving towards the Scorpion (if it is not did you remember to click the Go To pushbutton).  Notice that the RA and DEC indicators are changing.

When the telescope stops look closely at the center of the eyepiece, do you see the Scorpion?

If not, don't be disappointed, the Scorpion is difficult see with the Guide telescope.

Let's switch to the bigger telescope and take a look.  The telescope we will use is the 0.5 meter Zeiss telescope at the Auckland Observatory.

See 0.5 meter Zeiss.

Step 8	Select a Telescope

select telescope -> 0.5 meter Zeiss

Enter in your lab book:

Telescope: 0.5 meter Zeiss

You should now see the Scorpion in the eyepiece.

Next, let's measure the temperature of the five stars that make up the Scorpion.  The stars are numbered in the star chart.

The Zeiss telescope is equipped with a photoelectric photometer that we can use to measure the color magnitudes of the stars.

See Zeiss Photometer.

Step 9	Select the photometer

select telescope -> Type 2 Photometer

Enter in your lab book:

Instrument: Type 2 Photometer

Step 10	Set the Slew Rate

set slew rate -> 10 arc-seconds

This will make the telescope move slower for more precise positioning.

Step 11	Point the telescope at star #1:

go to object -> Star #1

The star should be located near/within the eyepiece reticle.

The star needs to be positioned within the circle.  If it is not use the manual drive controls to move the star into the circle.  The star does not have to be precisely in the center.

If you have already cleared window 2 and have no way to point the telescope, simple repeat step 11.  Do not press the OK button until you have used the manual drive controls to position the star.

Step 12	Measure the brightness

make measurement

The photometer measures the brightness of the star in U, B and V and then calculates the V magnitude and B-V color index.

For star #1 your measurements should be something close to:

V: 11.37

B-V: 1.64

U-B: 1.26

If your measurements are a lot different, go back to step #11 and check that you are measuring the correct star.

Step 13	Record the data

Enter in your lab book:

Star #1

B-V magnitude

Your lab book should look something like this [Lab book]:

Repeat steps 11, 12 and 13 for stars 2, 3, 4 and 5.

Here is a summary of the steps:

Step 11  [image]  Go To star n

Step 12  [image]  Measure

Step 13  Enter the star # and B-V in your lab book.

Continue


The Scorpion

The Scorpion is a small group of stars that resemble a scorpion.  The center of the Scorpion is located at coordinates:

RA  09h 12m 57s

DEC 13° 14' 34" N

The Scorpion is difficult to see in a small aperture telescope.

The stars range in brightness from 11th to 12th magnitude.

It is best viewed using a telescope with an aperture of 0.5 meter using a 40mm eyepiece with a 25° apparent field of view.

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Data analysis

Calculate the temperature of the five stars using the following formula:

T (°K) = 7,200 / ([B-V] + 0.64)

For each star, replace the [B-V] color index in the formula with the value you measured at the telescope.

For example, for Star #1, if you measured a [B-V] color index of 1.64 then calculate the temperature as:

T (°K)	= 7,200 / ([B-V] + 0.64)

= 7,200 / (1.64 + 0.64)

= 7,200 / (2.28)

= 3,160° K

Enter the temperature in your lab book:

	Star	[B-V] Color Index	Temperature
	------	----------------	------------------
	#1	1.64	3,160
	#2
	#3
	#4
	#5
Do the same for the other four stars.

Continue


Additional Study

1.	Is our galaxy populated more with hot stars, cool stars, or stars like our own sun?

To find out, use the telescope and photometer to measure the [B-V] color index of several stars at different locations in the sky.  Calculate their temperatures and count the number of stars at the different temperatures.

2.	Does the temperature of normal stars change over the years?

To find out, use the telescope and photometer to measure the [B-V] color index of several stars from the year 1900 to 2099 (200 years).  Graph your results.

3.	Measure the [B-V] color index of the five Scorpion stars on March 15, 1995 at 14:00 UTC.  Compare with the results measured at 10:00 UTC.  Is there a difference?  Why?

Done


Introduction

Astronomers learn about the universe by analyzing photons of light that have traveled thousands of light years to reach the earth.  In this lab exercise you will see how an astronomer uses the telescope to determine the temperature of a star.

Background material:

Star Temperature

Photoelectric Photometer

UBV system

Continue


Auckland Observatory

The Auckland Observatory, New Zealand, is situated in a pleasant pastoral setting on the south western slopes of Maungakiekie, more commonly known as One Tree Hill.

It is about ten kilometers from the center of Auckland, population of about 1,000,000.  Industrial emission control laws, introduced in the 1970's, give Auckland a very clear atmosphere which allows accurate photometry to be undertaken.

Winds are light and the night time temperatures range from 5°C in winter to 20°C in summer.

The Observatory receives just over 20,000 visitors each year and the telescope is open for public viewing two nights per week.  In addition, special groups are hosted on other nights.  School children visit during the day.

A Planetarium (also being built by Zeiss) with a 12 meter dome is to be built in the next year at which time the Observatory building will be significantly enlarged.

For general information contact:

The Director

Auckland Observatory

P.O. Box 24-180

Royal Oak, Auckland

New Zealand

Phone/Fax:  64-9-624-1246

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0.5 meter Zeiss Telescope

The Zeiss 0.5 meter telescope was installed in July, 1967 and was manufactured by Carl Zeiss of Zena.

The telescope was purchased with funds from a bequest from Edith Winstone Blackwell.

Optical specifications:

Zeiss Cassegrain:

50 cm aperture

f13.3 cassegrain

f5 primary

Finder telescope

110 mm aperture

f6.82 refractor

40 mm eyepiece

Guide telescope

150 mm aperture

f/15 refractor

20 mm eyepiece

Variable star research is the main observing activity.  In particular, UBV photoelectric photometry has been undertaken since 1969.

Measurements have been made of a large number of variable stars (mainly UBV) of many different types.

Studies have been made of dwarf novae during outburst, eclipsing binaries and short period pulsating stars of the delta Scuti class.

A variety of southern long period variables have been followed and many reliable sequences established for visual observers.

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Type 2 Photometer

This Type 1 Photometer is used for the study of variable stars.

Specifications:

Viewer

32 mm fl

20 arc-min f/view

Microscope eyepiece

25 mm fl

266 x

Aperture diameters

20 arc-secs

30 arc-secs

40 arc-secs

60 arc-secs

(30 arc-secs normally used)

Transformation coeff's

V	-0.075

B-V	1.076

U-B	0.985

Background Sky

V	18.9

B-V	19.6

U-B	19.5

Extinction coefficient range

k'v	0.18 - 0.25

k'bv	0.08 - 0.14

k'ub	0.25 - 0.45

k"bv	-0.035

k"ub	-0.01

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star chart
Scorpion Star Chart


Test Display

If you are using a 16 color display adapter you do not need to run the display test Skip Test.

For 256 color display adapters TS-24 uses a palette of 100 gray scale levels to draw the stars in the eyepiece and CCD camera.  The faintest stars visible are nearly black while the brightest stars are full intensity white.

How the sky looks through the eyepiece depends on:

the monitor brightness and contrast settings

the display adapter

ambient light surrounding the display

To adjust for this TS-24 has a Test switch that will display the 100 gray scale levels in the eyepiece.  Click the following icon to see the test pattern:

Adjust the intensity of your monitor so that the left edge of the test pattern is a very faint white next to the white vertical bar.  The right side will always be bright enough.

If the display is not set correctly you may not see the actual views you would see if looking through a comparable telescope.

When you are done, click the Test icon above once more to clear the test pattern.

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Null 1


Null 2


Null 3


Null 4

Done

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